GTA
Support Equipment For PCP/HPA/CO2 and springers ,rams => 3D printing and files => Topic started by: Mr.P on October 31, 2022, 12:37:07 PM
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I’ve been enjoying my .357 evanix Rex, which is powerful and quiet compared to my previous foray into .357 airguns.
It still puts a hurt on my ears and I don’t like wearing protection if I can safely avoid it.
The Rex has a really large shroud with lots of volume but it is purely cosmetic and non-functional, and this pisses me off more than it should. I think it is a missed opportunity. I have a can I can put on the end but there are a few reasons I don’t like that solution either so I am going to try making the shroud into something functional, allowing air to expand back into. 3d printer makes it possible!
Problems to fix:
1) front bushing isn’t vented
2) shroud has a bunch of large holes drilled in it for looks rendering it useless
3) end cap isn’t contoured to shunt air backwards
So I am printing a vented barrel bushing, a long tube-like insert to nest in the bottom part of the shroud and block the holes, and a baffle for the small chamber at the end which I hope will strip and force air backwards.
The baffle I think can be improved - I will likely redesign that part.
Two pics attached, one of the parts in tinker cad and another of the shroud. Right now I’m just printing prototypes to test for a fit. I may try to incorporate some O-ring grooves to get better seals against the shroud in future iterations
My hope is to get more of a muffled report, not aiming for anything other than “shooter friendly”.
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Marco,
Where does the barrel muzzle end relative to the shroud muzzle, stock?
There seems to be a lot of shroud volume ahead of the perforations. An internal sleeve to also shutter the perforations should add up to a huge reflex volume.
It looks like the shroud ID is large enough to make a smooth air stripper to reverse the air flow, and feed into the rear of the shroud.
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There is a small chamber at the end - about 33 mm between the end of the barrel and the Donnyfl end cap.
The first prototype print was effective fit well enough and my second iteration is printing. The report is greatly reduced by the reflex inserts, and the tank ping and mechanical / hammer sounds are now the most noticeable aspect of the report. Definitely did not expect it to be this effective because there is so little volume in front of the muzzle.
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Meant to say in that last post that I am unsure of the ideal design for the baffle that sits in that 33mm space at then end.
Right now what I have is contoured / curved at the end of the baffle and the “stripper” is a short tube the pellet passes through. Not sure what the ideal design would be as there is not a lot of room.
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I updated the design of the baffle and barrel bushing again to more effectively push air back into the shroud (or so I imagine). Here’s what they look like now:
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Marco,
The initial uncorking of the projectile is what produces the sharpest "pop". Your Evanix probably has quite a significant muzzle pressure and volume. Your modification has provided room for that air to expand near the source (muzzle), and put a wall in the way of the pressure wave to reflect it back. Because the Evanix was objectionably loud, reducing the blast by half would bring noticeable relief; even if it is still very loud compared to a stock Beeman R7, or Benjamin Marauder.
The thinned out barrel support should offer better flow, but the sharp edges at the OD look flimsy. I have modelled one that maximizes flow area, without becoming too thin at the OD, because a picture is self explanatory.
As for the best configuration for an air stripper; that depends also on the rest of the system design. I have attached a cross section of a two stage reflex air stripper that is intended to smoothly guide the air in front of the barrel muzzle, round towards the rear shroud cavity. There are two stages to limit turbulence at the muzzle, yet provide lots of flow area via the second stripper. This is made possible because of the diameter of the device. For a smaller ID shroud, I would attempt only one such air stripper. The design can be FDM printed as the barrel support spokes also serve as printing "supports".
There are many configurations that all work well enough, so I would be arrogant to suggest that one approach is always better. A flat wall with a sharp edge hole works by stalling the air and converting its momentum to pressure. That pressure then feeds back into the rear of the shroud. A conical air stripper should reduce turbulence right at the baffle bore. If it is curved, it can be used to convert the air's forward momentum to rearward momentum more directly. Note that my air stripper design partitions and guides the air flow on the inside of the bend, so to speak.
As for the length of the "pipe" at each air stripper. The longer that is, the more effective at holding air back. But the greater the risk of the baffle bore "steering" the projectile. The latter effect can be reduced by increasing the baffle bore diameter. But, making something so much better than you have to back off another aspect to prevent it from being worse seems silly.
I generally don't make the parallel section of a baffle bore more than a caliber long, or the bearing length of the standard projectile. I make the radial baffle bore to projectile diameter clearance 1 mm for close baffles, and increase the gap for forward baffles, if there is any doubt about the barrel bore to baffle bore concentricity or parallelism. Or if the vibration in the system is significant on firing. As measured baffle bore is what matters, rather than just the CAD dimension.
Your purple front air stripper looks like it should work. If I am seeing it correctly, the walls look a bit thin at the leading edge. That may be because it is in "ghost view". I find viewing "solid" parts that are sectioned easier to understand.
The slanted red lines in the bottom image represent a thread for the barrel stud.
I am pretty sure you are having fun designing your parts. Else I would offer to design some bits for you to try out. For that, I would need some key dimensions.
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Subscriber, thanks for your input - greatly appreciated.
Your bushing design seems optimal - I may try that on the next iteration because the centering was off on the fin bushing and I can’t use it… tinkercad determines the center of concave shapes like the cross section of that fin bushing in a weird and unhelpful way that i didn’t notice until I printed.
Believe it or not the fin bushing is not flimsy, though I agree it looks like it in the picture. The fin’s edge is about a mm thick and I printed at 100% infill so it’s “Lego brick solid” and I would not want to step on it in bare feet in the middle of the night.
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One mm is wide enough to be sturdy because of the taper. It looked like a knife edge to me in a previous image. I made the fins 2.5 mm thick and parallel, except for a 1 mm radius either side of the root.
I edited the above post, so you might have missed some statements I added later...
If you provide the OD, ID and length, I can attach such an STL here...
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Marco,
I have integrated the forward 33 mm long section with curved air stripper into my barrel support design. To make the part 3D printable with shorter bridged sections, I doubled the number of "spokes" and made them half as wide.
I assumed a number of dimensions, including a barrel/stud diameter of 18 mm. Please provide the correct dimensions and I will update the part. You can decide if you want to print it or not at your convenience.
The red lined image shows assumed dimensions. Please let me know what dimensions should be.
The part needs to stand on the printer platen, on its muzzle end, for printing so that the barrel support spokes act as printing supports.
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Hey there - that looks interesting.
The barrel is 14.5 mm diameter and my parts seem to print a little undersized so the parts should be 34mm diameter to fit well in the shroud.
33 mm from muzzle end to end of chamber.
My barrel bushings are fashioned like the one that came with the Rex - there is a 14.5 mm bore that the barrel slides into but for the last 3 mm it narrows to about 11 mm so the barrel stops and catches at the shoulder where the hole narrows. The bushing is 30mm long. It is press fit, no threads.
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Thanks, Marco
What is the length of the barrel channel? See image below
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27 mm, total length of part is 30mm
I don’t see any reason why the part is 30 mm and not 25 or 35 as it will center the barrel either way. I just went with the same size as the original
Also as you know the particulars of the “shoulder” don’t matter as long as there is some kind of stop to seat against.
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Updated part shown below, with STL print file attached (inside ZIP).
The total combined part length is 50 mm. The barrel seat is just over one diameter deep, using the 14.5 and 11 mm diameters provided. The barrel muzzle shoulder length to front face of part measures 33 mm.
For greater stability I extended the thin (1 mm) section of the 34 mm OD "casing" all the way to the rear of the part.
I could make a version where the 14.5 mm barrel socket tapers down to perhaps 14.45 mm, or any dimension you deem would take the slack out, without breaking anything.
Due to file size, I had to zip the STL.
Note print direction is a must...
For interest, the barrel muzzle face to leading air stripper cone face worked out to 21.25 mm. This is probably about a projectile length, or bearing length, depending on weight. A reasonable stand off distance, for the first air stripper that has to take the brunt of the blast.
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Version 2 of this part has a lightly longer airstripper cone, with a slightly sharper leading edge. The parallel bore is slightly longer now, at 9 mm. Also, the barrel muzzle face to cone face distance is 20 mm. If you have not yet started printing the first version, I recommend this version to try out first.
The difference is subtle, but closer to my original intent.
STL attached as a zip file.
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Version 3 is just like version 2, with more radial space for the air to expand just after it leaves the barrel muzzle.
Zipped STL attached
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These look great - will get one on the printer today!
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In progress. For some reason with this print the skirt didn’t really stick to the bed I guess I need to adjust my Z offset. I don’t think this will affect the print in a meaningful way.
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I get a strange satisfaction from seeing that thing "in the plastic", Marco! Hope it performs.
Now imagine adding an integrated nose to the part that contains another one or two baffle chambers, with the appropriate thread integrated into the OD. Or, one could use the thread adaptor to add nose section by screwing it on separately.
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I have a Rex-P in .357, same basic issue
I had been sketching out some ideas, but before I could do serious work on this, I bought a Bulldog. So the pistol has been side-lined for a while
My idea is based on getting rid of the inner shroud, outer shroud and making adapter rings to go over the existing shroud mount at the rear, while making from scratch the entire front mount
I purchased some steel tube that's almost the exact same OD as the Ronin I bought for this and was planning on having everything designed so that with the Ronin mounted, there would be no gaps or dips anywhere in the silhouette
I'd want some venting at the rear of the new tube, but otherwise no holes on that
I'll have to dig up the sketches I drew, but I think the front mount would be best with only three ribs, shaped for airflow
I have a stock Sidewinder X2 with some black Kodak PLA+, and I'm using Fusion360 & Cura
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An integrated design enables optimization, Phil.
I am amused by shrouds with vents at the rear to let air out - even if so many respected experts swear by them. Any holes in the rear of the shroud (or LDC) that reduce the noise coming off the muzzle in a meaningful way, increase noise to the rear and sides, in a potentially annoying way. Otherwise there is no way that enough air could flow out the rear fast enough to make much of a difference at the muzzle end. A handful of tiny shroud holes work like racing stripes and coffee-can exhaust tips to make Civics faster...
The above is not aimed at you Phil, but at the experts that measure sound level at the muzzle to prove their shroud holes reduced report by full 2 dB. The ones that claim more than that don't measure at the shooters ear; for good reason. Now, if the shroud had two concentric tubes, spaced perhaps 5 mm apart, with the inner tube 50% perforated with 2 mm diameter holes; then the holes would be useful. So long as they don't vent to the outside.
Do not be discouraged by my rant. Experimentation is a major route to discovery. Thanks for sharing your plan.
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Phil,
You are correct that three spokes to center the barrel are optimal. They are strongly self centering, with the least flow area loss, and least surface area skin friction.
The reason why I used 20 spokes in my design is because I combined two parts. In order to print them without internal supports turned on, the spokes themselves form printer support with short bridging spans.
I combined parts because that way there is no question about their alignment. Splitting the parts should enable better air flow designs, although the total reflex flow area in the above design is large, compared to many factory designs. More flow area is generally better, but it matter a lot more when flow area is a primary restriction.
My suggestion is that you explore making your custom shroud ID a little larger than stock. Not just to increase its internal volume, but because persuading the air leaving the barrel muzzle to make a "u-turn", without massive turbulence restricting reflex flow is much easier if design aspects have more room.
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No worries, I've already done the rear vent idea and I understand what you're saying. But even if it's a matter of re-directing noise and not simply reducing it overall, it can still be an advantage when done thoughtfully. In my case, I have a 1st gen .22 Gauntlet with the barrel band upgrade. I used a dremel and cutoff disks to place a pair of slots at the rear of the shroud where the airblast is directed down over the lower section of the action into the stock. There's a lot of room in that area. I do feel some air coming out of that area, and when I have my ear to the side of the gun, I can hear the discharge. But it's not very loud in comparison to the rest of the shot. I think that could be better if the stock wasn't wide open to the outside in that area. I also think the slots were much too big. I've been considering the vent holes in both the Ronin and a Emperor I have on a the Bulldog. Those holes are very small, yet seem to be flowing well enough
As for my Rex-P, rear venting will be put off until a decent baseline is established without it. And I think DonnyFL might have the advantage on R&D here, so I'll copy the Ronin's vent's. I'll need to buy a drill bit, it's smaller than anything I have at the moment
More important is whether or not I'll finally do anything with the tube I bought for the Rex-P. I'd start drawing up something right now, but I need to get that tube out of storage and measure it's ID
I'm attaching a sketch of my current setup, with DonnyFL adapter & Ronin. I'm not drawing the Ronin's end cap to detail, it has some beveled features. I'm also not drawing in the outer shroud, as that isn't going to be part of the final design anyways
One of the things I don't like about the Rex-P is that the inner shroud does not have a bushing to keep it centered to the barrel. The only thing keeping the front from drooping are a pair of setscrews at the rear of the shroud, and set on the sides so not even holding in a vertical plane. There's a few other things I don't like on it, mostly to do with the trigger group. But related to that, the instructions for the trigger adjustment screw are dangerously wrong
I don't know if the Rex is the same, but on the Rex-P, the trigger adjustment screw does not alter the trigger pull force. What it does is adjust how much sear engagement there is, so it's a way to reduce the travel on the release. But it's also a way to make the gun very unsafe when adjusted too far. I've done a few things to improve the trigger, but after setting a decent amount of sear engagement, I've epoxied the sear adjusting screw threads and cut it's head off. I added some washers on each side of the trigger to keep it from wobbling, as that changes how it interacts with the sear & thus the feel changes depending on how well you pull straight back. I've seen someone putting bushings into the holes for the trigger pivot pin, which could be a much better solution than what I did. If I were to re-do that, I would want to change too many things! I think I'd rather spend my time making my own design happen, than fixing someone elses
The insert I see sketched out above isn't really that bad, The thin leading edges of each rib wont reflect a lot of energy backwards as drawn & can always be rounded-off after printing. So it's a practical design. And if that resists flow to the rear, it can also resist reverse flow back to the front, though I would prefer an absorbent method to be used in the rear volume like using some ceramic muffler packing intended for rebuilding dirtbike exhausts
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Phil,
You are correct that three spokes to center the barrel are optimal. They are strongly self centering, with the least flow area loss, and least surface area skin friction.
The reason why I used 20 spokes in my design is because I combined two parts. In order to print them without internal supports turned on, the spokes themselves form printer support with short bridging spans.
I combined parts because that way there is no question about their alignment. Splitting the parts should enable better air flow designs, although the total reflex flow area in the above design is large, compared to many factory designs. More flow area is generally better, but it matter a lot more when flow area is a primary restriction.
My suggestion is that you explore making your custom shroud ID a little larger than stock. Not just to increase its internal volume, but because persuading the air leaving the barrel muzzle to make a "u-turn", without massive turbulence restricting reflex flow is much easier if design aspects have more room.
1st big change is that I want to move the DonnyFL adapter forwards about an inch or so. The tube I bought is long enough to move the end forwards over a foot if I wanted to, and I've been thinking about getting a longer barrel before I cut that new tube. So converting the Rex-P into a Rex. I've already asked the local factory rep about buying the Rex barrel, and it's too expensive. I've been pondering buying a Bulldog barrel. I need to do some measuring. Hopefully, the Bulldog barrel has a larger OD, well actually I'm hoping it's exactly the same - but know better! Then I'll need to buy a steady rest for my larger lathe
Just measured. Bulldog barrel OD is 0.643", Rex-P is 0.51". I think I might make some other changes so I wouldn't need to cut the entire barrel down. If I remember right, the Rex-P hammer spring had some problems with being too large. I think I made a bushing for it to keep it from binding on the inside of the hammer. It made 'crunchy' noises when re-arming the hammer and had very inconsistent velocities on the chrono until I worked on that
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I am printing v.3 of this (subscriber’s combined bushing and reflex baffle) tonight. I had some printer issues which I think I’ve solved. I tested a couple of different baffle and bushing designs of my own but all seem to have about the same sound reduction. Also judging by ear is challenging as the Rex is unregulated and each shot sounds slightly different, with the gun getting louder as tank pressure drops (due to valve staying open longer).
I guess I am not surprised the rear micro venting isn’t effective, but I had assumed they were there for a reason and I had imagined it was to maybe keep the baffles from “backing up” and the ldc flooding due to large air volume. perhaps without these vents the air trapped in the baffles would be reaching the same pressure as what’s in the bore and and so air would just be rushing through the the bore and out the end of the can in a less turbulent flow. I was thinking the holes kept pressure down and air moving into the baffles if that makes any sense. Sounds like it doesn’t. The other way it might work is if the “hiss” of air through holes is less perceptible during a shot than that same air popping out the end. Again, this is speculation not science.
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Phil,
That sounds like a big project, with potential "mission creep" making it harder to finish. The fact that you lay all the parts out in CAD should help prevent surprises.
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Phil,
That sounds like a big project, with potential "mission creep" making it harder to finish. The fact that you lay all the parts out in CAD should help prevent surprises.
Well, changing out the barrel is an 'extra'. The most critical issue is how little room I have to work with in getting air to u-turn & run back into the shroud. The DonnyFL adapter is going to get moved forwards no matter what I do with the barrel
There's only a half-inch spherical void between the muzzle and the throat of the adapter. The simplest thing I can do is add a tube inside the adapter, necked down to catch & strip the airflow. But that still needs some room to flow outwards. The measuring of all the parts is easy, and I'm using Deltacad for 2D shop sketches. That's a really quick & easy software to work with. I use Deltacad to draw profiles I then rotate inside Fusion360, for complex geometries. Fusion360's sketch tools are really crude and a bit non-intuitive
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I took a look at the V 3 STL & realized the barrel OD isn't the same as what I had drawn, so I re-measured my Rex-P barrel with a metric caliper. Seems I made a mistake a while back, darnit! It would've been really weird for the pistol to have a different OD. And somehow, I managed to get almost all the rest of it wrong. So I'm redrawing my sketch
I hadn't noticed the part had been refined like it is, guess I could've read the entire build to begin with. I would run that as it is, but it needs to be larger. The part OD should be approximately 47.2mm to fit the tube I have. It's placing the DonnFL adapter at a good distance forwards in comparison to what I have here, so If I cut my 2" tube to match, I think it'll work out just fine. I just have to get the part's OD larger
I've had pretty good luck printing threads, so I might just draw from scratch and elliminate the DonnyFL adapter, make the part do that also. Threads have been looking rough, but work just fine anyways
I'm going to test run the rear bushing, for fitting the tube I have, it's a shallow part that should print quickly
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Phil,
I made a 47.2 mm OD version of V3 for you.
STL attached below. Had to zip the file for GTA to accept the size:
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Phil,
I made a 47.2 mm OD version of V3 for you.
STL attached below. Had to zip the file for GTA to accept the size:
Oh, nice! I hope my estimate of the size works out. I'm printing a rear adapter right now. It has 27x1 internal threads, hopefully that will play nice with the threads on the barrel bushing that the inner shroud screws onto
If my rear adapter fits, then you've saved me a bit of work
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Don't know if I am confused, but my part for you above seems longer than required Phil. So, here is a shortened version of the 47.2 mm OD part, with the barrel seat one barrel diameter long.
STL attached as zip file.
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Don't know if I am confused, but my part for you above seems longer than required Phil. So, here is a shortened version of the 47.2 mm OD part, with the barrel seat one barrel diameter long.
STL attached as zip file.
Actually, that's probably better. There's only 63.8mm of barrel ahead of the barrel bushing. The bushing is about 25mm OD, so it uses up a lot of the volume available
Another thing, I had forgotten the tube I bought is actually aluminum, not steel. Which is why it has a 1.6mm wall
Just remembered, I'm going to need to mod that file so it has some 18x1 threads in front. Without the drawing history or original sketches, it gets a bit harder. I've only been drawing in 3D 8 months now. Fusion360 doesn't want to delete features on an imported STL, I think there ought to be a way to do that, but it's not something I've figured out yet
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This spiral baffle part is intended to be placed in the rear of the shroud. The idea is to damp reflected pressure waves like a depinger. This, while allowing air to flow easier from the front of shroud to rear, than from the rear to the front. Note vane slant asymmetry.
The spiral ID is 14.6 mm to slip onto the barrel freely. It has a 25 mm OD so air can also flow outside of the spiral. The part has aligning features to "clock" the vanes. One could then glue the faces for more integrity.
The length is 37.5 mm so that the print time for one part is not too long. You could use as many or as few in a stack as you like. Also, shorter parts are less likely to suffer adhesion problems. It should be printed with the three alignment holes down on the platen.
STL file attached.
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The spiral is an interesting idea.
I was working on my Rex reflex modification a bit yesterday. The monocore from subscriber definitely fits and is also effective in cutting the report of the Rex to the point where i don’t feel a need for ear protection.
Because I had the time, I designed some baffles that slide over the barrel between the bushing/baffle and the sleeve at the other end of the shroud. If stacked the right way these will allow air to flow into the baffle more easily than out of it. Since they will be taking up volume in the shroud I assume they might end up just making everything noisier, so I may end up not printing them.
So far all of the designs pretty much same loudness as the prototype so I’m not convinced that small changes are going to make a difference after the initial venting. I imagine to get an audible level of reduction of noise beyond that you would need to add a pickle on the end.
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Marco,
After thinking about the spiral some more, I figured that integrating it into the printed tube you used to mask off the large holes at the rear of the shroud would be better. Air entering the spiral would be spun up and hug the ID of the shroud via centripetal force. That air would need to substantially reverse its spin direction to easily leave the shroud. Such direction change would take time. Stretching the event duration to make it more gentle is the name of the game. Like a soft rubber recoil pad on a 12 gauge.
I would not fill the rear of the shroud with spiral, but leave a "chamber" at the back end where the air can dwell while spinning, before expanding forwards, where it should be subject to some delay. It would not be necessary to fill the forward section beyond the shroud holes with spiral either. So, not using up much more shroud volume than now.
That said, adding what amounts to a depinger to the shroud, is like seeking to decrease spur gear noise in a race car that still sports a "straight pipe" exhaust. Your system is begging for expansion volume ahead of the muzzle. Confining that blast to a small chamber that by definition would operate at significant pressure, and thus would need to be need to be strong.
By making that forward section one large expansion volume perhaps 2.5" long ahead of the end of the shroud would significantly reduce the pressure by virtue of the expansion volume.
My guess is that the reduction in noise would be as significant as that obtained by the air stripper setup you have now.
The "nose" section would have a 34mm OD, with a much thicker wall on the forward section.
The bore at the air stripper would be made on the wider side, rather than super tight. The section would provide a generous chamber before the air gets "out". To design such a thing I would need to know the thread diameter and pitch of the endcap to shroud used now. It may be something like 35 or 36 mm by 1 or 1.5 mm pitch. The thread depth into the shroud would be useful, so as to use as much as possible engagement.
It would be up to you if you want to try it. Of course, a thin walled aluminum tube could be slipped over and glued to the nose to contain the pressure with greater certainty.
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“ Air entering the spiral would be spun up and hug the ID of the shroud via centripetal force. That air would need to substantially reverse its spin direction to easily leave the shroud. ”
That makes sense.
“ That said, adding what amounts to a depinger to the shroud, is like seeking to decrease spur gear noise in a race car that still sports a "straight pipe" exhaust. Your system is begging for expansion volume ahead of the muzzle. Confining that blast to a small chamber that by definition would operate at significant pressure, and thus would need to be need to be strong.
By making that forward section one large expansion volume perhaps 2.5" long ahead of the end of the shroud would significantly reduce the pressure by virtue of the expansion volume.”
This also seems like solid reasoning and would be worth trying. I lack a suitable “can” to create that extra chamber and I fear plastic-only solutions at 150+ fpe
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One would need to "proof" a plastic only section before you can trust it. It is not the number of FPE that matters, but the pressure and to a lesser extent the volume of the air that is handled.
What is the barrel length on that .357? I will attempt some rudimentary calculations to see what the muzzle pressure is - compared to something like a 30 FPE .22. Yes, average and muzzle pressures are higher, as your air tank pressure comes down, compared to when the tank is full - paradoxically.
Scaled by bore volume, an 80 FPE .357 needs the same average barrel bore pressure as a 30 FPE .22, using the same barrel length. If your .357 is producing 200 FPE from a shorter barrel, then the average pressure has to scale up proportionally. Larger diameter "cans" need thicker walls than slim one, assuming the same pressure applied. Pressure applied drops with expansion chamber volume.
Muzzle pressure is normally less than average pressure (if the valve closes before the projectile reaches the muzzle), but making can parts strong enough to handle average barrel pressure is prudent. Muzzle pressure cannot be greater than average pressure.
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Marco,
I turned the inner spiral into an outer spiral, with a much slower helix - so the air can follow it more easily on the way in. Then combined the part with the tube insert you made, to mask the large vent holes at the rear of the shroud. I assumed a tube length of 165 mm and could easily adjust that, if needed.
The tube OD is 34 mm, with a 14.5 mm hole at the base end. The part is designed to be printed with the barrel hole end placed on the printer platen. The vanes start and end with slopes so they have no un-printable overhangs.
The radial gap between vane ID and barrel OD is 2.63 mm. While this seems tight, the open spiral has lots of space between vanes. The length of full section vanes each have one third revolution, so one vane only just shadows the others in terms of resisting direct pressure wave reflections.
The slant to the vanes more efficiently spins up the air as it moves back into the shroud; while also "wedging" the air into the narrow gap between barrel OD and vane ID on forward rebound, due to the conflict in rotational direction between air and vane spiral.
The open volume at the rear end is intended to take spinning air, and provide some distance and time for it to rebound into the rear of the vanes. Note that the linear travel direction of the bolus of pressurized air reverses, while the spin direction does not. Hence, that bolus of air rebounds into the spiral while spinning "backwards". This should make the spiral act as an impediment to forward flow, until the bolus spin direction stops and reverses.
If nothing else, the spiral vanes should act as effective shroud depinger...
STL attached
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Fusion360 doesn't want to delete features on an imported STL, I think there ought to be a way to do that, but it's not something I've figured out yet
I use Solidworks. The only way I can delete features from an STL is to convert it to a part file, then cut the features off using one of the regular forming operations. In the same way, I can add features. However, often the X, Y & Z axiis are shifted on such imported parts. That can limit some operations, such as revolving a profile sketch about the bore axis. A pity, because I find that the most efficient profile generation for the parts in this thread.
I would often create a new part from scratch, based on dimensions taken from an STL file. It is faster than reworking STLs, and I can more easily get the shape I want. And more easily modify it later.
Starting with a STEP file is preferred to starting from an STL; but modifying them is also a lot of work. How ever, STEP files can be created and read by all 3D CAD packages. So measuring key dimensions off them is easy.
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Fusion360 doesn't want to delete features on an imported STL, I think there ought to be a way to do that, but it's not something I've figured out yet
I use Solidworks. The only way I can delete features from an STL is to convert it to a part file, then cut the features off using one of the regular forming operations. In the same way, I can add features. However, often the X, Y & Z axiis are shifted on such imported parts. That can limit some operations, such as revolving a profile sketch about the bore axis. A pity, because I find that the most efficient profile generation for the parts in this thread.
I would often create a new part from scratch, based on dimensions taken from an STL file. It is faster than reworking STLs, and I can more easily get the shape I want. And more easily modify it later.
Starting with a STEP file is preferred to starting from an STL; but modifying them is also a lot of work. How ever, STEP files can be created and read by all 3D CAD packages. So measuring key dimensions off them is easy.
I think I'm going with, draw from scratch on that!
It'll have to wait, power went out for the area. 30mph gusts, and I'm hearing it could be another two days before I can power up the printer. All I have is my phone right now
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I know you are careful of PLA printed LDCs for 150+ FPE airguns, Marco. So, I did some calculations to "bless" or condemn the extended nose design I just did for your Evanix. That was for radial and axial loads and strength.
First, images below, so you can see what I am talking about. The nose section has a 5 mm thick wall, although I am probably going to increase that to 7.5 mm based on my calculations.
The worst case for muzzle pressure while producing 150 FPE is that the air tank pressure has dropped to 1250 PSI, and that valve stays open until projectile left muzzle.
There is no derating for the fact that air is expelled into the shroud, and from the bore of the device. So, a overly cautious mathematical model.
Assumed:
19" effective barrel length
average friction = 30 lb
PLA has a tensile strength of 7000 SPI along the length of the fiber, and a third of that between layers.
Thread is assumed 36 x 1 mm. Needs to be confirmed. Also length and placement need to be optimized.
As a 5 mm wall on the nose produces a bursting stress of 3000 PSI based on the above, and 971 PSI
Zipped STL for version with nose that has 5 mm wall thickness is attached below.
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This version has the nose wall thickness increased from 5 mm to 7 mm. Thus the radial burst stress in the plastic should drop from 3000 to 2000 PSI. Reasonable for material with a 7000+ PSI tensile rating. See conditional assumptions above, including 150 FPE performance at minimum practical reservoir pressure (valve closes as projectile leaves muzzle).
In the rear to front interlayer load direction, the stress would drop to less than 500 PSI.
The spacing between the barrel seat shoulder (1) and nose shoulder (2) needs to be refined. To meet the original design, (1) should make contact before (2), although no harm will come from making the thread stop screwing on (2). Also the actual thread used needs to be stated.
Zipped STL attached.
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Subscriber: your productivity is amazing and your willingness to help and offer your services just out of the spirit of shared enthusiasm is a great thing. Slick designs as well - I feel like a caveman using tinkercad… I just haven’t had the time or energy to climb the steep learning curve of better software. I may actually spend a few days going through the introductory tutorials for fusion 360 since I can get a non-commercial license without paying anything.
As far as my personal Rex rifle project goes, it is quiet enough as is.
I also have a rex p, which is way too loud to shoot without ear protection - that gun has a tiny expansion chamber and functional inner narrow shroud (no barrel bushings) but the volume is small and it can’t be helping much.
It occurs to me that if I remove the narrow inner shroud tube and create a sleeve to block the holes in the larger outer shroud, I could maybe quiet it down to “tolerable outdoors” level with minimal added length.
The sleeve could also extend beyond the muzzle and contain baffles - so it would be a monocore that slides over the barrel, and has reflex volume at one end and baffles at the other. Same diameter as the thinner shroud the gun came with so the only benefit would be adding a bit of length and a few baffles. Again, I am going for built-in design, not an add on moderator and only enough length to drop the noise to tolerable.
I think this is going to be the next thing I tackle with the printer. I’ll need to figure out how to firmly anchor the insert but the way the outer sleeve is designed and perforated suggests a couple of simple solutions that avoid having to design / print threads to match the Rex p receiver.
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Marco,
I don't consider myself as skilled or as efficient as a proper design engineer. Most of my working life, I had real designers working for me. That said, I tackle everything as if I am being paid for it. I am very much an ideas guy. Actual scale 3D CAD design just helps me to implement and evaluate spatial concepts in a way that is hard to do in one's mind alone.
I have developed many shortcuts to design, that might be possible in your design packages. For instance, I place more than one part in an assembly, then merge them into one body before saving as an STL. As long as two parts actually contact each other it is possible to seamlessly merge them into one part. While I designed the above 36 x 1 mm thread from scratch, I will often go to the McMaster-Carr site and download a nut or stud as a shortcut for the thread. Even so, I merged my 36 x 1 mm threaded sleeve, just like I do for imported nuts and studs.
That nut or stud is then "turned down", "bored out" or "milled" shorter in CAD to fit the custom design that required the thread. Then the borrowed threaded part is added to the assembly, merged into one body and saved as an STL. When saving the assembly as an STL, Solidworks asks if you want all bodies included. If the parts make contact or share some of the same volume, the original part boundaries are buried in the STL seamlessly. This is similar to movable assemblies printed together in one "part", except rather than actual clearance between parts in the STL, there is contact or overlap.
I generally take such borrowed threads and shave 0.1 mm off their flanks, OD and root diameter for 3D printing. Else, the threads are too tight.
I can see that your Evanix pistol needs help. It is likely that its muzzle pressure is higher than the rifle, even if the volume of air that needs handling is less. Your red line sketch should work to make a useful difference. Printing a thread with a 1 mm pitch over 10 mm OD is still "easy". My suggestion is that you explore how to do that. Threading only the "cap" from just ahead of the barrel muzzle would enable you to slip in a length of cheap plastic tube to cover the holes in the rear of the shroud, with just a short printed cap to block off flow around the barrel. Else, you end up with a tall narrow print, if you combine all features into one.
My biggest concern is the strength of the thread, and that the insert may blow off as a slow moving projectile. Now, I have used light friction to keep baffles in a 30 FPE .22 PCP, fully expecting them to go flying on the first shot. I was surprised by how little they moved on a single shot. My point is that the thread may not work as hard as we assume. Yes, larger calibers and higher pressures will generate more force; but a larger diameter thread has more area engaged.
It would be easy to print something to evaluate the thread strength. See image below. In concept it would be a part with the intended wall thickness and diameter, but with the forward exit (1) completely blocked off, leaving only the reflex space open to dissipate air pressure. Or, with both (1) and (2) blocked off, for a worst case strength test. Obviously the test would be via dry firing, as a projectile will punch its way out of the front...
If such a dry fire test with just the front (1) blocked off did not strip the plastic thread, then shooting one with a hole should be OK. The projectile is a loose "fit" in that hole and is moving fast, so not an equivalent plug. If you did such a dry fire plugged endcap test, I would point it at a trap from close by, until the pressure (no doubt a hiss) subsided. There is no O-ring at the thread, so there is at least a slow leak path. If you keep the reflex vents open, the leak around the barrel at the rear is significant (on top of the shroud expansion volume to diminish the pressure).
It is possible that the endcap might blow off after a one or two second delay. You certainly do not want to open the breech while the barrel is under pressure; nor allow the airgun to point casually. If the endcap flies off, it will not have as much energy as the projectile normally would, because the latter has the full barrel length to accelerate in - compared to the elongation of the material as it fractures (not much). Still, the cap blowing off has the potential to cause a bruise, an eye injury, or break something fragile - such as a window or light bulb or tube. Also, to potentially cause a scare that might undermine confidence in airguns in general, if the delayed event is not managed appropriately.
While my designs posted to this thread are motivated by your project, they are obviously available for anyone to print or modify into something else. I am aware that creating detailed "solutions" may not meet your goals, or if they do, may rob you of the satisfaction of making and using your own designs. So, perhaps helping you figure out how to design aspects for printing that seem daunting, would be more useful to you. It is just that doing a design seems the most convincing way of demonstrating that it can be done....
I don't use TinkerCAD or Fusion 360, but believe that you can find youtube video tutorials for almost anything. I find tutorial videos for Solidworks when I am stuck. I also have no problems finding or generating unofficial design shortcuts that would enable me to generate models of my ideas. Some of those shortcuts are probably also possible with other CAD packages. Merging parts into one piece is not actually a cheat in Solidworks. There is an official function to do that. Using that to add threads is a bit of a cheat, but who is complaining.
So, if you can tell me the endcap thread diameter and pitch I can design you a threaded sleeve that you can merge into your part design. Else, you may be able to add screws radially to hold your baffle stack in place. Similar to how valve bodies are contained.
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How to create threads in TinkerCAD?
https://www.youtube.com/watch?v=s1laczjEZOI (https://www.youtube.com/watch?v=s1laczjEZOI)
How to combine and separate parts in TinkerCAD?
https://www.youtube.com/watch?v=doStR4tUS_Y (https://www.youtube.com/watch?v=doStR4tUS_Y)
Splitting designs in TinkerCAD:
https://www.youtube.com/watch?v=ob3aSuATAcs (https://www.youtube.com/watch?v=ob3aSuATAcs)
https://www.youtube.com/watch?v=1SwSqROgLpY (https://www.youtube.com/watch?v=1SwSqROgLpY)
Merging by subtraction:
https://www.youtube.com/watch?v=NlHCvQrR_bk (https://www.youtube.com/watch?v=NlHCvQrR_bk)
Assemblies:
https://www.youtube.com/watch?v=_j6pcqLs5vw (https://www.youtube.com/watch?v=_j6pcqLs5vw)
Fusion 360:
https://www.youtube.com/watch?v=F1jZR10olss (https://www.youtube.com/watch?v=F1jZR10olss)
https://www.youtube.com/watch?v=N5parKTLVbY (https://www.youtube.com/watch?v=N5parKTLVbY)
https://www.youtube.com/watch?v=O1p7Sn1eMmE (https://www.youtube.com/watch?v=O1p7Sn1eMmE)
How strong is a 3D printed thread? Stronger than the bolt shank, apparently:
https://www.youtube.com/watch?v=ZiQek0wei1g (https://www.youtube.com/watch?v=ZiQek0wei1g)
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Marco,
I don't consider myself as skilled or as efficient as a proper design engineer. Most of my working life, I had real designers working for me. That said, I tackle everything as if I am being paid for it. I am very much an ideas guy. Actual scale 3D CAD design just helps me to implement and evaluate spatial concepts in a way that is hard to do in one's mind alone.
I have developed many shortcuts to design, that might be possible in your design packages. For instance, I place more than one part in an assembly, then merge them into one body before saving as an STL. As long as two parts actually contact each other it is possible to seamlessly merge them into one part. While I designed the above 36 x 1 mm thread from scratch, I will often go to the McMaster-Carr site and download a nut or stud as a shortcut for the thread. Even so, I merged my 36 x 1 mm threaded sleeve, just like I do for imported nuts and studs.
That nut or stud is then "turned down", "bored out" or "milled" shorter in CAD to fit the custom design that required the thread. Then the borrowed threaded part is added to the assembly, merged into one body and saved as an STL. When saving the assembly as an STL, Solidworks asks if you want all bodies included. If the parts make contact or share some of the same volume, the original part boundaries are buried in the STL seamlessly. This is similar to movable assemblies printed together in one "part", except rather than actual clearance between parts in the STL, there is contact or overlap.
I generally take such borrowed threads and shave 0.1 mm off their flanks, OD and root diameter for 3D printing. Else, the threads are too tight.
I can see that your Evanix pistol needs help. It is likely that its muzzle pressure is higher than the rifle, even if the volume of air that needs handling is less. Your red line sketch should work to make a useful difference. Printing a thread with a 1 mm pitch over 10 mm OD is still "easy". My suggestion is that you explore how to do that. Threading only the "cap" from just ahead of the barrel muzzle would enable you to slip in a length of cheap plastic tube to cover the holes in the rear of the shroud, with just a short printed cap to block off flow around the barrel. Else, you end up with a tall narrow print, if you combine all features into one.
My biggest concern is the strength of the thread, and that the insert may blow off as a slow moving projectile. Now, I have used light friction to keep baffles in a 30 FPE .22 PCP, fully expecting them to go flying on the first shot. I was surprised by how little they moved on a single shot. My point is that the thread may not work as hard as we assume. Yes, larger calibers and higher pressures will generate more force; but a larger diameter thread has more area engaged.
It would be easy to print something to evaluate the thread strength. See image below. In concept it would be a part with the intended wall thickness and diameter, but with the forward exit (1) completely blocked off, leaving only the reflex space open to dissipate air pressure. Or, with both (1) and (2) blocked off, for a worst case strength test. Obviously the test would be via dry firing, as a projectile will punch its way out of the front...
If such a dry fire test with just the front (1) blocked off did not strip the plastic thread, then shooting one with a hole should be OK. The projectile is a loose "fit" in that hole and is moving fast, so not an equivalent plug. If you did such a dry fire plugged endcap test, I would point it at a trap from close by, until the pressure (no doubt a hiss) subsided. There is no O-ring at the thread, so there is at least a slow leak path. If you keep the reflex vents open, the leak around the barrel at the rear is significant (on top of the shroud expansion volume to diminish the pressure).
It is possible that the endcap might blow off after a one or two second delay. You certainly do not want to open the breech while the barrel is under pressure; nor allow the airgun to point casually. If the endcap flies off, it will not have as much energy as the projectile normally would, because the latter has the full barrel length to accelerate in - compared to the elongation of the material as it fractures (not much). Still, the cap blowing off has the potential to cause a bruise, an eye injury, or break something fragile - such as a window or light bulb or tube. Also, to potentially cause a scare that might undermine confidence in airguns in general, if the delayed event is not managed appropriately.
While my designs posted to this thread are motivated by your project, they are obviously available for anyone to print or modify into something else. I am aware that creating detailed "solutions" may not meet your goals, or if they do, may rob you of the satisfaction of making and using your own designs. So, perhaps helping you figure out how to design aspects for printing that seem daunting, would be more useful to you. It is just that doing a design seems the most convincing way of demonstrating that it can be done....
I don't use TinkerCAD or Fusion 360, but believe that you can find youtube video tutorials for almost anything. I find tutorial videos for Solidworks when I am stuck. I also have no problems finding or generating unofficial design shortcuts that would enable me to generate models of my ideas. Some of those shortcuts are probably also possible with other CAD packages. Merging parts into one piece is not actually a cheat in Solidworks. There is an official function to do that. Using that to add threads is a bit of a cheat, but who is complaining.
So, if you can tell me the endcap thread diameter and pitch I can design you a threaded sleeve that you can merge into your part design. Else, you may be able to add screws radially to hold your baffle stack in place. Similar to how valve bodies are contained.
Just now got the power fixed, so the printer can run, and I can look at why my end cap didn't print with threads. It fits the tube I bought just perfectly, at least. I was expecting it might need some fine tuning, so I used some gray PLA+ that I have more of
That's a lot of tips there! Thanks, I'll definitely be busy looking into all that! The videos too
I've seen there's been some concern about plastic blowouts. Does any of that take into account the presence of a metal tube on the outside of the printed parts? What kind of burst pressure can a 1.6mm thick aluminum tube handle?
The rear end cap I drew up doesn't need to be as cut down as I did it. I was just testing the fit and wanted something quick to print. But the next one I print will be a solid puck. If this project fails, I would like it to fail forwards
Something I've been thinking about testing out, is wrapping a plastic tube with a low-stretch fiber, in order to improve it's pressure rating. Have you seen anything being done like this? Not counting carbon fiber tanks, but things more like these parts being discussed here
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Phil,
The concern is not about the plastic parts that are a snug fit inside metal tubes. The plastic won't fail because it will transfer load to the metal tube very early, at low pressure. The concern is about plastic pressure vessels with no metal tube around them (and perhaps capping them).
How much pressure a metal tube can handle depends on:
The material spec;
The ID;
The wall thickness.
So, the question about pressure and wall thickness cannot be answered without knowing the ID. The larger the ID, the thicker the wall required to stand a given air pressure.
To make my point, look at how high a pressure these small bore thin walled tubes can stand: https://www.mcmaster.com/pressure-tubing/extreme-pressure-miniature-stainless-steel-tubing/ (https://www.mcmaster.com/pressure-tubing/extreme-pressure-miniature-stainless-steel-tubing/)
Unless you wrap the plastic part very tightly, I think that any semi-stretchable fiber will give too much, to help a stiff plastic part. Wrapping the plastic part in a fiber that has greater stiffness should help, if the wrapping direction is in the right direction, and the fiber is then bonded (encapsulated), the way air tanks are wound. The fiber is stiff, but can be bent (wound) because it is so thin. The same principle is used to wind strong cable from thin flexible wire, that has great tensile strength.
Carbon fiber resin tubes are used often to stiffen barrels. For instance:
https://www.gatewaytoairguns.org/GTA/index.php?topic=202750.msg156382235#msg156382235 (https://www.gatewaytoairguns.org/GTA/index.php?topic=202750.msg156382235#msg156382235)
https://www.youtube.com/watch?v=SCT1st45JSE (https://www.youtube.com/watch?v=SCT1st45JSE)
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Thanks for the tips subscriber. You’ve shared a lot of designs and suggestions and I have some catching up to do.
I hope to put some time into the Rex P sleeve over the next day or two. You’re right that I enjoy trying to cobble together my own designs. Maybe I should get over it already but every time I send a design to the printer I am just amazed that the technology exists, and that I’m sitting here one minute whipping something up in some design software and the next watching a functional object grow on the build plate. It is just amazing and i feel lucky to be alive at this time.
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For the Rex P, A threaded insert is not needed. One detailed that didn’t come across was that the outer decorative shroud has a screw on collar that acts to center the inner shroud. that collar can be used to retain the sleeve and baffles.
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Phil,
I failed to make it clear that tube radial burst strength for a given material is proportional to the wall thickness and the inside diameter. While axial endcap retention strength is proportional to the wall thickness and the square of the inside diameter.
Thus, small diameter tubes fail in radial bursting before they fail axially; whereas after tube diameter reaches something like 2", axial thrust considerations may be the limiting factor. For even larger tube diameters, axial thrust dominates because D x D is always more than D. What also matters is that the resisting material areas also increase linearly for radial bursting, and based on the square of the ID and OD for endcap being blown off.
You asked specifically about 1.5 mm wall aluminum tubing. That wall thickness has only a few listings in McMaster, but look at 2 mm:
https://www.mcmaster.com/pressure-tubing/system-of-measurement~metric/material~aluminum/wall-thickness~2mm/ (https://www.mcmaster.com/pressure-tubing/system-of-measurement~metric/material~aluminum/wall-thickness~2mm/)
https://www.mcmaster.com/pressure-tubing/system-of-measurement~metric/wall-thickness~1-5mm/material~aluminum/ (https://www.mcmaster.com/pressure-tubing/system-of-measurement~metric/wall-thickness~1-5mm/material~aluminum/)
There are more examples if you include imperials sizes: https://www.mcmaster.com/pressure-tubing/general-purpose-aluminum-tubing/ (https://www.mcmaster.com/pressure-tubing/general-purpose-aluminum-tubing/)
The tubes in the sketch below, "A" shows material area resisting the radial bursting in red. The blue area in "B" shows the area applied for gas to act on, trying to cause radial rupture. "C" shows the resisting area to axial failure in red, with blue representing the area gas acts on to rupture the tube end to end.
I usually calculate safe working pressures from first principles in Excel, but am too lazy to do that right now. My post in a few pages back included such calculations, but it is not laid out to teach based on only the attached image. It would make more sense if explored as an active spreadsheet.
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417807;image)
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For the Rex P, A threaded insert is not needed. One detailed that didn’t come across was that the outer decorative shroud has a screw on collar that acts to center the inner shroud. that collar can be used to retain the sleeve and baffles.
That is useful.
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To continue about air pressure rupture strength, consider burst discs. They are mechanical fuses that prevent more serious, more expensive higher energy events. Note how thin the steel or copper discs are for the rated pressure. This is possible because of the small diameter, and thus low forces involved.
https://www.gatewaytoairguns.org/GTA/index.php?topic=162901.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=162901.0)
(https://photos.imageevent.com/wlleven/k10dtest/downloadphotos/Broken%20Disk%20sm.png)
https://northeastscubasupply.com/3-000-psi-sherwood-harrison-one-piece-burst-disc-assembly/ (https://northeastscubasupply.com/3-000-psi-sherwood-harrison-one-piece-burst-disc-assembly/)
This burst disc has a 3,000 psi working pressure and a 5,000psi blow.
https://www.actionvillage.com/Tippmann-Burst-Disk-3-000-PSI-p/tankpart-burstdisk-tip-3k.htm (https://www.actionvillage.com/Tippmann-Burst-Disk-3-000-PSI-p/tankpart-burstdisk-tip-3k.htm)
(https://cdn3.volusion.com/pgnrb.yxsdo/v/vspfiles/photos/TANKPART-BURSTDISK-TIP-3K-2T.jpg?v-cache=1553701468)
https://www.aplusmarine.com/Burst-Disk-and-Washer--3000-PSI_p_415.html (https://www.aplusmarine.com/Burst-Disk-and-Washer--3000-PSI_p_415.html)
https://www.pcptunes.com/airforce-burst-disk-5k-with-steel-body.html (https://www.pcptunes.com/airforce-burst-disk-5k-with-steel-body.html)
(http://)
https://www.silentexplorers.com/online-store/Burst-Disc-3-pc-kit-5000-PSI-p138150412 (https://www.silentexplorers.com/online-store/Burst-Disc-3-pc-kit-5000-PSI-p138150412)
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Phil,
I failed to make it clear that tube radial burst strength for a given material is proportional to the wall thickness and the inside diameter. While axial endcap retention strength is proportional to the wall thickness and the square of the inside diameter.
Thus, small diameter tubes fail in radial bursting before they fail axially; whereas after tube diameter reaches something like 2", axial thrust considerations may be the limiting factor. For even larger tube diameters, axial thrust dominates because D x D is always more than D. What also matters is that the resisting material areas also increase linearly for radial bursting, and based on the square of the ID and OD for endcap being blown off.
You asked specifically about 1.5 mm wall aluminum tubing. That wall thickness has only a few listings in McMaster, but look at
(deleting external links, which I can't post)
The tubes in the sketch below, "A" shows material area resisting the radial bursting in red. The blue area in "B" shows the area applied for gas to act on, trying to cause radial rupture. "C" shows the resisting area to axial failure in red, with blue representing the area gas acts on to rupture the tube end to end.
I usually calculate safe working pressures from first principles in Excel, but am too lazy to do that right now. My post in a few pages back included such calculations, but it is not laid out to teach based on only the attached image. It would make more sense if explored as an active spreadsheet.
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417807;image)
I made another mistake which I realized on taking the pistol apart. The inner shroud is actually pretty solidly set in place it's the decorative outer shroud that looks bogus. The inner shroud threads directly onto the barrel bushing with it's end solidly shouldered by the bushing, so it can't droop
I'll be changing all that, as I'm deleting all of the OEM shrouds
I think I should remove a flange that's part of the barrel bushing and turn a proper bushing out of aluminum. That way the rear end cap will be less of an issue for potential bursting. Only the complex air passages should be printed
And a reinforcing disk added to hold the front printed part from blowing off forwards
I've found some very interesting info on how to design a burst disk for industrial use. It's a huge jump in quality vs anything I've seen for airguns
Most of my design research was concerning NASA's light gas guns, for cross-over ideas to use on air rifles
I'm only recently seeing this approach being used on airguns. Can't recall offhand, and my phone isn't ideal for multi-tabbed research. I think it's the company that makes the Zeus. A two shot pepper spray pistol, uses a burst disk to control the shot cycle of a air cartridge. This solves the main issue with the Modoc rifles air cartridge design, which relied on a leaking o-ring on the pilot valve. That's why it would fire on it's own without warning
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Oh my goodness! One little detail I didn't notice ruined my test print on the rear bushing. Fusion360 has a checkbox to model threads, which is off by default. I hadn't realized even though they looked 'wrong', but otherwise looked like something had happened. Well, anyways, another test print. The 1st one wouldn't go onto the pistol because it was missing the threads
AEA Defender is the burst disk pistol I was trying to remember. That really needs to get applied to the Modoc air cartridge. Seems they've already done this, the AEA Harpoon. A whole 'nother thread topic I think. Anyways, the burst disk is being used as the 'pilot valve', to operate the spool valve inside the cartridge. An external hammer punches a hole in that disk to release air, which in turn allows the spool valve to open
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Phil,
You are lucky you can model threads. My basic version of SW does not have that ability. I have to model threads or import them.
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Phil,
You are lucky you can model threads. My basic version of SW does not have that ability. I have to model threads or import them.
You should try Fusion360. It's free with the limitation being that you can 'only' have 10 active projects at a time. I just mark projects 'read only' and then there's no limit to how many of those I have. Then I change one back to editable when I need it. I also don't get the fancy stuff like generative design, libraries and live support. But it's only $500 to get into that, instead of $5000 like everyone else
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I paid 4k for a stand alone license for Solidworks 2015. Until it is unable to do what I need, I think I have spent enough. I have more time than money...
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Currently printing a sleeve and baffle core for the Rex p. The section in front of the muzzle will be retained by the flange that screws on the outer shroud, but will be unsupported plastic beyond that… which I do not trust, I will be flinching every time I pull the trigger.
If any layers do not adhere well or form perfectly during the print, the part will eventually fail I think. The more I think about it the more I wish I had made thicker outer walls on this. Baffle design is as simple as can be for this first print.
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Marco,
Design up just the flange and first baffle section, with half the wall thickness you have on the full length version. Do another with a third wall thickness. Shoot the half wall thickness 5 times. If that does not fail, shoot the third wall thickness. Shoot it it until the pressure drops so far that the projectile velocity starts to drop. Repressurize and shoot it some more to check for fatigue.
If the half wall thickness survives, then your full wall thickness has twice the air pressure strength; and much greater bend stiffness (flop and flex resistance). If the third wall survives, then that should boost your confidence in the thicker version.
You could also make a short test piece with the muzzle cone blanked off solid. Then the expansion volume into the shroud and first baffle are all that provide pressure relief. Dry fire only on that version...
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Thanks for the suggestions. I wonder if, in a case like this, with a relatively slender (30mm) diameter, the function of the part that extends beyond the shroud might be more about “texture” and not so much about volume.
I imagine trying to create friction in the tube to resist airflow so air flows back into the shroud, as opposed to the baffles having space in which to divert and trap a quantity of air. Not sure if this makes sense in any fluid/aerodynamic sense however!
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Many reflex air strippers are flat. They stall the air that runs into them. That change in momentum increases the bulk air pressure in front of the wall. That high pressure air pocket then feeds back into the shroud. So, while a smooth curve is nice, a flat wall also works.
Even basic baffles act as restrictions to air exiting the LDC, over an empty tube. They also act as lenses that focus sound waves and flow.
There are reasons why LDC length "works" over an above adding volume. The simplest aspect is dwell time, but there are other factors that have to do with radial dispersion as a function of bulk pressure in any given zone of the LDC. The lower the bulk pressure, the less diameter helps and the more length helps. Very few suppressors taper down towards the muzzle, but I know Matt Dubber has one. Might be tricky to find which video has info on it...
There are overlapping mechanism for blast suppression. The basic even conical baffle stack with the distance between baffles about equal to the baffle "tube" ID, as used in the Marauder are a case in point.
What is better? That depends on how you assess the system, and how much you are willing to pay. Does a meter reading mean more than a subjective assessment of the tone? There is no single answer to that.
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Well here’s the Rex p with a shroud sleeve and that short baffle core I posted earlier.
This thing is loud - we will see if this makes a noticeable difference, I am not happy doing my breath. I also added a see-all sight, which I’m still trying to decide if I like or not on this air gun.
EDIT: proto type failed - some layer in the wall of the baffle section separated (no shattering of anything), launched the baffles which landed about 2 feet from the target 25 m away. Quite a bang.
I am re-printing with thicker walls that taper towards the front. I probably went too far the other way now…
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I paid 4k for a stand alone license for Solidworks 2015. Until it is unable to do what I need, I think I have spent enough. I have more time than money...
No need to spend more, just go with the free version
In addition to the default *.f3d, export also has: *.3mf, *.ipt, *.fbx, *.obj, *.skp, *.smt, *.stp/*.step, *.stl, *.usdz
You have to buy a license to get *.dwg, *.dxf, *.igs/*.iges, *.sat
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Well here’s the Rex p with a shroud sleeve and that short baffle core I posted earlier.
This thing is loud - we will see if this makes a noticeable difference, I am not happy doing my breath. I also added a see-all sight, which I’m still trying to decide if I like or not on this air gun.
EDIT: proto type failed - some layer in the wall of the baffle section separated (no shattering of anything), launched the baffles which landed about 2 feet from the target 25 m away. Quite a bang.
I am re-printing with thicker walls that taper towards the front. I probably went too far the other way now…
You're a step ahead of me, but I'll heed your experience accordingly! - at the moment, my threaded bushing goes on backwards perfectly fine, but it's cross-threading when I try it the right way around. I'm going to delete the rear portion of the threads because I can't run those in with the part on backwards, it bottoms out before it gets that far. Today I'll just cut the threads off on the 1st one with a lathe and see if that fixes it. I'm still planning on making the rear bushing out of aluminum, but that requires taking the barrel out so I can cut off a small flange. So until I get the front ready for testing, I'll just stay with the plastic rear just to hang the new tube in place
I have my SeeAll on a riser block set all the way forwards on the rail, and I put the reservoir under my cheek just like a cheek rest on a rifle. It works really well like that, the sight picture is perfect and the hold is much steadier on target. I'm getting inside one MOB (minute of beer can) at 50 yards
I cut an index mark into the SeeAll adjuster screws. But I keep forgetting which way they turn. I have the 1st gen version, which is opposite, and the company never thought it was necessary to indicate how to sight in. It was never intended to be used as a target sight, which is a shame because the sight picture is so much better than any other open sight. I'd like to find a riser block that has proper windage & elevation adjustments. But it might be better to just print one
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I appreciate your comments on the see-all as I have been trying to figure out where it would work best. I am going to try what you describe l because the Rex P hardly lends itself to a normal pistol hold and like you I end up doing a little mini carbine hold that is steadier than anything else I can manage. Having the gun close makes the “delta” appear pretty small, which actually
isn’t so bad for a couple of reasons, but I feel it is maybe a little smaller than practical or intended with the sight up close.
On the mark 2 they have a little arrow to show which way to turn the windage adjustment. For elevation I know clockwise raises impact point.
Minute of beer can at 50m with a rex p open sights sounds pretty danged good to me. I’ve just barely got it sighted in but I doubt I could pull that off. Would be fun to try however!
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I appreciate your comments on the see-all as I have been trying to figure out where it would work best. I am going to try what you describe l because the Rex P hardly lends itself to a normal pistol hold and like you I end up doing a little mini carbine hold that is steadier than anything else I can manage. Having the gun close makes the “delta” appear pretty small, which actually
isn’t so bad for a couple of reasons, but I feel it is maybe a little smaller than practical or intended with the sight up close.
On the mark 2 they have a little arrow to show which way to turn the windage adjustment. For elevation I know clockwise raises impact point.
Minute of beer can at 50m with a rex p open sights sounds pretty danged good to me. I’ve just barely got it sighted in but I doubt I could pull that off. Would be fun to try however!
Well, you have to re-air for each shot when shooting like that. I wish I had a regulated bottle for tethered shooting. And I'm using the FX hybrid slugs, which are very consistent & really nice for distance
It also helps that I worked on the trigger group. The pull is very smooth and much lighter than it used to be. I also rounded off the sharp edges on the face of the trigger so it fits nicer without distracting me. I added an insert at the front of the hammer spring to center it on the barrel, polished the outside contact areas on the spring and the inside of the hammer where the spring touches. That makes re-arming the hammer very smooth & the shot cycle is also smoother
I have the 2nd gen SeeAll, with the range compensating reticle. The windage marking is very hard to read. The triangle portion of the arrow barely looks like anything, so I have to keep reminding myself that it doesn't make sense for it to not be pointing away from the letter 'R'
In my Bulldog, the FX slug is dialed in at 30 yards and stays pretty flat from 20 to 70
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“ I added an insert at the front of the hammer spring to center it on the barrel, polished the outside contact areas on the spring and the inside of the hammer where the spring touches. That makes re-arming the hammer very smooth & the shot cycle is also smoother”
I have two rex receivers, one came with the Rex rifle and the other with the Rex p. The action on the pistol is much smoother, very nice smooth clicky feel like a well oiled machine. The rifle is grittier feeling, and maybe centering the spring would be a good fix. I’ve noticed that it’s smoothing out with time and I think it may just be that the pistol that I bought had been shot more times than the rifle. Got them both second hand…
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I am re-printing with thicker walls that taper towards the front.
That's the ticket. Add strength where required.
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“ I added an insert at the front of the hammer spring to center it on the barrel, polished the outside contact areas on the spring and the inside of the hammer where the spring touches. That makes re-arming the hammer very smooth & the shot cycle is also smoother”
I have two rex receivers, one came with the Rex rifle and the other with the Rex p. The action on the pistol is much smoother, very nice smooth clicky feel like a well oiled machine. The rifle is grittier feeling, and maybe centering the spring would be a good fix. I’ve noticed that it’s smoothing out with time and I think it may just be that the pistol that I bought had been shot more times than the rifle. Got them both second hand…
Best to skip all the shot cycles and polish the works right up front
I just took the pistol apart so I can cut the flange off the barrel bushing. The spring insert goes outside of the spring for centering, there's not enough clearance to have it on the inside of the spring. I'd forgotten, but I needed to take out a ton of slop. I'm not sure if a spacer was missing, or if the spring is shot, but it wouldn't fire at full pressure on the high power mode. My hammer spring spacer is about 38mm. The spring looks normal. It's free length is 104.5mm
I've been thinking about adding a regulated tube tank under the barrel, run a hard line back to the reservoir. I'd like to have the air line connect at the front of the reservoir. Then make a piston to go inside the reservoir so that the rear portion of the reservoir is a air-spring for the piston. The idea is to avoid a large pressure drop during the shot cycle until the exhaust valve can close. I think the hard line might require a metering jet so the front tank isn't wasting air
On the full-size Rex, I would place the main tank underneath the old tank and then the air connection could be a lot simpler
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Two things:
1) you mention an insert / spacer for the spring. You added this to add more compression to the spring correct? Are you doing this just to get the valve open at 250 bar, or are you trying to squeeze more power out of the gun? Also - how old is that spring? I know if you dig you will see people found various non-standard springs for this gun to replace the hammer spring. I am sure there is some risk of deforming the valve stem if a really heavy spring is added…
2) you mention your hammer was not hitting hard enough to open the valve all the way at full pressure. Does this tank have the stainless finish on the air cylinder near the top hat or does it have the darker blued finish?
I have one of each tank type and the silver one (which is newer) is harder to knock open for sure. When I first got it it was pretty much valve locked if I topped it off. It would shoot, but very underpowered for 1-2 shots. That’s no longer the case - something has smoothed out or broken in and now I get what feels like good power at 250 bar. I have not yet done any meaningful shooting over a chronograph with these evanix guns.
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I am re-printing with thicker walls that taper towards the front.
That's the ticket. Add strength where required.
This one works! Makes the Rex P much less noisy. Still loud, but I don’t feel a need for ear protection so that’s good. I can see a couple of areas to improve the seal in the shroud and will likely make another iteration of this even though it’s good enough.
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I'd like to have the air line connect at the front of the reservoir. Then make a piston to go inside the reservoir so that the rear portion of the reservoir is a air-spring for the piston. The idea is to avoid a large pressure drop during the shot cycle until the exhaust valve can close.
Unless you use a stepped diameter piston, you already have what you want without the piston - the air in the plenum is its own air spring. The best thing you can do is make the plenum volume larger, so that the pressure drop during the shot cycle is less.
If the air spring piston area is 10 times that of the bullet base, the piston has to accelerate and reach velocities approaching 1/10 that of the bullet. The piston has much more inertia than the bullet, so it lags in acceleration; then does not want to stop. The piston seal also has friction. Make a larger plenum as close to the valve as possible...
A sweeper piston has application in a SSP (but find one that is still on the market). In a PCP that expels only part of the plenum volume per shot, a larger plenum works better to maintain pressure during the shot, and is way less complicated. Yes, I have thought about this before....
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Two things:
1) you mention an insert / spacer for the spring. You added this to add more compression to the spring correct? Are you doing this just to get the valve open at 250 bar, or are you trying to squeeze more power out of the gun? Also - how old is that spring? I know if you dig you will see people found various non-standard springs for this gun to replace the hammer spring. I am sure there is some risk of deforming the valve stem if a really heavy spring is added…
2) you mention your hammer was not hitting hard enough to open the valve all the way at full pressure. Does this tank have the stainless finish on the air cylinder near the top hat or does it have the darker blued finish?
I have one of each tank type and the silver one (which is newer) is harder to knock open for sure. When I first got it it was pretty much valve locked if I topped it off. It would shoot, but very underpowered for 1-2 shots. That’s no longer the case - something has smoothed out or broken in and now I get what feels like good power at 250 bar. I have not yet done any meaningful shooting over a chronograph with these evanix guns.
My gauge red lines at 200 Bar, I've never had it at 250. The end of the tank is marked 'Max 220 bar 2017-05', and I have gone up to 220
Dark blued steel. I've had the exhaust valve apart several times and the valve spring is the one thing where results could vary, assuming the pressure gauges are all reasonably the same. The way it's assembled doesn't allow accidentally altering it's compressed length. The spring keeper is supposed to be firmly bottomed out and the dimensions of all those pieces should be the same from one batch to the next. But purchasing spring wire, winding technique, heat treating, and a few other things could introduce changes without spotting the difference on assembly
I would bet the valve spring may have settled in a small amount. Next time you have that issue, try swapping the spring with one off a tank without that problem
I have a couple chronys, but they have a few issues
There's something else I did with the Rex hammer spring. Springs like to rotate when they work. So I re-shaped & polished the ends so it can do that without binding. It was cut crudely and very casually finish ground, like someone was only concerned with deburring
I checked the amount of bind on the hammer spring, and I have it set with lots of room between coils
The cocking effort is very low, just a few pounds. I've never felt that it was 'difficult' after adding the spacer
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Huh… both of my guns specify 250 bar maximum fill. Must be a change from early models.
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Huh… both of my guns specify 250 bar maximum fill. Must be a change from early models.
I'm sure there's a long back story on it. If only the factory workers were on this thread
I think the example I have is better than when I first got it. But it would be nice if we were to meet at a range & compare the two
It's possible that if I were to get a new valve spring, that my spacer wont be needed
If the valve spring is changed such that it has greater closing force, the result will be that the same fill pressure will be too high
The opposite should be true. If the newer springs are much less in closing force, then the fill pressure could be higher
And, if some old springs get mixed up in the new production, results can be random
This is why I prefer dump valves with a plenum charge valve that only opens when the exhaust valve is reset
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Did some more testing of the Rex P, The simple modification is well worth the effort! Happy to provide the files for anyone who wants to print their own but it is such a simple 3d model to design - anyone with a printer and basic tinkercad could whip up something like this in short order.
Now I’m looking at this and even though I have achieved “good enough” I’m wondering if maybe adding another inch or two (and another chamber) to the baffles would be a good idea. I also like subscriber’s thought about having a tapered bore - with closer tolerances closer to the muzzle where any angular deflection has had less time to accumulate.
I wonder what principles as far as baffle spacing, number, angle, etc. would let you get the most back pressure from a skinny short ldc like this, and if my ear could even detect the difference between crude and more sophisticated designs.
Also for what it’s worth for these projects I’m trying to mainly use the shroud and avoiding screw-on solutions altogether for a couple of reasons. First, it presents a challenge, and working under constraints like this and trying to work around them is good entertainment. Second, screw on devices exist in some legal gray area it seems, and even though all I shoot / own are airguns it seems wise to have all of my noise reduction projects being built in the shrouds when possible or with customized shrouds that are adapted specifically to the airgun and effectively built-in when installed.
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Did some more testing of the Rex P, The simple modification is well worth the effort! Happy to provide the files for anyone who wants to print their own but it is such a simple 3d model to design - anyone with a printer and basic tinkercad could whip up something like this in short order.
Now I’m looking at this and even though I have achieved “good enough” I’m wondering if maybe adding another inch or two (and another chamber) to the baffles would be a good idea. I also like subscriber’s thought about having a tapered bore - with closer tolerances closer to the muzzle where any angular deflection has had less time to accumulate.
I wonder what principles as far as baffle spacing, number, angle, etc. would let you get the most back pressure from a skinny short ldc like this, and if my ear could even detect the difference between crude and more sophisticated designs.
Also for what it’s worth for these projects I’m trying to mainly use the shroud and avoiding screw-on solutions altogether for a couple of reasons. First, it presents a challenge, and working under constraints like this and trying to work around them is good entertainment. Second, screw on devices exist in some legal gray area it seems, and even though all I shoot / own are airguns it seems wise to have all of my noise reduction projects being built in the shrouds when possible or with customized shrouds that are adapted specifically to the airgun and effectively built-in when installed.
You should try bumping up to a 2", or just something larger. Maybe shape the LDC so it works better as a fore grip. Adding volume rearwards should give better results than adding length will
That's a good point about not having the detachable aspect interfere with the legal status. I imagine DonnyFL must be pushing the boundaries with his products! All of the DonnyFL LDC's have markings to indicate that they are for airgun use only. Should all of these prints include such markings?
For measuring your results, wouldn't it be good enough to just record video in a manner that keeps the microphone physically consistent in it's location from the muzzle? I don't think you need to know the true decibels. Just having a repeatable sound level that you can use to verify if it's going up or down should be good enough. If you see the sound level get cut in half, that's a meaningful result
Plus it's possible to get a fair approximation of your velocity if the video is recording a target strike. For that, I would shoot at both 10 & 50 meters
So far, I've cut the flange off and am planning on keeping the rear puck in place with bearing retaining compound. Last night, I cut a section from a flat bar for my rear puck, so today I'll be swapping in the 4-jaw chuck to bore that out. I ought to take a few breaks so I can take pics. A good reason to have my real camera, as it's less fiddly to work with
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Marco,
What about stepping up the diameter of the forward section to match the outer tube, at the same length you have it now, or with your added 2"? See crude sketch.
Asking what will reduce sound for a given narrow short baffle volume has to also ask what the side effects ight be. Other than clipping from the narrowest section of baffle bore being too tight, it is also possible to induce aerodynamic steering:
This is where barrel vibration causes the moderator section to move laterally before the projectile is out. The pressurized air can form an air bearing that tries to center the projectile in the "tube" to even out the leak path, thus causing the projectile to deviate from the path it acquired in the barrel. The risk of this happening goes up, if the parallel bore section of the baffles is both tight and long.
Why would you make the parallel bore section long and tight? Because it reduces air leaking past the projectile, thus reducing report. At the risk of steering the projectile. How does one reduce the risk? Open the bore first, because the parallel length affect the length of time that the projectile acts as a plug, while the bore affects how leaky the plug is.
Making the parallel bore or spigot long creates space for the air to go as the projectile enters the baffle bore. It also places the leading edge of the baffle bore closer to the muzzle, or to the exit of the previous baffle bore. Having the first air stripper cone edge near the muzzle works because the air pressure there is the highest (after projectile uncorking from the muzzle). Also, shielding the projectile from buffeting near the barrel muzzle is good for tighter groups.
The higher the pressure, the more air wants to flow radially off the bore line. As the bulk air pressure drops its forward momentum becomes more important. Thus the air takes on the shape of a more columnated jet. Such a jet tends to flow straight down the baffle bore more directly, and makes cone lips placed close to the baffle exit (or barrel muzzle) less effective at diverting flow. This effect exists at forward baffles because the average air pressure there is lower (the whole idea). The effect also affect the effectiveness of the first baffle cone or air stripper, as the pressure decays from the barrel muzzle.
The latter may matter less because the loudest noise comes from the peak muzzle pressure immediately after the projectile exits the muzzle. This is where the downstream air stripper cones do more work.
I mentioned Matt Dubber had a suppressor that tapered to provide more radial space near the barrel muzzle and less further forward. He just happened to use it in a video he posted yesterday. I grabbed an image of it - see below.
I mentioned before that there are many overlapping principles at play and that there is no one best way to design a LDC. Taking a good principle too far can make the device less effective: The number 4 sketch below has a very long parallel tube, place very close to the barrel muzzle. It would allow a lot of air to follow he projectile straight down the tube because there is only one narrow path between air stripper and barrel muzzle to divert air into the body of the LDC and potentially the rear of a shroud. Breaking the long tube up into several stage with more radial vent area would be more effective, despite shortening the parallel section of the inner tube. The later would also have maximum potential for steering the projectile aerodynamically.
How long should the parallel section of the air stripper be? One half to two calibers should work well enough without causing trouble. The tighter the bore to projectile radial clearance, the shorter the parallel sections should be, or expect more fliers on target. A radial clearance of 1 mm is good, while 0.5 mm is asking for trouble. More than 1 mm clearance is better when there is uncertainty about alignment, and to allow for sideways motion of the moderator while the projectile is moving through it. Or, yaw of the projectile for that matter.
What about the other sketches? # 1 is a better geometry for the first air stripper near the muzzle. #2 is better for later stages where the pressure has already dropped. #3 avoids a excessively long parallel path, but in a narrow LDC tube, reduces the forward air capacity (green bars). So, a whole bunch of interdependent factors at play.
How large should the first chamber be? For PBs the first or "blast chamber" is usually 1/3 of the total expansion volume, simply to lower the bulk gas pressure immediately. This is so that a thinner wall outer tube can be used without bulging it. Reflex shroud volume on a PB or airgun allows the first baffle volume in front of the muzzle to be smaller.
Then why do we see so many airguns with all the baffles exactly the same "medium" length? Cheaper to manufacture and simpler to install. A reasonable compromise where more length can be added, if more sound reduction is required.
Your latest design is approximated by #5. I suggest #6 may be better suited. Place the air stripper closer to the barrel muzzle, but grow it out of a tube with a 3 mm radial clearance on the projectile diameter. That way, the distance between the first and second cone is greater, which makes more sense to allow the now lower pressure air to spread more before reaching the second air stripper lip. While you are at it, make use of the potentially larger nose diameter on your "insert", without it looking like a pickle. Or, perhaps, you prefer the stock .50 BMG look, with a smaller barrel pocking out of a "heat shield". You could make the nose section of your insert larger in diameter, short of matching the heat shield diameter...
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Lots of good stuff to digest!
A couple of replies:
1) The diameter projecting beyond the shroud is determined by the collar that is retaining it. There is a collar that screws into the end of the outer tube and it has to slide over the projecting tube. The photo attached should clarify.
2) I had the baffles much closer to the muzzle on my prototype and the plastic split at the first chamber. When it exploded I made two changes - thicker walls and moved baffles away from barrel to allow more room for air to expand. Same rationale as a large expansion chamber in powder burner ldc’s. The plastic is fragile compared to aluminum and it seemed like a good idea to alleviate pressure. Attached is a view of the design with the additional baffle.
I just printed a longer one with an additional baffle and chamber otherwise of similar design. Adds an additional 2 cm to the length and cut sound down quite a bit compared to the first (non exploding) model.
I will incorporate some of your ideas into Mark 3 before printing it and share whatever the result is here.
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Looks good to me, Marco.
The small ID does not allow for a long spigot on the first chamber, so what have is as good as can be made in the available space.
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Little bit of progress today. Practicing the art of pounding square pegs into round holes
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Took a look at that video with the tapering suppressor: https://youtu.be/ct5g6LC1BsQ
I think I've seen that before, on various 2-stroke exhaust systems. The taper is intended so that there's no end wall for a energy pulse to bounce back off of. It needs to be narrower than a certain cutoff point where the taper stops acting like a resonant chamber and gently squeezes the energy pulse into a very small area. So the idea is to get it into the smallest area/create the highest pressure peak possible. Having that much energy focused into such a small area has several different things happening
The squeezing process introduces a delaying effect and also increases the contact time & total area being heated from the energy pulse, thus absorbing energy & indirectly reducing the energy level of the sound wave. Thermal transfer is key to this process
Reducing the bounceback effect of a end wall or end chamber, means that the energy return will be much slower. So the peak of the reflected shockwave will be more averaged-out - thus quieter
Some of the more advanced factory exhaust systems that use this trick, will cascade these tapered chambers in such a way that the flow will move from one to the next in a concentric pattern, either beginning at the center & exiting at the outside, or the other way around
At any rate, I'd like to see that tapered suppressor get cut apart and torn down. I'm curious to see if it's just one single chamber or if it's hiding a cascade
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Little bit of progress today. Practicing the art of pounding square pegs into round holes
Nice!
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Got a cheap Lee mold for 148 grain “trash cans” - I read somewhere that these are decent projectiles and the mold was only $45 or so. Also got a simple magpul ak grip to replace the odd one contoured one that comes with the Rex.
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Got a cheap Lee mold for 148 grain “trash cans” - I read somewhere that these are decent projectiles and the mold was only $45 or so. Also got a simple magpul ak grip to replace the odd one contoured one that comes with the Rex.
Nice score! I've tried my 175 Pro Slugs in the Rex-P, and the recoil is pretty fun to play with
But I'd have to wonder if that grip will play nicely with the grip screw. Unless they changed that in newer production? On mine, the screw is perpendicular to the action, so it doesn't hold AR grips correctly. The outer edge of the screw only touches in a very small area, so it works loose quickly
Does an AK grip have it's screw vertical?
I've just finished drawing up a retaining nut to take the place of the rear bushing I was playing around with. Now that I'm going with solid aluminum for the rear bushing, all I need is something to brace that so it can't work it's way forwards. So I'm 'cheating' this build. For keeping the tube locked to the rear bushing, set screws could make more sense. I'm still considering that
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The grip fits but the angle is a poor match as you predicted. it did take a little finesse to get the grip screw in. Basically had to start with the grip rotated 90 degrees and then turn it in line once the screw was started in the hole.
i have that same grip on the Rex and Rex p now and much prefer it to the stock grip.
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The grip fits but the angle is a poor match as you predicted. it did take a little finesse to get the grip screw in. Basically had to start with the grip rotated 90 degrees and then turn it in line once the screw was started in the hole.
i have that same grip on the Rex and Rex p now and much prefer it to the stock grip.
Maybe a small wedge could be printed, to fit it in better?
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Rear nut partially finished when the printer stopped working. Got 6.5mm complete on a 10mm part. Probably still work for test fitting the new bushing, but I haven't printed the front yet!
Looks like it's either the temp sensor or the heating element. Nozzle is running cold
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Marco,
I presume that after blowing off the nose on one of your recent LDCs you set you printer to 100% fill?
Just something to consider.
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Good advice. Indeed I have been printing at 100% infill for these. The current model has held up to many shots at this point so i feel reasonably confident.
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I was a bit inspired to do some work on the grip. Rasp on a dremel, followed up with sandpaper, scraping & more sanding. I flush fit the top of the grip to the upper block
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All right - that looks great.
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Printer is working again, so I've printed the long version of the 47.2mm baffle, because the tube will be needing that support & the reduction of volume isn't that great. I went with 60% triangle fill. I'm going to be adding a stainless washer, cut to fit, to the front face to help prevent the front of the print from blowing out
I was thinking it would be nice if the rear portion of the ribs were curved to give a spin to the airflow, with every other rib ending short of that transition & the trailing edge ending at a 14° angle
A bit more advanced than what I've been doing so far in Fusion360, but that's the direction I want to work towards. There's a few other items I'd like to draw up, with geometry like that
The STL's are the wrong end up, I'm rotating them inside Cura which isn't a real issue. The bridging just as the ribs collect into a joined circle seems to be working perfectly for me with this gray eSun PLA+
Just for the fun of it, I tried this out without the outer tube, as that still needs to be cut down & assembled. I actually thought I was gonna hold that on there...
Running a 175 on full power, cylinder at 220. Holding as tightly as I could, couldn't even feel the print sliding out of my hand, knocked the insert out of the bottom of the grip, print went flying like that's what we were planning on doing - oh man, if only this thing had fins!
So I have the impression that there's going to be a lot of force pushing the tube forwards off of the rear bushing, and that's basically all that's going to be keeping this together. I might revise my plan on how to attach the outer tube to the rear bushing
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Phil,
He front support washer is a good idea.
The internal fins are thin. If you angle them the have to do work that was not intended. I predict they will crack and than rattle... Or am I mistaking your design for one of mine?
What caliber is that pistol?
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Phil,
He front support washer is a good idea.
The internal fins are thin. If you angle them the have to do work that was not intended. I predict they will crack and than rattle... Or am I mistaking your design for one of mine?
What caliber is that pistol?
The Rex-P I have is a .357
For the moment, I'm only considering drawing my own design from scratch, but I'm also wondering what else you might think of it, if not try it out too
There had been some talk about this earlier in this thread. Was that put aside because of the ribs not being strong enough?
Good point about the ribs, need to make them thicker and maybe reduce the height of the passage for the transition to spin. Centripetal force will play into that to some degree. If the rib height is reduced, it's bending moment will also be reduced
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There had been some talk about this earlier in this thread. Was that put aside because of the ribs not being strong enough?
I think that Marco simply wanted to make and test his own designs.
As for spinning air with vanes, Maxim had a patent that included that in 1909: https://patents.google.com/patent/US916885A/en?oq=916885
I have had a few designs that use vanes in the "blast chamber", with toroidal spirals that also delay air via centripetal force, down stream from there.
The attached images further down are for inserts into a robust tube or a shroud (if scaled down). Did that design for another GTA member (for free) about two years ago. Have many more, for the same person; and others. Just wanted to show you that most good ideas have already been thought of. Not meant as a snub to you: I have had thousands of "good ideas", but have only 25 US patents.
(https://patentimages.storage.googleapis.com/44/70/1f/6b9f135b360d07/US916885-drawings-page-1.png)
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There had been some talk about this earlier in this thread. Was that put aside because of the ribs not being strong enough?
I think that Marco simply wanted to make and test his own designs.
As for spinning air with vanes, Maxim had a patent that included that in 1909: https://patents.google.com/patent/US916885A/en?oq=916885
I have had a few designs that use vanes in the "blast chamber", with toroidal spirals that also delay air via centripetal force, down stream from there. The attached images further down are for inserts into a robust tube or a shroud (if scaled down).
(https://patentimages.storage.googleapis.com/44/70/1f/6b9f135b360d07/US916885-drawings-page-1.png)
All interesting. Maxims idea is a totally different kind of flow, I'm not sure how I could work with that. If it was changed from a torus shape into a pair of rectangles, then there's an opportunity to vent the center of each spin chamber to avoid over-pressure
There's a circle track muffler that does this: https://spintechmufflers.com/ - they started out by telling everyone how their baffles worked. I got a brochure from the company back in 1996 that explained it in detail. Back then, the company was called 'Spin Trap'. Thermal transfer is a very big part of how that works. Of course, airguns don't provide as much heat energy, so that's not as usable. Hot & cold zones are too close in temp. But the chambers also had holes stamped into them with the stamping intentionally done so each hole would project into the chamber. That would make it harder for exhaust flow to turn & enter the hole until the velocity slowed enough. These holes were placed in a small area where they could vent into a large exit chamber that would then lead out of the muffler. You can see the spot welds where they place the spin chambers, but they're not talking about the other little details like they used to
Yours is a bit closer to what I was thinking of, with the main exceptions being that flow isn't guided into the final trailing edges with quite as much control, and it's not a 'pumping' method with flow separating into high & low energy states. My idea wouldn't work as well in a forward chamber like that anyways. What I want to do is more like a cyclone dust filter, but with a tube in the center
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I found my design that uses both toroids, and vanes in the "blast chamber". The attached STL is a half shell for printing flat on the platen, because else the last expansion chamber has a floating ceiling - along with the toroid "nozzles". The idea is to glue two shells together and place inside an outer tube.
Zipped STL attached for half shell.
The mind boggles at the possibilities...
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I found my design that uses both toroids, and vanes in the "blast chamber". The attached STL is a half shell for printing flat on the platen, because else the last expansion chamber has a floating ceiling - along with the toroid "nozzles". The idea is to glue two shells together and place inside an outer tube.
Zipped STL attached for half shell.
The mind boggles at the possibilities...
Also interesting. I think the spin chamber is too small, but if it were to be reversed and located back over the barrel, then a lot more room is available
Have you considered printing lost wax cores & casting with zinc/aluminum alloys?
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I have not considered any material for my own use that would be robust enough to use with PBs. What I would design for myself does not limit what I might design for others. One could probably use lost wax to define the shape of a cavity that is filled with reinforced resin. After that cures, the wax could be melted out.
I designed vanes for a shroud not long ago, posted in this very thread: https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383702#msg156383702 (https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383702#msg156383702)
I prefer the latter vane design that lives near the ID of the shroud, rather than the earlier ones near the OD of the barrel: https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383950#msg156383950 (https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383950#msg156383950)
The vanes for supporting the barrel behind this reflex air stripper could be made into a spiral to get closer to what you might have in mind. That said, making it harder for air to get into the rear shroud volume (because the vanes will increase back pressure, if they are too steeply angled) could be counter productive.
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417798;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417800;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417802;image)
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I have not considered any material for my own use that would be robust enough to use with PBs. What I would design for myself does not limit what I might design for others. One could probably use lost wax to define the shape of a cavity that is filled with reinforced resin. After that cures, the wax could be melted out.
I designed vanes for a shroud not long ago, posted in this very thread: https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383702#msg156383702 (https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383702#msg156383702)
I prefer the latter vane design that lives near the ID of the shroud, rather than the earlier ones near the OD of the barrel: https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383950#msg156383950 (https://www.gatewaytoairguns.org/GTA/index.php?topic=203675.msg156383950#msg156383950)
The vanes for supporting the barrel behind this reflex air stripper could be made into a spiral to get closer to what you might have in mind. That said, making it harder for air to get into the rear shroud volume (because the vanes will increase back pressure, if they are too steeply angled) could be counter productive.
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417798;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417800;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203675.0;attach=417802;image)
Lost wax cores are printed using a filament that melts at a very low temperature. It's called a 'wax filament', but it's really just a thermoplastic with a low melting point, intended to be used with investment
I guess there's enough complications already! The most important thing I should focus on is keeping the tube from flying off the front
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I guess there's enough complications already! The most important thing I should focus on is keeping the tube from flying off the front
Simple; use a front and rear washer, connected by a few thin threaded rods (bicycle spokes?). The rear washer is also used to prevent a Heli coil from pulling out of the printed thread at the rear.
https://www.mcmaster.com/91732A533/ (https://www.mcmaster.com/91732A533/)
https://www.mcmaster.com/99055A110/ (https://www.mcmaster.com/99055A110/)
https://www.mcmaster.com/92856A107/ (https://www.mcmaster.com/92856A107/)
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There had been some talk about this earlier in this thread. Was that put aside because of the ribs not being strong enough?
I think that Marco simply wanted to make and test his own designs.
As far as I am concerned I ended up with a working solution after just a couple of iterations. Subsequent changes weren’t making any noticeable difference that I could hear so I decided I had achieved what I needed to. For example I tried two radically different approaches to filling the shroud space and really experienced no noticeable difference when shooting the gun.
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I guess there's enough complications already! The most important thing I should focus on is keeping the tube from flying off the front
Simple; use a front and rear washer, connected by a few thin threaded rods (bicycle spokes?). The rear washer is also used to prevent a Heli coil from pulling out of the printed thread at the rear.
https://www.mcmaster.com/91732A533/ (https://www.mcmaster.com/91732A533/)
https://www.mcmaster.com/99055A110/ (https://www.mcmaster.com/99055A110/)
https://www.mcmaster.com/92856A107/ (https://www.mcmaster.com/92856A107/)
I like the idea, and I have used bicycle spokes for things they were never meant for
But the rear washer is actually a 10mm thick aluminum puck which needs to go on 1st before attaching the outer tube to that. There's pretty much no access to the rear face of the puck for adding hardware, so the spokes would need to be in place already with some method to prevent them from rotating. One possibility is to form a T-head on the end of the spoke that fits a slot cut into the puck. But that still leaves a small issue with running the spokes through the front baffle section
What I ought to do is cut threads to attach the tube to the puck, but I've never actually done that before, and it's a bit too late for that now as the material needed to form threads is not there anymore. Of course, I could just cut another puck. In it's rough size, it's pretty close to the same OD as the tube
For now, what I'm thinking is to just continue with what I have for now as this build isn't intended to be a final version. I already have decided against powder coating or painting the tube, instead I'm going to wrap it with a fabric cover held on with velcro. One of the other tools in my shop is a sewing machine
Set screws would have issues with the threads cutting into the sides of the holes they would be restraining, and that would introduce slop. I've considered putting in a snap ring, but the access is very limited. I would need to move the tube forwards about 2mm and cut a clearance step in the puck so the snap ring has a resting place before it's put in place. And accessing the ends of the ring would be a bit too much fun for me! At some point I will want to take this back apart and that probably means welding part of a socket to the end of a tube so I can remove the nut holding the puck. I've already decided against setting that puck in place with retaining compound, because I do want to try having threads at some point
So my plan is to pin the tube. The holes in the tube will be counter sunk, and the pins will be peened a small amount to give a lip I can use later to pry them back out. I'll use some scrap to make a steel jig for the peening operation. Then all the pins will be held in place with shrink tubing. I happen to have some 3" shrink tube. All of that being under a fabric outer wrap, so how it looks isn't an issue
What I'm revising is how many pins I'm thinking of putting in. I had been thinking of only putting in four. But now I'm thinking I should see how many I can put in with something like 20mm spacing between them
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There had been some talk about this earlier in this thread. Was that put aside because of the ribs not being strong enough?
I think that Marco simply wanted to make and test his own designs.
As far as I am concerned I ended up with a working solution after just a couple of iterations. Subsequent changes weren’t making any noticeable difference that I could hear so I decided I had achieved what I needed to. For example I tried two radically different approaches to filling the shroud space and really experienced no noticeable difference when shooting the gun.
Nothing wrong with that!
I'm an inherent tinkerer, project snowballing is something I'm always getting into if I'm not careful!
This pistol is a back shelf piece nowdays, so I'm not too concerned if it goes through many changes. I think I spent way too much on it to begin with. Bought it for $500, and now that I've seen how well it's designed & built, I wish I had only paid $300. I honestly wouldn't feel right trying to sell this thing for what I paid without making improvements here & there. It's already many times better than how I got it
Seems like I don't buy airguns to shoot them, I buy them to work on...
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I have a carrying case to fit inside of, so I'm going to draw up the rest of the inserts, starting with the very end, adding in the thicknesses of the washers I have yet to source & then working with whatever room remains
So this is what I'm going to put on the end
There's a complicated area that needs to be rounded-off. I decided it would be easier to just print it with a sharp edge, then round it off in post-processing. Going to be some supports on this thing too - the rearmost portion of the vent passage inlet openings, leaving the forward portion sharp-edged to divide airflow
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Interesting brake design. Will the overhang in red be able to print without adding supports - below? Perhaps there is another orientation that will work. Or supports will be easy to break out.
Threaded dowel pins, so you can pull them out: https://www.mcmaster.com/pins/pull-out-dowel-pins-6/ (https://www.mcmaster.com/pins/pull-out-dowel-pins-6/)
Bicycle spokes have an "L" shape that will prevent rotation. Select length:
www.amazon.com/SENQI-36pcs-Bicycle-Spokes-Nipples/dp/B01GZPWE2A (http://www.amazon.com/SENQI-36pcs-Bicycle-Spokes-Nipples/dp/B01GZPWE2A)
https://www.amazon.com/Motoforti-Silver-Thread-Motorcycle-Nipples/dp/B093WQKNJ6/ref=sr_1_24?crid=22FT2OWE35U22&keywords=short+bicycle+spokes&qid=1668728089&sprefix=short+bicycle+spokes%2Caps%2C133&sr=8-24 (https://www.amazon.com/Motoforti-Silver-Thread-Motorcycle-Nipples/dp/B093WQKNJ6/ref=sr_1_24?crid=22FT2OWE35U22&keywords=short+bicycle+spokes&qid=1668728089&sprefix=short+bicycle+spokes%2Caps%2C133&sr=8-24)
www.amazon.com/SALUTUYA-Spokes-Convenient-Installation-10pcs/dp/B08YP1WSPB/ (http://www.amazon.com/SALUTUYA-Spokes-Convenient-Installation-10pcs/dp/B08YP1WSPB/)
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Interesting brake design. Will the overhang in red be able to print without adding supports - below? Perhaps there is another orientation that will work. Or supports will be easy to break out.
Threaded dowel pins, so you can pull them out: https://www.mcmaster.com/pins/pull-out-dowel-pins-6/ (https://www.mcmaster.com/pins/pull-out-dowel-pins-6/)
Bicycle spokes have an "L" shape that will prevent rotation. Select length:
www.amazon.com/SENQI-36pcs-Bicycle-Spokes-Nipples/dp/B01GZPWE2A (http://www.amazon.com/SENQI-36pcs-Bicycle-Spokes-Nipples/dp/B01GZPWE2A)
https://www.amazon.com/Motoforti-Silver-Thread-Motorcycle-Nipples/dp/B093WQKNJ6/ref=sr_1_24?crid=22FT2OWE35U22&keywords=short+bicycle+spokes&qid=1668728089&sprefix=short+bicycle+spokes%2Caps%2C133&sr=8-24 (https://www.amazon.com/Motoforti-Silver-Thread-Motorcycle-Nipples/dp/B093WQKNJ6/ref=sr_1_24?crid=22FT2OWE35U22&keywords=short+bicycle+spokes&qid=1668728089&sprefix=short+bicycle+spokes%2Caps%2C133&sr=8-24)
www.amazon.com/SALUTUYA-Spokes-Convenient-Installation-10pcs/dp/B08YP1WSPB/ (http://www.amazon.com/SALUTUYA-Spokes-Convenient-Installation-10pcs/dp/B08YP1WSPB/)
Weird, the red area should be all solid - Oh, ok I see what you mean. I'd recommend using tree supports
Those dowls are nice, but they don't go small enough, however I'll try to find smaller versions - and then I see they list 3/16 diameter. That might work ok, just not sure I want to spend $33 a piece for each one of those!
I'll need to put holes in the brake so there's something solid to run holes into. Probly print a drill guide so I can get the holes in the tube in the right locations
Bicycle spokes need to be cut to length, so putting new ends on is a basic task when using them for things they weren't designed for
I'm printing that with the front face down, all the supports should only be inside the vent passages
And I'm already re-designing the darn thing! Just thinking about it anyways, while it's printing right behind me right now...
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Bicycle spokes need to be cut to length, so putting new ends on is a basic task when using them for things they weren't designed for
A few of those links for spokes had a pull down menu to select spoke length. Pick the nearest length you want and design your plastic parts to work with that. Cutting spokes is difficult, and then you are missing useful features.
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Even though all was working fine I tried another print just for the heck of it. This model has a little extension of the first and last baffle - a short tube for the pellet to pass through. I felt the last chamber was not really doing much with the large bore, small diameter, and simple end wall.
I think this may actually make the report a little louder. Need to test a few more times because it basically works as well as the last one but it seemed that with these extensions the report has more of a “pop” and less of a “chunk” sound.
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The longer "tubes" increase dwell time, while the pellet is in near contact with the tube wall. This should make the system quieter.
However, the longer air stripper "tube" places the entrance nearer the exit of the previous stage, enabling more air to take a "shortcut", rather than fanning out and being blocked by the downstream "wall".
So, you win and you lose. Which effect dominates is the question...
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In this case, MUCH quieter with the simpler baffles. I just retested a few times. Makes me wonder if the best approach in this case might just be a single large empty chamber…
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Do you think a design like this has any merits? I was imagining a large chamber and then some short section at the end of the ldc to resist flow and create back pressure. I doubt the spacing and angle of those cones in my design here are optimal or even close and it might be that air will just go straight out the end here… this is just a quick mock up of the idea and I am wondering what suggestions you might have for how to create the most friction / resistance to air flow in a brief section like that?
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Bicycle spokes need to be cut to length, so putting new ends on is a basic task when using them for things they weren't designed for
A few of those links for spokes had a pull down menu to select spoke length. Pick the nearest length you want and design your plastic parts to work with that. Cutting spokes is difficult, and then you are missing useful features.
I'll have to admit, I hadn't looked at those spoke links. However, cutting spokes isn't hard for me. I usually use an abrasive cutoff disk on a dremel, or a thin cutoff disk on my angle grinder - which really makes short work on spokes, probly could do 100 at once in 20 seconds
If I need to do any bending, I use a propane torch to get them just barely a red glow. Full-on glowing red is way too hot & the spoke usually breaks later. After bending, I anneal the spoke, reheat and air cool
True though, it could be easier to use the OEM toggle ends they come with & design parts to allow threading spokes into place. For a project like this, maybe a T-slot that the end can just slip into? I could place the hole far enough in to allow running the bent end outwards. Turn a step to clearance the heads, then cut slots to run inwards to each hole. But this again, has the issue of running spokes through all the inserts. I could run the spokes on the outside, seeing as I'm planning on covering the thing with a fabric wrap. I'll consider that for the next build
On the other hand, I like putting new ends on spokes too, because then I get to decide how the end is designed. Essentially, trading off old features in order to gain new ones. It's really not hard at all to peen a head onto the end of a spoke. I use a ball ended drift I made just for this out of a hardened center punch and a very small ball peen hammer. The drift allows very precise placement of each strike. I can get a mirror smooth finish doing this, even better than the OEM head and the end is hammer forged, which improves it's strength
Now that I'm thinking about it, I'm all out of spokes. I think I used the last ones as welding rod
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Even though all was working fine I tried another print just for the heck of it. This model has a little extension of the first and last baffle - a short tube for the pellet to pass through. I felt the last chamber was not really doing much with the large bore, small diameter, and simple end wall.
I think this may actually make the report a little louder. Need to test a few more times because it basically works as well as the last one but it seemed that with these extensions the report has more of a “pop” and less of a “chunk” sound.
The last chamber probably has the most effect on the sound quality
Why not make that a cone like the rest? The end wall makes a great spring board to bounce off of. What about a energy pulse capturing shape like this?
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Do you think a design like this has any merits? I was imagining a large chamber and then some short section at the end of the ldc to resist flow and create back pressure. I doubt the spacing and angle of those cones in my design here are optimal or even close and it might be that air will just go straight out the end here… this is just a quick mock up of the idea and I am wondering what suggestions you might have for how to create the most friction / resistance to air flow in a brief section like that?
Possibly, it's a lot like more traditional designs
I would want the 2nd chamber to be a lot larger though, and then each one after progressively smaller. One idea is to have the resonance of each chamber at a different frequency, with the intention that they avoid coupling effects such as if the frequency is an even multiple. This is the basic design of all world class three-chamber hair curler moderators. You may have noticed those use chambers of three distinctly different volumes
I have a Weihrauch hair curler style for their HW77. It has very interesting ratios between it's three chambers, no two combinations of which will match each other
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That makes sense - I will make some changes along these lines before I print. Thank you
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That makes sense - I will make some changes along these lines before I print. Thank you
Well, if you try an anti-resonant approach, keep in mind the frequency of the 1st chamber will affect the rest. It might be easier if you only have two forward chambers
When trying to use multiple small chambers, it's very difficult to avoid overlapping edge resonance effects
The weihrauch uses a Small/Large/Medium progression
Something I do when picturing how this stuff works. Each shot cycle is just like striking a tuning fork one single time. What happens next depends on how you adjust the fork leg lengths & weights
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Marco,
Stacking a bunch of short chambers near the front end is probably not going to be very effective for an airgun, because the air pressure is not very high, compared to a similar caliber PB. The higher the pressure, the more the air wants to flow radially into available space. The lower the pressure, the more it wants to travel forward, straight down the central hole. You need to allow the air more forward distance to travel radially, the further forward in the device.
Right near the barrel muzzle the air pressure is high enough that it wants to take up space radially, and can flow into more closely spaced baffles near the beginning of the shot pulse. As the event takes some time to finish, the second half of the pulse occurs at lower pressure and could make closely spaced baffles less effective. While this seems "bad", the bulk of the noise comes from the leading edge of that volume of air, so dulling its effect is more important than after the bulk pressure has trailed of.
The idea is to remove the crash, while recognizing that smooth flow does not make that much noise; as long as the pressure ripple superimposed on it can be filtered out - or allowed to dissipate. Long chambers near the front end tend to drive a deeper tone, that seems less unpleasant; but is more likely to be audible.
I am not sure what your absolute dimensions are, but would suggest you try leaving about a third of the part length open as the first chamber. Then have two baffles with simple conical air strippers spaced about 1.5" apart. Followed by a third chamber that takes up the rest of the length (hopefully more than another third of the total length). The front end air stripper can have a tubular air stripper, about twice the length of the LDC bore.
I understand the "make the chambers unequal length, to avoid driving a resonance" argument; yet Marauder baffles are all the same, and that PCP is the standard for quiet. I think it is because the 3/4" long chambers would resonate air at ultra-sonic frequencies; so no one notices. Yet, the principle Phil mentioned works on tire tread patterns, where the block lengths are varied around the circumference to avoid driving a single tone resonance that would be heard as an annoying drone with a constant block lengths.
It would be interesting to compare just one long chamber, with only a single tubular air stripper at the front end. It may have a pleasant sound, but my prediction is that it will be louder than a similar design with at least two more baffles somewhere near the part's mid length.
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Marco,
Stacking a bunch of short chambers near the front end is probably not going to be very effective for an airgun, because the air pressure is not very high, compared to a similar caliber PB. The higher the pressure, the more the air wants to flow radially into available space. The lower the pressure, the more it wants to travel forward, straight down the central hole. You need to allow the air more forward distance to travel radially, the further forward in the device.
Right near the barrel muzzle the air pressure is high enough that it wants to take up space radially, and can flow into more closely spaced baffles near the beginning of the shot pulse. As the event takes some time to finish, the second half of the pulse occurs at lower pressure and could make closely spaced baffles less effective. While this seems "bad", the bulk of the noise comes from the leading edge of that volume of air, so dulling its effect is more important than after the bulk pressure has trailed of.
The idea is to remove the crash, while recognizing that smooth flow does not make that much noise; as long as the pressure ripple superimposed on it can be filtered out - or allowed to dissipate. Long chambers near the front end tend to drive a deeper tone, that seems less unpleasant; but is more likely to be audible.
I am not sure what your absolute dimensions are, but would suggest you try leaving about a third of the part length open as the first chamber. Then have two baffles with simple conical air strippers spaced about 1.5" apart. Followed by a third chamber that takes up the rest of the length (hopefully more than another third of the total length). The front end air stripper can have a tubular air stripper, about twice the length of the LDC bore.
I understand the "make the chambers unequal length, to avoid driving a resonance" argument; yet Marauder baffles are all the same, and that PCP is the standard for quiet. I think it is because the 3/4" long chambers would resonate air at ultra-sonic frequencies; so no one notices. Yet, the principle Phil mentioned works on tire tread patterns, where the block lengths are varied around the circumference to avoid driving a single tone resonance that would be heard as an annoying drone with a constant block lengths.
It would be interesting to compare just one long chamber, with only a single tubular air stripper at the front end. It may have a pleasant sound, but my prediction is that it will be louder than a similar design with at least two more baffles somewhere near the part's mid length.
Good point about the pressure regime. I keep trying to treat the flow like it's much denser than it is. Most of what I've done is on gasoline exhaust systems, and the operating conditions are much more challenging. I've made my own design mufflers, some which work really well, in very interesting ways. And some which failed in interesting ways. My best design is quiet until you open up the throttle, then as the engine speed picks up becomes ricer-loud! That's where I get my experience with over-driving baffle systems
The single baffle/single chamber approach is how my .22 Origin works. It's pretty effective like that
I think a key part is having that one baffle far enough forward that it doesn't get over-driven by the initial pressure spike
The Origin barrel is fully free-floating inside the shroud, so I'm very careful about not knocking the shroud against anything. I made that mistake on a .22 Gauntlet. I had to remove the extra brace they added because the shroud was pushing the barrel off to one side. With the Gauntlet, there's a bushing at the front of the barrel to center the shroud. In the Origin the end of the shroud could easily wind up out of alignment with the bore if that tube gets bent. And the tube is the exact same soft aluminum, in the same wall thickness
On the other hand, the Gauntlet has a multi-chamber monocore which never worked well. I bought a 3D printed core that's designed to vent each chamber backwards into the shroud. Seems to work ok. Someone on ebay who doesn't sell them anymore. I think they didn't get enough business to keep putting that one out. I went to their listings to see what other guns they covered, and there weren't any others
I'd forgotten about tire treads. I have a small motorcycle that I've switched over to dual-sport tires. It has that droning effect because the tread pattern is a lot of the same size shapes, so the air pockets are all the same size. I hadn't caught on to that right off, and changed out the front wheel bearings because it sounds like they're shot
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Marco,
If your .357 produced as much energy by area ratio as a 30 FPE .22. then that would be about 80 FPE. Assuming a similar barrel length and average pressure, thus a similar muzzle pressure at 2.63 times the air volume. However, if your .357 used a barrel around 11" length, and produces 120 FPE, then the job you are trying to accomplish becomes about 60% more difficult (depending on similar air tank or reg pressures being used), because the muzzle pressure would be that much higher than for a 30 FPE Avenger .22.
The point is that the airstripper spacing and cone angles that help for an Avenger, may be longer and shallower than would still benefit your Evanix pistol. Or, that the Evanix could work with closer baffle spacing and shallower cone angles, due to its higher muzzle pressure.
The attached image is a crop from a rather effective design I did, posted on the thread "Taming the bark of an Avenger". The reason for showing it is not for you to copy it directly for your Evanix, but for you to understand the principles of increasing stand-off distance and shallower cone angles as the bulk air pressure in your LDC drops towards the front end. It so happens that as the pressure drops towards the front of the LDC, the same air needs to occupy more space, because it is less compressed.
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Something like this tunable configuration:
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=203825.0;attach=418897;image)
From here: https://www.gatewaytoairguns.org/GTA/index.php?topic=203825.msg156387958#msg156387958 (https://www.gatewaytoairguns.org/GTA/index.php?topic=203825.msg156387958#msg156387958)
Then scroll down to see the configuration shown in the above image.
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fascinating, some of these little details! I probably could've been here more, sooner. I've missed a lot
Has anyone here tried allowing the baffles to slide freely inside a tube, so they can find their own spacing? Maybe damped with silicone grease & spring-loaded drags
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I know of a number of people who have contemplated sprung baffles. My concern is slamming them into the end of their travel, and making noise; then breaking them. As for active elements, these videos might amuse you. Second video is slowmo, and shows more:
https://youtu.be/VZ06GAsIR2A
https://youtu.be/t9I-Mm1tCUw
Hardware made and Videos owned by GTA Forum owner, Travis.
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Then this really is the place where I should have been!
That bullfrog LDC is funny! I'm actually tempted to print an actual bullfrog to fit onto my Daisy 822 & have the diaphragm set at the throat so it looks like the real thing. The 822 will often make a sort of croaking noise while pumping, if it gets a bit dry
I wasn't thinking of using springs to control the fore/aft position of the baffles, but to help keep them from sliding around. More like spring-loaded skis that ride along the inside surface of the outer tube
And each sliding baffle would need to have a large mass, so it doesn't react too quickly
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I think I'm going to call it on the pistol. I really want any moderator experiments to be done on a more purpose-built platform than this. It's just too annoying working on this thing while ignoring a few things!
So I decided to try a single baffle, single chamber. That's not working as well as I was hoping, but I'm not changing it out because it does work better than stock. And I think the muzzle brake is going to actually work properly. I'm going to switch gears here and just go for looks now
Part of the reason is my idea for 'temporarily' holding the rivets with loctite retaining compound doesn't actually release with heat like I had thought it might. And I think I may have melted the printed insert to the tube...
Anyways, case will have a storage compartment cut in, a bit more 3M outdoors tape slapped all around and I'm posting this for sale
But 1st some target shooting to see if it's really working right, get a better idea on how poorly this moderates & hopefully get a group worth showing on the listing
I have some loose barrels that need to get put to work, and it's about time to try out one of my valve ideas as something that works with a trigger, instead of hitting it with a ballpeen hammer
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That thing looks purposeful.
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That thing looks purposeful.
Even better in person. I'll get some pics of the range when the weather gets a bit nicer. Snowing right now
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I did a modification to a previous design and printed yesterday.
In the two photos below, you can compare the new design to the old. The old was not especially quiet. On the image of the older print, I have circled the main areas that I changed. The extensions are now shorter and taper to a thinner edge.
Clearly more effective than both of my previous designs.
When these are fresh off the printer, it’s interesting to force a lung-full of air through these. You can clearly feel the resistance when you blow in the intended direction (and lack of resistance when pushing air backwards). It’s a strange sensation - always makes me feel like I have to clear my throat because it is oddly similar to breathing when there’s a lot of phlegm in your lungs.
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The proof of the pudding is in the eating. Next installment; results. I wonder how different the new one will be from the previous one.
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It is noticeably quieter. Deeper pitch as well
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Nice!
I'm really wishing I'd went with setscrews...
But I'm beginning a scratch built in .32 caliber
It's just a valve test, so I'm not so sure I should be doing much for moderator design on it. I ought to just go with a threaded adapter to run the DonnyFL Emperor
But later, I want a custom tank that can hide inside a straight stock like a early 1900's style and that's when I'll be looking at a reproduction of antique moderators, but with modern baffles inside
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It is noticeably quieter. Deeper pitch as well
Did you do a "double-blind" in changing back and forth between two configurations in your above post, more than once? If so, the effect of such a subtle change is amazing.