.22 LDC bore clearance study print files

[subscriber]

Gavin,

Some airgun air strippers are narrow perforated tubes, such as used by Cometa - image below.  So, venting air from behind the pellet in the long skinny tube by means of perforations into a secondary cavity is an option.

Don't the perforation change the tube's resonant characteristics?  Not the fundamental frequency perhaps, but the strength of the waves?  On the other hand, damping waves bouncing in that tube is surely a good thing.

[mrbulk]

Thanks, Charlie

Any chance you could test the 7.5 mm one too, if you are using some sort of dB meter on the 6.5 and 8.5 mm versions?  There appears to be something interesting going on with bore size that was not expected; hence my question.

Just saw this, will see. I now permanently reside at a difficult-to-shoot venue ... but if I can figure it all out will meter them then. Thx.

[subscriber]

To study the effects of pellets travelling down "skinny tubes" with high pressure air behind them, print up a series with ID's varying from 0.01" radial clearance, up to pellet caliber radial clearance.  Somewhere in that series I predict "blown groups" due to the air from behind trying to flip the pellet in the tube.   I am not suggesting the pellet actually flip, but that it is likely to wobble greatly.  It should be obvious that a pellet cannot actually flip inside a tube that has a narrower bore than the length of the pellet.

This effect drops off once the clearance between pellet and tube ID is so large that the air velocity down the tube past the pellet is reduced.  Certainly, when the air flow in the tube matches the pellet velocity, then spin stabilization should be enough to keep it stable.  This speed matching of air happens when the pellet is a tight fit in the barrel (or tube), and when the tube area is large enough to accommodate the volume of air at a lower  flow rate.  As we can't make a printed plastic tube as close and smooth sliding fit with the pellet, the trouble starts when a significant volume of air leaks past the pellet, at much higher speed than pellet velocity.

Most of the skinny tubes in such a series of IDs are likely to shift the point of impact too, by aerodynamically steering the pellet; relative  to bare muzzle. 

If the printed tube is long and flexible such that it whips around, the effect of steering to a different POI will be worse.  It may be repeatable, but should act like poor barrel harmonics on a whippy barrel.   If the skinny bore tube is stiff due to a large OD, then the mass of the device could still shift barrel harmonics and POI.

Now, if you mount the skinny tube to a PCP that has a shroud and its own air stripper up stream, the negative effects of a skinny tube will be smaller, than if it were mounted directly to the barrel muzzle.  This is simply because the volume and energy in the air directly from the muzzle is much greater.

If you doubt that this skinny tube pellet instability effect exists at all, shoot them pellet from a smooth bore tube that has an ID that is 0.001 or 0.002" smaller than the pellet head.  This smooth bore barrel then steps up to the skinny tube where the tube no longer supports and guides the pellet.  Then the skirt still acts to direct the pellet nose forwards, except the airflow in the second tube passes the pellet and the skirt tries to correct the pellet's attitude by flipping it over.  Yes, we have rifled barrels, but that does not remove the conflict that occurs between air flow direction and spin stabilization.   

[WobblyHand]

Perforation of the tube can change resonance depending on the hole size.  But I'd think it would be a perturbation of the frequency if the holes are relatively small.  The holes should act as high frequency sonic leaks, or perhaps even as whistles.  The energy that leaks out is removed from the main slug of air.  Having not run any numbers, couldn't say if it is a good or effective strategy to pursue.

I'd agree that too narrow tubes aren't good, as there could be unwanted guiding even without contact.  Would be nice to know the clearance, but that's the whole point of this thread!  I suspect that even without tilt error that the clearance could depend on both the bore (duh) and the length and maybe the properties of air.  A helium LDC might be a lot different, even at the same pellet velocity.

[Back_Roads]

 On what  subscriber is saying , I have found a large tube on the end of a barrel that is open say 1/2" or there abouts, like the cocking aid on a springer pistol, or a ldc with the muzzle end removed along with its baffles, throws accuracy out the window.

[WobblyHand]

On what  subscriber is saying , I have found a large tube on the end of a barrel that is open say 1/2" or there abouts, like the cocking aid on a springer pistol, or a ldc with the muzzle end removed along with its baffles, throws accuracy out the window.

Weights, like LDC's or large tubes at the end of a muzzle, definitely can affect accuracy.  I've heard that sometimes other weights strategically place on the barrel can help tame the barrel harmonics.  Our long skinny barrels are prone to issues like that. 

Are tensioned barrels prone to the same effect as you describe?  Is the effect less or more?

[WobblyHand]

Finally got my LDC to print!  I have two skinny tubes, separated by a large wide expansion chamber.  This is the bigboy, at 40mm OD.  I also added 24 1.5mm whistle vents that go from the expansion chamber out away from the shooter.   The area of the whistle vents is comparable with the bore area.  I also used the area that I previously left solid.  They are radial sectors, with some rounded ends to use up some air energy.

Been playing with adding vents in the narrow bores, but haven't finished that - sort of an air stripper idea.

One thing that I have on my 1/2-20 adapters are rather large flanges.  I turned the adapters on my lathe, so they are guaranteed perpendicular to the bore.  I seat the LDC's to the flange.  My PP700SA flange is 25mm in diameter.  Another thing that can help, if you have the equipment, is to face off the end of the LDC in a lathe.  Even with a decent print, there are little bumps and things on that surface that can slightly cant the LDC.

In case you are wondering, the bigboy is quieter than my previous try.  I now need to get some relative quantitative acoustic measurements. 

The bigboy is at my absolute limit for size.  Now I have to do internal stuff, to further reduce the sound.  Tone isn't bad indoors, but I think it could be improved a bit more.  The bigboy has a 7.25mm bore.  I shot through it a couple of times with a pellet, but I don't know if there's any steering going on, I need to set things up in a vise to check that. 

Hmm, close examination shows a small chip at the muzzle exit.  Arggh.  I need to measure carefully the bore ID.  My digital caliper says 7.00mm, not sure I believe that.  The CAD file says 7.25mm.  I'll go get my micrometer and my inside hole tools.

Also haven't faced off the thread area yet, which might alleviate the issue.  The thread area has a slight rock on a flat surface, whereas the muzzle area does not rock.  This means the bore of the LDC was not quite mounted true.  Or, the pellet was processing or tumbling.  Or something else entirely!

[WobblyHand]

With the small hole gauges and a micrometer, near the exit I get 7.24mm.  But further in the bore it seems smaller, around 7.23mm.  This could be measurement error, deep hole measurement isn't easy in metal, and in plastic with layers it's harder to gauge what is the proper feel.  Going to say this is not conclusive.  When I set it for size at ~5mm deep, and then slowly moved the gauge downward, the gauge did catch on an apparent shelf or narrowing.  It's quite repeatable.  Playing around some more, I'd say the bore is not uniform.  Setting the gauge to slightly slip at 5mm deep, also catches on the way up.  In other words, the very end is smaller.  I get 7.00mm and it was compressing, so it is narrower than that.  If I wanted it to be dead on, I guess I would have to run a reamer through it.  At least my FDM printer (and filament) is not quite good enough to maintain the correct bore size.

But from a conservative approach, I'd say this is too narrow of a bore for this length, with a non guaranteed flat bottom mating surface.  7.5mm might work.

Or I could get an "L" reamer at 0.2900", which is 7.366mm.  At least an L reamer is 6" long.  The cheapo 7.3mm reamers from China are only 100mm long total. 

[WhatUPSbox?]

Cleaning up the bore of the 3D print may be a challenge. You could line it.
Example: https://www.grainger.com/product/APPROVED-VENDOR-Tubing-Seamless-4NTD5

But I think the general concern remains. A pellet being pushed down a loose fitting tube is likely to suffer in accuracy. The barrel bore and the tube are unlikely to be concentric. The pressure in that eccentric annular gap is probably not uniform so the pellet will likely see some side load, and it may vary from pellet to pellet.

[WobblyHand]

I am thinking these LDC's need more clearance for printing defects.  There is something in the first narrow bore that could deflect or alter the pellet path.  I could easily see this being a large contributing issue. 

As printed, the narrow hole did not pass a J drill, which is 7.04mm.  That means the bore was narrowed by 0.21mm from the design point.  I started the J, then worked up to L, going through K.  The bore is clean now, except for a sort of hanger of filament on the inside of the entrance tube.  It does protrude into the bore.  Will need a special deburring tool to cut that off.  Maybe I can get a Noga debur tool in there.  Something like an RC2200 might work if it has the reach.

I lucked out, the Noga was just long enough to do the job.  All clear all the way through.  Bore is roughly 7.366mm, give or take what ever spring back the plastic has.  I'll measure it when I get a chance.

[WobblyHand]

Cleaning up the bore of the 3D print may be a challenge. You could line it.
Example: https://www.grainger.com/product/APPROVED-VENDOR-Tubing-Seamless-4NTD5

But I think the general concern remains. A pellet being pushed down a loose fitting tube is likely to suffer in accuracy. The barrel bore and the tube are unlikely to be concentric. The pressure in that eccentric annular gap is probably not uniform so the pellet will likely see some side load, and it may vary from pellet to pellet.

I think with sufficient clearance this is not an issue.  The real question, which has yet to be answered is what is the minimum sufficient clearance that maintains the original accuracy.

These printed devices are tough to chuck up in a lathe and be straight.  This particular size is outside of my collet range, (max collet I have is 32mm) so I need to deal with 3 jaw, or preferably 4 jaw chucks, which take a while to set up.  I did a quick and dirty set up in my 3 jaw and found it wobbled by 2-3 mm as it rotated.  I used a bump wheel to straighten it, but as soon as I put any load, like a light touch with a sharp cutter, the cylinder went out of true again.  I could make a collar, sort of a clamp around the LDC and clamp that.  It would need to be about 2" in diameter, maybe a 2" long, and bored out to 40mm in diameter.  Then I would slit it so it could clamp on the plastic.  Straight chuck jaws on plastic tend to just mash the plastic.  I had wanted to true the threaded end, but so far my attempts have failed, due to the cylinder run out (for what ever cause).

[TorqueMaster]

With the small hole gauges and a micrometer, near the exit I get 7.24mm.  But further in the bore it seems smaller, around 7.23mm.  This could be measurement error, deep hole measurement isn't easy in metal, and in plastic with layers it's harder to gauge what is the proper feel.  Going to say this is not conclusive.  When I set it for size at ~5mm deep, and then slowly moved the gauge downward, the gauge did catch on an apparent shelf or narrowing.  It's quite repeatable.  Playing around some more, I'd say the bore is not uniform.  Setting the gauge to slightly slip at 5mm deep, also catches on the way up.  In other words, the very end is smaller.  I get 7.00mm and it was compressing, so it is narrower than that.  If I wanted it to be dead on, I guess I would have to run a reamer through it.  At least my FDM printer (and filament) is not quite good enough to maintain the correct bore size.

But from a conservative approach, I'd say this is too narrow of a bore for this length, with a non guaranteed flat bottom mating surface.  7.5mm might work.

Or I could get an "L" reamer at 0.2900", which is 7.366mm.  At least an L reamer is 6" long.  The cheapo 7.3mm reamers from China are only 100mm long total.

The bore issue has been discussed many times -- maybe not in this thread though.  It's a side-effect of FDM printing that small circular holes in the XY axis will have some shrinkage.  Half a nozzle size reduction in diameter is about what's expected for these bores.  Smaller holes, 1mm and under, may seal up completely.  Larger holes are less affected.   I've read that square holes, hex holes, etc., are also less/un affected by this shrinkage, since they are made up of straight lines, not a constant curving. 

Cura has a setting to try to compensate (hole expansion) which might solve it with no side-effects.  I haven't tried it.  I know my models are printing on FDM, so I increase the hole size a bit in the CAD to compensate.

If your "tube" is not a constant ID that's tougher to diagnose -- it should be pretty steady, only off by some backlash error.  It's possible on layers with more or less material surrounding the tube, the holes could close in more or less.  Finer tuning (less overextrusion) might help, or it could be another of those FDM peculiarities to watch out for if fit is critical.

[TorqueMaster]

Deleted repost.

[WobblyHand]

@TorqueMaster Bob, I was less surprised about the narrowing of ID, since I've experienced it before.  But more surprised about the uniformity of the bore.  Using a Yuasa small bore gauge (the kind that look like expanding wine glasses) and setting it to the current bore, and then dragging it showed that there were sections that varied in ID, a bit more than I would have expected.  I suppose this is all due to the mechanical slop in the printer system.  It's not huge variation, but in bores, where we are probably cutting it too close perhaps, the non-uniformity might matter.

At least for my future LDC's, I'm going to run a drill bit or reamer down the bore to ensure it is the right size, just like I do with a tap for my 1/2-20 threads.  It doesn't take much time and ensures there's fewer problems.

[subscriber]

For what it is worth, the 6.5, 7.5 and 8.5 mm bores of the mufflers attached to the OP have CAD dimensions of 6.8, 7.8 and 8.8 mm, in anticipation of shrinkage.

Non-circularity would seem to come from the X/Y movements, or the smoothness of the filament feed.  Try printing a 7.5 mm rod so you can look at the surface on the outside.  If it is X/Y axis movements causing non-uniform tube ID, the same movements should occur producing an OD of that diameter.  If there is mechanical ringing, then the same ripple should show on the OD.  If it is random layer variation, it might be filament extrusion rate variation; or the filament itself.

If the printer is belt driven, placement of the part on the platen might affect how badly the mechanism vibrates in one spot VS the other, as different lengths of tight VS slack belt would be involved.

Of course, I am not printer expert, so the above diagnostic is speculation.

[TorqueMaster]

@TorqueMaster Bob, I was less surprised about the narrowing of ID, since I've experienced it before.  But more surprised about the uniformity of the bore.  Using a Yuasa small bore gauge (the kind that look like expanding wine glasses) and setting it to the current bore, and then dragging it showed that there were sections that varied in ID, a bit more than I would have expected.  I suppose this is all due to the mechanical slop in the printer system.  It's not huge variation, but in bores, where we are probably cutting it too close perhaps, the non-uniformity might matter.

How much variation did you measure?  My ender 3 is no high end machine, and I expect mechanical slop of ~0.05mm.  Printed uniform tubes, inside and out, shouldn't be much worse than that.  I think it may be how much "stuff" is exterior to the bore affecting the hole diameter.

I'm going to try printing this -- actually just the top part -- to see if I feel any "steps" in the bore.  I expect it may be narrower where the print is widest, as shown by my crude lines on the cutaway view.  My text disappeared -- it's CAD at 7mm bore diameter.

[WobblyHand]

Bob, I destroyed the evidence on the bigboy, by drilling it out.  But I still have two failed prints (just fished out from the trash) that should have similar issues.  I will measure them and see what I get.  The exit dimensions should be similar.

In my case, the small diameter tube near the AG muzzle had some multiple strands of filament cutting across the bore near the edge, as a WAG, it protruded maybe as much as 0.5mm into the bore.  I hadn't noticed it before.  This could have caused some instability in the pellet flight. 

Also the bore at the exit of the LDC was less than 7mm, possibly due to first layer elephant foot.  Further in, the bore was larger.  After the second shot, that was clipped.  After drilling out the holes to 7.3mm, (and internal deburring) I have not seen any evidence of clipping.  The filament creating the chord (mentioned above) was not removed by drilling.  I had to insert a Noga inside debur tool to remove those strands.

[TorqueMaster]

Elephants foot and loose strands are two more potential issues for sure.  I do my first layer with a bit of "horizontal expansion" to counteract e-foot caused by getting a good squish.

I printed the test part, and I do feel ridges inside it as depicted in my lines drawn on the cutaway.  Moving in from one direction, it gets hung up on them, and from the other direction I feel the bumps, but they do not impede movement.  Similar to trying to go up a step vs. falling down a step.  I do not have tools to measure the size of the steps.  I may cut it open.

Anyway, the suspicion is correct  -- the hole diameter is affected by the thickness of material outboard of it, and can contribute to uneven/unsmooth bores.

[WobblyHand]

Elephants foot and loose strands are two more potential issues for sure.  I do my first layer with a bit of "horizontal expansion" to counteract e-foot caused by getting a good squish.

I printed the test part, and I do feel ridges inside it as depicted in my lines drawn on the cutaway.  Moving in from one direction, it gets hung up on them, and from the other direction I feel the bumps, but they do not impede movement.  Similar to trying to go up a step vs. falling down a step.  I do not have tools to measure the size of the steps.  I may cut it open.

Anyway, the suspicion is correct  -- the hole diameter is affected by the thickness of material outboard of it, and can contribute to uneven/unsmooth bores.

Thanks for doing the test.  Interesting results.  Most of the time, this minutia doesn't matter.  But as they say, in pursuit of performance, nuances can make the difference.

[WobblyHand]

I was able to save one of my failed prints, the one with long skinny tubes that died on the variable layers (where the threads were).  There was enough meat in that design to drill the correct hole for tapping 1/2-20 threads.  So the side that screws onto the muzzle is 10mm short.  Nearly all the print was done at 0.3mm spacing, so called draft mode.  As you might imagine, the squish was a little less, and a J drill only cut minor nurbs off, but not the bore.  A K drill actually took off minor amounts of material.  The L drill did remove material.  There were also filaments cutting a chord across the bore on the AG muzzle end.  Once again, had to use the Noga to internally debur the material.  Since the failed print had blobs and a gouge in it, I chucked it in my lathe and turned the blobs off, and faced the end.  I was able to tap the hole about 11mm deep.  My adapters are a little long than that, but I was able to test my straight tube design. 

It is a little higher pitched than the bigboy, but not that bad.  It has a bunch of vents out the front.  From an acoustic point of view, they should only pass ultrasonic frequencies, no matter how many there are, as long as they don't resonate by some other means.  I could test that by simply blocking the small holes to see if it was quieter.  Duct tape might work.