GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rkr on August 01, 2021, 04:57:03 AM
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I've been thinking, as we do the the chambers and leade in for barrels we need to use special reamers for that. Now we don't we use generic 1:50 reamers for the leade in and make a separate thimble for the chamber/transfer port like FX does? That would allow us to experiment with different length and ID chamber parts for different bullet seating depths. Granted it makes the barrel support inside the breech shorter but with barrel bands that's not an issue. Is there a downside to that approach that I haven't figured out?
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I would think it's just more work is all. I think the combination of a thimble and a replaceable probe (similar in design to a Flex's bolt) would be an outstanding idea.
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One of the most crude things on the modern air gun is barrel retention by grub screws. A few years ago I made a 2240 with a threaded barrel to engage with a threaded breech. That required a bit of finesse and the barrel-breech became a matched set that would not be able to interchange with any other gun.
I purchased one of the original Exclusive Shop 22xx side-lever breeches which has a "thimble" arrangement, but the thimble has nothing to do with the transfer port. After building up the 2250 with the ES side-lever breech, I began to think of the possibilities of changing the internals of the breech.
Those changes would include, as the current topic by the OP suggests, a thimble that contains the transfer port, and a separate barrel which would be threaded to engage the breech assembly, a part of which would be threaded.
The "project" was still-borne and became secondary to many others, but this topic has modestly rekindled interest.
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What you are describing, I believe, is how I built my sidelever breech. Mine also incorporates a load tray but doesn’t have to. My bolt extends past thimble/tp then retracts back out of the way.
https://www.gatewaytoairguns.org/GTA/index.php?topic=178164.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=178164.0)
Dave
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What I like about a thimble is that it makes it possible to index the barrel... there are other advantages also...
the downside is the work needed does need to be pretty precise... each barrel/thimble would have to be a set as stated...
https://www.gatewaytoairguns.org/GTA/index.php?topic=52432.20 (https://www.gatewaytoairguns.org/GTA/index.php?topic=52432.20)
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I requested that Travis make my .20 JSAR Raptor with a thimble configuration.
I liked it and it was very interesting to watch that impact point rotate as the barrel was indexed. Seems like I settled on the 12 o clock position.
The concept sure can't (or maybe even CANT is at play here) seem to hurt and potentially even help with keeping impact points in a vertical line, especially as distance increases.
Not sure why more manufacturers don't go that route though.
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The thimble breech is one area where more parts actually makes making the parts simpler.
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The thimble breech is one area where more parts actually makes making the parts simpler.
Exactly my thoughts, also if you make a mistake you just need to make one new part. I've been doing some rimfire BR and got interested in bullet seating depth and it's effects on accuracy. Having the actual leade in on the barrel and chamber + TP in the thimble would allow experimentation with seating depths and we could even try different thimbles for different length bullets. It would also allow experimentation with chamber tightness. I think I'll give it a go, experimentation is always interesting.
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Guys
Here is some interesting info on bbl indexing, it is showing with the 22 LR,
but will work on just about any rifle or I would think air rifle etc. Look at the groups size/POI impacts photo's etc.
https://www.6mmbr.com/gunweek080.html (https://www.6mmbr.com/gunweek080.html)
This is only 1 of the articles dealing with indexing/thimbles, listed on this site.
Don
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I built one PCP using a thimble, getting the alignment 100% straight and concentric between the thimble and the barrel is the hard part.... If it isn't perfect, you will find tight spots when you rotate the barrel to index it.... Mind you, I didn't just butt them up like in the photo above, I machined a spigot on the barrel and a matching recess in the thimble, to make sure they lined up.... This was on my 7mm Hayabusa....
(https://hosting.photobucket.com/albums/oo221/rsterne/Hayabusa PCP/.highres/Thimble_zpsxzzhas8x.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/60abf6ff-337f-4270-86ba-4dd32dede705/p/6ee00110-a984-4a65-8d6d-335934f99d07)
There was a long threaded sleeve (with the flats) which screwed into the receiver to hold the barrel tight against the thimble.... It pushed on the sleeve in the above photo, which is loctited on the barrel threads....
(https://hosting.photobucket.com/albums/oo221/rsterne/Hayabusa PCP/.highres/Barrel and Bolt_zpsbthnumwp.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/60abf6ff-337f-4270-86ba-4dd32dede705/p/d7c6e16e-9331-421d-a11b-c152f3c78d4c)
Here is a pic with the retaining sleeve threaded into the receiver.... A CF shroud slid over the boss on the receiver....
(https://hosting.photobucket.com/albums/oo221/rsterne/Hayabusa PCP/.highres/7mm Breech Top_zpsuvxrgl0j.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/60abf6ff-337f-4270-86ba-4dd32dede705/p/131ad3de-654c-40f6-b2c4-d52b3dd6462b)
There was an air stripper at the muzzle, and a stack of Belleville washers to tension the barrel by putting the shroud in compression....
(https://hosting.photobucket.com/albums/oo221/rsterne/Hayabusa PCP/.highres/Barrel Tensioner Sleeve_zps4btf2mcf.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/60abf6ff-337f-4270-86ba-4dd32dede705/p/bcd5701d-7828-4ff2-817a-02332ea32ac0)
(https://hosting.photobucket.com/albums/oo221/rsterne/Hayabusa PCP/.highres/7mm with Shroud_zps4i4eugfs.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/60abf6ff-337f-4270-86ba-4dd32dede705/p/b99802e6-8862-4ef7-b052-56d75ce5275b)
Bob
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Make the barrel and thimble meet on a self centering taper...
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Would that not wedge together and make it difficult/impossible to rotate the barrel to index it?....
Bob
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How stuck the taper gets depends on the angle. Think Morse taper (3 degree included) on the chuck or live center stem in your lathe tailstock. The "friction sticking" range of angles are well known for steel on steel. If you use a 60 degree included cone it will come free as soon as you stop pushing on it. Ditto for 30 degrees. If you use less than 6 degrees will start to wedge. So, you design with your user intent in mind.
Note the "fast taper" designation for automatic tool changer collets that use a drawbar while working, but must release freely when the drawbar tension is removed: https://en.wikipedia.org/wiki/Machine_taper
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Cone clutches exist so that less clamping force is required to produce a certain level of friction coupling torque. Yet the cones must release immediately the axial force is relaxed.
The cones are coated with friction material, rather than being steel on steel, but the principle of releasable cone angles helps ballpark a conical thimble: https://roymech.org/Useful_Tables/Drive/Cone_Clutch.html
The cone angle α is always above 8deg and is normally between 12deg and 15deg. if the angle is less than this value than the clutch is liable to jam in engagement.
(https://roymech.org/images/cone_clutch.gif)
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I don't quite understand why the thimble and barrel need to be very tight against each other. My plan is to do it like JSAR did in the photo above, a completely separate thimble and o-ring to prevent the airblast from escaping. From bullet loading point of view a tiny gap of 0.1-0.2mm shouldn't matter as long as the barrel and thimble are concentric.
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A slight oversize id on thimble and a smooth taper into chamber mouth will afford some forgiveness in the pellets travel to the chamber.
Dave
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I think those who assume a thimble and barrel can just be put into a drilled hole in a receiver and located with set screws.... need to make one first before they have a lot to say about how easy thimbles are to align with a barrel to allow indexing and accuracy.... YMMV, of course, as your machining skills and equipment may far surpass mine.... ;)
Bob
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Done it….
I guess you are assuming set screws, not the case. Lots of ways to skin a cat.
Dave
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I envision a threaded collar holding the barrel in the receiver. The barrel stub can also be tapered to always sit in the receiver without slop. The thimble can meet the barrel on a mating taper on one end and sit in slight parallel clearance at the other, inside the receiver. O-rings placed strategically can be used to minimize dead volume near the TP. The barrel to thimble seal can have an O-ring sit adjacent to the barrel to thimble taper; or nearer the TP.
Something like an AR-7 takedown rifle, with or without barrel indexing feature (so you can find the best barrel orientation for grouping). Rather than drive the barrel stub into a shoulder, stop it on a mating taper. AR-7 barrel attachment images below.
If all this sounds too complicated, just making the projectile to thimble bore clearance slightly larger, than the expected barrel bore to thimble bore alignment, should provide enough eccentricity tolerance. How does FX manage its thimble concentricity, and sealing to the receiver and to the barrel?
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I prefer to use a thimble when I do a re-barrel. I've done it on 4 different guns. It's easy to change thimbles and transfer ports. Less chance of messing up on the barrel since I don't need to port it. When I turn the breech end of the barrel, I use a dial indicator and a 4 jaw chuck to center off of a pin gauge fit inside the bore. Not a perfect fit, but no taper needed. Chamfer the barrel just enough to accommodate any mis-alignment and that will help guide the pellet in. If there is no setscrew at the thimble, I add one to hold the thimble in place. Sometimes, I'll add an extra setscrew to hold the barrel. Other times, I've gone with a threaded barrel. Threaded barrel is more secure. Setcrews can allow easier indexing. I have tried indexing, but I usually don't worry about it. I use an o-ring between the thimble and barrel. Sometimes another o-ring on the thimble, depending on how I want to seal the transfer port.
I usually leave the barrel as long as possible, so a thimble gives me another 3/8" of usable barrel. Easier to redo the thimble than the barrel.
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I like it, Scott
Does what it needs to do, without risking an expensive barrel, with an oops at the TP.
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Yep, excellent description Scott and works out great. Also like that you mentioned it saves every possible bit of barrel for its intended purpose.
Dave
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I have 360deg index able barrel with more than bore size ports around to the bullet at any given degree of barrel rotation.
Barrel is secured with pressure fitting, four bolts tensions the barrel to the breech. Just like Anschutz does it with 22lr match rifles.
No thimbles or nonsense needed, one action for any caliber, just remove the barrel and probe.
Marko
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Marko,
Do you have pictures you can share?
Thanks
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Guys
Here is more info using a .177 air rifle bbl and thimble,
http://benchrest.com/showthread.php?96044-Barrel-Indexing&highlight=indexing (http://benchrest.com/showthread.php?96044-Barrel-Indexing&highlight=indexing)
Good photo's on post #15 etc.
Hth's,
Don
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Barrel is secured with pressure fitting, four bolts tensions the barrel to the breech.
Marko
Would I be correct that you mean a compression fitting (as they are known here)?
Jesse
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A little digging into the Anschutz was justified. I see it, but don't fully understand it:
https://www.ssaa.org.au/?ss_news=classy-anschutz-doesnt-disappoint (https://www.ssaa.org.au/?ss_news=classy-anschutz-doesnt-disappoint)
The receiver ring is precision bored to take the shank of the barrel in a tight, slip fit, the barrel accurately located by a short pin in the shank mating with a slot in the front of the receiver ring.
Inside the ring is a virtual V-block mounting system made up of two shallow C-shaped locking clamps housed in recesses in the bottom of the ring, internal curve of the clamps exactly matching the shank diameter. Screws through the bottom flat of the receiver are wound up to push each clamp against the shank and hold the barrel in place, a simple but precise way of locking everything together while still allowing barrel interchanges.
(https://i1.wp.com/www.ssaa.org.au/wp-content/uploads/2021/02/IMG_7318_2.jpg?fit=1200%2C800&ssl=1)
https://www.anschutznorthamerica.com/1761-details.html (https://www.anschutznorthamerica.com/1761-details.html)
(https://www.anschutznorthamerica.com/uploads/1/9/9/7/19975381/published/action-and-barrel-detail-1761.jpg?1557440551)
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Marko,
Do you have pictures you can share?
Thanks
They should be in the forum in the Lucky shot build thread if they are not deleted already.
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Barrel is secured with pressure fitting, four bolts tensions the barrel to the breech.
Marko
Would I be correct that you mean a compression fitting (as they are known here)?
Jesse
English is not my native language so thanks for correcting.
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They should be in the forum in the Lucky shot build thread if they are not deleted already.
Thanks. Do the pictures in this post look familiar? : https://www.gatewaytoairguns.org/GTA/index.php?topic=190039.msg156200207#msg156200207 (https://www.gatewaytoairguns.org/GTA/index.php?topic=190039.msg156200207#msg156200207)
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That thread is this one I think.
Dave
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Barrel is secured with pressure fitting, four bolts tensions the barrel to the breech.
Marko
Would I be correct that you mean a compression fitting (as they are known here)?
Jesse
English is not my native language so thanks for correcting.
Didn't mean it as a correction; I figure that when folks are having a conversation with different base languages that bridging any terminology differences is a shared task-so no need to thank me.
I like that idea-you are basically holding the barrel in a collet... A self-holding taper with a lock nut might be another interesting method...
Thanks,
Jesse
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That thread is this one I think.
Yes. With pictures.
I could have said look up at post number #, or provide a link :)
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I like that idea-you are basically holding the barrel in a collet...
Some engines use split tapering poppet valve retainers, that appear to have similar functional properties to the Anschutz barrel retainer:
(https://i.ebayimg.com/images/g/z2AAAOSw65FbVZkl/s-l1600.jpg)
(https://www.bimmerworld.com/Valve_Keeper_11341461405.jpg)
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I like that idea-you are basically holding the barrel in a collet...
Some engines use split tapering poppet valve retainers, that appear to have similar functional properties to the Anschutz barrel retainer:
I wouldn't agree that the Anschutz retainer is at all alike to a split bushing/collet/compression fitting system... By the description/pictures of the Anschutz breech, it is nothing more than a (very well done) set-screw mount-and will give not anything more than two lines of contact between the barrel and the breech... A compression fitting gives (almost) guaranteed concentric clamping/contact; a collet or split bushing should (done right) do the same-done less accurately they will/should (or may-in the case of the split bushing) give three points/lines of contact-and thus be more stable than the two lines of any set screw configuration-barring one that pushes the barrel into a lobed ID that results in the aforesaid three lines of contact... I believe Scotchmo's solenoid-driven build uses a V-block and U-bolt type configuration to accomplish 3 lines of contact-will try to dig up that thread...
Jesse
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Here it is:
https://www.gatewaytoairguns.org/GTA/index.php?topic=170568.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=170568.0)
Jesse
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Thanks Jesse,
Scotchmo's thread is brilliant. Pictures don't display for me in the thread, but I can open them in a new tab. It will take some work to find the barrel mount images. Here is the TP, as a for instance.
Have to click on the ling. Can't imbed the image: www.scotthull.us/photos/Armada/IMG_2379.JPG (http://www.scotthull.us/photos/Armada/IMG_2379.JPG)
One comment about the collet barrel mounting sytem. There is no need to copy the Anshutz or engine valve system exactly. The point is to understand the concepts and designs, then come up with a simpler system that is as good, or better. Or easier to make. Whether it is based on one or all of the observed principles.
Quite frankly, Schotchmo's bottle clamp could be used for a barrel mount, providing it starts with a slipfit (else centering will be offset when tight): www.scotthull.us/photos/Armada/Armada-sleeve-bottle.jpg (http://www.scotthull.us/photos/Armada/Armada-sleeve-bottle.jpg)
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I never took it that you meant that the Anschutz/any other system had to be copied-I merely was pointing out how (to me) the way that it worked, or didn't... In the context of a separate thimble; I find the various concentric clamping methods the most intriguing-as the thimble, being a much smaller and more readily worked/replaced piece; could be made to ride in a reamed bore, with precision-the barrel(s) clamping surface needing only be concentric with the bore to achieve alignment between the two...
Jesse
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Anything to get away from the typical set screws bearing on a thin inner barrel. Thinned some more by creating convenient dimples to locate the screws. There is no way such airgun barrels are round to 0.001" when the grub screws are tight. Even worse when they are overtightened.
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How about using something like a ER-20 collet?
https://littlemachineshop.com/info/er_collet_sizes.php
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How about using something like a ER-20 collet?
https://littlemachineshop.com/info/er_collet_sizes.php
Great idea. The cheap collets may have over .001" runout, but can be trued to a larger ID with hard tooling or grinding. Decent collets, may have a one or two ten-thousands of runout.
I think there are collets made for machinating to a custom size too.