Shrink fitting end caps

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speed6
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Shrink fitting end caps

Post by speed6 » Tue Jul 05, 2016 10:33 am

Has anyone tried shrink-fitting end caps on a rifle suppressor? I want a fairly light can (say, 18 oz max) but I want to be able to make it with just my hobby lathe (grizzly G8688), which doesn't like machining anything but free-machining steel or aluminum that much. I had an idea: 0.035" wall 1.5" OD 4130 tube (I can machine this if it's just squaring up the ends and maybe some chamfers...otherwise it gets really tedious and the surface finish isn't the best) shrink-fit onto 416 stainless end caps. I would heat up the tube with either a torch or in an oven, cool off the end-cap with dry ice, then press them together. I DO have access to a decent shop press, but have been thinking of just buying my own.

The rest of the can would probably be 416, with either 416 bar turned/bored into spacers with lightening holes OR something more exotic...304 tubing (I haven't tried this but am not hopeful it will work...at best squaring up ends etc. on the little grizzly will be all I can do) or titanium tubing (this involves some kind of coolant system and is probably in the same boat as the 304 tubing as far as machinability goes). Worst case I would use some 4130 tubing and turn it down to the right OD then drill copious lightening holes on my drill press with a cylindrical part drill jig.

IF this is feasible, would I need to machine the mount threads after the mount-side end-cap is pressed in? I have to use a steady rest and although I have had good results keeping things straight using it I would like to avoid this if possible.

a_canadian
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Re: Shrink fitting end caps

Post by a_canadian » Tue Jul 05, 2016 2:42 pm

You haven't stated a caliber, but at a guess anything larger than .22lr and shooting subsonics only seems likely to blow a press fit end cap right out of the tube. Even with a liquid nitrogen soak for the end caps I'd not want to trust compression and friction to hold it together with significant internal pressure, as well as the heat in the rear of the can on firing. At a minimum you'd want to add green Loc-tite to the surfaces to bond better. Perhaps a TIG weld bead around each end would be a better idea to avoid watching your baffles fly downrange. Or just buy one of those sdtactical or other pre-made cans with end caps. There are Ti versions available.

vaeevictiss
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Re: Shrink fitting end caps

Post by vaeevictiss » Tue Jul 05, 2016 3:27 pm

i couldnt imagine building anything larger than a .22 out of a tube with a thickness of .035. Thats dangerously thin for anything else. 1.5" OD would also be huge for a .22 can.

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CMV
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Re: Shrink fitting end caps

Post by CMV » Tue Jul 05, 2016 6:26 pm

Do as described & then drill 13/64 or #6 holes and use 3/32 roll pins to secure the caps?

I think you'd want to use a little thicker tube so you could undercut a square shoulder for your end cap to butt up against to make sure it's square when pressed in. Maybe that wouldn't matter - never done it, but picturing just a cap being pressed into a tube on a shop press it would seem there's a lot of margin for error for not getting it perfectly perpendicular.
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alordnapa
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Re: Shrink fitting end caps

Post by alordnapa » Tue Jul 05, 2016 6:59 pm

I have visions of baffles being launched out the end of the tube...Consider how much your tube is going to heat and cool in actual usage, and the idea of a straight interference fit seems less attractive. I would consider a couple of set screws to back-stop the end cap, in addition to, or in place of, the shrink-to-fit concept. If this is a sealed can, never to be opened again, you could build a roll-crimp style die ( The old-fashioned way they closed shotgun shells) to roll the end of the tube to keep the end cap captive. If your tube is aluminum, "aluminum solder" is pretty good for soldering on caps, but you have to be careful not to smurf your material with overheating, even though its done at propane torch heat ranges. Good luck, and try not to create an arms-length pipe bomb!

speed6
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Re: Shrink fitting end caps

Post by speed6 » Wed Jul 06, 2016 8:43 am

I had a 5.56 rifle can in mind for this. This is really just in conceptual phase right now and I wanted to see if anyone had successfully done this before. Threaded inserts for the mount cap ARE done this way. I have seen posts where people on this forum have done it, and I'm pretty sure I have a gemtech multimount end cap for 300 blk that is done this way. It seems like the blow-out forces involved would be the same for an insert or an end-cap, but the end-cap has much more contact area to counteract this force. I have not seen this done on a rifle can though. Also, I had assumed that 400 series stainless and alloy steel had approximately the same rate of thermal expansion but I was wrong. This is probably enough to abandon this concept by itself.

I took a look at ASME B&PVC section II table 1A (SA or allowable stress for ASME materials) and found the closest material to 4130 (SA-213 I think) at 700F. This is only ~15 ksi, so even with a large blast chamber (~2 inch length) I would exceed the allowable but still be under yield at this temp (26 ksi). I am going with a thicker tube...say 0.49...SA is a pretty good maximum to shoot for (no pun intended). I originally used some interwebs data that showed 60 ksi yield and was taking 0.6 of this, which is about what SA ends up being, but this is way high according to ASME. I also assumed that 4130 material properties do not drop off that much up to 600 or 700 degrees, which they really don't, but I still trust ASME way more than the internet number.

I think I am going to take a closer look at the set-screw idea. Thanks for the replies!

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Re: Shrink fitting end caps

Post by a_canadian » Wed Jul 06, 2016 10:21 am

I've seen aluminum start to pull and stretch around set screws on a .22lr can. Sure that was just 1" OD 6051 with a 0.65" wall, with a press-fit 7075 rod for the rear plug, but it was also just subsonic ammunition and started sliding at under 100 shots. Your 5.56 is going to rip through set screw holes like cheese. Think about the contact area of a couple or a few set screw holes compared to half a dozen or more threaded turns of proper end caps. Something like a 100:1 ratio more contact with threaded caps. Set screws are fine for a low powered air rifle but that's about it. Even for that I've stopped using set screws as I trust threaded caps more.

vaeevictiss
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Re: Shrink fitting end caps

Post by vaeevictiss » Wed Jul 06, 2016 12:32 pm

speed6 wrote:I had a 5.56 rifle can in mind for this. This is really just in conceptual phase right now and I wanted to see if anyone had successfully done this before. Threaded inserts for the mount cap ARE done this way. I have seen posts where people on this forum have done it, and I'm pretty sure I have a gemtech multimount end cap for 300 blk that is done this way. It seems like the blow-out forces involved would be the same for an insert or an end-cap, but the end-cap has much more contact area to counteract this force. I have not seen this done on a rifle can though. Also, I had assumed that 400 series stainless and alloy steel had approximately the same rate of thermal expansion but I was wrong. This is probably enough to abandon this concept by itself.

I took a look at ASME B&PVC section II table 1A (SA or allowable stress for ASME materials) and found the closest material to 4130 (SA-213 I think) at 700F. This is only ~15 ksi, so even with a large blast chamber (~2 inch length) I would exceed the allowable but still be under yield at this temp (26 ksi). I am going with a thicker tube...say 0.49...SA is a pretty good maximum to shoot for (no pun intended). I originally used some interwebs data that showed 60 ksi yield and was taking 0.6 of this, which is about what SA ends up being, but this is way high according to ASME. I also assumed that 4130 material properties do not drop off that much up to 600 or 700 degrees, which they really don't, but I still trust ASME way more than the internet number.

I think I am going to take a closer look at the set-screw idea. Thanks for the replies!

damn, i would be willing to bet building a 5.56 rifle can out of a .035 wall tube will have the same effect as taping an explosive to the muzzle. .049 would be better but i dont know if thats still safe. From what ive read here is most production cans are at least .065 and F1 builders try to go no lower than that. I did .055 on my recent .223 can and was very nervous about it but its been great. Doing .049 safely would probably warrant the use of a sizable reflex chamber.

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LavaRed
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Re: Shrink fitting end caps

Post by LavaRed » Wed Jul 06, 2016 12:40 pm

I would not safely go less than 1/16" wall thickness. Also, I would not press fit. Four allen screws radially placed around the endcap will do just fine and much better than a press fit.
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Re: Shrink fitting end caps

Post by Historian » Thu Jul 07, 2016 8:20 am

LavaRed wrote:I would not safely go less than 1/16" wall thickness. Also, I would not press fit. Four allen screws radially placed around the endcap will do just fine and much better than a press fit.

+2!

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Re: Shrink fitting end caps

Post by 300sniper » Thu Jul 07, 2016 11:33 pm

I'm thinking if you had to do this, it would be best to make the end caps fit over the tube instead of inside. At least this way the expansion from pressure and heat would be working in your favor.

I'm not looking any of this up now so it's pretty much coming out my ass, but say your end cap is .003" larger than the ID of your tube. When you fire, the tube expands .002" while at the same time has the most pressure pushing the end cap out. Doesn't sound like a situation I'd intentionally create.
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policetruck
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Re: Shrink fitting end caps

Post by policetruck » Fri Jul 08, 2016 7:27 am

Also remember, every rifle that has a pressed in barrel also has a cross pin to lock it in place.

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gunny50
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Re: Shrink fitting end caps

Post by gunny50 » Sat Jul 09, 2016 9:19 am

policetruck wrote:Also remember, every rifle that has a pressed in barrel also has a cross pin to lock it in place.
What about the STEYR SSG 69 rifles?
Can't remember a cross pin there. :shock:

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policetruck
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Re: Shrink fitting end caps

Post by policetruck » Sun Jul 10, 2016 7:20 am

Doh! That's what I get for generalizing.

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