Low plasticity burnishing to increase service life of AR/M16 bolts

[cmshoot]

In the LPB process, a smooth, freerolling spherical ball is pressed against and rolled along the surface of the workpiece to be burnished, deforming a surface layer into a state of compression.

LPB costs less than does laser shock peening and offers greater depth and stability of the compressive layer, relative to shot peening.

In a single-weapon test conducted with a US military-issue M4A1 carbine, the service life of the current mil-spec bolt with M855 ammunition was approximately 13,000 rounds.

After the application of the LPB process, the service life with M855 ammunition was approximately 26,000 rounds.

The primary cause of bolt failure is cracked locking lugs.

 

[bambam]

Is it common practice to replace the bolt without placing the barrel extension?

 

[cmshoot]

Is it common practice to replace the bolt without placing the barrel extension?

You don’t typically replace a barrel extension; you replace the entire barrel. The throat will wear out before the barrel extension will.

When replacing the bolt, you confirm headspace and that’s it.

 

[Jambo]

All ball burnishing does is cold work the surface of the part. Cold working is a common method to increase hardness and strength, since it increases the density at the surface. Kind of like shot peening, only you can do it in the same turn / milling machine that makes the part. Just a tool change and some high pressure coolant to drive the ball.

I'm not sure how one could ball burnish into the corners of the locking lugs; the ball has a non-zero diameter, so it can't reach the sharp corners of the locking lugs. It just can't get in there if it's a ball of any size at all. The locking lugs create a stress riser because it's a change in geometry, and sharp corners, so that's where I would expect the failure to start, just due to fatigue. I don't see how this process could prevent that, unless it's a very small burnishing ball.

It could improve the strength and surface hardness around the cam pin opening, so it could help prevent failures there.

A few years back we manufactured a ball burnished product, using the high pressure coolant in the machine at 3,000 psi, if I remember correctly. But the ball was about 10mm (about 3/8") in diameter, if I remember correctly.

 

[cmshoot]

I have a PDF document on the process, along with the results of preliminary testing. Bolt life was approximately doubled after application of LPB. Testing was conducted by the U.S. Army Research, Development and Engineering Command.

 

[cmshoot]

 

[cmshoot]

 

[cmshoot]

 

[cmshoot]

I’m not any sort of expert on LPB, and I did not stay at a Holiday Inn Express last night. I only came across the process fairly recently. The stats I read sounded very interesting, and I’d enjoy talking about it with anyone that knows about it more than I……..which wouldn’t take much.

 

[Jambo]

I would be happy to share what I think I know, but I'm certainly no expert. I have done a little research into cold work surface hardening, but clearly not to the level of these guys. It looks like what they're doing is basically the same as shot peening, only with more force and more control, as it's probably done in a turn / milling center. It is a way to get more strength and fatigue life out of part through "low temp" surface hardening.

We've done ball burnishing on parts for the company I work for in the past. That's not exactly new technology (we were doing it 15 years ago, and it was not cutting edge then), but doing it on something as small as an AR-15 bolt, especially around the lugs, is impressive to me. How close you can get to the locking lug is basically controlled by the burnishing tool's diameter. A smaller ball can get closer to a sharp inside corner; however, since force is pressure multiplied by area, and a small burnishing ball has a small area, a small burnishing ball can't generate the same force as a large one.

It certainly looks as though they figured it out, if their report is to be believed. I still want to see some independent testing.

It's probably only possible because of the accuracy, precision, and power of modern CNC machines. Some of them are pretty impressive; how about 1200 ft lbs of torque at 0 rpm and all the way through 6,000 rpm? We have one of those, and it's not even the most powerful the manufacturer offers. Couple that with positional accuracy measured in microns and a high pressure coolant pump that could crush a small submarine and things like this are possible - if you can imagine it.

Thank you for posting this. I might keep these guys in mind in case we have problem parts at work; maybe the process could benefit other parts.