Recoil lug thickness studies

[aschler]

Not really. It takes a fairly large amount of force to shear a piece of steel 3/16" thick. Even if the stock started out flat and parallel, there is always distortion, especially at the edges, when shearing/stamping. I guess it depends on what a person considers "flat". I don't consider .001-.002 flatness/parallel acceptable. There is a difference between the two. The only way to get something truly flat and parallel is through lapping and grinding operations which, like you said adds more operations, drives the cost up and lowers production. When I rework a lug, I grind as flat as possible, lap the best side as flat as I can get, then skim grind the opposite side a 2nd time to insure parallelism. They're usually within .0002 or less. When I make one from scratch, the blank gets rough ground flat & parallel, the hole drilled and bored square to the faces, the outside profile milled, deburred and then check-lapped one side and final grind on the other to a thickness of .200. I pick that thickness for 2 reasons...one it adds a bit of beef over the standard 3/16 factory lug, and .200 is an easy number to work with when setting head space.

I remember trying to get a 7/8"x2"x3" piece of steel flat with-in 0.0003". You had to turn down the power on the surface grinder's magnet or you would distort the block. If you turned it down to much the block would go flying.

 

[ntsqd]

Sounds like the mag chuck itself was out of true. BT, DT.

 

[Doom2]

To bend a recoil lug is going to require poor bedding and/or a very rigid shooter to avoid absorbing the recoil energy. One of the quickest ways to destroy a rifle years ago was to put the wood stock against the trunk of a tree and pull the trigger. Dumb stunt but it happened (no, not me). I cringe when I see people come to the range and put high a recoiling rifle with a wood stock in a lead sled with 50 lbs of weights.

 

[aschler]

Sounds like the mag chuck itself was out of true. BT, DT.

I'm sure it was and I'm sure the teacher knew it was was. When the scale gets small enough things you realize a lot of the things you take for granted are not true. Almost nothing in the world is flat, square or perpedicular and a 7/8" thick block of steel is pretty flexible.

 

[jpndave]

Strength of parts generally can be increased in three ways, more mass (thicker part in this case), stronger material such as going from a mild steel w/strength of ~40kpsi to a high heat treated alloy that could go over 200kpsi, and third better design which would be difficult with the constraints of the existing system. Better bedding would absolutely add to the strength. If the recoil force is applied in sheer right against the action that much different than a bending force lower on the lug.

Steel is NOT all the same strength and the difference between annealed (dead soft) and a high heat treat can be substantial depending on the alloy. Generally the higher the heat treat the more brittle the steel becomes. The yield point (bend) gets a lot closer to the ultimate or failure point (break).

I am going to try a Titanium Alloy on a Kimber 84M I am building because I want to see what it does. I'll use a 145ksi titanium alloy which is stronger than most steels, even some alloys with a decent heat treat. 416 as below is 120kpsi max which is probably what is in there now and not up to the upper end heat treat either.

I'm not certain what the factory ones are, likely 416 & 4140 or 4340? Aftermarket are all over the place 416 (75-120kpsi), 4340 (ultimate 100-190kpsi), 4140 (ultimate 100-160kpsi), 17-4 (130-200kpsi). For reference 1020 mild steel (ultimate 62-67kpsi). So yes, material and heat treat do matter. In this use, the bedding is far more important though.

 

[Rflshootr]

I remember trying to get a 7/8"x2"x3" piece of steel flat with-in 0.0003". You had to turn down the power on the surface grinder's magnet or you would distort the block. If you turned it down to much the block would go flying.

I'll bet I can tell you which part of that you remember the most.

 

[Bob Wright]

I remember trying to get a 7/8"x2"x3" piece of steel flat with-in 0.0003". You had to turn down the power on the surface grinder's magnet or you would distort the block. If you turned it down to much the block would go flying.

In our development shop the guys on the surface grinders would throw down a paper towel wetted with water/coolant then the part on top of that. It would take up the out of flat condition of the part on the magnet side. Kiss grind the top, then flip it over for finish grind. Parallelism was excellent.