I guess I wonder about the "inducing stress" remark.
My understanding:
If I take a softball made of rubber bands and begin to randomly snip at it, before too long it will no longer be spherical. By doing so I actually removed the tension (stress) that was inherently present in the mass.
It's been my experience that machining an object with cutting tools behaves much the same way. The classic example that I fall back to is when I was making reactor parts for General Atomic in the mid/late 90's. You toss a slab of 17-4 on a vise and fly cut it, then watch it turn into a banana when the vise is loosened up.
-Intro here to "stress relieved" or "normalized" materials. This is a heat treat process executed well ahead of the material going inside a machine. If you buy the good stuff, it's far less likely to push around on you. The example I use here is if we take the same ball of rubber and cook it at an appropriate temperature for a known period of time, it's much less likely to morph into another shape after I whittle on it. It may not stay exactly the same coming out of the oven as when it went in, but it will be closer than when the scissors went after it like in my first example.
The summary here is to begin a job with a material condition appropriate for the desired outcome. Good barrel companies have been doing this for decades. So long as a barrel has a round hole, decent surface finish, and a uniform rate of twist from one end to the other, chances are good it'll print nice groups on paper. We as a shooting community see this all the time.
C.
Amazing you bring this up.. because yesterday and today I had to cut a huge piece of brass to make a part that got lost at a chrome shop.
And as the stocks weren't available for the size I want, I got over-sized and figured it would be easy to fly cut it down.
Well no. Cut 1/8" off in several passes and the bar (28" long) bowed up like a banana. This is naval brass, purchased from MSC (Probably chinese shit.)
I straightened on an arbor press and got it back flat. Then saw cut off another piece... and watched it banana out almost 3/8" over 12".
Again, straightened on an arbor press. I was thinking that I was taking too-aggressive cuts and work-hardened one surface and caused the bow. Sort of like an English Wheel. Harden one side and it bows the metal into a compound curve.
This morning, called a buddy at a company in VT where they make the machines that cast stuff like that... to ask him about it. He and I were in school together... he knows his shit! They make the machines to do all kinds of metals in continuous strip casting... and asked his opinion. Which was interesting.
He said it was definitely stress in the bar. Not from the casting process, but from the rolling process... with the rollers set up just downstream from the strip casting machines. The more aggressively they roll... the more stresses that get introduced to the bars. The more they 'banana' when machined. Back a bunch of years ago, most mills ran many rollers to gradually shape the final bar. Might be as many as 10 rollers. These days, the 'cut rate' mills in, shall we say, certain countries that undercut American Steel production... run things through 3 rollers at most. Jamming the metal to shape. And introducing massive stresses.
Well, I can tell you that cutting a 1/2 x 1/2 piece out of my part to make a feature... that 'cut out' part banana'd almost 3/8" of an inch over a bit more than 12 inches! Talk about stress.
I read this post this morning... while making the part.
Very informative and timely!
Cheers,
Sirhr
PS. Ruger has been EDM-ing barrels for years. Saw the machines when doing a plant tour about a decade ago. Nothing new. The chemical stuff... that is some cool technology! Again, Theis, and the rest of the folks on SH are on the bleeding edge. This place is THE home for a lot of advances in, not just shooting, but in metallurgy, barrel design, CNC... so much amazing stuff goes on in this community. The ELR folks are at the bleeding edge all the time. Awesome!
