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Paul Alciatore
03-06-2005, 06:00 PM
I am making some arms out of 360 brass; 1/2" square. They are about 3 5/8" long and have a reduced section for about 2" at one end. The reducded section is 1/2" x 3/16" and in the finished part will have a 1/8" slot in it. I konw it would be best to have this reduction placed in the center but the design requires that it be on one side.

I saw cut the bulk of the material off for the reduced section and am milling the remainder with a 1/2" end mill. After saw cutting, I noticed that the reduced ends were bent outward. I figured that there were stresses from the rolling mill and when I relieved them by cutting half of the metal away, it just had to bend. So I carefully bent them back to as close to straight as I could. The part is not high tolerance so this should be OK. I'm sure 10 or even 15 thousanths will make no diff. in the end and I was able to get them back to +/- 0.005" or so.

When I continued with the milling to get to the final dimension, about 2/3 of them bent again and had to be straightened again. Seems to be about 50% - 80% as much as the first time in spite of the fact that I removed only about 15% as much metal.

What concerns me is what they may do after they are in service. Can I expect more semi-randum bending? If I have to straighten them 4 or 5 times, will I induce stress cracks? Do I have a bad situation?

Paul A.

Tony
03-06-2005, 06:55 PM
paul,
a subject near and dear to my heart..
straightening them by hand a few times,
especially if only 15 thou or so, shouldnt
induce any cracking.. though i'm not that
familiar with 360 brass.

after saw cutting, before milling, throw them in a rotary tumbler or deburrer for an hour or so, if you have access to one. see if that doesnt reduce (if not eliminate) the distortion.

you could try lightly peening one of them with a light hammer. peen them on the surface to be milled so you can cut off any marring. the peening could get tiresome.

alternatively, you could 'normalize' them in an oven. check the specs for your brass and see what the tempertures are like. you may be able to do it your kitchen oven.

let me know how it works out.
-tony

Paul Alciatore
03-06-2005, 07:26 PM
Tony,

Not sure I completely understand either of your suggestions.

Should I pean on the concave or convex side? I must mill on the convex side - no choice there. How does this work? Is it the shock acting on the crystaline structure or does the peaning spread the metal?

On the heat treatment, would I do that before or after milling?

Paul A.

ricksplace
03-06-2005, 07:51 PM
I'm not that familiar with 360 brass, but cartridge brass anneals quite well. Heat it to cherry red and quench in water. This makes cartridge brass ductile and very resistant to cracking (good for forming wildcat cases) but may make the brass too ductile for your use. When I anneal cartridge brass, I hold the case with bare fingers at the case head and heat the shoulder in a propane torch flame until red, then dump the case in a pan of water. Holding the case with bare hands ensures that the cartridge case head does not become annealed, and consequently, too soft. A too soft case head will rupture when fired with disasterous consequences.

Tony
03-08-2005, 02:56 PM
paul,
vibrating (either in a tumbler, or by peening) will reduce the residual stresses in your brass .. though i'm not sure how well brass responds to this. at any rate, even if it only normalizes (and not reduces), it will reduce the amount of bending you get when cutting the stuff.

the best bet is to "mechanically excite" the work before you do any cutting. that is, stabilize it as early in the process as you can.

you can peen either side. even the thicker end. the idea is to impart some energy into the workpiece. you're trying to "shuffle" those stresses around.. move them towards the surfaces to balance them out. that way, cutting one side off will have less of an effect on the other side.

even with heattreat, your best bet is to do it before you begin cutting if the stock isn't too large.

you dont want your work to move/bend on you, so do it as early in the process as possible.

(maybe even buy it already normalized)

-tony