I'm wondering if anyone has experience with the structural stability
of brass after it's been machined? example: If I formed a cylinder out
of solid brass round stock on the lathe, and milled and opening in one
side, similar to a clarinet or saxophone mouthpiece -would the part
warp or move over time? Is it necessary to apply heat to the part, ie
stress relief to lock it into its machined form?
My concern is that the force of the tool bit would twist the bar stock
in the lathe and then after the part is made it would slowly find its
way back and warp. Also the wall of the cylinder would be thin and
large amounts of force and heat would be applied to remove the stock
in order to form the part.
Any help would be appreciated, Thanks
You have it wrong. The distortion one experiences when altering bar stocks
of all kinds has little, if anything, to do with tool pressure. Materials
that have been worked, especially when cold, contain considerable stress
from the cold flowing of the alloy. When you machine on them, you tend to
relieve the inherent stresses. Generally, if you machine on both sides
equally, such as a round piece in a lathe, the stresses tend to be relieved
somewhat equally as well, so you don't experience much movement. However,
when you alter materials unequally, you can expect considerable movement.
Those of us that have worked in the trade know to always rough pieces before
finishing. That means you go over the *entire* part, removing the bulk of
the unwanted material, but leaving an amount for finish cuts. A rule of
thumb is to leave 1/16", but you can alter that depending on the particular
circumstances. The less you leave, the more likely you are to have finished
parts that do not distort, but you also risk having parts that don't clean
up when it's time to take finish cuts. That way you generally remove the
vast majority of the pent up stress with the roughing cuts, and the finish
cuts then return the finished part to being relatively straight, flat, etc.,
by removing the now altered (from stress relief) surfaces. Does this make
sense? If not, lets talk about it some more.
Don't heat your brass parts unless you want them soft. Brass is sold in
various states of hardness, all created by cold working, (work hardening).
It's a desirable property, depending on how the material will be applied.
All you have to do to lose the work hardness is get the part red and quench
it. Dead soft.
You should be able to successfully turn and bore a thin cylinder of brass
very easily without losing physical size and configuration unless it's not a
free machining stock. Some brass/bronze materials are tough to machine. I
strongly suggest you do it in progressive steps if it will have a wall that
is very thin-----say down around .030". Rough it inside and out, then take
equal passes, maybe .010" off the diameters, alternately, on the bore and
OD until you achieve the desired size. You may have to slow down the
spindle to prevent chatter as it gets thinner and thinner, or run your
finger on the outside, which will also dampen the chatter. If the wall is
*very* thin, it may require a perfectly sharp tool, too, so be sure to hone
your tool to a keen edge. Too much positive rake can be a bad thing,
depending on the alloy. It will permit finer finish cuts, but tends to hog
badly. Let us know how it turns out.
Especially in brass. Don't use *any* positive rake with brass unless you
know exactly what you're doing. It's OK for you, Harold, but it's not
something I'd want to see a beginner try to do.
Chuckle! It's not always good for me, either. For finish cuts it is
usually OK, for the depth of cut and fine feed generally don't permit
hogging. Roughing, it can be (usually is) instant death. I should have
been more explicit in my comments. Because turning very thin cross
section bushings sort of changes the rules, I still suggest a small amount
of positive rake, especially for the thin cross section. It reduces cutting
pressure considerably and assures a good outcome, unless the lathe being
used is a toy. I would normally not recommend any positive rake for brass.
One job I did for Litton Guidance Systems, many years ago, involved turning
a 1/32" diameter pin 7/8" long on a 5/8" diameter piece of leaded brass, a
part of a tool I was building for them. I had to turn a half dozen of the
pieces, which were probes of sorts. Piece of cake. Care to venture a
guess how it was done? How would you approach it?
LOL. That's an easy one. Just take the entire cut at one whack.
Start out with the 0.625 bar and turn the final diameter in one
cut. The brass will cut fine, and if the machine is at all rigid
a cut like that in brass is no trouble at all. I've seen the
same trick used in turning cryogenic parts in OFHC copper.
I doubt that would have worked in this instance because the remaining piece
is too fragile. It would have been destroyed by the chips coming off, if not
by other means. . Keep in mind it was only .031" diameter when finished.
It was done differently.