Any guidelines for "ideal" finish passes? When is a cut too lite?
Also, if you want to save doing a whole 'nother pass, what's a good
compromise of material AND finish?
Is it impossible to generalize, or are there rules of cnc thumb?
I've noticed that in lathe cuts (manual), I can sometimes get a mirror
finish on alum at .010, but wrinkles at .005.
Part of the Q is the concern about "rubbing".
However, I have also been told that a great method for holding tolerance on
a part is to do second passes at zero finish cut, iow, the same coords as
the previous cut.
Which I have noticed can sometimes still seem like a .005 cut!
When I was making a fixture recently (V grooves), I noticed that I could do *numerous* passes at the same coords, and hear cutting! The bar was held
pretty well in two kurts, or so it seemed to me.
Someone told me that in this case, I should have V'd just one face at a
time, altho I don't quite see how V'ing both faces could have required so
many identical finish passes.
I'm thinking regular HSS endmills, 6061 alum, but discussion of any combo of
cutters/material would be illuminating, I'm sure.
Does carbide make a diff, ito finish passes, esp in soft-ish mat'l?
I'll give you some ball end mill help, let the radius to be finished
determine the smallest finish tool. If you us a larger dia ball, then
you are ok if you have rest milling capabilities, if not finishing gets
hairier.If the smallest radius is .125 or larger use the nearest dia
ball to fit the smallest fillet radius. Some jobs on larger molds, I can
rough with a .750 flat and go directly to a finish pass with a half inch
ball. The material determines being that aggressive. The finish tool
scallop height is where your finish feed line answers lie. We could go
on for days on that one. Bench work is always your friend.
Taking too light of a cut will come back to haunt you as well as
having deflection in a finish tool by cutting too heavy. If you get a
chatter pattern forming you can reduce the rpm's a bit and push the feed
rate a bit and usually it will load the tool enough to get away from bad
finish patterns forming. That strategy works in lathe and mill
programming. Pushing like hell will give more problems than backing off
slightly and chipping tools. In mold work I refer to having a machine
"Cadillac", that is a balance of not too many rpm's (burning up tools
while doing nothing) and a just right feed rate that allows the tools to
work properly. Cycle times are wayyy over rated, IMHO, in mold work.
There is a fine balance where you can run well and where you get
deflection and tools pushing up into the holders. I detest collet
holders for end mills. They should be banned from mold shops, 'cept for
occasional tool usage where light cutting is done. Reamers belong in
floating reamer holders.
I don't get much time in here due to work load but this evening I got a
few minutes to stop and read a few posts.