With long parts, sometimes you have to do the unthinkable and go against all logic. I used to run a cnc lathe that was 120" between centers and there were times when I had to cut an od much faster than I typically would to get rid of vibration.
Long part turning is a different animal at times.
My long part story if anybody cares, it wasn't a thread, but it was 321.. and 321 just blows.
Its like 304 alloyed with Ti.. Because it is 304 alloyed with Ti.
1.125 stock. I think the part was 32 inches long, and 28 of it had to come down to .750..
I fought with that sumna bitch, And I was only running half at a time.. Tolerance was wide
open. So I was cutting about 14 inches of 18 sticking out. Tail stock of course..
Had plenty of extra material, and was fed up with it. I was at some god awful huge amount of run
time, and it still wasn't working right... So I took a huge swing from the conservative side
to the stupid side..
Cranked up the surface speed. I'd guess around 400. 2 passes.. Almost .100 each, no finish, and
the faster I fed it.. The better it did. I ended up at I think .014" a rev, just shy of a .100
depth.. Not too bad for a DNMG 432. A couple of minutes instead of a substantial portion of
an hour just dinking with it. The insert had to be changed/rotated every run, but.. Whatever..
I saved about 15 hours, but I ate up a box of inserts.
Sometimes you just have to take a wild swing.. See what happens, it doesn't always work,
actually it usually doesn't work..
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Thought on this one. I think I would run whatever your running now.. Leave the PD a little big.
Then come back in and run 2 separate cycles. Just by changing the start position. A thou or two
z plus, a thou or 2 z minus. Single pass, same depth, maybe a little less.. Take out just the
left and then just the right. Halve your tool pressure and hopefully put it at a point that its
not bouncing around like a meth head that wants to steal your scrap.
The tolerance of the root of a 4 pitch lands in the "who cares" territory. As long as it doesn't look
bad.. who cares.
Edit: thinking about this. Maybe.. Maybe.. having the tool tip on the finish passes buried
a few thou might make things better.. Maybe.