On Thu, 18 Aug 2016 17:27:26 -0000 (UTC), Doug Miller
wrote:

I needed to shorten the tailpiece for a bathroom sink drain: chrome-plated brass tube 1.250
OD with 0.030" wall. Obviously the tool of choice would be a tubing cutter, right? Except that
my tubing cutter has a maximum opening of 1.220", despite being labeled as "1-1/4". Grrrr.

OK, no problem, I'll chuck it up in the metal lathe and part it off. Should be easy, right?

No sooner does the parting tool start removing stock, than the tube begins slowly but
inexorably sliding out of the chuck. Not crooked or anything like that, still concentric, just
creeping slowly in the direction of the tailstock. OK, maybe I didn't tighten the chuck enough.
Stop the lathe, reseat the tube in the chuck, make sure it's darn good and tight, try again.

Same thing. I get maybe 0.0005" (that's right, half a thou) removed, before the tube begins
sliding again.

I tried low rpm, high rpm, slow feed, rapid feed, lots of oil, no oil -- no difference that I could
see.

After 45 minutes, I finally got it turned down far enough to slip it into the tubing cutter, after
which it took about 45 seconds to complete the cut.

Why is this happening? And, more importantly, how can I prevent it? This was number 1 of
two pieces that I have to shorten by the same amount.

My bet is that the tool is distorting the tube enough that the
pressure is reduced bewteen the tube and each jaw in turn as it's
approaching the tool. As that that jaw leaves the tool the tube
springs back. The result is a squirming of the tube in the jaws. The
cure is a plug inside the tube, a collet, a chuck with more jaws, a
bullnose center, a piece of sacrificial wood between the part and the
tailstock, or a hacksaw.

--
Ned Simmons