"Bob La Londe" wrote in message
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"Jim Wilkins" wrote in message
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"Bob La Londe" wrote in message
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"Doug Miller" wrote in
message . ..
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.

Which way was it creeping? If out, then just set the lip on the
tube up against the back of the chuck jaws, and use a bullnose
center in the end. If slipping in, put a block of wood inside the
chuck behind it, and tighten down the screws of your spider to
keep the block from moving. If you don't have spider bolts on the
back of your lathe spindle it might be time to add them. Never
know when you might need to do some turning on a rifle barrel
next.

I would suspect it was slipping in because the end with the lip
would be more rigid effectively making your tube into a cone with
the large end at the lip when you clamp down.

Personally I would have probably cut it on the bandsaw or with a
hacksaw, and then debuirred it with a file, or deburring tool
depending on what was handy. There is a huge amount of overlap in
the joints of that type of fitting, and it doesn't have to be very
precise at all.

Lots of tools could have done the job. Tail pipe cutter. Pipe
cutter. Tubing cutter. Hacksaw. Bandsaw. Abbrassive saw.
Dremel tool. Jigsaw. Sawzall.

That style fitting and assembley is designed with ease of use in
mind. Hence the huge overlap. As long as it reaches past the
geasket, and doesn't have any burrs to catch hair and clog its
fine.


http://www.autozone.com/loan-a-tools/tail-pipe-cutter

That is an option I never think of. Its a good one too. I've used
my tail pipe cutter maybe a half dozen times in 25 years, and most
of those times I could have used something else. The one or two
times it was really the right tool for the job I could have just
borrowed one from Autozone. Doh!

Tubing is difficult to part with a larger lathe too, because it flexes
too easily. I sometimes score it with the lathe bit and then saw along
the groove with a 24 TPI or 32 TPI hacksaw.

Whenever you are turning wood on the lathe you could make a pair of
cones with holes through the center for threaded rod, to hold hollow
tubes. Just don't try parting work held at both ends because when it
becomes flexible enough to deflect away from the cutting pressure, the
near side will close and grab the bit.

--jsw