Roy wrote:
I am in the process of making an item for my sal****er setups, and
have to thread the inside diam of a 3" socket joint (happens to be
correct bore needed for my application and only needs threads) which
are 13 tpi. My biggest problem is if I tighten the piece up in the
chuck it distorts it. I ruined a bunch of pieces trying to find a
sweet spot n chucking prressure with out distortin oor without having
it slip. Even with light cuts its gone wrong. Perhaps I need to put it
all away and give it a try on anaother day, unless someone can suggest
something that may work to hold this piece in the chuck without
distortion or slipping. I have played with super glueiing it, double
faced tapes, but one thing I have not tried net is to glue two or
three lugs on the outside of the piece I am turning and allow them to
hold against the chuck jaws sides.........hmmmmmmmthat idea just hit
me, is there anything else that may work?

PVC is neat to machine, but it can get awfull flimsey and such when
chucked up.....

Make a custom "collet" for it. Hopefully, these parts are
quite cylindrical (probably so if you were chucking them).
So, you get a piece of pipe, aluminum or steel, that has a
slightly smaller ID than the PVC part's OD. Turn in the
lathe until the PVC part is a very tight press fit in the
ring. Then, slit the ring at one point and clean up the
edges. When you put the PVC part in the ring, and then
clamp the ring in the 3-jaw, it will close up the slit
and grip the part. If the ring is stronger than the PVC
it will distort much less, and put the PVC part in
circumferential stress, so as to minimize distortion.
The PVC part would bulge out between the chuck jaws without
this ring. I've made this sort of fixture a number of times
when turning flimsy parts, and it has never failed to fix
the problem.

Jon