According to Clayton E. Cramer :
I have recently acquired a Sherline lathe, and I am learning how to use
it. I have run into a problem that I sure those of you with more
experience (or practically any experience) can help me solve. I needed
to center a 3/8"-16 threaded hole in a Delrin cylinder.

[ ... ]

first two times that worked well, I was doing this to a cylinder that
was only about 3/4" long.

Then I tried to center a hole in a 3" long cylinder, using the exact
same mechanism--and it didn't work. The hole was way off--about .05"
difference from one side to the other.

[ ... ]

2. Is it possible that that the 3 jaw chuck can't hold a piece of Delrin
that long without high speed rotation causing it to wiggle slightly
off-center? Is the solution to drill the center drill and/or twist
drill at very low speed?

What is the diameter of the Delrin cylinder? Perhaps 3/4"?
Even with steel, you don't want to support something purely in a chuck
which extends as much as four times its diameter. And Delrin is a lot
less rigid than steel.

What you need to do to start is to set up a steady rest to
support the free end, and keep it from bending. (I know that the
Sherlines have steady rests as options -- but I don't know whether you
have one.)

Once the Delrin is supported, start with a center drill, and
follow up with a drill bit appropriate as a tap drill for your 3/8-16
threaded hole to be.

If it were much larger, I would suggest drilling it undersized,
and using a boring bar to enlarge it to the proper size for tapping. I
don't know whether you have a boring bar small enough for this, however.
But a boring bar can bring a hole back on center even if the drill
wanders a bit when starting -- as long as the starting hole is not too
far off center for its size.

Good Luck,
DoN.
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