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.

The first couple of times that I did this, it worked great. I put the
workpiece in a 3 jaw chuck, faced each end, and then used a center drill
in the tailstock to make a small hole. Then I used a 5/16" twist drill
in the tailstock to enlarge the hole, and then used a 3/8"-16 tap. The
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.

Possible sources of difficulty:

1. I know that twist drills are intrinsically less accurate than center
drills. Should I look for a 5/16" center drill to make the pilot hole
for the twist drill? Or should I start with a twist drill the diameter
(or slightly smaller) than the pilot hole of the center drill, and
gradually move up in size?

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?

Clayton E. Cramer

You stated that on the "first" pieces, you faced them off on each end,
then center drilled and bored. Okay so far. If you faced off the end
of the bigger(longer) piece the little nipple left would be centered
and your center drill should just touch the center of the nipple when
getting ready to center drill it. If not, something wrong possibly
with the piece being too long. Do you have a steady rest?
Ken.