Clayton E. Cramer wrote:
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?
What diameter is this part? I assume too large to fit through the
chuck and spindle center holes. If forced to do this work that far
from the chuck, you would want to use a steady rest to support and
center the outer end of the workpiece.

You can put the part in the chuck and slowly rotate the chuck by hand
while the drill bit is very close to the end of the workpiece. If it
..025" out of center, it would be very easy to see this with the naked
eye.

Jon