You need to isolate your problem. Your problem could be that your lathe isn't
aligned correctly, or it could be that the ways are right but that the tailstock
needs to be aligned to the headstock. Could also be that your chuck isn't true.
Start with this: put a center in the headstock and another in the tailstock, and
scoot the tailstock right up so the centers almost touch. Do they look perfectly
aligned?

It isn't easy to drill a hole .050" off center in a rotating piece. It's much
more likely that 3" out from your headstock, the workpiece was .050" out of
center. Really think about it, asking yourself the question how could a piece be
centered at the headstock end and .050" out 3" towards the tailstock. Thinking
like this is how you really learn about a lathe.

You may find the book "How To Run A Lathe" by the South Bend Lathe Works to be
useful. Many, including me, have.

GWE

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?

Clayton E. Cramer