Three things: After making the center hole, use a smaller drill for the
initial pilot hole, say 1/8" and make sure the pilot drill registers on the
conical sides of the center drill hole. Use 2 or more other drills,
graduated in size, up to the final size.

Run slowly when working plastic. Plastic will soften under the generated
heat causing drills to wander in the hole and seek the wrong path.

Use stiff drills such as screw machine drills, esp. for the pilot holes.
You will appreciate the extra clearance on your Sherline lathe. Jobber
length drills are just too long for Sherline equipment, esp. in the sizes
around 1/4" and above. Try this for the 1/8" pilot hole. Shorten the drill
by cutting almost all of the shank area off. This will give you a much
stiffer drill for the initial pilot hole. Drills can be cut by nicking them
on a grinder and breaking at the scored spot in a vice. Use a rag around
the drill and strike it with a hammer.

Bob Swinney


"Clayton E. Cramer" wrote in message
...
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