Clayton E. Cramer wrote:
I've moved up from "guy whose bought a lathe and a vertical mill" to
"rank amateur machinist," and I have a question about facing on
lathes. I USED to be able to do this--and now I can't, and I don't
know why.
I'm using Delrin (machines nicely, but not too soft). As I understood
how to face a cylinder, you take something that is approximately a
cylinder, put it in a 3-jaw chuck, then turn the cutting tool across
one end. Then you turn the cylinder around in the chuck, and turn the
cutting tool across the other end.

I was doing this a week ago, and it worked great! When I was done, I
had a cylinder with a perfectly smooth face on each end, and two right
angles--very important for my next step, which involved centering a
hole in one end of the cylinder.

Now, suddenly, my Sherline 4000 lathe won't give me right angles on
the cylinder--and the surfaces are tremendously difficult to make
come out smoothly. I've also noticed that I am having some
difficulty getting the cylinder to spin smoothly in the chuck--and
instead of getting a cylinder that is the same diameter over its
entire length, I now get a cylinder with a bit of taper.

First obvious question? Is the chuck and headstock still properly
square? I removed the chuck and put a dead center in the headstock.
Then I brought the tailstock, with a live center in it, point to
point. Sure enough, if I turned the headstock slowly, I could see
the dead center making a little tiny oval around the point of the
live center in the tailstock. Okay, it is not a lot of runout (is
that the correct term?) but once you put a 3" long piece of plastic
in the chuck, that little runout becomes a somewhat larger runout.

My next step was to dig through the Sherline manual where it explains
how to change the settings in the headstock that control runout.
Apparently, there's a trade-off between stiffness and runout.

After tightening the spindle nut in question, I can no longer see any
runout in that dead center in the headstock--but I can still see the
chunk of plastic in the jaw wiggling all over the place--and I suspect
that this has something to do with my inability to get a right angle
on the end of the cylinder, and why the facing operation doesn't
produce a nice smooth surface.

So, what's gone wrong here?

Is my cutting tool dull? (I was trying to cut aluminum in the
meantime, and it was apparently some form of torture--at least the
cutting tool screamed in agony.)

Is there something else that is not properly aligned here that I
should be checking?

How, exactly, does the method I described above for facing a cylinder,
work (assuming that I have the method correct)? I suspect that the
only way that this can work is that the workpiece, when placed in the
jaws, is exactly aligned with the axis of rotation of the lathe. Perhaps
this has worked well for me in the past because my workpiece
was close enough to a right angle that the jaws properly located it
relative to the axis of rotation--and now I am starting with a
workpiece that is far enough from a right angle at both ends that the
chuck doesn't hold it exactly on-axis.

Help me, Mr. Wizard!

If you can *see* a wobble of the dead center in the headstock you have a
serious problem!
Seems like you solved that by adjusting the spindle bearings.
I would suggest that you examine your whole approach to your machine as the
loose spindle bearings should have never been allowed to happen.
Now - to get the workpiece to run true.
Assume that the end of the workpiece in the chuck is *not* at right angles.
With a firm - but not tight - grip on the workpiece tap into running true. A
dial indicator helps here.
Once it is running true tighten up on the chuck.
Double check to make sure that it is still running true.
Now you can face off the outboard end.
Reverse the workpiece and again make sure it is running true. It should but
lots of inaccuracies can await you if you do not male sure!
Now you can face off the other end secure in the knowledge that it will be
at right angles to the cylinder.
Running a cut down the length of your cylinder should now result in a true
cylinder with no taper
and a hole in the end of the cylinder (drilled fron the tail stock) should
be dead centered.

Hope that helped.
Ken. (not - by a long shot - Mr. Wizard)