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!