...........Willbird, you must be talking about a 6 jaw independant chuck? As a 6 jaw scroll chuck would have a very hard time clamping to anything other then round stock. Sorry about the following llinks as I can't yet figure out how to download images directly from my host site, LIKE I USED TO BE ABLE TO !!!!!
The above will open a photo of a 6 jaw Buck Adjust-Tru I have, and was using as part of getting my lathe leveled and set up.
And turning a test bar. You can use an Adjust-Tru to get right down into the tenths, but just like a 4 jaw independant, everytime you put in a new workpiece you're back with your dial indicators setting up again.
In using a 4 jaw to hollowpoint bullet moulds, at first I wore myself to a frazzle chasing thousandths back and forth across the parting line of the blocks. Mould blocks aren't always as square as we'd like, and the chuckjaws may not apply even pressure, especially with a 2 cavity set as the jaws may be well across the parting line, as in the photo below:
What I've gone to is to place copper wire (ground wire stripped out of old Romex) between the jaw and block. You also have a 'bell mouth' situation with the jaws able to tilt a bit in the chuck body. The copper wire compensates to a degree.
After doing a couple moulds, I have come to understand that having the test indicator bounce a thousandth and a half, or two across the parting line is just something to expect, so the REAL alignment measurements are taken opposite them on the clean cavity surfaces. Naturally you want everything to be as perfect as possible, right? Early on I would stick the pointer of a .0005" TI down into the cavity to ride a lube groove. That was pure idiocity. I spent hours re-setting, flipping the mould blocks back and forth from cavity to cavity, etc & etc. Oh my aching back!
I even set up block halves on a granite surface plate to try and find out what was happening, only to accept that cavities aren't always cherried straight. I told one guy when I'd first started that I was unable to get the cavity to run straight to the blocks. I was chasing a thousandth back and forth all over the place on a cherry cut block. I have a piece of paper stuck on the cabinet over my reloading bench, opposite the lathe that says, "Remember the RCBS".
This was a 7mm-168 that was oval. I guess the blocks didn't completely close on the cherry when it was made. Trying to get THAT abortion to run true would turn a saint into a blithering lunatic and have you sitting in a dark corner sucking your thumb. I have become much more serene and can accept a .001 to .002" bounce across parting lines if I can get the opposing surfaces to run within a thou.
It's still spooky to 'do the deed' and I always lean against the bench and have a smoke reviewing what I saw before advancing the spotter to centerpunch the nose.
...........A rotary table used to cut varying diameter cavities with one cherry sounds interesting. Sounds kind of like using a router? I hope someone does that and reports back. I'd sure be interested in hearing about how it worked. Remember that whatever minimum level of accuracy the rotary table has in 360* rotation will reflect X2 in the cavity you cut that way.
.............Buckshot