On Tue, 5 Jul 2005 21:28:19 +0000 (UTC), wrote:

Use a 4 jaw chuck to hold the work, then when you've scribed the circle,
use a vertical prop ( piece of brass bar/rod is ideal) between the
underside of the 9 -o-clock jaw & the front shears of the lathe bed. Turn
the chuck anti clockwise( is that counter clockwise over there?) to trap
the prop. Now use the cross slide to move the tool ( at centre height) to
scribe a radial mark intersecting with your circle at 9 o-clock & 3
o-clock.

OK, that's brilliant. I have the degrees of rotation inscribed on my
chuck, but wasn't sure how to locate with them, as there's no obvious
way to make a reference point. But, I could either do the prop thing as
you're suggesting, or since I have the degrees shown on the chuck, I
could use my magnetic base to hold a pointer just off of the inscribed
degree markings, and do whichever angles I want with that - without
trig. So, scribe the circle, let the chuck decide where on the round
piece we're going to call zero degrees, mark the part with the
cross-slide and tool, back off the tool, rotate to next target location,
um... lock the spindle, and repeat until I run out of points to mark.

Thanks, I wasn't thinking in that direction at all, but that might be
easiest - all done on the lathe, except for the drilling of the outer
circle of holes.

Rotate the chuck to the next jaw & repeat using the same prop.
Result 4 marks evenly spaced on the PCD. Use a 3 jaw chuck for three
holes, marking once per jaw. 5 holes - use a 5 jaw chu.... now you are
back to trig GG

Well, maybe having those degree markings on the chuck makes the trig
unnecessary... however, none of these methods involve making or buying
more tooling, so I'm not sure I've hit on just the right one yet