As many of you may know I made and use a centering jig (pic 1).
I have adapted this jig by drilling pairs of holes (pic 1) to draw (pic 4) a circle (Pic 5) to allow a cut insert or spigot to hold a piece to turn.
This is done by preparing a blank (pic 2) placing and holding the the jig against the blank with the tail stock (pic 3), then a pencil is inserted (pic 4) to mark the blank (pic 5) creating a circle when rotated.
This gives a very easy and accurate position for cutting a recess or a spigot.
This idea was taken from a recent flyer from Axminister woodturning speed sizer.
Cheers Bill Shankland