He uses the disc to establish his chucking point by starting the lathe and scribing a line around the disc that he can feel.
I thought this exercise would save me time measuring and setting callipers every time I needed a chucking point. The first photo is a set of discs for my primary chuck (Versa chuck from Toolpost.) The discs were done with a compass and cut with my scroll saw. I had various attempts to do the centre locating hole before I landed on the centre finder drill bit, it makes a tapered hole for the live centre, the wood is 3.5mm ply. I suppose this could be done on the lathe with a glue gun or double sided tape.
I was going to start on the discs for my other chucks when I had a thought, It takes quite a long time to mark and cut the discs, could I think of something easier?
I came up with version 2 after a bit of fiddling about.
Cut an oblong to suit the chuck jaws inside and outside measurements, mark and drill the centre so that the live centre point just protrudes into the work piece.
Mark a line along the side of the rectangle of the size you want, remove the live centre and the jig.
Set the tool rest on the centre line then put the pencil on the centre of of the line previously marked. Hold the pencil on this point and rotate the work piece by hand. You should have a perfect circle that just “kisses” the straight line and this will be the size of your required chucking point.
I should say at this point, I had to make minor adjustments to the jig rectangles to get the exact size I wanted. The thickness of the pencil lead can make a mill or two difference. I just measured the circle with a vernier and sanded the rectangle 'till I got the right fit.
This whole thing took me about a day, but if you were to go for the rectangle version, an hour or two at the most should do.
Finally, as I said I got the idea from Chris Fisher the Blind Woodturner. You can see him on his Youtube channel, but if you get a chance to see him live, you wont regret it, it was one of the most amazing things I have seen for a long time. And the turning wasn't bad either!!
All the best, keep turning !!