Square peg in a round hole

[mklotz]

I think the maths for this might be quite complicated

Accounting for the shape of the jaw tips would make it complex indeed !

If the jaw tips were flat, I think it might be easier. Since the 3jaw is self-centering, the bottom jaw must be at the same distance from the center of the square as the two top jaws are.

Let:

S = length of the side of the square stock

Then S / √2 is the distance of the top jaws from the center of the block and the shim needs to be S / √2 - S/2 = S ( √2 / 2 - 1/2) = S (√2 - 1) / 2 = 0.2071 S

Oh, that it were that simple. :-)

Regardless, shimming isn't nearly as safe as a collet arrangement along the lines of what I described.