On Mon, 26 Jan 2004 21:58:48 -0500, BRuce BRuce wrote:

I plan to use 2 - 1 1/2" for a tail vice, you are looking to hold the
wood not squeeze it into submission.

It's a question of helix angle. You need at least a certain pitch on
a vice, or else you'll be winding it in and out all day. If you cut
this onto too small a diameter, the angle of the thread is too steep
and it doesn't hold.