On Feb 12, 6:07*pm, Lew Hartswick wrote:
John Martin wrote:

That said, I've never heard of a 7 TPI lead screw, and can't imagine
why you'd want one.

John Martin

Well I never heard of a 9 tpi thread but that is what the
Kurt vise on one of the mills at school has in it.
* * ...lew...

Big difference between a vise screw and a lead screw, though. All the
vise screw has to do is move the jaw - it doesn't matter where it is
and it doesn't have to be repeatable.

The lead or feed screw is different. If it is a 9 TPI screw, one turn
will move the tool or part one ninth of an inch. Great if you happen
to work in ninths of an inch, but most people don't. Most work in
thousandths. So you graduate the dial in thousandths. There will be
111 of them. Plus a "remainder" of .111111....... You can graduate
the dial with 111 OK, but it's pretty tough to handle that remainder.

John Martin