In article ,
says...

3. What is this kind of gear called? I.e., when I call Boston Gear,
what should I say I need? Is "To fit an 8 tpi Acme thread sufficient"?

Ignoring the tooth form, which Nick has responded to, you
won't be able to buy a stock gear of the proper pitch. If I
did the math right, you'd need a 25.133 diametral pitch
gear (8 x pi), or a 1.01 Module (metric) gear. Huh, maybe
you could get away with a 1.0 Module.

I don't think McMaster carries Module gears, you can get
them from Ametric if you're willing to give it a try.

http://www.ametric.com/home.htm

Since the tooth form will be wrong, you might consider a
design that would allow disengaging the thread dial when
it's not being used, especially if this is a lathe that
doesn't have a separate shaft for the regular feeds, i.e.,
the lead screw is always turning.

Ned Simmons