Turn down shaft of tap

[Buffalo21]

All of the pulley taps, I have have larger shanks than the tap size, but the nut taps are all smaller shanks than the tap size.

 

[Illinoyance]

A nut tap has a shank smaller than the minor diameter of the thread. A pulley tap has a shank equal to the major diameter of the thread. T tap a hole longer than the flute length requires either a reduced shank tap or a nut tap.

 

[ericc]

This solves a mystery for me which has been bugging me for quite a while. I have a commercially made hand tapping block, and the reduced shank taps are unpleasantly wiggly. I think the block is made for full diameter shanks. When I tap with those fat-bottom taps and they wobble on me, I just sing to myself "fat-bottom taps you make the rocking shank go round."

 

[macardoso]

I see IRWIN brand on the tap shank.

That must be a carbon steel tap (not HSS).

Accurate, HSS would probably be a whole lot less fun. CBN tipped inserts would machine it fine though.

 

[extropic]

Accurate, HSS would probably be a whole lot less fun. CBN tipped inserts would machine it fine though.

True, except the OP says he's got a 7" x 12" lathe.
I've never seen such a machine stiff enough to hard turn.