At 18 tpi there is no reason why you couldn't screw cut this under power and also undo the half nuts wind back and drop in again at any numbered mark on the thread dial.

That's if it's a thru hole which isn't clear from the picture.

If it is a blind hole or with a step in it then yes go for the handle.

I've done this a few times but on a hole that is not much larger than the tap eg 1/2" tap to cut a 5/8" thread. Is there a rule of thumb as to when the helix angle on the tap becomes too steep compared with the one being cut and starts to rub and push the tool away?

J

Edited By JasonB on 15/05/2014 07:29:22

If you have any die head chasers they are great for cutting multi start threads. Just grind them back to two teeth for a double start, three for a treble etc and set the gear box/change gears to two or three times the pitch and screw cut. Avoids any problems that can occur cutting the thread two or three times by advancing the compound slide.

I'm using this method now to do 5/8" x 14 BSF for a topslide tee nut to anchor me new warco QCTP onto me 1236 lathe by using a 7/16" BSW tap. Jolly good in the absence of the correct tap to thread it by hand.

Rick