View Full Version : Has anyone made a Thread Mill out of an old Tap?

12-31-2013, 10:18 PM
I was shopping for a 'Single' Thread Mill to use for various pitches, and I was shocked by the prices.
Most single Thread Mills are $40 and up. ZOINKS!! :eek:
So... Have any of you guys made a Thread Mill out of an old Tap?
My thinking is to grind the end back past the lead-in, to get a full 60 degree thread form.
Then grind the other, un-needed teeth, down to the minor diameter using my Spindexer on the surface grinder.
Would this work? It sounds feasible.

J. R. Williams
12-31-2013, 10:38 PM
A tap has the helix angle of the thread ground into the teeth while a thread mill has a zero angle plus clearance to make the thread. I have seen comments where people have used a single tooth on a tap to cut a thread but it is not correct.

12-31-2013, 10:44 PM
It will work to a certain extent. But, you must grind the flank on the upper side (shank end) for relief. Otherwise it'll rub. Because of the tap's helix, there's no relief on that flank side. Taps also have no radial relief. Basically you have to regrind the complete tooth.

Still easier than starting from scratch so worth doing. Use the largest diameter tap that'll fit your application as a starting point. The larger diameter the less helix to grind away.

Whenever I've seen this topic come up someone always chimes in saying they've been making thread mills for years without the grinding I'm describing. I can tell you for sure it won't work. The cutter will squeal and screech from the rubbing (lack of relief) and get quite warm until it fails.

12-31-2013, 11:14 PM
Oh yeah, I didn't think of the helix aspect of taps.
So in reality, I'd only be able to use ONE tooth, since the teeth on the other flutes are slightly 'higher' in pitch as the tap rotates.
And that ONE tooth would also need topside and backside relief.

12-31-2013, 11:28 PM
Correct, you can only use one tooth unless you have a way to do some very fine grinding to eliminate the helix on adjacent teeth.

01-02-2014, 08:20 PM
The easy way is to buy a lay down or on edge lathe insert and make a holder for it to screw on to. Also you can buy a 60 degree double angle cutter to do outside or large inside threads. Ideally the holder would be made of drill rod and hardened, but CRS will work.

01-02-2014, 10:46 PM
Haven't tried making one like this, but it looks pretty straight forward.... http://www.cnczone.com/forums/tormach_personal_cnc_mill/75806-wanted_make.html