help making a tap

[Mcgyver]

I'm making a left hand tap and wanted your guys opinion on...do i cut the flutes then thread, or thread then cut flutes. I'm thinking cut the thread after makes more sense as then the proper thread profile is in place whereas the flute cut mess up the profile a bit from burrs etc

how have you done it and how do think it should be done?

[rkepler]

Thread then cut flutes with a ball mill offset so the ball gives you some hook (or not, if you're making a tap for brass or bronze). ACME tapes are strange as the major diameter of the tap if about .010" over the male thread major diameter.

[John Stevenson]

As Russ says, cut thread first then flute.
If you do it the other way round the screw cutting tool will bounce on the cut flutes and the OD will be all over the place .
If you ever have to cut a thread on a shaft where there is already a keyway you finish up with the thread next to the keyway smaller than the rest so for a tap you are starting with a blunt bit.

Look at a tap and you will see that the cutting edge is usually over centre again as Russ says to provide a 'hook' remember though you want to mimic this on the opposite side for a left hand tap.

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[rockrat]

As noted above, but, just give it a go. I have done a few and after the first, they were easy. If you can get the "hook" on the cutting edge in there your better off.

I now polish everything behind the cutting edge as well. Tap does not seem to bind if I have to reverse it.

rock-

[japcas]

Are you guys filing the top of the thread on the tap to make for some radial clearance? I made two once and they would start but felt like they were binding up in the hole. They were 1/2-32 if I remember correctly but I could be wrong.

[SGW]

Well, I think the tap SHOULD be hardened first, then the flutes ground, but that's beyond me.

For my (two?) attempts to make a tap, I've always cut the thread first, then milled the flutes. I cleaned off the burrs as much as possible, then used anti-scale compound when hardening and then touched up the cutting edges a bit with an oilstone. The taps worked, but they were pretty fine thread (one of them was 10-56, as I recall), not going very deep, in free-cutting brass.

The burrs can be a bit of a problem though.

[Mcgyver]

thanks guys, made four, taper and bottoming in 1/2 20 L and 3/8 24 L. as suggested, i cut the threads first. made them so the taper cuts half the depth the bottoming cuts the other. I used short stubby ends of drill rod i had lying around, were i to do it again I'd re-think it so that i end up with a proper round part to chuck and then a square section afterward, hopefully they'll work well

John, you are right on the 'make sure the hook is positioned properly' , the point occurred to me just after I'd cut the first flute (I was making them when you posted) I lucked out and cut it on the correct side for a left tap.

SGW, hadn't thought of anti scale treatment, in the far reaches of my memory i recall something about covering with soap? or is that wrong?

here's a shot before hardening



I'll post back and hardening and trying them out

[SGW]

Soft soap is the classic anti-scale trick. My couple of tries with soap have produced sort-of okay results, but I find the proprietary anti-scale compound works better.

[Swarf&Sparks]

I've had more luck with borax than soap. Mix with water to a consistency that will stick to the job after pre-heat, and fire away.
Oh yeah, you can buy borax in welding supplies at 5 times the supermarket price.
They call it "brazing flux" or some such.

[Rich Carlstedt]

Mcgyver
A hint for you from my perspective.
You have too much metal left for thread..
Try going deeper with the ball mill, and do not have it on center!
Have it offset, so the 'cutting" flutes have a straight section below them, allowing the use of a flat stone for touch up.
On commercial taps, when you see as much thread as you have, they are 'backed" off so the drag is less. I think having a 1/16 to 1/8 inch long tooth on the major diameter will do a better job
IMHO
Rich

[Mcgyver]

hey Rich, thanks for the tips. I thought the profile would show better in the photo than it did, the ball end mill was .1875 off centre on this 3/8 tap ( drew it out to figure what looked right re offset and depth), but i see your point....the rake i created is part of the curved surface left by the ball mill, whereas if I'd gone deeper I'd have a rake face that was flat for easy stoning. I thought that i was probably not allowing enough for chips, but tried to balance a stronger tap against having to back out more often...probably I was too cautious, 3/8 taps are plenty strong

Here's a shot showing the milling - guess the offset should have been less but depth of cut greater.



unfortunately they're cut off so can't re-mill. The other mistake i made was i made them back to back on 2 short pieces of drill rod and didn't end up with enough to have both a chucking length and sq end - one of the hazards of blundering forward, but I'm learning. since they're are made and can't easily be corrected, i''ll give them a whirl, they only have to work on a couple of turnbuckles.

I do appreciate all the advice, next time they'll be perfect

[John Stevenson]

You don't need much thread, just enough really to give strength to the cutting edge.
Some times pays to increase number of flutes to reduce the exposed thread sooner than do deeper and risk the chances of breaking.




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[Fasttrack]

I just had another one of those "aww cool" moments


you guys amaze me... i kinda want to get some drill rod and try making my own tap just for kicks. Of course then i need to buy a ball end mill, a collet block, a ... well maybe i better wait until i have money (hah money? )

[JCHannum]

I have made taps from drill rod as well as mild steel and casehardened with Kasenite. You don't need a ball nose mill or index either.

I have cut the flutes with a plain end mill, set to cut on the centerline and indexed by eye. The plain end mill method will require filing some relief on the back of the threads, but is easily sharpened with a handstone.

I made a set of model pipe thread taps & dies this way. I made the dies from a used countershaft from a Muncie four speed. I annealed it and sliced it into die sized slices.

[John Stevenson]

I'm 100% with JC on this.
It's only been the last few taps that have been professionally looking.
All the early ones were just endmilled set up on a guessing block Different length flutes, different depths and odd spacing.

All worked well for what they were.

In fact odd spacing is an advantage as it supports the tap better in the hole and stops the threepenny bitting.
It's not a bodge but an industry fact. There are even tables set out for flute cutting on reamers to offset some teeth to stop this bouncing.

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