Making a Tap

According to Dave Young :

Hmm ... making something to screw onto a Unimat SL-1000 spindle or tailstock? That's what my M12x1.0 tap was bought for all those years ago.

0.0009" error per thread. Assuming 8 threads, that would be 0.0073" error from start to finish. 2.23% error, or 18.2% of a thread total error for the eight threads. If you have more than eight threads through the thickness of your workpiece (8 threads is about 5/16" thick in this case), the cumulative error would be greater.

Neither CRS nor HRS have enough carbon to harden -- so it does not matter much which quench you use. They will remain soft.

You *might* be able to make a deep enough case hardening with Kasenit or something similar -- but you'll need to cook it for quite a while to get a deep enough case to do you any good.

However, if you make it from a hardening steel, such as drill rod, you need to use the right quench for the particular drill rod which you purchase. They come in water hardening, oil hardening, and air hardening.

A little more complex than that, if you want it to cut well. You need just a little rake on the flutes, which calls for a conventional milling cutter (for a horizontal mill) with the right profile for the flutes -- though you might be able to approximate it with a ball end mill of the right size -- perhaps 1/8" radius would get you close enough. And once it is hardened, you will want to grind the rake into the flutes, along with the starting taper as needed. (Don't grind too hot, or you will anneal the steel and it will be soft again.)

A truly nice tap has the threads ground as well, using a properly shaped grinding wheel on something like a toolpost grinder on the lathe.

Oh yes -- you also don't say what material you are planning to tap. That could make a difference in your chances of success.

Good Luck DoN.

I should have mentioned the material in the original post. The quill stop that I'm trying to rethread appears to be aluminum and is .67" thick. In case you missed an earlier post regarding the need for the tap, I'm pasting it below:

Regarding my "close enough" need; I recently bought a cheap quill stop for my mill. It's the type that you squeeze the two knobs, it opens up and you place it where you want your quill to stop. The threads on it are 12mm x 1.25mm. My Grizzly mill's depth stop threads are 12mm x

1.0mm. So the threads don't actually revolve on anything. It only clamps onto existing threads with relatively light pressure. I'm figuring that if I clamp the two halves shut and drill and tap it, it should work fine. Thus "close enough" should be "good enough". It actually works now, but is a bit cocked to one side because of the thread differential.

I'm us> According to Dave Young :

Turn the thread on the lathe, then turn the starting taper, then mill the flutes, then harden, then anneal, then use.

;-)

Nick

How about just remake the part altogether? This would justify the cost of the tap and let you have a little more fun in the bargain.

Jim Harvey

If you are just repairing a thread in aluminum, you don't need flutes! Just grind 4 flats on the bolt, leaving threads on the corners. I used to repair threads in motorcycle cylinder heads this way. It's an old watchmakers trick that dates back hundreds of years. They used to thread carbon steel wire with a screw plate, and file the flats to make taps. A right hand thread filed to an oval shape can be used to cut left hand threads.

Steve R.

According to Dave Young :

I saw it later on in the thread -- after I posted my answer.

Hmm ... I'm not sure about how good a fit you will get when starting with another 12mm thread. How much gap is there when it closes on a standard M12x1.25 thread?

Why not make your own clamp from scratch, using that as a guide. That way, you will have enough meat for the full threads.

And directly threading the clamp on the lathe -- with the clamp held in a 4-jaw chuck (if you have one -- if you don't you should get one) should given better results. With the independent jaws of a 4-jaw chuck, you can adjust for the existing thread being properly centered and then (re)thread (with a small enough internal threading tool).

Hmm ... make sure that you support the free end of the bolt with a live center if you are going to be turning an interrupted thread like that.

Good Luck, DoN.