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Dave

You are having fun.

I think making a tap would be difficult without specialist tooling. Even for a tap for brass I would start with silver steel. First do the machining, then harden and temper and finally sharpen by grinding the cutting faces. Easy, no, but this is a training exercise. And don't use cutting paste with brass.

I would thought a better exercise would be to screw cut an internal thread. It would be easier if the diameter was larger and the thread pitch finer. You should make male gauge first. I will not give any further advise other than think a lot about the job.

All the best and I will enjoy reading the replies.

JA

Edited By JA on 12/05/2017 18:07:02

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Dave,

Looking at the photo; two things spring to mind

1. The taper on your tap looks rather steep … making it at least 50% longer would probably help.
2. Making the hole [say] 8.2mm instead of 8.0mm would almost certainly help.

It seems evident that the tap is jamming, and then stripping the thread, in preference to continuing to cut.

MichaelG.

Is it a left hand thread ? The slot shown in the CAD model might work for left hand thread, not for right hand.

For encouragement it's perfectly possible to make usable home made taps. These are my first attempt at tap making:

Cutting the thread and tapers is simple. For cutting the flutes I used the horizontal mill, as I had a suitable cutter. But as JasonB says it could equally well be done on a vertical with a ballnose cutter. I spent some time twiddling in CAD to determine the best position and depth of cut to get a slightly positive rake on the cutting edges:

It also helps to have some sort of relief on the cutting edges. I used a Clarkson T&C grinder, but a few strokes with a needle file would be more than adequate:

Hardening and tempering is straightforward, except that I bogged it up for one of the taps, and it wasn't as hard as it should have been.

These are the final threads cut in bronze:

Andrew

Dave, check this video, skip through with fast forward or use of the mouse, flutes ground at about 9mins in.

**LINK**

Emgee

You could probably dress the edge of those milled slots to the correct cutting angle by careful use of a Dremel tool or similar. I use it to sharpen old taps and it works quite well if you sit quietly and comfortably and take your time.

The real death knell for taps is when the edges of the threads start to visibly round and peen over. It tends to happen after they've been used on harder materials.

But once that happens it's only worthy of the bin at that point, otherwise it just kind of "extrudes" the thread into the material rather than cutting, you can feel it's just too hard going to still be good. The ones you get individually like from tracys also tend to be better quality than the ones that come in sets.

For silver steel/carbon steel tool treatment, Tubal Cain (Tom Walshaw) "suggests 770-790C heating slowly and holding the temperature for one hour per inch of thickness. This may be followed by tempering to improve shock resistance (and reduce hardness)…. Final light grinding will then produce a keen edge"

I admit I misquoted the time reference, but the other thing is if you were to hold the temperature for this long in a furnace of some kind presumably, then you'd be better off quenching in oil, because I'd imagine it would be much more likely to crack after being in there for a while.

You can quench in water of course but I would plunge straight down, hold it there for a couple of seconds, to allow a steam jacket to envelop the tool, which would retard the cooling slightly, perhaps just enough to reduce too much shock, then slosh it around from side to side so that it quickly cools.

Michael W

Looking at your CAD mockup. Even if you had machined it the right way around surely the angle of the cutting face used on the back turn of the tapping motion would push the swarf into the thread rather than cutting it off?

Martin

john carruthers:

Thanks for your response and for reminding me that I, too, have one of those useful torches.

There has been some discussion on another thread about amateur heat treatment of HSS and I was wondering how hot you went and what you did by way of quenching, tempering, etc?

So far as tempering silver steel and gauge plate is concerned, I have had some success using a deep fat fryer.

I would assume because there is no demand. Offhand the only common use of a square thread I can think of in the workshop is on flypresses. Presumably in that application the fact that some of the axial load is not resolved as a radial load is useful.

From a practical point of view the need for sharp corners would be difficult with a sintered component, and prone to damage or wear.

I think it would have been darn near impossible to tap the thread without using three taps. It was hard enough as it was. Mine own fault mostly as I boo-boo'd on the tapers on the second and third tap. They're supposed to start from the OD of the previous tap, not from the core diameter.

Unlike V-threads a square tap is cutting full width on all teeth all of the time. So the DOC increment between teeth needs to be small. One could of course use one very long tap, but it is easier to make multiple taps with increasing OD, ie, serial taps.

Andrew