Home-made Screw Tap - Advice Please

As an exercise I'm making an M9x1.0 threaded component in brass using my lathe only. I've successfully cut the M9 male and moved on to the M9 female.

My first attempt at the female was to develop an M9 tap. This I did by starting by making an M9 bolt out of silver steel and then converting it. The steps were:

1. The M9 thread was cut on my lathe
2. The hex head and relief slots were milled.
3. The 'bolt' was tapered in the lathe to look like a tap
4. Burrs produced by tapering were gently filed off.
5. The 'tap' was heated up, held bright red-hot for 2 minutes and then plunge/stirred into a bucket of cold water.

The test piece is a short length of brass rod with an 8mm hole in it. I held the ring in the 3-jaw chuck and attempted the tap. It didn't go well!

After producing some swarf the tap stopped cutting. It feels as if the thread is being ground and progress is very slow. The cuttings are paste.

The picture says it all. Surprisingly the poorly defined thread produced by the tap mates with the male for a couple of turns. It's main fault is that the thread isn't deep enough.

One problem is that the tap is still magnetic, ie not properly hardened after heating and quenching. How did I mess that up?

The other issues must down to the way I've designed or made the tap - the teeth aren't cutting. How should a tap be made?

As always, all advice and comments welcome.

Thanks,

Dave

Edited By SillyOldDuffer on 12/05/2017 15:51:13

Buy a set of three from Avon Engineering, only £15.21 + vat

Well there are many ways to make the cutting edges. A way that works well is to grind a pyramid end, as for example **LINK** .

Steel only looses its magnetism at high temperatures, it of course returns after cooling down. Test the hardness of the tap by trying to file it. The way the tap seems to be cutting, it doesn't feed itself and is recutting. Perhaps if you had a bigger hole to begin with, 8mm is a bit on the small side. How does the taps cutting edge look like? Negative or positive cutting angle?

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

What profile did you mill the slots to, if the edges have ended up with a lot of negative rake you won't get the tap to cut. If you used a ball ended mill then set it to one side of ctr don't mill on the same axis as the tap.

.

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.

Thanks chaps. I hadn't thought the taper would be a problem, but it could well be.

In answer to Jason's profile question here's a CAD mock-up of how I sliced the thread. I used a flat bottomed 4-flute end-mill to cut a slot mid-point on the bolt. I'm not sure that provides enough relief, or enough rake. It's certainly not how real ones are made!

I dunno what I was thinking when I said the tap wasn't hardened because it's still magnetic. Of course it is; the magnetism only disappears when steel is red hot. Doh.

Dave

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

I think you've got it David. Righty-tighty means I've cut the slot on the wrong side!

Oh dear...

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

<quack, quack... oops!>

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

**LINK**

Emgee

I've made a similar tap to Daves, except I did set the edges to one side and used 3 flutes rather than 4 to make an M8 tap, it worked fine enough with a bit of a shove to start because the angle was a little steep. I could've done with a sharper threading tool at the time I think or maybe feed in a little deeper. 

One thing I know now that I didn't know then is that from the workshop practice series on the tempering and hardening of silver steel that the temperature needs to be suspended for 40 minutes per inch of thickness for full hardness.

Michael W

Edited By Michael-w on 12/05/2017 22:52:20

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.

In extremis I made a tap by turning a thread on a bit of old drill shank, then filed a flat on both sides rather than fluting, and hardened / tempered it. It got me out of trouble, it's still in the 'odd taps tin' somewhere.

I'm encouraged! I'm out today so it will be tomorrow before I can have another go after watching emgee's video.

It occurred to me that the amount of burring on the thread would be reduced if I cut the taper before cutting the thread. Apart from light deburring I didn't clean up my Mk1 tap at all. That's worth doing too especially if I'm in a beauty competition with Andrew's professional efforts.

Michael's comment about heating for 40 minutes per inch for full hardening has got me thinking. That's much longer than I heated the tap. A happy accident perhaps. Would leaving the core of the tap unhardened compared with the teeth make it tougher?

Playing with screw-cutting has added several new toys to my wish-list. 5C collet chuck; quick-change tool-post; carbide threading tool with inserts; and now a small ball cutter as well. Fortunately I already own a Dremel and needle files...

Thanks,

Dave

john carruthers:

Assuming it was an HSS drill shank, how did you harden it?

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

EGA:
I use a carbon arc brazing torch for local heating on tools, my little gas torch just wouldn't have the temperature.

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