Making a DIY tap

Hi. I have a project which involves cutting a number of M10x0.7 internal threads - I 'm sure that the pitch I need is indeed 0.7, rather than the standard 0.75mm metric fine thread. So I thought I'd make a tap from silver steel. What I did was to cut the thread in 10mm SS on the lathe, then cut four flutes along the centreline to a depth of 2.2mm using a 4mm diameter ball-nosed slot drill. I then hardened the piece and ground a leading chamfer to 3-4 threads.

My reason for going in 2.2mm was just that I thought it would guarantee a positive rake at the cutting edge, but I didn't calculate what that would be - as the material I'm tapping is aluminium alloy, I hoped it would be forgiving.

The tap works (sort of) but requires inordinate force to drive - more than a commercial coarse M12 tap in the same material I'd guess.

I'm drilling out to 9.3mm (Major Diameter minus Pitch) before tapping.

Any suggestions as to where I'm going wrong?

Regards, Robin.

I have found this with home made taps - it's almost as if the tap is forming the thread rather than cutting it. I have assumed it's because cutting the flutes with a ball end cutter leaves burrs. I'd try to grind the flutes with a Dremel or similar to get a cutting edge that is sharp and clear of burrs.

I made a 3/8" acme tap many years ago to clear a thread in a PB bush for the cross slide on my antediluvian Grayson lathe. I used tool steel which was then hardened. I ground the flutes between centres and made sure the cutting edge attack angle was sufficient.

A chap at work first roughed out the thread in the bush (he told me he was not good enough to machine an accurately fitting thread) then we used the tap to clear out the bush to finish size.

It easily did a very nice job but I am fairly certain it would not have managed a complete thread in a plain hole.

If I had to guess, I would say that your tap might not have sufficient attack angle or if it has, then Otley is on the ball when he suggests the flutes need grinding to give your tap the sharp edge it needs to cut.

Rik

I think your lead of 3-4 threads is way too few. A commercial taper tap has a far more gentle approach. What you are doing is equivalent to lathe plunge cutting the thread in 10 passes simultaneously, with a depth of 14thou per pass. (that's 10mm-9.3mm /2 x 40thou / 10passes)

Perhaps reducing the amount of thread on the tap in contact with the metal being cut will help. Being a turned thread its surface will be rough and not like a ground tap. The undercut of the leading edge of the thread just needs touching with an oil stone to take off the burrs.

Clive

PS. I make 20mm dia. taps with a fine thread with maybe 5 flutes and they work fine. These I use for cleaning out threads that have had loctite in them.

I doubt this will be very helpful for M10 threads, but when making specials for smaller threads in ductile materials consider making a thread forming tap - use a gradual taper and increase the hole size to allow for extrusion of the thread form.

For fluted taps (and I have only made a couple) consider making the flutes wide but shallow. You may need to clear the tap more often and it may be less self-guiding, but it helps reduce that area of contact Graham describes.

Neil

If you machined down the centre line of the tap it will have zero rake. You must go over the centre line to have a positive rake. To achieve a 7 degree rake then you must mill 0.61 mm past the centre line (x =5 tan 7).

On a home made tap there is no relief on the cutting edge so all the cut threads will rub the tap and this also causes a high torque requirement. You should try to keep the threaded portion of the tap narrow with plenty of land in between the threaded portions. You can achieve this by milling the flutes deeper. However, there is a compromise here because if you mill the flute too deep then the tap become weakened.

Why did you use a ball nosed mill? I would have used a normal 6 mm endmill to ensure a good square cut.

I have only made a few taps to date but I have a feeling, not based on any concrete evidence, that odd numbers of flutes make better taps than even numbers of flutes.

Mike

G'day.

There is an excellent sketch of the clearance on the tap lead in Harold Hall's 'Workshop Practice Series #38' (page 76. SK5). To quote; "The clearance is ground about a different centre to that of the tap itself "

Quite a tricky operation.

Regards * Danny M *

Edited By Danny M2Z on 08/04/2014 09:24:16

Your answer in in your first statement.

You went 2.2mm deep with a 4mm ball cutter so in theory left a side flat of 0.2mm but your depth of thread is 0.35.

As others have said, normal end mill, go deeper and be over centre on the cut, cut 3 or 4 flutes, then reset the tap so you can chamfer the beack edge off to leave a thin land of cutting teeth.

If it's in alloy harden it right out, don't temper. Then sharpen the cutting edges of the tap with a slitting disk in a dremel.

The heat from grinding will be enough to temper a tap this size for use in alloy or brass.

I have had an identical problem with tapping plastics (I want an M7.4 x 0.55 thread for pen making) and have burst several plastic tubes I was trying to thread. I have not yet cured the problem - but will describe what I did to make the tap & mitigate the problems, none of which worked, so it may save others time!

The tap (of SS) was turned on the lathe, tapered to 2.5 degrees at the entry & threaded. Then on the mill I used an end mill to cut a right angled notch out of the side of the tap below the centreline of the tap to give the positive cutting angle. The thread depth was 0.27mm, and the groove went the same depth into the core of the tap. The reason for this is that I'm a bit ham fisted and didn't want the tap to break.

I used a tiny triangular file to re-instate the threads, then hardened and quenched the tap. Tempering was 30 minutes in the oven at gas mark 9 (light straw). Did it this way for 3 reasons - 1) the tap was black & well nigh impossible to clean anywhere to see the tempering colours and 2) It was a Sunday and I was cooking the roast, so had to have the oven warmed up to gas 5 anyway, so bumped it up to gas 9 an hour early. 3) The wife was at Church & wasn't there to complain.

Anyway. I then drilled the tapping hole (6.8mm) and fed the tap in. Two threads bit before stripping the thread off the plastic as it bound up. So, I went and ground a bit of relief on the taper section. Which meant it didn't stay at 90 degrees to the surface, so turned the end of the tapping hole into a countersink. Then on using a fair bit of force to get it in and bite, the thread was formed more than cut, none the less the groove filled up with swarf rapidly and burst the plastic tube.

On re-machining the grooves I wrecked an end mill as I forgot that it'd be really quite hard after the tempering.

After annealing the tap, then I milled the groove deeper with the new end mill at 0.25mm higher up so as not to damage the thread cutting edge. This time the groove was 1mm deep not 0,6mm. Didn't harden or temper the tap because plastics are soft... And tried to tap another tube. The first 5mm were not successful as it just reamed the plastic out to 7.4mm from the tapping dia. Then the thread bit, and started cutting for 3 threads, when the plastic tube burst again.

Then I tried lubricating the non cutting area with silicone grease. Not successful as it stopped all cutting action entirely, creating forming only. The force increased to turn the tap until the tube burst.

Tapping compound didn't help, with the same predictable result.

When I stopped inventing Esperanto on the spot, I put it to one side until I can find out more.

Which may be now.

Regards,

Richard.

> I used a tiny triangular file to re-instate the threads

I wouldn't do that - any slight reduction of the cutting edge will impair the cutting action. Always clean up by stoning the cutting face.

Plastic swarf is springy and curly and will rapidly fill your 2mm deep grooves. With no-where for swarf to go that's almost certainly what's bursting the barrel.

No need to temper for plastic.

Neil

Robin,

I'm drilling out to 9.3mm (Major Diameter minus Pitch) before tapping.

Any suggestions as to where I'm going wrong?

Your tapping drill is too small so you are trying to get 100% engagement of the thread which is not practicable or necessary. All tapping operations are a mixture of cutting and thread forming. With soft metals you need to aim for only 65% thread engagement so I suggest you use a larger diameter tapping drill. All this is fully explained in Tubal Cain's excellent book Drills, Taps and Dies.

Eric

Gray,

I stand corrected.

Eric

Thanks to all for your replies. I should have thought the rake angle thing through a bit better obviously! I think John S diagnosed the disorder in my thinking well - I was focussed on the rake angle at the cutting edge (which I think has to be positive if you go in to greater than d/2 with a ball nosed cutter of diameter d on the centreline, but I might be wrong!) and neglected to think about how the angle changed over the depth of the thread. Doh!

I'll have another bash at this over the weekend, taking on board all your suggestions, and let you know how it goes.

Regards, Robin.

One way to get relief on a home made tap using a ball nosed cutter is to form the cutting edge using the ball and not the side of the cutter.

Altering the radius of the cutter alters the clearance angle and the diameter of the cutter controls how much land is left on to reduce drag.

Upside is as you can see from the drawing [ shown from the bottom of a RH tap ] it produces a far more professional cutter.

Remember before cutting the first flute what hand you want.

John,

That picture is worth the proverbial Thousand Words.

Thank you.

MichaelG.

Richardandtracy, I'm not sure if you can apply this strategy to pen making, but perhaps you could tap your hole while the plastic stock is a larger dia, then turn to your finished size. Also multi flutes on your tap would help as you don't require a great deal of strength behind the cutting edge.

Roy

Richardandtracy, another way might be to put the plastic tube inside a tight fitting metal tube to support it while it is being tapped. Ian S C

Plastic when tapping a thread, unfortunately has a tendency to ball up and it does not cut chips like steel. This means the pressure builds up and has nowhere to go except into the body of the plastic and it generally ruins the thread. The tap should have good deep flutes and be extremely sharp. Multi fluted will give the best results and assure that the thread clears itself better. Try beeswax a a lubricant, perhaps softened with a bit of turps. Relief of the start of the thread is important too, so maybe leave a spigot that tap can ride on.

You can get this balling of swarf/chips when boring on the lathe taking fine cuts in a small hole.

Clive

Remember that the flutes iof the tap have to be large enough to take all the material removed as it turns. With only a short lead-in taper, this can be a serious problem - as all the swarf is heaped up in the first couple of threads.

Cheers, Tim