Making a screwcutting tap

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Making a screwcutting tap

This topic has 7 replies, 5 voices, and was last updated 20 February 2010 at 15:27 by Richmond.

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19 February 2010 at 21:46 #48876
Peter G. Shaw
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@peterg-shaw75338
Hi folks,
A recent project with my grandson resulted in an attempt to make a UNF 3/8 24tpi tap out of silver steel. Lots of things went wrong but we did eventually manage to cut a thread in a piece of aluminium. This is what we did, along with what we know we did wrong, but I’d like to ask, for the future that is, for any other thoughts.

I should point out that one of the reasons for this project was to let the grandson do as much as possible, and to see the rest.

First we took a piece of half-inch silver steel and turned it down to 3/8ths. Unfortunately the only way that we seem to be able to get a smooth, and hence accurately measureable, surface is to rough turn first, then finish turn using high speed and slowest possible manual feed. So of course, the finished surface ended up slightly less than it should have been, and a repeat attempt the same. As it happens, the device that was intended to screw in was itself slightly less, so we continued.

Next we screwcut. In retrospect we should have produced the taper first, so that was yet another mistake. Anyway, I’ve never had much luck with trying to calculate depths of cut etc for screwcutting and so had to resort to using a nut as a test piece. So this is what we did, and produced a thread.

At this point we tapered the job. That was ok, we used 8° included as per Tubal Cain’s instructions.

Then we tried to create a cutting edge using a minidrill with a grinding disc in the chuck. We had looked at other similar sized taps and realized that we needed to produce an undercut under the cutting edge and so we tried to set the minidrill appropriately. We weren’t particularly successful and finished up using a file to try to improve matters. Thoughts on how to do this would be most appreciated. Also on how to produce, say, three cutting edges.

Having got this far we then hardened it. Ok we didn’t bother with the one hour per inch as per Tubal Cain and others, but neverthe less got it very hot and then quickly dunked it in cold water. This was followed by tempering to light straw, although here again I suspect we may have heated it too quick and possibly had some of it nearer brown.

We tried it out on a piece of aluminium, slowly increasing the drill size until we could get it to cut, which it did after a fashion. After cleaning up, we found that the original device it was meant for could actually be screwed in, albeit tightly, so in that respect it was more or less successful.

Grandson, by the way, was absolutely delighted with the result, even though it wasn’t perfect. To him, it was the act of screwcutting in the lathe and then hardening and tempering the silver steel, none of which he has ever seen before.

So, thoughts please.

Peter G. Shaw
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19 February 2010 at 21:46 #15455
Peter G. Shaw
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@peterg-shaw75338
19 February 2010 at 21:48 #48878
Peter G. Shaw
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@peterg-shaw75338
Oh dear, I’ve managed to enter it twice. Please accept my apologies and ignore this entry.

Peter G. Shaw
19 February 2010 at 22:38 #48882
mgj
Participant
@mgj
Peter I have just done a 1/4 x 16 square castle thread tap, and that worked fine in bronze.

One ought to form relieve on the trailing edge of the cutting face. Sadly I don’t have a form relieving tool, though the Quorn will do it. Thats a tragedy I just have to live with.

So, I just turned the thread to size (very slightly oversize) to give clearance when I cut the real thread to dimension. In silver stell. Then I put the taper on, and then I milled the cutting faces.

I set the milling cutter to cut dead to centre, looking down from above and depth of cut was to bottom of thread. I cut 4 cutting faces. 3 would have been better . So I had 4 cutting faces at zero rake.

Hardened to bright red, didn’t temper, because the cutting edges temper soft and the centre stays hard (I don’t have a sandbox)

Then I tapped the bronze nut. Bit of a rachet action, 1/8 turn fwd and click back (no form relief!) and slow progress but it did thread the bronze fine and to hte size required.. and the “bolt” of the brake mechanism which had been cut to size fitted fine, so I had no fiddling. Lots of RTD!!

So the only difference is the way the edges were put on – mine was cribbed from GHT dividing and graduating – how to make a spotfacer.

Without proper form relief, and proper grinding these taps will always be imperfect and more suited to single object runs, but in general does that matter to the modeller.

Jason did a similar exercise for his TE, so it would be interesting to hear how he did.

Edited By meyrick griffith-jones on 19/02/2010 22:40:45
19 February 2010 at 23:12 #48886
Ramon Wilson
Participant
@ramonwilson3
Hi Peter,

Albeit you have experienced difficulties you have just carried out what to me is one of the most satisfying parts of machining – making a cutting tool that will sucessfully cut another piece of metal.

You ask for thoughts, so taking it a stage at a time —

Silver steel is not the easiest of materials to work at the best of times but it’s readily available, does a good job and is what most only have acess to. Finish turning it to good tolerance needs fine finishing cuts with a sharp tool (preferably touched on the grindstone before those final cuts) The finishing tool (HSS) needs a good top and back rake – about 10degrees or so – better to be more angled than flatter. Use coolant even if it’s only brushed on. Take the spring out of the cut – either cut on the backcut or take the same cut twice. For the size you were quoting you certainly should not need to be in slowest speed to obtain a decent finish.

Screwcutting silver steel can be a very ‘bitty’ operation. Personally I would not use a carbide tipped tool for this however desirable the perfect section. (I know others will differ but that’s my view based on a lot of screwcutting) HSS will give you a much better finish in Silver Steel. Keep the speed right down and use a good cutting fluid – Rocol or similar. You do need to know the depth of cut for the thread if you want to make an accurate tap – not much point if it’s not (accurate) unless its for a one off never to see anything else screwed into it other than a mating part.

Cut the taper first and preferably reduce the diameter for the tool to run into rather than stopping ‘blind’.

The flutes and cutting edges really need to be put in with a milling cutter. Either a ball nose endmill or a radiused circular cutter. To do this whilst still in the lathe is possible but you would need some form of cross slide milling attachment. Simple, three division, dividing can be carried out by using the three jaws of the chuck. Either align them with something ‘fixed’ or place a short rod vertically under one jaw. Big problem is preventing the chuck from rotating – at a push you can engage back gear. Failing this you can still use the chuck to obtain the divisions and set a tool on edge and scribe a line down the shank and use this to guide setting the blank up in a mill vise if that’s all that’s available. The divisions do not have to be perfect as no doubt you have found. The main thing is not to leave the threaded faces too wide. The cutter should be off-set over centre to give a rake to the teeth. Debur only by filing down the flutes the heat treatment will normally take out any whiskers left in the teeth. This way the teeth edges do not get deformed.

Heat treating – Don’t over heat it – a dullish side of bright red is all that’s neccessary. (This comes from doing this in a contolled environment) Despite the fact that it is recommended to quench silver steel in cold water I always do it in oil. The thermal shock in water can cause cracking and a tap takes a lot of work. Oil quenching makes virtually no difference to the intial hardness. Tempering is best done by heating a small container of sand- I use a tobacco tin – heat the sand up first before putting the item in and bury it under the sand. Keep turning it and checking the colour build up. This will bring the colour out much more slowly – you have much more control. Quench again – If you don’t get it right and you want to reduce the temper a bit more you must remove the colour – if you just put it back in the sand the colour stays as it was and the temper disappears altogether!

I hope these views help and will no doubt stimulate a few more.

Have a great time with your grandson experimenting again.

Regards – Ramon
20 February 2010 at 00:08 #48888
mgj
Participant
@mgj
This heat treatment game- the tempering temperatures are important. You do need to hit them more or less right.

The red on the way up for first treatment is much less critical.

Basic metalurgy. Steel is an alloy of iron and carbon (in a small %age) and is a solid solution. This means that it can alter its crystalline structure without melting. However it takes time to do this, so that’s why you hold for so long per inch – usually 10 minutes. Its also why you dunk it – because by cooling faster than it can transform, you can “lock” it in a state it would not naturally hold at that temperature. (Some steels notably some grades of stainless will transform with at room temperature! Most can be locked though)

Precisely what happens depends on the carbon content and the other alloys which make up the eutectic, However, as a generalisation, heat it up to red and it transforms to a very hard needle like form – very hard, very brittle when cooled (not when it’s red hot of course). You then quench to lock it in this martensitic phase. Then, having got it in a known condition, you heat it to a temp at which it starts to transform back to other phases, of which there are various but austenite and pearlite are the two most common. (The differences depend on how much carbon is dispersed in the steel, and and the form that the carbides take) They are ductile and soft(er). So the tempering temperature controls how much of this hard material transforms into softer forms, giving you whatever qualities you need.

Other alloys have different effects, controlling a number of actions, and crystalline activity. However essentially once the thing has got to the transition temperature, getting hotter is of no real consequence – it cant get “more” martensitic though it will increase the rate of transformation, so in crude terms, a bit hotter is a bit better as it were, but not essential. Beyond that, it melts.

That by the way is how these hard spots in cast iron form – through rapid cooling, and instead of forming a nice nodular iron with carbides interspersed, the carbides hold uniformly as this very hard martensite. Which can be a bit of a sod.

Edited By meyrick griffith-jones on 20/02/2010 00:12:09
20 February 2010 at 12:11 #48902
Circlip
Participant
@circlip
Meyrick, you can add all the Martensitic or Austenitic or Spheroidal graphite or altropies you like, but unless one has the ability to ACCURATELY measure the temperatures, Dull red or Bright red or Cherry red??? are the easy ones for us general dobbins to use. As far as tempering is concerned, I keep throwing this one in, a VERY efficient source of a tempering “Oven” sits in most kitchens and providing that the bits are CHEMICALLY clean and no trace of oil or other detritus. You can set the thermostat on the domestic cooking oven and dangle the bits on wires from the racks and WATCH them go the right colour.

Sundays works best in our household as the chamber is already up to speed (Or just below it) after the luch is prepared.

Did all the theory bit in the lab where I worked when an apprentice Meyrick and had the crazy paving diagram stuffed under me nose, but I don’t have access to the salt baths, Lead baths tempering furnaces OR the measuring and inspection tackle to check it with anymore.

Regards Ian.
20 February 2010 at 15:27 #48921
Richmond
Participant
@richmond
Hi,

Due to a duplicate thread, I am going to lock this one. No more additions will be able to be made here. Unfortunately, I am unable to merge the threads. So, my sincere apologies.

Please use the alternative thread.

Rgds

Keith
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