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Making a screwcutting tap

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Ramon Wilson23/02/2010 22:01:48
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Hullo Peter, That's the spirit.
 
Regarding Michaels post above on fluting it appears I lost a bit of text in mine regarding moving the cutter over centre. It reads that 'too far this will reduce the diameter'. It should read - 'too far this will reduce the depth of flute relative to the diameter'. I'm sure you've already seen that the further you go past centre will of course give a more positive rake and reduce the width of the teeth but the core needs to be as big as you can get away with for safety. (That's 'tap breaking in the hole' safety!) You need to be over just far enough to create that positive rake situation
 
Mike's comment on keeping those thread widths to a minimum is a valid one as is the depth for clearance.The smaller the tap the more important it is to keep as much of the force required applied on the actual cutting! When heat treating don't forget also that the smaller the cutter the quicker it will heat up and therefore the more important it is not to create a brittle condition.
 
I have never carburised a cutter so can't comment on the technique but I do know that a local, well revered model maker, sadly now departed, was a big advocate of it. I would imagine it's main advantage is the more ductile core the only disadvantage being the time it takes to heat treat it. Mikes time of an hour is a long time to keep a flame on it if that is the only means of heating you have. I wonder if that could be reduced considerably and still give enough depth of hardness for the job in hand ie for a one off situation. Any thoughts Mike?
 
Keep us posted of your progress.
 
Regards - Ramon
Michael Cox 123/02/2010 23:22:14
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Hi Ramon,
I have a small furnace that I use for melting aluminium that I use for case hardening. I seal the part in a copper tube with charcoal and heat in the furnace for 1 hour. I think the depth of case is a "few thou". The advantage of this is the core is soft and not brittle. The tap works fine for brass but I have never tried it in steel. Probably you could get a reasonable case with a commercial  case hardening compound in a much shorter time and still have a tool that would last a reasonable time in soft metals.
With regard to the threaded land width it seems the smaller the better. This could be achieved, after cutting the flutes,by milling off more of the threaded part at quite a shallow angle so as not to reduce the core width too much. When I make another tap I will try this.
One detail I omitted in my earlier post was that I used 4 flutes to make the taps.
Mike
Stub Mandrel24/02/2010 22:38:32
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Regarding tempering small objects in silver steel. If you need then fairly hard, put them in the chip pan on 180-190 degrees. Tubal Cain suggested 160 degrees in the chip pan was fine as long as you avoid interrupted cuts. I would temper a tap more than that though.
 
It can take some explaining before your wife appreciates that a clean piece of metal is no threat to the health of the household or the quality of the food...
 
On the other hand, i've found that an ordinary over at gas mark 6 gives a nice dark straw temper. I haven't had any problems with blungting or chipping from tools hardened these ways - this one was done in the oven, note the even colour:
 
Neil

 
Ramon Wilson25/02/2010 10:09:56
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Hi Mike, thanks for your further up date.
 
I do actually have a small bench top electric furnace given to me sometime ago but have never used it. Mainly because I'm a bit nervous of the rather Heath Robinson electrics! Theres plenty of Casenite too so perhaps I should bite the bullet and get it working though I hate to think what it will do to the electric bill!
 
Do you use yours in the workshop? - Casenite smells quite a bit.
 
Neil, The oven is ideal providing there is domestic appreciation and approval! I guess for some that might not be available! Your tap though has a lovely even temper and shows the benefit of using this way.
 
I used the oven to colour the steel lagging on a double diagonal engine. I was able to use spring steel which was much easier to work than the planished blued steel sold for the purpose. This enabled all the work to be done before 'colouring' a nice blue/grey.
 
Regards -  Ramon
 
 
Michael Cox 125/02/2010 14:44:18
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Hi Ramon,
My furnace is a gas fired furnace and I definitely would not use it inside my workshop for two reasons. Firstly, the workshop is in a large wooden outbuilding with wooden floors so the fire risks are high. Secondly gas fired furnaces have the potential to produce large quantities of carbon monoxide and I am too young to die!
Mike
 
Stub Mandrel26/02/2010 21:20:04
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Intersting footnote: I read LBSC's advice on making piloted taps for threaded boiler stays. He said if you can't flute the tap, just file four flats on it and take things easy!
 
Bet it works too, even on mild steel - I'll give it a try.
 
Neil W.
Ramon Wilson26/02/2010 23:08:27
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Hi Mike, well errr.... Quite! I think its the dog kennel then! I'm getting a bit side tracked at the moment but will try when I can to see if this furnace works (if only to see if I can get the electric meter disc to self distruct). Thanks.
 
Yes it does Neil, particularly on very small taps where fluting would prove difficult. It won't exactly be a 'production' tool but it does get the job done surprisingly well. It helps too if the flats taper very slightly inwards (towards the point).
 
Ramon
 
 
 
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