In my recent attempts at tap making; I've come across a puzling problem which I suspect is due to faults in the heat treatment department (an old propane torch, firebrick and plastic bucket).
A correcty working tap, in use on brass, just sheared off, not near the top of the work which is what usually happens when I have trouble with blunt taps, but in the threaded portion well above the work. The shear looks like a sheared off allen key! My suspicion is that a crack developed there during hardening, since it happens to coincide with a damaged "tooth". I don't know how plausible this is, and I have been wondering if it could have been incorrect tempering.
The silver steel taps which are two fluted for use on brass, due to my not having any dividing gear are supposed to be hardened and tempered to dull yellow, which is easier said than done, but in this case the cutting part was yellow all over and the shank blue. It then went straight in the water and I wondered if the metal in the centre of the cutting portion would have had a chance to become hot enough to be tempered or if it would have remained dead-hard? Tap diameter is about 3/16".
Suggestions as to what I might have done wrong would be most welcome as this doesn't seem to be a subject anybody I know is knowledgeable about!
After trying this procedure a few times commercial ground thread taps seem very cheap!
Alan Bain