Somewhere on this forum there's a thread where the use of taps for thread milling was discussed with several contributions from John Stevenson, though I can't readily find it. I did contribute myself and John was very helpful. For my M14x1 internal thread I use two cutters (different trials). One was an M8x1 tap, with all the rows of teeth ground away except one and the tip ground square. That worked well but is limited to conventional milling down into the hole, and unless you also grind away all the teeth but one it will only do one pitch. A single point cutter (like my internal threading lathe tool I discussed) can start at the bottom of the hole and spiral out, climb milling all the way which is quieter and gives a better finish. To generate the code I used a little program provided by Chestnut Pens which Richard kindly modified to allow the use of a tap.
After some though I decided that the helix angle of the tap isn't really a problem. You have to remember that the tap is significantly smaller than the hole you are threading. Yes, in principle as the tap rotates the helix may cause some rubbing, but both the smaller tap radius and its form relief are quickly taking the cutter surface away from the work, as it were.
Well this will be the last adventure on this version of the forum.
A name plate that will be bent and then let into a recess on a cylindrical part to represent cast letters. They are raised and tapered though hard to tell if they are raise dor cut into the metal from the photo
Plate is 50mm x 32mm letters 3.5mm tall and stand 0.4mm out of the background. Apart from the contour cut around the edge all was done with a 60deg engraving cutter with 0.3mm end width, 0.15mm stepover running at 5000rpm and a feed of 200mm/min. I left it running while I went out to do other things as the run time was just short of 4 hours to get through the 83,000 lines of code.
The machining marks that are just visible on the background cannot be felt and will have disappeared by the time I have silver soldere dit into place, cleaned things up and probably bead blastsed for good measuer.