making a tap

I need to make a tap for a British shotgun screw. The screw is .150" in diameter and is 38TPI with a Whitworth 55 degree profile. Since this conforms to no standard that I know about I think that the maker must have made his own taps and dies. Any thoughts on how I should proceed? I could anneal another tap, turn it down, cut threads and flutes but then what? How do I reharden it to the right hardness. The hole to be tapped is just drill rod but I've broken enough taps in my life to know that hardness is critical.
Regards, Nial

Haven't had the need to make one yet, but here's how I would do it. Start with some oil hardening drill rod, turn to diameter and taper. Grind your own HSS thread tool, and thread. Mill a couple grooves. Heat and oil quench to harden. Basically, just examine and copy a tap. Note that the grooves (flutes) are not centered, but offset a bit to give the right rake angle.

after hardening , put the tap in ur wifes oven @400 deg. for an hour to draw it a tad...greatly increases toughness @ little cost in hardness.........easiest way to quench w/ minwarp is in drill press @ 200 rpm ,& bring can of oil up to tap to quench ... & blow out flash ..this leaves tang less than full hard which is GOOD....ATF works well ...u need a lite oil.....tap efficiency /ease will be increased if u back off some/all the the thrds leaving 1/32 to 1/16 land on thrds ....u can mill rounded grooves w/ ballend/horiz.convex. cutter or just mill like end mill setting up for light rake( but zero rake will work )

best wishes
docn8as

Man, I really liked doc's idea, and tried it a couple years ago when (I think) he first mentioned it. It was a near disaster for the house =:^( But the tool came out pretty good:^)

It was a long 7/16" x 14tpi modified square thread tap.

I'll only say, won't try this in the house (cellar) again. But if you aren't doing anything of too great a size (I had my regular 5gal oil bucket under it, and was using a portable mag drill for the driver), it shouldn't be too bad as far as smoke, and flames when you go to beat them out with a towel in one hand while still holding the drill handle and running the tool with the other so it and the drill doesn't get ruined, and then the flaming oil gets splashed out and the oil soaked towel catches....

I regularly heat treat O-1 "near" the open garage door to the cellar. The complication and sort of casual set up of the mag drill and having too many things and not enough hands is what caused the problem.

It works. Be careful.

BTW, I am tempted to use S5 or S7 (forget which is on hand here) for a tap next time. It doesn't get quite as hard, but probably hard enough for mild steel tapping, and is tough. Also, would probably use A2 otherwise. There isn't much real savings between A2 and o1 in small diameters. A2, if handled properly, eliminates the possible bending and distortion form oil quenching. But best if the A2 is done in foil in a furnace as opposed to torch.

smt