This was in reply to someone who was needing to chase so old threads on something. “Since you have done your own machining, make your own tap. It’s not hard to do. The easiest way would be to take something like 4140 steel rod (a bolt might do), machine it down to the correct diameter for your particular thread and then cut threads on it to the correct dimensions. You can also find thread dimensions in the Machinists’ Handbooks. After the threads are confirmed to be correct, machine flutes along the sides like all taps have. A minimum of two, but three or four flutes would probably be better and not be as likely to break. Make sure to make the flutes off-center to create some “hook” for each section of thread. Then take it and put it on a fire-proof surface like a metal plate (that’s not sitting on something flammable, like I did!), heat it up to cherry red and quench in water. You will then have a pretty hard tap, for a one-time use that will work very well, to your exact dimensions. I have made three of them like this. Two were for the 3-1/2” threads on the nose of an inert M-117 750 pound bomb. I don’t remember the thread pitch off hand. I literally broke the first one by torqueing it too hard during use. I had accidentally created a stress riser with a center drill too near the hole in the top used for an 18" piece of pipe to turn the tap. The second one worked well, but I was lazy so I only put two flutes in it. It drags too much when you use it and I’m pretty sure it’s too hard to put more flutes in without doing one or more additional heat treatings. This is NOT guaranteed for cutting new threads, just chasing old ones. I also had an old RCBS “A” press that 1-3/8" threads in it. I include this description in case someone wants to know more about how this system works. While they used to make some dies with those threads, they haven’t in a loooooog time. So I converted it to 1-1/4" threads for my dies of that size by taking a 1-3/8" bolt, drilling a cross-hole in it so that I could put a 1/4" square HSS lathe bit in it. I then cut a hole perpendicular to that that I threaded for a set screw to keep the 1/4" bit in the bolt. The cross-holes were more than 1" up the side. I then set the bit in the bolt to the correct depth to create a correct-size tap hole for a 1-1/2" thread. I then screwed the bolt down into the press. When the bit started to touch the cast iron, it started cutting (in one pass) the tap-drill size for 1-1/2". I then continued screwing the bolt down in and the 1-3/8" thread that was in the press aligned the new (for want of a better phrase) manual lathe until it was almost all the way through the press. If the last thread or so was off-center, no matter. I then took an 1-1/2" tap, drilled a hole in the end with a carbide drill and VERY CAREFULLY tapped it to at least 3/8" thread, about 1/2" deep. I then made a pilot from some soft steel I had around that I put 1/2" (outside) threads to go into the tap and the large diameter of the pilot was a few thousands of an inch smaller than the tap-hole diameter in the press. I then inserted the pilot into the hole until the tap’s threads contacted it. Then I tapped the hole to the 1-1/2" size. At this point the bit tip shattered. No flute at the cutting edge of the tap, dummy, and so no place for cast iron cuttings to go. I took it back out, cut one flute where the cutting edge was and put it back in. Note that I had never sharpened the bit, it was just raked at the end as it came from the manufacturer. It worked just fine this time. After I was done with that, I made a bushing with outside threads 1-1/2" and inside threads 1-1/4". Someday, if I ever see a need for it, I’ll make another bushing with 1-1/2" outside threads and 7/8" on the inside for normal dies. Maybe for the .30-.378 Weatherby cases that my Lee press doesn’t care much for. If anyone wants photos of the taps and pilot, just email me and I’ll find them and take some photos. My email is Mignuc@mediacombb.net."
Just to add to this, its really worthwhile using a cutting lubricant or compound when using taps. I used to use a paste called Trefolex. Smells like s…t but really works well.
Drilling 1/8" holes through 5/8" stainless steel rod, I could get maybe two holes before the drill was useless adding Trefolex the one drill would drill through about a dozen holes.
Also cuts easier and faster, there are other cutting compounds which may work as well.