Strange shop made tap

I had a need to make a nut for a rotary electrical switch.



The thread measured .270" and 36tpi. I tried measuring it in mm and it didn't match anything there either. Anyone know what threads these are? McMaster does carry a 1/4-36 tap but this is definitely larger. They are cast threads so not sure how precise they are.

So I made a tap to match it.



Worked like a charm.



Jeff

I would bet it is a 9/32-36 thread.

Nice job on the tap there.

awander said:

I would bet it is a 9/32-36 thread.

Click to expand...

funny you should mention a 9/32-36tpi --I have about 200 threaded drill bits, and a 1/4-28tpi is the standard aircraft size, but these that I have must be 9/32-36tpi size..--I have searched for ever for any tool that takes anything but 1/4-28 size to no avail.--I can't find any nuts of that size or special taps either.--I am wanting to make some small short adapters from 1/4-28 to 9/32-36 so I can use all these nice bits--I am thinking I will have to do the internal threading myself (I've never cut threads yet on the lathe is why I've searched in vain for a 9/32=36tpi tap ) It is funny that just now you mention this size!!!!--Dave--* has anyone else ever heard of a aircraft style drill with that size?--there must be some somewhere otherwise I wouldn't have all these nice drill bits--thanks if anyone has :thinking:

What kind of steel does one make taps out of?

Can you explain more about the tap? Material used, did you harden it, steps involed?
Thanks, Larry

The tap is made from a piece of O1. I turned the threads, then cut the flutes out with an end mill.


This view is looking at the tap point on. When I cut the flutes the point of the tap was facing away from me and the cutter was on the right side of the tap. You have to get the flutes cut on the correct side or the tap will not cut. Ask me how I figured that one out. :banghead::banghead: After I cut the flutes and put the tap back on the lathe. I ran a file on the end to give the threads a little tapper. Maybe the first 4 or 5 threads. Then I match the threading tool back up to the threads with the chasing dial matched up and the half nut engaged then did a last pass on the threads just barely taking anything off to clean up the burs left by the end mill. Then Heated it with torch and quenched in oil. I used a hand stone to dress the edges of the flutes. I didn't go back a temper it. Mainly because I'm lazy but also because I was cutting brass. I guessed at the tap drill size and now can't remember what I used. :thinking: There are others here who have made taps they can probably post some more help. I think that awander is right that this is a 9/32-36 thread and mine is made a little to small (by .011") But it worked well enough for this project. If I have to use it aging I will redo the tool to the proper size.

Jeff

I've never heard of 9/32-36 but MSC carries all sorts of odd sizes like 9/32-32 and 9/32-40. There are lots of oddball sizes out there used for special purposes... I routinely use 1/4-80 and 3/8-80 for fine adjusting screws on the machinery I design. Machinery's Handbook defines every possible UN size even if most are rarely used. Lots of odd sizes used in the optical world, too.

You must have filed some relief on the tap or it would not have cut.

I make quick taps by some old time tricks. They work perfectly well. When I make parts for antique mechanisms they always need special threads.

A suitable tap can be made by just filing them square or triangular. In the 18th. C.,taps were made that way commercially. They cut very well. I have made taps by filing a tapered flat spot on 1 side of the tap. The flat at the nose is 1/2 the diameter of the tap,tapering out quickly to nothing in about 3/8". The important thing is to then file the threads into a relief so that the foremost cutting edge is higher than the area behind it,or it won't cut. As long as the threads themselves are correct,these quick taps leave a threaded hole just as good as any other tap will. They are not for high production,just for a few special threads. I harden them,and draw to a dark brown color. Some temper to purple,which is just below blue. Blue is 52 Rockwell,and getting too soft to hold up on steel.

I used the same process when I had to make a special tap for a dividing head I was working on (5/16-40 IIRC). I used a file to taper the end threads, and then I hand filed relief at the tapered end. It's not hard to do, just takes a little time and patience (and care). Just file till you can barely see any "land" left at the thread peaks. I used W-1, because that is what I usually used. Tempered to a dark straw (I believe I did it in the kitchen oven at around 450F). Sharpened with a diamond hand lap.

I hear a lot of people complain about the quality of high speed steel taps, but I was amazed at how well this cut. Better than any commercial tap I had at the time. It seems daunting, and it is a lot of work (much easier to buy if you can), but it's definitely something anybody can do.