I made a tap today. More like a clean-up tap if there is such a thing and not to cut metal but dried oil.
I’m working on an old Singer Model 27 that needs some TLC. By the serial number it was made around 1909 but for the last forty years or so has belonged to one of my older sisters. I know she never used it and maybe had intentions of fixing it up, I don’t know, but that ship has sailed and it’s here at my place for some rehab.
The tap comes into play for cleaning out two threaded holes in the bed casting. The holes are for fixing one of the supplementary attachments and almost never used, so consequently they get filled with bits of lint and then somebody thinks they’re oiling holes so puts a few drops of oil in there, etc etc. And after a hundred years the oil turns to varnish reinforced with lint and it’s impossible to get a screw in there anymore.
So why make a tap? Because the holes are smaller than a #8 but larger than a #6, and I couldn’t see a #7-40 tap within easy reach from anywhere else. Besides, it only needs to de-gunk the existing threads so I figured I didn’t need a real cutting tap.
I used some W1 rod I had in the rack and copied the 0.140” major diameter from the thumbscrew belonging to my own machine, then turned the threads to 40 tpi .
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I was originally going to cut the gullets using a small slotting saw but decided to just use the corner of a regular end mill instead. There’s obviously no relief on the trailing edges of the threads but I thought it would still do what I wanted. I had also thought that I wouldn’t need to use a handle with it but after the first attempt I brought it back down to the shop and milled a turning square on the end.
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And here’s the finished effort. Works great, fits the holes very nicely and runs nice and smooth so I know it’s not removing any metal, just the dried gunk. And all that for two holes. But hey, that’s what we do, right?
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Thanks for looking!
-frank