Not exactly common thread
19mm x 2mm Tap
© Frank Ford, 2005; Photos by FF

Recently I was asked to design and develop a special little fixture that would screw on the nose of a standard Dremel tool.  I looked in a catalog that sold other screw-ons for Dremel, and it said that the threads were metric, namely 19 mm x 2mm.  On my day off, a Friday, I decided to dig into this project, and immediately ran into a snag trying to locate a tap that size anywhere near my neighborhood.  Just as my wife was taking off for work she asked me what I'd be up to, and I told her I'd planned to work on this little device, but couldn't because I'd have to order the tap from MSC or some such, and that it would be after the weekend before it could get here, and I'd hoped to work with it on Sunday for a while as well. Her reply was typical, "So why don't you just make one?"  Duh, since I was only going to tap into acrylic or aluminum, why not?

Here's how it went:

I had some nice 1" diameter W-1 drill rod, so I turned down a section to 19 mm, and put on some nice threads:

Sticking it in my spin index, I milled four flutes with a regular ball end mill:

I hadn't decided what to do with that little section at the end quite yet, so I just plowed through it as I milled, thinking it might serve as a guide in the hole as I used the tap.  Later, I changed my mind and turned it down to 5/16, figuring I could stick on a pilot or some such if needed later. While I had it back on the lathe, I also tapered the threaded flutes at the leading end.

Sticking the threaded section in a collet, I turned down the shank to an appropriate size to fit my tap handle:

Then I was able to load the tap into another collet from the rear:

And mount it in the spin index to mill the square drive end:

A bit of hand work to file some clearance for the tapered end finished up the tap:

I used a flat file for the tapered section, and my ground triangular thread restoring file inside the threads.

All done, and ready for a try:

Later I'll harden and temper the tap for use in aluminum, brass or whatever, but for now I'm going to try it out in acrylic.

I drilled through a thick piece of scrap acrylic and used the drill press chuck to stabilize the tap handle as I tapped by hand:

One cool thing about tapping acrylic is that you get a great "outside" view of the new threads to check your work:

It fits right on the Dremel, so I guess now I'll have to get on with the original project!

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