This forum is using up a lot of my spare time. I keep running across interesting ideas.
So.
A chunk of 1/2" aluminum plate from my scrap bin, and a truly horrible and dull 4-flute 3/8'-24 UNF tap from a $12 Canadian Tire tap and die set. No coolant. No measuring and the plate isn't set true to the quill. No effort spent in trying to make it pretty, just chucked the tap into my mill (using a drill chuck), and scrubbed it across the plate a couple of times at 500 rpm, 0.010" depth of cut per pass. Lots of tearout, the tap wouldn't cut butter.
My ancient eyeballs and a 32 TPI thread gauge says 96 LPI is about right.
This could be an interesting surface finishing method.
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