On Jan 9, 11:50*am, Jim Wilkins wrote:
...

You can make a crude quickie tap for soft metal by grinding the end of
a screw conical and filing lengthwise notches or grinding a flat on
either side, like the end of a chisel.

Jim Wilkins

I thought of a variation on that during lunch, tried it, and it worked
pretty well. A home-made tool will probably cut with only one edge
anyway, so why make more?

I hand ground a bevel about 2/3 of the way across the end of a Grade 5
1/4-20 screw, trying for a 30 degree angle (it measures 33).
After filing the burrs off I used it to complete a hole tapped 1/4"
deep in 3/4" aluminum, with Tap-Free lube. It acted like a spiral
point tap should and spit out one long curled chip. The exit wound is
tighter than standard but another screw will go in with finger
pressure.

Jim Wilkins