Joe AutoDrill wrote:

Okay... I can understand that concept... Seems to make sense.

It makes me pucture a 24" long 1/4-20 tap, but the concept is at least
clearer now...

I would imagine the taps are silicon carbide, diamond, etc...

Abrasive shaping of glass I've seen. Most do it "wet" rather than dry -
presumably so they don't breathe in glass dust.
No, the grinding creates hot spots, the thermal stress cracks
the glass. When wet, the grinding area is kept cool, and the
glass doesn't crack.

Jon