On Saturday, February 23, 2019 at 4:08:43 PM UTC-8, Cydrome Leader wrote:
I've been playing with tapping plastics like HDPE and so forth and not
surpisingly discovered that sharp taps and drill bits and not taking small
cuts seems to help.

So the question is how does one source "sharp" taps in the 6-32 to 3/8"
sizes?

Most mass-produced taps are for steel, and the tooth geometry isn't good for
plastics. You can hand-stone a tap for brass (by establishing a circa-90-degree
edge, and that's about right for plastic, too. This is a good use for
an old carbon steel tap that hasn't got a good edge, because you're making
a new edge anyhow.

The 'sharp' steel-cutting edge is more acute than you want, because steel doesn't
stretch when being cut; plastic does (elastic snap-back is the unfortunate result).