Bill Schwab wrote:
Space is tight, so that has
pushed me toward 4-40, which is a pain to tap.
I do 4-40 all the time. I use these $8 machine taps with "spiral
flutes", they look a lot like twist drills. In fact, there are thread
drills that will drill the hole and tap it in one pass. They are a
little tricky, you have to do the drilling at a lower feed than the
threading, so I guess you can't do that on blind holes. Anyway, with
the spiral flute taps, you tap the full depth of the hole in one pass,
then back all the way out. The chips are sent mostly up the flutes,
just like a drill bit. I use "alum-tap" as an aluminum tapping fluid,
it is quite amazing. I recently stepped my 4-40 threading up to 1200
RPM, it works even better than 600.


In such a situation, how close to the edge of an Al part would you
consider it "safe" to tap for 4-40 and 8-32, respectively?
Oh, you can get down to about .1" easily on the 4-40, and about 1/8" for
the 8-32.

Jon