Thread checking

[mklotz]

I had considered getting the metric nut and bolt checkers on a cable from ENCO but put it off. It seemed to me that the cable would really get in the way; your comments seem to confirm that. Plus, the advertised item doesn't cover a lot of the really small metric threads I encounter. I suppose I'll just make another board like the one shown when a find a nice round toit.

I'm glad you liked the formula. Too many people want cumbersome tables when an entire table can be condensed into a single formula. Every shop should have a scientific calculator - they're dirt cheap now - and with one even the most math-fearing has no excuse for not using a formula.

Here's another one. Thread-forming taps, which displace rather than cut metal to form threads, need larger tap drills. There is a formula for this too and it includes an allowance for depth-of-thread...

Tap drill size for a thread-forming tap can be found from:
tap drill = OD - 0.0068*(DOT)/pitch
For example, with a 1/4-20 tap and a desired depth of thread of 65%, we have:
tap drill = 0.25 - 0.0068*65/20 = 0.228 in. (#1 drill)

65 % DOT for a conventional, metal-cutting tap would yield a tap drill of 0.208 or a #4 drill.