OldTools Archive

Recent Bios FAQ

274836 Erik Levin 2021‑11‑25 Re: Adjustable Knob for a Johnson Combination Square
The screw appears to be 1/4" nominal, but the threading is unclear from your
measurements. 1/4"-28 is common (fine thread), but 1/4"-32 isn't unknown.
Measuring the way you did with the caliper isn't great, as the tips of the jaws
don't match the thread form well. Try this:

Set the tips in two threads or so apart, then zero the caliper.
Open it five more threads and measure.
Divide the measurement by five to get the pitch.

If the value is 0.03125", you have 32 thread/inch.
If it is 0.0357", the threading is 28 thread/inch.

Of course, the best way is a thread gauge.

The reason to zero the unit with the tips set two or three threads apart is to
get the zero where they actually seat.
The reason for measuring five threads (or better yet, ten) is to reduce the
error that remains, due to variation in the individual threads and due to the
imperfect fit of the jaw tips, as well as get an extra digit of precision, as
the difference between 0.0313 and 0.0357 is a little ding, or a speck of dust.

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Recent Bios FAQ