"Jim Pugh" wrote in message
...
Alan, while awaiting the answer from those more knowledgeable, see if you
can borrow a Machinery's Handbook. Most machinists and mechanical
draftsmen will have a copy. It will explain more than you want to know
about threads. And, the American Standards Association (ASA) sets the
standard on screw threads and a whole bunch of other things as well.

Alan Horowitz wrote:

I've got an old screw pitch gauge that I inherited. There's no mfr's
name marked on it, just "Number 137".

the individual leaves are marked with two numbers each, but these
numbers are not reciprocals. The first number is always a small
integer which I presume is threads-per-inch.

the second number is always a decimal.

Any idea what the second set of numbers are?

pairs of numbers are
~~~~~~~~~~~~~~~~~~~~
treat the first number as TPI
I have added witworth form thread depth in inches.....
doubleing it gives the second number.

60 - 0.021 0.0160009
48 - 0.026 0.013340
40 - 0.032 0.016008
32 - 0.040 0.020010
30 - 0.043
28 - 0.046 0.022869
26 - 0.049 0.024628
25 - 0.05
24 - 0.063 0.026681
22 - 0.058 0.029106
20 - 0.064 0.032016
19 - 0.067 0.033702
16 - 0.080 0.040020
14 - 0.091 0.045738
13 - 0.098
12 - 0.107 0.053360
11 - 0.115 0.058212
10 - 0.126 0.064033
9 - 0.142 0.071147
8 - 0.160 0.080041
7 - 0.183 0.091475
6 - 0.215 0.106721
5 - 0.259 0.128065
4-1/2 - 0.284 (sic) [ 4.5 ] 0.142295
4 - 0.320 0.160082

N.B. unified thread depth is diferent, at 40 tpi for exampe it's 0.015336.

Hope this helps

looking at the leaves, "depth of thread" in inches seems the most
plausible to me. This item would date back to the 50's, I don't
believe there was too much metric action in American machine shops at
that time.

I'd try _measuring_ the depths with a micrometer, but I don't know
where to measure to/from. How is thread depth defined? And by the
way, who sets the standards on how these things are measured?


--
Jonathan

Barnes's theorem; for every foolproof device
there is a fool greater than the proof.

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