2 Sets of Numbers on Imperial Screw Gauges

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Well-Known Member
May 9, 2009
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I found 2 sets of blade type screw gauges, that I have been wanting for ever, a NNB metric with some insanely fine pitches and a an old L. S. Starrett imperial one. The smallest size I can read is marked 24 - .072 which is useful for me, and there are 4 sizes smaller, stamped quite poorly over the whole set, but I think the blades must be very hard. the 1st number is obviously TPI ...
Other numbers are e.g. 12 - .114 , where .114 is obviously twice .072, but other than that I can see no relationship, and 14 – 123 and 11 - 157 etc.
What is the meaning and/or practical application of the second set of numbers? Tried some numbers to see if it was thread depth or something ...
Another one of my interminable Questions ...
Hey ant.

I think that second number is the double-depth of thread, the depth of the thread on both sides of the thread section. Subtract it from the major diameter (outside) and it'll give you what the minor diameter should be.
Useful when using a thread mic or wires when thread fit is important.
The 2nd No is the amount you remove from the diameter to get the minor diameter = dot x 2

NB this is for American threads where Dot =0.866/tpi.

Best Regards
Thanks guys. On the coverplate is proudly stamped Pat May 19 1886 (I can barely see) I love that I got something so neat for so much less than the local or imported price new