Here is a vintage pitch diameter comparator made by the Standard Gage Company. I really don't much about this other than it came from an old machinist's shop. I'm not entirely sure what the purpose of this unit is either, other than to tell the difference in diameter/thread pitch of bolts and etc.
DIAL TEST INDICATOR & MINI MAGNETIC BASE This Is A"Brand new" Measuring Tool used In Milling And Lathe Applications And For Other Measuring Tasks. It Is A Small And Handy Instrument At A Reasonable Price.
Balancing Machine Parts and Miscellaneous Mechanisms, Constructions and Data, two chapters from Handbook for Machinery Designers, Shop Men and Craftsmen, by Frederick A. Halsey, B.M.E., originally published by McGraw-Hill Book Co., New York, NY, 1916. Reproduced by Nation Builder Books, Mebane, NC, 2001.
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