Waltham watch screws

One curious characteristic of this series is that the screw diameters were defined in millimeters, but the pitch in threads per inch. The Waltham Watch Company, in Waltham, Massachusetts, was a major manufacturer of watches.

No. of tap	No. of threads per inch	Diameter of thread, millimeters	Diameter of tap drill, millimeters
1	110	1.50	1.32
3	110	1.20	1.02
5	120	1.10	0.95
7	140	1.00	0.85
9	160	0.93	0.71
11	170	1.34	1.22
13	180	1.00	0.85
15	180	0.83	0.71
17	200	0.65	0.54
19	220	0.55	0.45
21	240	0.45	0.34
23	254	0.35	0.27

Source: Machinery, vol. 22 (March 1916), page 590.

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