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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