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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Last revised: 18 January 2011.