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UNM
Unified Miniature thread series

In April 1955 representatives of Britain, Canada and the United States agreed on a standard for miniature threads, which the Americans had never addressed. In the manufacture of watches the situation of the 1850s lived on. The Waltham Watch Co. had a metric series of 12 screws peculiar to itself, and the Elgin National Watch Co. produced 27 miniature screws based on the inch, replete with left-hand thread sizes.

The scheme adopted for the Unified Miniature Threads was based on the meter and on the recommendations of the ISO regarding sizes. Unlike other metric screws, the designation is the major diameter of the screw, in millimeters, followed by UNM.

Size Designation
nominal major diameter
Threads
per inch
(approx.)
Pitch,
millimeters
1st preference 2nd preference
0.30 UNM   318 0.080
  0.35 UNM 282 0.090
0.40 UNM   254 0.100
  0.45 UNM 254 0.100
0.50 UNM   203 0.125
  0.55 UNM 203 0.125
0.60 UNM   169 0.150
  0.70 UNM 145 0.175
0.80 UNM   127 0.200
  0.90 UNM 113 0.225
1.00 UNM   102 0.250
  1.10 UNM 102 0.250
1.20 UNM   102 0.250
  1.40 UNM 85 0.300

B1.10-1958, R1988. American Standard for Unified Miniature Screw Threads.

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