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	Threads per inch (approx.)	Pitch, millimeters
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.