Screw thread designations on metric drawings are preferably shown in either ISO unified inch or ISO metric sizes. ISO is the abbreviation for International Organization for Standardization, which is concerned with establishing standards for industry worldwide. (See Unit 26 for more information.)

ISO has recommended certain of the American screw thread sizes for use with SI. These include a selected assortment of National Coarse (NC), National Fine (NF), and National Extra Fine (NEF) thread sizes. They are identified by the addition of the capital letter “U” (meaning unified) to each of the abbreviated thread classifications:

UNC for Unified National Coarse

UNF for Unified National Fine, and

UNEF for Unified National Extra Fine.

It should be noted that certain American screw thread sizes not classified as unified (for example, -13NC) will also be found on prints during the transition period to metrics.

The elements applied for designating an ISO inch (unified) thread size on a metric drawing are illustrated in Figure 22.3.

Metric threads have the same basic form as the American National Thread. That is, each thread form has an included angle of 60°, Figure 22.4. The principal difference in the forms is that a metric thread has a greater depth in the cut of the thread. The manner in which an ISO metric size is designated on a metric drawing is illustrated in Figure 22.5.