Specifying Threads


One of the most critical items in the design and production of a precision machined product is threads. The selection of standard thread designs and size aids in securing a satisfactory part at an economical price. Special threads require special tools and gages.

Information on thread forms, sizes, major, minor and pitch diameters and tolerances can be obtained for both U.S., Canada and Metric threads from the American National Standards Institute. Thread standards for Federal Services are described in H-28 Screw Thread Standards for Federal Services available from the National Technical Information Service.

The standard method for designating a screw thread is by specifying, in sequence, the nominal size, number of threads per inch, thread series symbol, and thread class symbol, optionally supplemented by pitch diameter and its tolerance. An example of an external thread designation and what it means would be:

1/4 – 20 – UNC – 2A

Where 1/4 = Nominal Size
20 = Number of Threads per inch (Pitch)
UNC = Thread Series Designation
2A = Thread Class Designation

PD 0.2175 – 0.2147 (Optional – Required If Special Threads) PD .xxxx” – .xxxx” Before Plating PD .xxxx” – .xxxx” After Plating – Necessary for Checking

Metric Threads are designated by specifying, in sequence, a capital “M” for metric followed by thread size in millimeters, an “x” to separate the size from the pitch, the pitch in millimeters, a dash and then a pitch tolerance symbol (to designate the tolerance grade and tolerance position) and a major diameter tolerance symbol (to designate the tolerance grade and tolerance position). An external thread, then, would be shown as:

M – 6X – 0.75 – 5g – 6g

Where M = Metric Thread
6X = nominal size in millimeters
0.75 = number of threads per millimeter
5 = pitch diameter tolerance grade
g = pitch diameter tolerance position
6 = major diameter tolerance grade
g = major diameter tolerance position

When drawing either internal or external screw threads, the correct dimensioning of effective (usable) threads or full threads is important. Of equal importance is noting whether or not partially formed threads or tool marks can be allowed on the part. The unthreaded length should also be dimensioned, especially when this is essential to the function of the piece.

See Drawing #31 for an example of a properly dimensioned thread.