Unified Thread Standard
The Unified Thread Standard (UTS) specifies a standard thread shape plus allowances, tolerances, and designations. UTS is for screw threads and most often used in the United States and Canada.
UTS threads have the same thread shape specs as the ISO metric screw thread, the difference being use of inch based diameters (Dmaj) and pitches instead of metric values. Like ISO threads, UTS threads have a V shape with flanks at 60 degrees (in a plane intersecting the thread axis). The outermost 1/8 and the innermost 1/4 of the height (H) of the V-shape are cut off from the profile.
Definitions (see the thread glossary):
P = Pitch = the distance along the axis the screw moves for one full rotation.
Dmin = Minor diameter
Dmaj = Major diameter
Dp = Diametrical pitch
H = Height = distance between peaks and valleys prior to above mentioned cutoff of the outermost and innermost portion of the V shape.
TPI = number of threads per inch.
Dp = Diametrical Pitch.
The following formulas apply:
P = 1 / TPI
H = P * SQRT (3) / 2
Dmin = Dmaj - H * 5 / 4
Dmin = Dmaj - P * 5 * SQRT (3) / 8
Dmin = Dmaj - P * 1.082532
Dp = Dmaj - 3 * H / 4
Dp = Dmaj - P * 3 * SQRT (3) / 8
Dp = Dmaj - P * 0.649519
UTS threads are normally indicated with the major diameter and TPI. Under 1/4 inch the major diameter is typically specified by an integer whose meaning is specified by the standard (see formula below). These two numbers may be followed by the letters UNC, UNF or UNEF if the diameter-pitch combination is from the “coarse”, “fine” or “extra fine” series, and may also be followed by a tolerance class. For example:
6-32 UNC 2B means major diameter 0.1380 inch, 32 TPI, coarse series, tolerance 2B
The conversion from number screw sizes to major diameter is:
Dmaj = Screw Size * 0.013 + 0.060
For example for a #6 screw:
Dmaj = 6 * 0.013 + 0.060 = 0.138"