Editing Screw thread (section)

===Thread depth===
Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, yielding a final ''thread depth'' that can be expressed as a fraction of the pitch value. The UTS and ISO standards codify the amount of truncation, including tolerance ranges.

A perfectly sharp 60° V-thread will have a depth of thread ("height" from root to crest) equal to 0.866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle — a direct result of the basic [[trigonometric functions]]. It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V.

The nominal diameter of Metric (e.g. M8) and Unified (e.g. {{frac|5|16}}&nbsp;in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by {{frac|0.866|4}} of the pitch from the dimension over the tips of the "fundamental" (sharp cornered) triangles. The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation {{frac|1|8}}''p'' or 0.125''p''), although the actual geometry definition has more variables than that. A full (100%) UTS or ISO thread has a height of around 0.65''p''.

Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 75% of the 0.65''p'' value. For example, a 75% thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that [[tap and die]] wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed.

This additional truncation is achieved by using a slightly larger [[wikt:tap drill|tap drill]] in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread's [[#Major diameter|major diameter]]. In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases, the [[#Pitch diameter|pitch diameter]] is not affected. The balancing of truncation versus thread strength is similar to many engineering decisions involving the strength, weight and cost of material, as well as the cost to machine it.