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The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries.
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is an inclined plane wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.
The nominal diameter of Metric (e.g. M8) and Unified (e.g. 5 ⁄ 16 in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by 0.866 ⁄ 4 of the pitch from the dimension over the tips of the "fundamental" (sharp cornered) triangles.
The across-flats definition is the common standard today, and has been for many decades. The larger spanner in this photo is from the 1920s or earlier. Its face was polished to allow the size stamp to show well in the photograph. This example is American, but it illustrates the way that spanners for Whitworth fasteners were typically labelled.
United States Standard thread (USS thread), also known as Sellers Standard thread, [1] Franklin Institute thread [1] and American Standard thread, [2] is a standard for inch based threaded fasteners and washers. [3] The USS standard is no longer supported. It, together with the SAE fastener standard, was incorporated into the Unified Thread ...
The Thury thread form had the crests rounded at 1 / 6 p and the roots rounded at 1 / 5 p so the thread angle was close to 47.5° but not exactly. This was simplified in the BA thread definition by defining the thread angle to be 47.5° exactly and the thread form to be symmetrical with a depth of 3 / 5 p.
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25).
The Unified Thread Standard (UTS) originates from the United States, while the ISO metric screw thread is standardized worldwide. In turn, these thread standards define preferred size combinations that are based on generic units—some on the inch and others on the millimetre.