Editing Screw thread (section)

===Form===
[[File:Different Thread Types Update.jpg|thumb|right|260px|Different (and incompatible) threads including (from left) M12 left hand, standard M12, M12x1.5 (fine), M12x1.25 (fine), 1/2" UNF, 1/2" UNC, 1/2" BSW, and 1/2" BSF]]

The cross-sectional shape of a thread is often called its ''form'' or ''threadform'' (also spelled ''thread form''). It may be [[Square (geometry)|square]], [[Triangle|triangular]], [[trapezoid]]al, or other shapes. The terms ''form'' and ''threadform'' sometimes refer to all design aspects taken together (cross-sectional shape, pitch, and diameters), but commonly refer to the standardized geometry used by the screw. Major categories of threads include machine threads, material threads, and power threads.

Most triangular threadforms are based on an [[isosceles triangle]]. These are usually called ''V-threads'' or ''vee-threads'' because of the shape of the [[V|letter V]]. For 60° V-threads, the isosceles triangle is, more specifically, [[Equilateral triangle|equilateral]]. For [[buttress thread]]s, the triangle is [[wikt:scalene|scalene]].

The theoretical triangle is usually [[wikt:truncation|truncated]] to varying degrees (that is, the tip of the triangle is cut short). A V-thread in which there is no truncation (or a minuscule amount considered negligible) is called a ''sharp V-thread''. Truncation occurs (and is codified in standards) for practical reasons—the thread-cutting or thread-forming tool cannot practically have a perfectly sharp point, and truncation is desirable anyway, because otherwise:
* The cutting or forming tool's edge will break too easily;
* The part or fastener's thread crests will have [[Burr (metal)|burrs]] upon cutting, and will be too susceptible to additional future burring resulting from dents (nicks);
* The roots and crests of mating male and female threads need clearance to ensure that the sloped sides of the V meet properly despite error in pitch diameter and dirt and nick-induced burrs.
* The point of the threadform adds little strength to the thread.

In [[ball screw]]s, the male-female pairs have bearing balls in between. [[Roller screw]]s use conventional thread forms and threaded rollers instead of balls.