ISO metric screw thread

The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.

The "M" designation for metric screws indicates the nominal outer diameter of the screw, in millimeters (e.g., an M6 screw has a nominal outer diameter of 6 millimeters).

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter, D (D_maj in the diagram), and its pitch, P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The thread depth is 0.614 × pitch. The outermost ¹⁄₈ and the innermost ¹⁄₄ of the height H of the V-shape are cut off from the profile.

The relationship between the height, H, and the pitch, P, is described by the following equation:

$H={\frac {\sqrt {3}}{2}}\times P=\cos(30^{\circ })\times P\approx 0.866\times P$ $H = \frac {\sqrt 3}{2} \times P = \cos(30^\circ) \times P \approx 0.866 \times P$

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