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ISO 68-1 Metric Thread Profile Specifications and Equations
ISO 68-1 Basic Metric Thread Profile Specifications and Equations
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).
Each thread is characterized by its major diameter, D (Dmaj in the diagram), and its pitch, P. ISO metric threads consist of a symmetric Vshaped 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:
In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.
The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as: