The difference between imperial fasteners and metric fasteners

The thread specification for inch thread fasteners always shows the number of thread teeth directly after the thread diameter, such as 1/4-28, where 28 refers to 28 teeth per inch, that is, the tooth spacing is 1/28 inch. In the metric thread fasteners, there is no such a parameter in the thread specification, the pitch is specified as the distance between the top of the adjacent two threads, such as M1*1.25, where the distance defined is 1.25mm, and there is no number of teeth per unit distance. So there can be some confusion here, because the imperial fastener does not describe the actual tooth tip distance, but is defined as the number of threads per inch. If it is necessary to convert between the two, for example, to convert the metric representation to the imperial representation, first change the pitch in the metric size from millimeters to inches, and then find an inverse.

For the length representation of external threaded fasteners, the imperial and metric measurement methods are the same, for flat head fasteners, the length includes the head height, for convex head fasteners, the length is calculated from below the head. The only difference between them is that the units are different, the metric system uses millimeters, while the imperial system uses inches. For metric fasteners, the larger the diameter number, the larger the "bulk" of the fastener, the thicker the thread, that is, the less thread in the unit size; The larger the length value, the longer the fastener.