The diameter of the thread is calculated metrically

When naming the diameter of a thread, basically two values come into question. The diameter that is measured at the thread tips and the value that indicates the distance between the indentations. The international standard for metric ISO threads defines the diameter at the thread crests as a nominal value.

Metric standard threads

For so-called metric coarse threads, the nominal value is given as a number marked with an M. With the exception of three nominal values with a small diameter, 1.2, 1.6 and 2.5 millimeters, all other sizes consist of natural numbers. The relationship to the thread diameter in the notch or thread determines the thread pitch.

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Like the determination of the nominal value and the thread pitch, the flank angle of the metric standard thread is firmly defined. The angle of two adjacent thread flanks is sixty degrees. The nominal value minus the assigned pitch difference results in the theoretical diameter of the screwed object, nut or screw.

Non-metric thread types

In addition to the standard threads, there are other thread types with different helix angles. The thread flanks of pipe threads, mostly with sealing function, are 55 degrees. Metric trapezoidal threads have flank angles of thirty degrees. Buttress threads have flank angles of thirty to fifty degrees, depending on the design.

The determination of the nominal value of the diameter at the thread tips is the same for all thread types. However, the calculation of the diameter for screwing devices such as nuts and bolts must be carried out using other mathematical formulas.

Movement difference

There must be a difference between two and three tenths between the determined nominal value of the diameter of a thread and the screwing device. This "play" allows the screw in the thread or the nut on the thread to move and turn.

Tips & Tricks Extensive lists and configurators are available on the websites of many manufacturers. They enable the determination of secondary diameters assigned to the nominal values. After measuring the nominal value, you can, for example, assign the diameter of suitable screws and nuts.