How to understand screw threads?

16th August 2021

How to understand screw threads?



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			All threaded items have a measurement, threaded inserts included. When working, it is important to check that all pieces and tools involved have corresponding thread measurements.

			There are several systems by which a standardisation has been tried, and, whilst some are still around, there are now two major systems which are becoming standard. One for metric measurements and one for imperial: the ISO metric thread and Unified Thread Standard (UTS).

			The pitch, diameter and length are usually included in a thread measurement, whether it be the metric scale or the UTS. Both systems are similarly formatted, the only difference being the unit of measurement; one being in millimetres (mm) the other inches. The major and minor diameter swap for internal and external threads.

			Pitch is the length of one whole thread from any starting point (root, crest or flank) and this is the measurement used in the metric scale. For the imperial scale, threads per inch is used. This is a reversed version of pitch (equivalent to length per thread). Diameter is the cross section of the threaded item and length is the measurement of the shank.

			The way in which the different scale measurements are written can be shown by one formula (at the top of the image). n1 is the diameter, n2 is the pitch or threads per inch and n3 is the length. Not all the measurements are always included as there are some standard sizes available. The first example is metric and the other two are imperial.

			All threaded items for a single fastening should have corresponding measurements for both diameter and pitch. In the case of threaded inserts, this also means that the tools used to install them should have the same measurements, like the tap, drill bit and driver tool. This is to ensure the threads interlock for the best strength and do not cross-thread. Cross-threading is where the threads intercept at the wrong point and damage each other.



			Threaded items are often referred to as coarse or fine – this is referring to the pitch or threads per inch. Coarse thread has less threads per inch or a larger pitch. Fine threads have a smaller pitch and more threads per inch. The type you have depends on the application, coarse and fine do not refer to the quality of the thread, they are just another measurement, (size of threads relative to diameter).