Metric screw threads are measured in millimetres, and are determined by measuring the distance between two adjacent screw threads. The problem with this measurement system is that such minute distances can be difficult to measure accurately, particularly with smaller fasteners. For a more reliable measurement, it is advised to take a measurement along a larger sample of threads, such as 10, then divide this number by the number of threads chosen, which will calculate the average thread spacing across this range.
Metric machine screw threads are defined by ISO standards. For more information on ISO screw threads, see our lookup table for ISO Screw Thread Dimensions.
Measuring Imperial Thread Pitch
Imperial screw threads are measured in threads per inch, or TPI. As the name suggests, TPI is measured by counting the number of thread peaks along a 1 inch length. The TPI measurement is then often affixed to the diameter of the thread to form a common imperial thread size, e.g. ‘1/4-20’, which would denote a ¼ inch diameter, and 20 threads per inch. This measurement system can cause problems when measuring fasteners which are shorter than 1 inch in length. When measuring the TPI of shorter fasteners, a shorter length, such as ½ inch, is used – the number of threads in this section is then multiplied by the appropriate factor to find the hypothetical number of threads if the fastener was 1 inch in length. Imperial thread pitches are generally divided into one of two categories; UNC (Unified National Coarse) or UNF (Unified National Fine). The UNF version of a fastener will have finer, tighter threads than its UNC counterpart, and is slightly less common. Most imperial fastener diameters are available with the choice of either a UNC or UNF thread. UNEF (Unified National Extra Fine) is a rare variant which can also be specially manufactured on request.
Thread size and pitch can also be identified using a basic trial-and-error method of simply attempting to use a component of a known size with the unidentified product – if the two fit together with some (but not too much) resistance, they are likely the same size. This method is widely used, but is not recommended, as it relies on having properly-sized fasteners at hand, and can cause damage if performed repeatedly. Additional problems such as cross-contamination of metric and imperial threads can further complicate this method. Specialist tooling is available for accurately and safely determining the correct thread size for a chosen application. A variety of thread gauges are available for many metric and imperial thread sizes, and can be used to identify both male and female thread types. Ring gauges can identify male threads such as screw or bolts, and plug gauges can identify female threads, such as those found in nuts or pre-tapped holes. Thread gauges are available in ‘go’ and ‘no-go’ varieties – if the thread being measured fits the ‘go’ gauge, and does not fit the ‘no-go’ gauge, then the thread has been correctly identified.
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