Thread pitch is one of the key classifying units used to specify threaded fasteners in the metric system. Thread pitch refers to the distance between threads and is expressed in millimeters. This measurement is taken along the length of the fastener. For example, if the distance between two threads is 1.6mm, this means the thread pitch is 1.6. A steel rule, caliper, or comparator are the most common measuring instruments used to determine thread pitch.
The thread pitch for fasteners is the axial distance from one thread groove to the next. Another way to measure thread pitch is to place a steel rule down into the axis of a screw and then count the number of thread crests in a given length from that point. The pitch can be calculated by dividing this count by the length.
One of the common mistakes engineers make is to start their count of threads at “one” instead of “zero” when using this counting method. This results in a reading that is off by one pitch which leads to a faulty measurement of the fastener which may not fit with its mating component. To avoid this mistake, start with a count of “zero” for the first thread. It is advisable to check the pitch determination made by counting against the actual pitch measurement.
Thread pitch standards
For different screw thread sizes, there are standard values for the pitch. The thread pitch standards vary from pitch diameter standards, which are meant to measure the distance from the center of the screw to its pitch line. As was already defined, a screw’s thread pitch is the distance from one of the thread’s ridges to the next ridge. Thread pitch standards are measured in threads per inch, while pitch diameter standards use inches.
A screw’s pitch is closely related to the screw’s lead. Lead is the distance along a screw’s rotational axis that the screw traverses during one full turn of its threads. For most screws, lead and pitch are equal, as most screws wrap only one ridge around the screw’s shaft. Some screws, however, wrap multiple different ridges around the shaft. For these screws, the lead and pitch are different values.
International standards organizations assign thread pitch standards to different screws based on the nominal diameter size. Nominal size is the common size engineers and builders use when referring to different types of screws. Nominal size is only the common value by which people refer to screws, and this value often differs from the screw’s actual size.
A single nominal screw size can have several different pitch standard values based on how many threads per inch each individual standard screw has. Likewise, screws with the same nominal size and pitch value can have different pitch diameter sizes based on how shallow or deep their threads are.
If you want to know more about threaded fasteners, consider taking our online course in Fastener Fundamentals.