Threading (manufacturing) - Inspection

Inspection

Inspection of the finished screw threads can be achieved in various ways, with the expense of the method tailored to the requirements of the product application. Shop-floor inspection of a thread is often as simple as running a nut onto it (for male threads) or a bolt into it (for female threads). This is plenty good enough for many applications (e.g., MRO or hobbyist work), although it is not good enough for most commercial manufacturing. Higher-precision methods are discussed below.

Commercial-grade inspection of screw threads can involve most of the same inspection methods and tools used to inspect other manufactured products, such as micrometers; vernier or dial calipers; surface plates and height gauges; gauge blocks; optical comparators; white light scanners; and coordinate-measuring machines (CMMs). Even industrial radiography (including industrial CT scanning) can be used, for example, to inspect internal thread geometry in the way that an optical comparator can inspect external thread geometry.

Conical micrometer anvils, specifically suited to resting on the sides of the thread, are made for various thread angles, with 60° being the most common. Mics with such anvils are usually called "thread mics" or "pitch mics" (because they directly measure the pitch diameter). Users who lack thread mics rely instead on the "3-wire method", which involves placing 3 short pieces of wire (or gauge pins) of known diameter into the valleys of the thread and then measuring from wire to wire with standard (flat) anvils. A conversion factor (produced by a straightforward trigonometric calculation) is then multiplied with the measured value to infer a measurement of the thread's pitch diameter. Tables of these conversion factors were established many decades ago for all standard thread sizes, so today a user need only take the measurement and then perform the table lookup (as opposed to recalculating each time). The 3-wire method is also used when high precision is needed to inspect a specific diameter, commonly the pitch diameter, or on specialty threads such as multi-start or when the thread angle is not 60°. Ball-shaped micrometer anvils can be used in similar fashion (same trigonometric relationship, less cumbersome to use). Digital calipers and micrometers can send each measurement (data point) as it occurs through an interface (commonly RS-232) to storage and as input to software, in which case the table lookup is done in an automated way, and quality assurance and quality control can be achieved using statistical process control.