Thread Gage

a device for measuring and checking screw threads. A distinction is made between gages used for comprehensive checking and those used to measure particular parameters, external and internal threads, straight and taper threads, lead screws, and so on. Gages measuring external threads are the most varied. Internal threads are usually measured by test molds.

Among the comprehensive checking devices used for inspection of finished parts are go and no-go gages, which determine whether the dimensions of the screw surfaces being connected (nut and bolt) are within tolerances for the screwing length. The go gage, which must be screwed in during the test, checks what is called the adduced mean diameter, an artifically created control parameter that ensures interlinking of the thread coupling. Indicator gages with thread-measuring elements are also used in comprehensive checking.

Thread gages that measure the principal parameters of an external thread (mean diameter, profile, pitch) are used to determine the precision of the manufacturing process or to evaluate the operating characteristics of special precision threaded parts, including lead screws, micrometer screws, and threaded gages. Micrometer calipers with inserted pieces that have a thread profile are used to measure the mean diameter. One way of determining the mean diameter of a precision thread is to measure the diameter with wires or rolls set between the turns of the thread and some measuring device, such as an optical or micrometer caliper. The measurement is based on the height to which the wires protrude above the external diameter of the thread. Special attachments with one, two, or three wires are also used. The mean diameter of the internal thread is measured with a specially designed inside caliper or with instruments that have replaceable spherical tips.

In lead screws, worm gears, and other parts with relatively coarse pitches, thread profile is measured by an instrument whose measuring unit is turned by the angle of profile of the thread as the tip moves along the thread’s lateral surface. Specially designed goniometers are sometimes used for this purpose. The pitch of a thread is usually determined in axial cross section on tool and universal microscopes and projectors. Instruments that measure the pitch of the helical line continuously as the part turns are used to check precision threaded parts, such as lead screws. The measurement is done by comparing the actual helical line with a theoretical line reproduced in the instrument by means of a master-pattern screw or by means of pulsed linear and angular sensors that produce pulses with a frequency proportional to the linear displacements of the screw surface beyond a certain angle of rotation. When pulsed sensors are used, the data are processed by computers, which are part of the instrument.