18.2 Over-Pin. Gaging
The equations relating tooth thickness and a measurement over cylindrical pins or rolls inserted between the teeth were given in Paragraph 4.6. This is a widely used method for gaging gears during fabrication (while they are still in the gear generating machine) and during final inspection. Accuracy of the over-pins measurement is on the order of .0001 inch.
A major disadvantage of over-pins gaging is the inability to correlate precisely with variable-center-distance measurements. This is because over-pins gaging is insensitive to pitch-line runout. On the other hand, rolling a gear necessarily involves the TCE and its runout component. The best correlation is obtained by equating the over-pins measurement to the average value of center distance found in the roll test.
Apart from the correlation problem, over-pins measurements by themselves are inadequate because the undetected runout can be out-of-control causing interference with its mate. It is necessary, therefore, to control and to inspect runout.

18.3 Other Inspection Equipment
In addition to the basic inspection methods and equipments described in Paragraphs 18.1 and 18.2, other special-purpose equipment is available. This includes involute-profile form checkers, tooth-spacing gages and runout checkers. Also, for high precision gears, equipment is available for inspecting the position error of individual gears and the transmission error of a gear train.

18.4 Inspection of Fine-Pitch Gear.
Because of their small dimensions, fine-pitch gears do not easily lend themselves to the kind of detailed tooth measurements suitable for large, coarse-pitch gears. Hence, fine-pitch gears are almost exclusively inspected by functional testing on a variable-center-distance fixture.
Over-pins measurements are also used, but generally are restricted to a reference measurement, This is primarily used in the fabrication process as a set-up dimension, and in inspection departments which are not equipped to roll test gears.

18.5 Significance of Inspection and its Implementation
The inspection operation is essential to obtaining a quality product. In effect, it is a policing operation that ensures conformance to dimensional tolerances and other drawing specifications.
The effort, care and cost of inspection are related to the quality level. Precision-gear inspection demands a much greater effort than that for low quality gears. Equipment must be of the best grade, calibrated periodically and restricted to use by qualified personnel. Control of temperature environment is essential for measurements on the order of .0001 inches. The cleanliness of equipment, gears and working area are also very important.