From AGMA 917-B97, Design Manual for Parallel Shaft Fine-Pitch Gearing
"Tooth thinning coefficient is a normalized measure of the amount by which the tooth thickness of a pinion or gear is reduced from its nominal value at the standard pitch diameter. The tooth thicknesses are reduced by feeding the cutter deeper into the gear blank. The tooth thinning coefficients are considered independent of the profile shift in order to have the outside diameter independent of tooth thinning for backlash."
.
.
.
"The maintaining of a desired minimum backlash at the tightest point of mesh is the first objective of the design procedure. The minimum backlash is desirable for one or more of the following reasons:
-- To provide room for the fluid lubricant film that separates the loaded surfaces of the gear teeth;
-- To provide a flow channel for the fluid lubricant trapped in the root areas, especially in high-speed, wide-face spur gears, thereby relieving the pressure that adds load to the shaft and bearings and reduces efficiency;
-- To permit the passage of small dirt particles carried by the lubricant;
-- To provide additional clearance between the gears as an allowance for possible gear and mounting dimensional variations and thermal effects not considered elsewhere in the design procedure.
An interference condition, or negative calculated backlash, should be avoided. Even when it does not prevent assembly of the gears, the interference will cause deflections of the gear teeth and of the shafts, bearings and other supporting members. Such deflections often result in unpredictability high forces, greater than the forces from the design load, and lead to reduced efficiency and premature failure. The value for minimum backlash is used as input data in the design calculations. It should be selected after review of the above considerations and their importance to the application."