Crowning is a modification that results in te flanks of the the tooth to be relieved slightly on each end. The purpose is to ensure that the center of the flank carries the full load even if the gear and pinion is slightly misaligned.
The amount of crowing depends on the load carried,precision, alignment and pitch/module of the pinion. Generally the amount of relief will be in the range of .0004-.0015 inch relief.
Crown is generally put in when the tooth flank is ground. The manufacture of the tooth generating machine will have a setting to include crowning.
__________________
"You are never competent in any subject until you know what you don’t know." David Eugene Parker