It has been mentioned that the involute curves begin at the base circle and extend outwards to form the gear tooth profiles. Obviously, there is no involute inside the base circle. We know that the line of action of the two inter – meshing gears is tangent to the two base circles. The two points of tangency represent the two extreme limiting points of the two base circles. The two points of tangency represent the extreme limiting points of the length of action. These two points are called the “interference points”.
It has been emphasised that to have and maintain conjugate action, the mating teeth profiles of the gear pair must consist of involute curves(when involute curves are used as teeth profiles). Any meshing outside of the involute portion will result in non-conjugate action. That portion of the tooth profile which lies between the base circle and the root circle comprises non-involute curve.
The tooth is already at the weakest region in the vicinity of the root. By undercutting,the tooth becomes further weakened. Hence, although interference can be avoided by the generation process because the corresponding recess is made at the tooth root by the cutter with the consequent absence of fouling, this is not considered an acceptable solution because of its tooth weakening effect. The problem of interference is simplysubstituted by another problem caused by undercutting.
The above fig. shows detailing of a interference zone.
While generating gear teeth, if there is interference of the cutter, then a recess is cut at the root of the tooth. The profile thus generated deviates from the theoretical tooth profile. This happens when the cutter extends beyond the base circle of pinions having small number of teeth. This removal of material at the root of the gear tooth is called “undercutting”.