Why do spur gears with small numbers of teeth have a corrected PCD ?
In cases where the number of teeth on a small pinion is low (< 12 teeth), the bottom of the hollow between the teeth is very narrow. Consequentially, it is difficult, or even impossible, to engage these gears: the heads of the teeth of the larger gear can not get into the gaps between the teeth of the smaller pinion. This is known as interference. In the same way, during cutting, the cutting tool must work right to the bottom of tooth hollows, also causing cutting interference. It should be noted that this cutting interference is higher than the operational interference and considerably weakens the tooth. It is in order to avoid this interference that the correction of teeth was introduced into the manufacturing of gears. As a bonus, this process also brings about improvements in the teeth's resistance to wear.
