Most gears have gear teeth with involute faces. An involute can be defined as the curve traced by the end of a taut string when it is unwound from a circle. Such gear teeth provide smooth operation and allow greater tolerance in the distance between the gear centers.
A characteristic of involute gears is that one involute gear can be used to form other gears of an interchangeable series of involute gears having a common pressure angle and thus common pitch. By illustration, an involute gear could be rolled against a plastic gear blank to mold the desired shape of gear teeth in the blank. A gear of a particular pitch can be used to form any other gear of the same pitch by proper selection of blank diameter and speed of rotation of the master gear and gear blank. Pitch can be defined as the number of teeth per inch of pitch diameter.
A more typical form of gear cutter which is based on the method described above utilizes an involute gear having cutting edges which are pulled axially across the peripheral face of a gear blank. As the cutter and blank are rotated the gear is formed by successive cuts by the cutter gear. That approach allows for very precise cutting of gears but suffers the drawback of requiring expensive equipment.
Another form of gear cutter utilizes a rotating cutter blade shaped as a tooth of a rack gear, a linear gear with flat faces. When a circular gear blank is rolled along the rotating rack gear cutter, the gear cutter forms involute faces of adjacent teeth. The blank is then incremented and another pass of the cutter blade is made to cut the next tooth space. A difficulty is in precisely incrementing the cutter blade and the work piece and controlling their relative speeds. To overcome that problem another system includes a series of cutter blades and the gear blank is rolled over those blades. A difficulty with such a system is in providing a large number of precisely positioned gear cutters for cutting uniform involute teeth about the gear blank.
Another form of gear cutter utilizes a rack gear cutter in the form of a worm gear and the blank is rolled past the worm gear. Again, there is difficulty in precisely manufacturing the worm gear cutter.