Over the years, gear detection technology has been continuously improved to accurately measure most of the errors in gear cutting processes. Scanning measurement of involute tooth profile usually only detects a few teeth in a circle. Plastic gears are processed on gear machines such as hobbing and slotting, and the tooth profile of each tooth is basically the same. The injection gear may have a large individual error at a certain position on any tooth surface of the gear. What’s more, the injection moulding process may also introduce many types of errors that are different from traditional machining.
Since any injection gear has to shrink, the involute tooth profile is a target tooth profile, not a given value. Whether using diametral pitch, modulus, base section, pressure angle or any other involute parameters to control the gear geometry, these parameters are variables for the actual machined parts. For these changing parameters, it is necessary to set practicable tolerances.
The only way to determine that the injection gear is qualified (in the tolerance zone) is to scan and measure the involute tooth profile to determine the actual physical geometry of the gear. However, there may be the following situations: the size of the injection gear has completely exceeded the tolerance requirements, but the rolling comprehensive test results are still qualified. For example, the tooth profile detection result of a gear, the involute base circle has deviated far from the specified value. The measured gear has 64 teeth, and the measuring gear used also has 64 teeth. In the rolling detection, the gear meshes with more teeth at the same time, and there is almost no comprehensive deviation of one tooth in the measurement result. Although this kind of gear looks larger, the base circle is small. As the tooth thickness of the gear is reduced, good technical indicators can be achieved in rolling detection. Once these injection-moulded gear parts are provided to users, they will fail immediately when they mesh with metal gears of the correct size.