The essence ofhob is a worm meshed with the worm gear to be machined, but its outer diameter is larger than that of the working worm, so that there is a certain radial clearance when the worm gear to be machined is meshed with the working worm.
The hob commonly used is of the incomplete cutting type. Only the top edge and the side edge of the non full cutting hob participate in the cutting, while the bottom of the tooth groove does not participate in the work. In addition to the top and side of the cutter teeth, the bottom of the tooth slot of the full cutting hob also takes part in cutting. In the processing of small module gear, the full cutting hob is often used. Although the cutting amount of the hob increases, the cutting force will not increase much and vibration will not be produced because the gear teeth are small. This method can make the top gear and the ring gear develop at the same time, keep the top circle and the base circle concentric, so that the gear has a more uniform radial clearance when meshing. The method of machining gear with full cutting hob can reduce the accuracy of the outer circle of the gear blank, and there is no burr on the outer diameter of the gear. At the same time, the tooth thickness can be ensured by measuring the diameter of the tooth top circle.