In the process of tooth profile generation, the tooth profile of automobile gear hob works in conjugate form. The formation of conjugate hob tooth profile is that the hob and the workpiece rotate at a certain angular speed. Therefore, the automobile gear hob and the workpiece rotate at a certain angular speed on the gear hobbing machine, and the automobile gear hob must have the correct conjugate form to generate the required tooth profile.
Because the tooth profile of the generated automobile gear hob is not completely copied on the workpiece, a clear relationship must be established between the automobile gear hob and the workpiece to ensure the tooth profile of the workpiece before part processing. The tooth profile and pitch circle diameter of the workpiece ensure the design requirements of automobile gear hob teeth.
Generating pitch circle diameter can most effectively establish the tooth profile of automobile gear hob, that is, the tooth profile required for workpiece production. This is the diameter generated on the tangent and indexing circle of the automobile gear hob and the base circle used as the tooth of the automobile gear hob. In the case of forming an involute, the pitch diameter generated by the involute is usually the same as the standard pitch diameter. Therefore, the automobile gear hob can be used to cut all the teeth of the workpiece.
The diameter of the generated diameter pitch is established between two circles, or in the normal pitch circle of two adjacent teeth of the part. On the normal section of adjacent teeth of automobile gear hob, the distance is equal to the normal pitch circle generated by the pitch circle diameter on the part. The same distance can be based on the normal diameter of the pitch diameter. For example, spur gear is the number of teeth per millimeter on the pitch diameter.
The normal circumferential pitch includes tooth thickness and tooth groove. On the generated pitch circle diameter, the normal tooth width of the part is equal to the normal tooth thickness on the pitch line of the automobile gear hob. On other reference lines, the hob tooth thickness is a function of the hob tooth shape.
Pressure angle is the most common standard pitch diameter with involute tooth profile and design requirements. The pressure angle of the generating line of the pitch circle of the automobile gear hob is the same as that of the gear pitch circle. The self generated pitch diameter is usually the same as the standard pitch diameter, and the pressure angle of automobile gear hob teeth is usually the same as the standard pressure angle of gear.
The full tooth depth of the whole automobile gear hob is related to the full tooth depth of the part and the tooth top modification of the hob. When the full tooth depth of the whole automobile gear hob tooth is equal to the full tooth depth of the part, the hob tooth top is modified. When the full tooth depth of the automobile gear hob is greater than that of the part, the top of the automobile gear hob is not modified or semi modified. Both types of hobs need to pre process the tooth surface.
The upper part of the pitch circle generating line of automobile gear hob is equal to the tooth root part of the part. When using Archimedes hob profile involute, the tooth surface can be set at different cutting depths, but the standard pressure angle and standard pitch diameter can still be obtained. All other automotive gear hob teeth must be ground to a fixed depth to produce the correct tooth profile.
Many special requirements can be incorporated into the specific design and processing requirements of basic automobile gear hob tooth profile. Some of the most common special requirements are convex corners, antennae, chamfering of the tooth top and complete arc of the tooth top belt. Convex corner is a kind of root digging treatment for part teeth by increasing tooth thickness at the tooth top of automobile gear hob. It is mainly used for semi finishing gear hob, leaving a certain gap at the top of shaving cutter.
The antenna is the tooth profile depth of automobile gear hob, which is required to exceed the basic full tooth depth [24]. Usually, their function is to eliminate discontinuous cutting edge errors. When chamfering is required at the top of a part’s tooth, the chamfering design must be carried out for the hob tooth of automobile gear. Some gear designs specify a full arc tooth root, so it is necessary to have an arc design at the tooth top of the automobile gear hob. Other special functions can also be specified in the design of automobile gear hobs.
In order to eliminate the eccentricity of automobile gear hob teeth, enough clearance must be left on the cutting edge. With the continuous wear of automobile gear hob in the machining process, the tooth profile of automobile gear hob must be consistent from beginning to end. There are several factors that affect the calculation of wear in the manufacture of automobile gear hobs. The type of processed part material affects the grinding and cutting amount of the tool, which should also be made clear to the tool design engineer.
Compared with gear hobs, non gear hobs have at least one tooth with the same tooth profile when working. Because all non gear hobs are unit set hobs, they are usually used when the part shape cannot be generated by involute. They are applied to special shapes, such as ratchets or other special automobile gear hobs.