Gear semi precision hobs are mainly used in the hobbing process before shaving or grinding. Hobs of any accuracy grade can be selected, and special requirements can also be made for hobs, such as hobbing tooth top, hobbing part of tooth top or no hobbing, etc.
Gear semi precision hob is mainly different from rough hob. Gear semi precision hobs are usually not finely ground, and the machining process is at least shorter than that of precision hobs. At the same time, the accuracy of gear semi precision hob before precision grinding is basically the same as that before precision hob machining. The precision of tooling and the minimum clearance of gear semi precision hob shall also be ensured during grinding. Therefore, most of these hobs belong to class A or class B accuracy. However, many gear manufacturers will also use C-level precision tools, hoping to reduce the tool cost of each part.
In general, Preshaving hobs are recommended to ensure tooth profile and size requirements. When designing the gear hob, the tooth thickness will leave an allowance on the basis of the finished parts. The full tooth depth of the part must ensure that sufficient clearance is provided for the razor. Convex angle is another important feature in many Preshaving hobs. The generation of convex corners will dig the root at the bottom of the part teeth, provide the clearance during the processing of the gear shaving cutter, and prevent the stress concentration at the tooth root caused by the sudden change of the tooth shape. In the parts with few teeth, the hob without convex corner will often cause the root cutting of the part and reduce the bearing capacity of the gear. In gears with a large number of teeth, it is generally required to increase the size of convex angle to ensure the clearance between razor and gear root. Generally, the shaving allowance shall be at least 0.025mm in the direction of tooth thickness. In addition, the minimum meshing point of gear hob shall be lower than that of matched gear.
For a given modulus, the size of the convex corner is generally fixed. Therefore, in this case, parts with small number of teeth may have large undercut. Some gears will require a certain amount of undercutting. At this time, the gear hob should be designed according to the number of teeth of a specific gear. In a given convex corner, the point of undercutting is related to the involute tooth profile and the number of part teeth. Ensure that this intersection does not exceed the involute form, and the true involute forming diameter should be determined in the gear design. Therefore, a special protruding angle is usually designed in this case.
Because a large number of gear grinding processes are used in factories, the pre grinding hob is also widely used. At the beginning of the part design, the tooth profile of the part and the grinding process must be determined. Accordingly, the tooth profile of pre shaving hob should also be confirmed. Since the grinding allowance is larger than that of shaving, sufficient tooth thickness must be reserved. At this time, the convex angle is much larger than that of the preshaving hob.
If chamfering is required for the tooth part of the part, another modification must be made to the tooth profile of the gear semi precision hob. Chamfering should be completed during gear hobbing, not by razor or grinding wheel. This design requires that the chamfering function must be added in the design of gear hob. The chamfer will change with the number of teeth and helix angle. One of the main reasons for increasing chamfering is to remove the burrs on the tooth top in batch. Because many semi-finished gears are produced on a large scale, the meshing part of the tooth groove must be guaranteed. In addition, the size of chamfer will affect the effect of burr.