The size of automobile gear hob mainly includes diameter and length. The diameter of automobile gear hob usually depends on module, hole diameter (shank diameter), equipment conditions and part structure. Usually, tool manufacturers will recommend the use of standard automobile gear hob diameter to reduce the preparation cost of raw materials. However, not all auto gear hobs can be directly selected from the standard. Due to the limitation of processing equipment, we have to reduce or increase the diameter of automobile gear hob, which can improve the processing efficiency and tool life.
In addition, improperly increasing the diameter of automobile gear hob will also affect the machining efficiency, such as increasing the cutting time. As shown in Figure 3-1, the axis of the cut gear and the cross-sectional view made at point a. When cutting gears, the automobile gear hob should be inclined at an angle ψ， Its value is equal to the thread rising angle of automobile gear hob λ 0, therefore, the cross section of automobile gear hob is ellipse, its short half axis is Ra0 and its long half axis is. It is calculated by the ellipse equation of point A:
Simplify this formula to:
It can be seen from the formula that the larger Ra0 is, the greater the cutting distance y is and the longer the cutting time is. Automobile gear hobs sometimes have a taper of about 35 ° in the length direction. The reduced length provides a greater full tooth depth. Other non cylindrical hobs include spline hobs, worm hobs and some fixed hobs.
The recommended length of automobile gear hob is based on the diameter pitch and the maximum size that can be clamped on the machine tool. Usually this is the most economical size. For some special process schemes, the automobile gear hob is required to exceed the standard length, or the tool adopts large spiral angle. There are also many automobile gear hobs. It is not necessary to choose standard length hobs, such as flower key hobs, worm gear hobs and fixed installation hobs, as long as they can be used. The economy of automobile gear hob should also be considered when selecting the length of automobile gear hob. The longer the length of automobile gear hob is, the better its economy is. However, when the number of processed parts is small, it is not suitable to order too long auto gear hobs.