A gear hob is a gear cutting tool composed of different parts. The main parameter characterizing a gear hob is the module, which is the result of the division between the pitch gear diameter and the number of the teeth.
Another element of the hob is the pressureangle which coincides with the nominal pressure angle of the gear. The normal pressure angle αn – measured on a section perpendicular to the direction of the thread – and the axial pressure angle αt – measured on an axial section of the hob – are easily distinguished.
Besides, helix angle β, also called threadangle, is measured on the pitchdiameter of the hob depending on this value and the number of starts. The angle is direct proportional to the diameter, pitch, module and number of threads. So, when this increase also the other parameters increase consequently. It is important to highlight that the helix direction of the hob is chosen in agreement with the helix direction of the gear.
Gashes are another fundamental geometricalelement of a gear hob. These are serrations that cut the threads either in an axial or in a perpendicular direction. The number of gashes should be chosen during the design phase on the basis of many factors, including final quality and working conditions. They depend on the hobdiameter and on the number of starts. The current tendency is to design hobs with a larger number of gashes due to the fact that the hobbing time is reduced and the feed speed is higher. Moreover, the gash determines the cutting face by intersecting the thread. The cutting face is radial in the majority of cases. In any case, to realize a positive or negative rake angle, the hob must be reshaped with a grindingwheel positioned out of axis by a distance that depends on the rakeangle itself and on the hob diameter.
There are also other important gash’s elements which influence the cutting efficiency of the tool, just like:
- the tooth back angle;
- the bottom radius of the gash;
- the gash depth.