During the standard shaving processes, the workpiece and cutter rotate together as a pair of helical gears with crossed axes. The shaving cutter receives the motion from the motor and drives the workpiece which is coupled onto a shaft moving freely. The meshing is free and this is the main reason why indexing errors are not easy to improve.

The reasons why axes are crossed is to create an action of reciprocal sliding between the cutter teeth flank and the gear tooth. Let’s imagine the cutter prolonged along its axes in such a way to form a cylinder; if b axes were free to move, the wheel coupled on it would have a given transition movement around the cutter axes in the V direction linked to the cross angle γ.

shaving cutter
How to obtain the sliding on the teeth flanks in a crossed axes mating

Speed V is the resulting of two components and precisely Vl and Vs. Vl is compensated by the natural cutter rotation, while the Vs component would move the workpiece along the cutter axes. In effect, wheel b cannot move in that direction because it is clamped on its axes and the axes, on its turn, is fixed. It is quite clear that in order to compensate such missing movement a sliding action will occur between the two teeth flanks.

For a complete essay on the kinematics related to the mating of two gears with skew axes you can go and make reference to G. Henriot (Gears. A Practical and Theoretical Essay – Ed. Tecniche Nuove 1978 – volume 3 – chapter 5). Some tracks, normally called serrations, are usually manufactured along the cutter flanks. These serrations form a series of cutting edges. These are the edges that, because of the sliding effect between gear tooth flank and cutter tooth flank, generate the removal of metal giving it the shape on tiny chipping. We are going to deal with this at later stage about shape, dimensions and manufacturing method of these serrations.

The cross-of-axes angle is the difference between the cutter helix angle and the gear helix angle. The range of the values of such angle is normally between 10° and 15°. If the value is higher than 15°, from one side, gives a higher capacity and easiness of cutting, but from other side, the increased sliding speed, induces a consequential shorter life of the cutting edges, and could lead in to the possibility to lose control of the profile and the lead direction because of the reduced guide action.

Instead of the above, if the angle decreases, starting from 10°, a progressive upsetting will have effect with the tendency to reproduce the conditions of parallel axes. In case of gears that because of their shape won’t allow a regular cross-of-axes, you can go as down as 3° thus obtaining a shining surface, due to the intense upsetting action. When you mention shaving you normally mean the finishing operation of an external cylindrical gear, but is also possible to shave an internal gear, even if, in general, working conditions are less favorable.

shaving cutters samputensili
Two Samputensili made shaving cutters

First of all there are limitations linked to the internal diameter of the gear, due to the fact that it is not possible to go below a certain value with cutter diameter. It is possible to determine the minimum cutter outside diameter of the shaving cutter around 100 mm. There is also the problem that, usually, internal gears are not through and therefore it is not possible to freely choose any cross-of-axes angle. When it is possible to choose a cross-of-axes angle of at least 10° the operation can give good results; in case of small cross-of-axes angles results are poor.

Internal shaving can be performed onto internal gear wheels of big commercial vehicles and on gears employed on large size geared reducers (turbines and naval motors). Internal shaving requires quite special machines.

Advantages compare to the grinding process:

  • Easily performing short cycle time and high productivity
  • High versatility of integration with any automatization
  • Reasonable set-up time
  • Easiness of involute profile and lead twisting compensation
  • Possibility to finish gears close to shoulders
  • Low tool cost per piece
  • Low investment for the purchase of the shaving machine

Disadvantages against grinding process:

  • Necessity to shave before heat treatment
  • Distortions, and therefore errors, due to the following heat treatment
  • The shaving results is highly conditioned from the pre-machined condition of the work piece
  • Occasional impossibility to use the shaving tool onto a gear with a small number of teeth

This process is still the mostly used for gears assembled into automotive gearboxes and automobile industry in general.