Page 364 - Gear Technology Solutions

Figure 11: Side effect length crowning (left) and flank twist (right)

An effective possibility to reduce the twist in the measurement result is to ad-
just the 3-face blade geometry closer to the standard 2-face blade geometry.
This requires maintaining the original effective cutting edge hook angle, how-
ever the length crowning error still remains in the flank surfaces without any
available correction freedoms.

26.4 Influence of Blade Stepping

If the outside and inside blades in a face hobbing cutter are built to equal
height above the cutter head, then a degradation of the tooth root geometry
will occur if a gearset member has length crowning which is created by a cutter
head tilt k as shown in Figure 12. Figure 12 also aids in the understanding that
if the lower outside blade was built to the same blade height above the face of
the cutter head as the upper inside blade, then the root of the work would be
severely stepped between the two flanks. In order to eliminate stepping in the
work gear root fillet, a stepping between the outside and the inside blade
height above the cutter head face is required.

The initial blade stepping calculation uses the length crowning tilt component
and applies the objective to keep the blade tips within the vertical generating
plane in Figure 12. After the stepping amount in direction of the cutter head
axis is determined, the cutting-edge tangent vector is used to calculate the
precise extended or shortened blade tip locations by maintaining the correct
radius and offset at the blade reference point. The effective blades with their
original calculation point locations and their stepped blade tips, as shown in
Figure 12 are used in the generating software to calculate the correct effective
tooth slot and root fillet geometry.

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