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Figure 6: Kinematic link of two meshing teeth with the generating rack
The graphic in Figure 6 shows a two-dimensional top-view onto the green
generating rack with the blue pinion tooth being generated with undercut, and
the red gear tooth (without undercut). The graphic shows that the edge corner
of the red tooth comes very close to the generating rack corner which creates
the undercut on the blue tooth. If two mating gears are generated according to
the graphic in Figure 6, then there will be a small amount of clearance between
the undercut root and the top edge corner of the blue mate [2].
6.6 Possibilities to Eliminate Undercut
Gear engineers like to find ways to avoid undercut conditions. If the attempt is
made to eliminate the undercut for example with a larger edge radius of the
hob cutter blades, then an interference is created as shown in the top section
of Figure 7. If the top edges of the mating tooth receive an adequate topland
chamfer (bottom section), then the interference is eliminated. This is a
procedure which is applied in cases where the undercut is not acceptable, for
example because of the reduced root bending strength.
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