Page 84 - Gear Technology Solutions

6 The Undercut Phenomenon

6.1 What is Undercut?

The first questions are, where does the undercut come from, and which
physical or kinematical effects lead to undercut? Then the following questions
are, can it be avoided and how, or can it be avoided only in certain cases?

A comparison between a tooth profile without undercut and a similar profile
with undercut is shown in Figure 1. The profile at the left side of Figure 1
shows a healthy profile with a root fillet radius which blends perfectly tangential
with the involute profile. In contrast, the right-side graphic in Figure 1 shows a
severe undercut resulting in a ridge on both sides of the tooth which weakens
the root and reduces the amount of profile depth, where the mating tooth can
mesh.

Figure 1: No undercut and undercut comparison

6.2 Rule of Involute Generation

The term “undercut” comes from “undercutting” which is a hollowing out of an
area because the cutting take place below a certain depth, where no involute
exists. In Figure 2 the author of this book demonstrates the classroom

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