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Bevel Gear Question

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  • Bevel Gear Question

    This is not a question on how I am going to produce a Bevel Gear , but rather a question of tooth form.

    I am aware of involute gear cutters and that they are of different shape depending upon the tooth count.

    My assumption is the cutters are proportional as the Diametral Pitch changes. That is to say, for example, a # 4 cutter will have the same geometric shape whether it is a 24 DP or a 5 DP cutter.

    My second assumption is based on the first one. The tooth shape of a Bevel Gear remains the same but its size diminishes as it proceeds toward the point that defines the apex of the cone.

    The last assumption would be that the tooth shape, actually the space between the teeth, would be at 90 degrees to a line from the Pitch Circle to the apex of the cone.

    Are these assumptions correct?

    Am I missing some information that would help with answering my question?

    Thanks in advance

    Have a Safe and productive New Year


  • #2
    For the home shop bevel or miter gears can be cut with involute cutters. While not having a perfect tooth geometry when made properly they can function acceptably. I would assume that the tooth geometry for a set of involute cutters would be similar but larger as the pitch gets larger but I can't say 100%. As bevel or miter gears have their cones intersecting at each axis centerline they have a conical shape. The proper tooth form would have the involute curve change shape as the tooth got smaller toward the center of the cone but for home work it's a compromise using the curve at the small end of the tooth to cut the entire length. This leaves the thick part of the tooth at the pitch line with a narrower contact point. With a first pass through the gear blank the root of the space is a parallel as is the sides of the teeth. When the second space is cut the tooth that has been formed now has a conical side wall. This side wall of the tooth needs to be trimmed to give the tooth a parallel shape so it will fit in the corresponding gear space. There are formulas to do this trimming. It involves offsetting the cutter from center while rotating the blank a specific amount, usually 1/4 of the tooth pitch.