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Posted by Bill Simpson on February 03, 1999 at 07:33:56:

In Reply to: SAP & EAP posted by David Robinson on February 01, 1999 at 13:48:06:

If active profile means involute portion of the tooth profile, then the first consideration is whether undercut occurs or not. Undercut is the formation of a surface between root and involute profile generated by cutter tip edge (aka radius or corner). the undercut surface is not part of the active profile. The upper end of involute profile is the gear's OD or start of tooth edge chamfer, if any.

Tooth bearing or contact profile is determined by both members in mesh. Length of contact profile would be no greater than the smaller involute profile, which is, for high ratios, the pinion's profile.

Small numbers of pinion teeth and sharp cutter tip edges contribute to undercut. Reducing a pinion's undercut usually requires larger pressure angle or increasing tooth count or holding the cutter "out" to produce a long addendum.

The geometry (and equations) of gear tooth profiles differs between hob and shaper cut teeth.

My source of information about the geometry of gear tooth profiles is Scott Atkinson at Ash Gear & Supply (248)357-5980. His program GCP-2 computes TIF diameter (diameter at intersection of involute and trochoid curves) and plots a graphical image of tooth profile. I believe he could help you further.

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