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Re: Internal vs. External Gear teeth - same profile?
I have to disagree with Jack on this one. The proper tooth form for an internal involute gear is indeed the inverse of the external involute. The involute is formed on the common tangent between the base circles of the exterior and interior gear. This is identical to the involute formation for two external gears, except that the tangent does not cross the line of centers between the base circles. This configuation causes the relationship to invert, but in all other respects the formation action parallels the formation of the standard external involute.
So your idea of "if I took a spur gear and (analogously) put in in a bucket of plaster, and then the plaster hardened and I pulled the spur gear out, would I have an internal gear of correct dimension/shape?" is theoretically correct. In actual fabrication, the addendum of the internal tooth is slightly shortened to prevent interference and reduce the need for undercutting on relatively large pinions (thus, the note above regarding having internal teeth - external teeth > 15).
-dave
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Last edited by dlavery : 28-02-2005 at 11:03.
Reason: aye learnt to spel
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