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Originally Posted by dlavery
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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And I have to disagree with Dave. If made that way, the gears would mesh perfectly, but only in one position - locked in that position unable to rotate. When you were done filing not only the addendum but the whole depth and possibably pitch angle (involute) to get them to mesh, then you'd end up with the proper profile for the internal, which is the mirror image - not the negative - of the external gear.