[Joe Johnson]

From my experience, the main issue with the idea of making an internal gear by boolean subtraction of an external gear from an internal gear is the clearance issues.

There are 3 clearance issues to think about.
The tip of the pinion clearing the valley of the internal gear
The tip of the internal gear clearing the valley of the pinion
There being enough room along the pitch diameter to fit both teeth and enough backlash to allow things not to bind.

#1 and #2 often can cause problems because of naming conventions. What is the "addedendum" for an interal gear? What is the "Dedendum"? There are definitions but folks don't always say what they mean yet alone look up the proper definitions in an AGMA book or a DIN standard.

#3 basically means thinning one or both of the teeth from the line to line dim.


All three of these problems can cause problems if you use a boolean of an external gear subtracted from block.

This is what you must do to the "tool" (external gear) that you subtract from the block to make the internal gear:

THICKEN the gear teeth by the amount of backlash you want (0.002-0.005" say)
Add to the addendum (1.25 / DP rather than 1/DP)
Subtract from the dedendum (1/DP rather than 1.25/DP)

From my experience, if you do this, you will be just fine.

Joe J.