problems making a ring gear

Quote

So I made an animation of an internal gear meshing with a spur gear (clearance set to 0) and it looks to have a perfectly correct tooth shape.

I have come to think of it like this: Consider the small spur gear that goes inside and place an imaginary gear the same size next to it so it meshes. Make a larger spur gear that meshes perfectly with the imaginary gear. The inside contour of that large gear must then mesh with the first gear. I have attached a picture. The red gear meshes with the green imaginary gear so the inverse of it mashes with the blue gear.

To get clearance between the roots of the teeth you need to set the clearance of large internal gear to 0, as it works the wrong way. To get clearance between the tips is seems better to reduce the addendum of the small gear rather than increasing it on the large gear as jag has done.