Hi there,

I need to make some internal gears and have decided to make a broach to do this job.

What I would like to know is whether the shape of an internal gear is the same as a spur gear. In other words, if I make a broach that is effectively just a plain old spur gear using an involute cutter and then broached an internal gear, would this give me the correct shape?

Thanks
Warren

Probably not, Gearcutters are usually Modular made
for a small range of Diameters they can be used on.
The shape of the tooth profile for smaller Diameters
is very different then the profile for larger Diameters.
You will have to take this in consideration when grinding
your own profile.
Good Luck

The shape of the tooth space for an internal gear is quite a bit more "closed up" than is the tooth space for external gears.

Imagine taking a rack gear and rolling it into a circle. This would bring the tips of the teeth closer together. This means that the tooth space is narrower than any you would find on any external gear, all the way up to and including a rack. That precludes you from using standard gear form cutters to make your broach.

Because the meshing gear "rises up", the tooth lifts out of mesh as it rolls inside the internal gear, this is why the reduced tooth space does not cause interference.

HFD is resourceful with the description of imagining a tooth on a rack and rolling it up. Likewise its the same as bending a rack to make an external gear, the tooth has a convex[/U][/I][U] involute surface. If you roll a rack to make an internal gear the tooth has a concave[I][U] involute surface.

Here is one of the ratios of planetarys I drew in Autocad. Notice how the ring gear teeth are concave on an internal gear as dictated by the involute curve.

I use the external gear and offset the dedendum and addendum cicles the distance of the clearance

Clearance = 0.25 * Module

Then it will be an internal gear. This will also show why the internal teeth are concave. (I hope I've remembered this correctly.)

You do not state what size internals or the ratio you are considering. There are some limiting factors i.e. the mating gear can not be any bigger than half the size of the internal gear for satifactory running without modification of the tooth form.
I have used stub tooth gears i.e. 7/9 pitch to improve the running. There are internal gears with concave faces but this is more of a conjugate action (more difficult to maintain a rolling action) If you are just using hand operation or low speeds you may not have any trouble.