Internal gear cutting attachment - video

[ductape]

Great job. Is the cutter able to do external too or is it internal only? Thanks.

Internal and external gear teeth have very slightly different profiles, so it depends on the application. The planetary gears in an automatic transmission would require the proper cutters for each set. Something that doesn't need that kind of speed and torque could probably get by with just using a regular involute cutter for both.
There are some good folks here with far more machining experience who could explain it better.