Thanks for pointing that out! I have run through some calculations in inventor now, the 8mm total facewidth is actually fine if using a hardened mod4 gear to drive it.

However the gear is too big to be hardened easily since its >100mm so if I get it from EN24T (250-300 brinnel) it will be fine with a 12mm face width. (Safety factor 1.36) If I increase the facewidth to 16mm its 1.56.

Rather amusingly the stacked 4mm thick internal gears are plenty strong (Safety fac 2.7) its the drive gear that gets ruined due to it being very soft.

Does anyone know any places that do case hardening? Cheaply..

I guess I could get the drive gear waterjet too which would solve that problem. Although the taper might cause havoc.