Using RC gear to drive a grain mill

[BrianG]

Are there any inline planetary gearboxes that will fit? Something like this? They usually have ratios somewhere around 4:1 to 7:1. Two 7:1 drives will get you a 49:1 ratio.

With a large reduction ratio, you should have TONS of torque at the output! Using your 50k rpm and 833w figures (which I assume means you are using 4s?), that would be ~6ft-lbs of torque (assuming 50:1 ratio).