Well here you go Calum... following Andy's lead, here V3 of the tunnel grinder...
I tried it out this morning with the wire bristle heads, it worked ok, but rather slow. It took quite a bit of force to stall the worm drive gearhead motor, so at 160rpm it does have good power.
These #40 sprockets are 90mm diameter, I'll fab them up similar to how Andy did & give that a go... if the 160rpm g/h motor don't have the ponies, then I have another g/h motor just like it, but @55rpm to swap in.
A vid request from another forum had asked how I dug out the ceiling...
https://www.youtube.com/watch?v=BJOJD6g-CM8