Cardboard Shredder
I think I can shred cardboard with wood. I've designed the shredding wheels. I think I can build a simple model an run it with the corded drill.
A simpler option may be to put nails into wood and have the nails actually shred the cardboard. I could also use disposable razors. They're cheap and it would make it easy to replace the blades.
- https://www.youtube.com/watch?v=AheXQ9-SVn0
- https://www.youtube.com/watch?v=6VS_sHrUdJ0
- https://www.youtube.com/watch?time_continue=20&v=bJ78IgmuWyE This one actually looks really good.
The general design seems to be a first stage that sort of tears up the cardboard breaking down it's strength. Then a second stage that actually does the shredding.
Looking at that last one, I could build it so the top is blades and the bottom is nails. They could be connected with a geared drive train so the bottom rotates at 1/2 the speed of the top. The blades can be made of sharpened metal strips.
Crusher
It may be easier and more effective to build something to crush the cardboard. Motor drives a lead screw, when the current through the motor rises above a certain level, we've reached max compression, reverse the motor.