There is an update: https://www.thingiverse.com/thing:4658514

Things you need

8mm rod: you need one long and two relative short. I have a 500m long one and two about 200mm.

8mm linear ball bearing

sharpening stone

m3 nuts and screws also four m4 screws. I would buy them at your local hardware store.

Who ever had a really sharp knife for chopping, cant life without it anymore. There are many things on Thingiverse, who did a great job with it, but there was always Something, wich Bothered me. The desings requierde a ton of filament and a printing time of several hours. Also there were two desings:

Some really small desings, with a kind of sled to remain the knife in the right angle. The stone stays still and the knife is beeing moved.

Some really big desingns, with rods and clamps with a fixed knife and a movable stone. This desings seemed superior to me, but I was concerned about the stone could crush my feet, when it comes off.

So I did a kind of remix the desings, with the stone fixed and the knife moving. I am sharpening my knifes for a couple of years now and I can tell, that I am a lot quicker now. And my knifes are sharp like a razor.

There are some issues with my desing too, but I will keep working on it, to make it easyer and cheaper to print.

Update 21.03.20: I made a new attachment for knifes. It uses a magnet 60x10x3 in mm. It is much better than the clamp. Stay tuned!