Hi guys,
I am building the shredder according to the drawings. The motor that i am using is very strong(2,5 Kw). However the shredding gets very aggressive. I am thinking that this is caused maybe by 2 thinks.

  1. The total distance between the moving knives and the fixed knives is 1mm which i believe is too big. In order to cut easily the knives should be very close together. If we think the example of scissors cutting paper, if the scissors’ cutting edges are not close together they can’t cut paper very well and the paper is chewed up. The solution to this would be the small fixed knives and the moving knives to be the same thickness and reduce their thickness accurately by some hundreds of a millimeter with a surface grinder or a milling machine so the total distance between the knives is very close but they don’t hit together. I understand that this may cost and Dave wisely designed the machines to be easily built with accessible materials and tools. However, I think this would decrease dramatically the cutting forces and increase the efficiency and shredded plastic’s quality.

2)My second and biggest concern is that the aggressive cutting is caused because the motor is not directly connected through a coupling to the shaft but through a chain and sprockets in order to double the speed and archive the desired RPMs as my motor is outputting 48RPMs.

What do you guys think? Did you had any experience with anyone of the problems? I think the 1st problem may not be so critical because Dave Hakkens and many others built it like that and it seems to work. I really want to have the shredder running nicely and i believe i am close.
Thanks, Antonis.

Aggressive cutting?

What do you mean?

Got a video of your shredder shredding?

Hi guys thank you very much for your support. I have modified the shredder. I removed the chain and conennected it directly to the motor through a coupling joint. The problem of aggressivenes was reduced. However now my problem is that the rotating knife at the one side is grinding on the wall and has a lot of friction.

The distance between the fixed knives and rotating knives was designed to be as close as possible but at the same time to reduce the chances of grinding themselves. I have personally faced the problem you mentioned with plastic getting in between the knives but only when using really thin plastic such as pet bottles. Thicker plastic does not exhibit this issue.

If you share a video of what you refer to as “aggresive cutting” we might help you pinpoint the root of the problem.


I dont think it is a problem to connect the motor via a chain, think a motorcycle for instance. the main problem as you describe it seems to me that the fixed blades are too far apart.
The reason i say that is because there is no inherent problem with a chain driven system apart from the relatively low efficiency compared to other things like a direct drive, that shouldn’t be a problem in this case, as long as the chain is reasonably tight