Shredder design has major flaw

I have come to the conclusion that the shredder design as published has a major flaw.


I have observed that when the cutting edge passes the plastic and does not catch it prevents the next blade from catching the plastic because of the geometry.


I have changed the blade placement which has improved that issue a little bit, but overall the shredder blade design needs some tuning up or a re-design

PS the capcha on this sucks

@btmetz sorry for the captcha, but it was necessary to help stop those chinese spammers we had for such a long time.
On regards of the shredder design, yes, I agree we need a new version with some improvements. In the meantime, @anne-barbier posted a small upgrade and seemed to make a big difference, take a look a this topic starting page #2:

Hi !

Yes not false, but like @anne-barbier says big things aren’t very used so for the bottles I think you can cut them before, but if you have a lot of bottles you cand keep your system but for ltittle plastic objects or very thin (like bags) I’m not sure that will work properly.

After, for myself : I haven’t a shredder for the moment and I want to create one. But I don’t want to pay a lot (cause I’m a bit poor XD) and laser cuts are too expensive for me, I made a quote to an artisan near to my place and he would 700€ !!! So I’d like to know if there is an other way more cheap ? With maybe thinner blades ? or other way …
(This will be for a temporary use, so if it cut only bottle, small plastics, … it will be sufficient for the moment)
Thanks in advance !

right, thanks @btmetz ! yes, larger bodies like bottles of course can’t get under the knife. that’s why people use ‘straight hoppers’ (rectangular shape), in addition you can put a ‘pusher’ into the hopper. that worked quite well.

that larger bodies however represent only a fraction of the plastic i could find and I would actually consider to recycle, i think Dave’s knife’s design, despite all the flaws, is still the best approach to shred common household plastic waste of all sorts 🙂 I somehow wish he would have uploaded more footage of his trials.

i am no expert in this but lets say you want to shred bottles a lot, i think the best is still to cut the bottles in half,  at least, and then use the mentioned ‘pusher’ in a straight hopper, my tests went fine after all.


look at the model.  The ball represents a plastic bottle.


You see how the blade only touches the plastic and then how it will prevent the next blade from catching it?


Now if you change the blade to number 2 picture, you will see how it will work better.



The problem with staggered blades is they tend to move the product but not cut it. A well known shredder manufacturer had staggered blades in a complete spiral or helix and it created a lot of wear at the end where the plastic was pushed to. Attached here is a pic of an industrial low speed shredding machine.

@btmetz : and the files of course 🙂

@btmetz: please, i’d love to have a picture at least too 🙂

@btmetz could you share a picture of the knives pattern of your shredder?

It is really obvious watching it in action that the blades hold the material away from the shredder cut area.

If the first blade does not catch the material it passes by and then it subsequently holds the plastic away from the cut tooth of the blade next to it.


I changed the blade orientation using just one pattern file for all the blades vs the 3 different files and now it catches the material in several teeth simultaneously.

Problem solved.


Next step is to use laser cut tool steel for the blades.

When you say that the ” blades hold the material away” you mean the plastic ?
If the plastic it’s ejected by the top, I think its’ because your motor turn too fast.
If they are ejected by the bottom, I think you should add a sieve.

Well I spent a lot of time and money on making these files.  4 sets so far.  So yes my criticism is valid.


What I am getting at is that when one blade comes in to cut and passes by the material, the next blade that comes in can not grab it to shred easily because the previous blade is now holding the plastic away away.  I instead changed the design to cutting all the blades the same design and staggering the blades around the shafting so that several teeth grab at once vs the spiral pattern recommended.

I understand it’s not very cool.

But check that you have a good motor. Maybe it is too fast ? How many rpm have you got with it ?
Or maybe you could put a grid under if it isn’t ?
Post a picture of your machine there is maybe a problem that the community could resolve freely.

Hello @btmetz,

It’s nice to criticize (I do this all the time) but so, you can try to share a better shredder using an other system. You can also sen pictures about how you did you’re trick.

And for the captcha, I think ReCaptcha works well, why do you think that ?