Machine Development……Shredder – Problems and Questions

Hello everyone
We’ve managed to build the shredder and make it work. sort of.
This post is for all those that have built it, or are building it, and are having problems with the shredder.

There are many other forum posts that relate to the shredder

and many others.

– How many people have actually built the shredder?
– Can you please share how it works for you? we need benchmarks (does it get stuck? does it chew everything or not? is it slow? what motor are you using? problems you had with assembling the shredder body? etc)


Our shredder works, but it clogs often and even with “simple” parts like plastic bottles, it sometimes can’t chew the top harder part.
I’ve built a circuit to allow it to go both directions (and will post a specific post and video about that)

I’ve made two videos of our current shredder that works 85% of the time 🙂

We are unsure about what the problem is.

  • is it the motor that is not powerful enough?
    Theoretically we’ve got a powerful motor that satisfies what other people have said works: 1.1 Kw (1.5HP) 220V motor with a Gearbox reducer whose output is 70 rpm and, most importantly, a torque of 68 Nm.
    It is very likely that our motor was a 3 phase motor converted to single phase because it has a starting condenser, which also means that the actual power might be lower than the nominal 1.1 Kw and therefore the Nm might be lower than the 68 Nm that the manufacturer stated (although the Nm is a factor of the gearbox more than of the motor).

    Q: Anyone know a way to measure the exact KW and Nm output of the motor?

  • is it the shredder that is not well assembled?
    we had several problems with it and we had to assemble and reassemble it a few times.
    – The laser-cut parts were microns bigger than 6,5 or 3mm, which meant that when adding all the parts, the shaft was 2 mm shorter and the rest of the body parts didn’t close very well.
    – we also had to clean the inside of the hexagonal holes since they didn’t fit the hex bar, and unfortunately by doing so we made them too large, and now many pieces seem to wiggle a bit.
    – the borders of the laser-cut are rugged and I’m not sure if it is supposed to be completely smooth, if it is normal, if it is due to a cheap laser cutter, if it normal and all laser-cut parts are like that, or if it is supposed to be completely smooth. (see picture below)

    Q: can the parts wiggle (move) a bit or they must be perfectly stiff?
    Q: what should the laser-cut parts look like? can we cleanup the rounded parts?(I didn’t want to do it fearing that it would not make it cut afterwards

  • is it an alignment problem between motor and shaft of the shredder?
    it was hard to align the shaft of the motor and the shaft of the shredder: when trying the machine on a table both the motor and the shredder body would wiggle in every direction (x,y,z) as if they were not aligned. Of course when screwing both the the structure of the table that movement would not be visible, but it probably is acting upon the shaft making it more resistant. We think we’ve solved the issue by putting both the motor and the shredder on top of thick rubber that should give it some elasticity.

    Q: is it normal that the shaft and motor wiggle a bit when activated (and not screwed to the table), or is that a sign that there is something wrong?
    Q: @paulfreed what is that black thing you have between your motor and the shaft of the shredder? is that a silent block of sorts? any suggestions?

Well, I hope these questions and the answers that you might be able to give me can help all those that want to build the shredder.

Any thought on this? @davehakkens, @keesdeligt, @paulfreed, @andyn @xxxolivierxxx


Hi Guys,
Sorry i didn’t answer your questions earlier, though it looks like you sorted most of the problems out between yourselves.

I also had alignment problems but like you said the Jaw or Oldham couplings are more forgiving.

Are you guys still getting jams?

The tips you gave the others are pretty much what I would have recommended, particularly regarding extra friction and drag – smooth smooth smooth!

I had the same issue with stainless, but chose to get 0.9mm shims cut for the stationary knives & spacers. I like your idea though of just shortening the shaft and 3mm parts. Did it work out okay in the end?

I live in South Africa and it seems 5mm Stainless steel sheets are not available. Instead I’ve only got access to 4.5mm sheets.
For those with a similar problem who still want to use the laser cut body, I’ve altered the 3mm cuts to accommodate for the thinner material.
All other measurements should remain the same, except the length of hex bar will now change from 148mm to 141mm.
Hope this helps


Acetal is an engineering plastic often known by the brand name Delrin. Nylon would probably work just as well in this case. I used acetal because I happened to have a piece the right size.

@luisa we went old school and used a hand file and some sanding paper to clean the pieces. Especially for the inside part, this will give you more accuracy.
We where careless at the beginning and filed away abundantly, so some pieces ended up being sligthly bigger (way smaller than the 1 mm you say you now have)

it is better for the parts to be snug, but when you close everything together the parts will tighten a bit.

very important is to clean the border from roughnesses so that it becomes smooth, also in the spacers: our pieces had all the markings of the laser cut and even a thicker line/ blob on the side where the laser probably started to cut the metal. if you don’t clean those you’ll have a lot of noise and mechanical resistance once the shredder is assembled

You can choose to do this by hand, with file or sanding paper, but if you have power tools such as a band sanding machine that is going to be faster. Just be careful not to overdo it and also, not to round the cutting edges under the S tip.

Very Very important: check with a caliber the thickness of your pieces. If you have pieces that are not exactly 6 mm or 5mm, you can either choose to clean them until they have that size (with the band sanding machine) or to add the extra 1-2 mm plates like in my previous post.

good luck 🙂

@xxxolivierxxx many thanks for the fast reply and tips! I will check which one I can find! Best regards!!

@luisa I used an angle grinder for the exterior and a dremel with a grinding stone for all the internal grinding. I actually recommend using a diamond grinding file instead of a dremel to polish the internal sides of the hexagon holes, but I did not have one handy so just decided to use a dremel. Depending on how dirty the laser-cut job was for your parts, you might have to do a lot of grinding to make the knives and spacers fit on the hex bar.

I´m also struggling to build the shredder, I found a used powerful motor with reduction (but still have to convert it from 3-phase to 1-phase), got the hex bar and also order the laser cut pieced…but the laser cut they do not fit! I saw you guys also had this problem…


you mentioned you: “had to clean the inside of the hexagonal holes since they didn’t fit the hex bar, and unfortunately by doing so we made them too large, and now many pieces seem to wiggle a bit.”

How did you do this cleaning? manually? I´m worried it will also be a wiggling too much after I cleaned this…Is this problem only due to imprecision of the laser cutting? I was checking the cad files, they also have different values in the measures of the hexagonal holes…


“I cleaned the parts and the hex-holes of my blades are 1mm bigger than the Hex shaft on every side of the hexagon.”

Which was your cleaning method? Do you recommend me do to it? or should I maybe ask a professional to guarantee it wont be loose?

Great job on your work, guys! And many many many thanks for the awesome tips and technical support!
As soon as I assemble together the shredder I´ll detail all infos/trouble here!

@andyn looks awesome and professional 🙂
by “Acetal” do you mean Nylon
or Acetate (the material used also in x-ray prints)?

I’m waiting for a 26€ replacement spider that can withstand 96 Nm.
This is the simplest way. Next maybe I’ll fabricate a spider like you.
Or try the double cardan, as that is probably going to be the simplest solution even for the whole precious plastic community

Yes, an Universal Joint will work just fine, probably better than a Jaw Coupling since the Universal Joint allows for a little more misalignment.


the machine would jam when you tried to tighten the bolts located on the “B” side of the machine, because B was 1~2mm shorter than “A”, correct?

yes correct.

I determined it the first time we assembled the machine:
– the two lateral boxes (the ones that you assemble like a jigsaw puzzle) where shorter than the body suggested.
– Also the cutting blades and spacers would be slightly longer than the hexagonal part of the shaft, even if it was machined to the exact size of Dave’s blueprints.
At first I thought that the files had weird units but then I simply measured with the caliber the thickness of the cutting elements and I could see that they were not exactly 6mm or 5mm, but roughly ~5-10 microns bigger each.
10 microns x 27 elements that is almost 3 mm off … enough to be causing problems and bending the 2 plates when pulling everything together.

I think this might be also the solution to the issue that you had pointed out with your machine in another post.

thanks for your tips. they were very helpful. 🙂
However I haven’t yet fully solved the issue with the coupling. a new jaw coupling is quite expensive (± 120 euro) so I was thinking of joining 2 cardans like the ones visible in the videos in this wikipedia page.

what do you think about those?

Here’s a picture of the jaw coupling I made for my shredder, it’s 50mm diameter, mild steel and the spider is acetal. I’ve run it over 100 Nm and it shows no signs of wear/damage.

I think the double cardan would work great. Another simple, cheap and very strong coupling can be made by mounting two identical sprockets on the ends of each shaft and wrapping a turn of duplex chain around the teeth.

@lyricalpolymath it’s great to hear that your machine is now working as expected, I know jaw couplings were the solution to your machine alignment problem, they are awesome pieces of engineering 😉
You probably had a rush of joy after watching the machine spinning that smoothly.

And thanks for the tip, I’ll try doing something similar with mine and see what happens. Quick question: How did you determine that your machine had 1.5mm-2mm extra thickness? Did you measure each knife/spacer/fixed-knife individually, or just measured the lenght of “A” (see picture below)?

If I understand correctly, the knives, spacers and fixed knives had a measure of 2mm~ (represented as “A” in the picture below). Since all that parts are located on one side of the machine only, the machine would jam when you tried to tighten the bolts located on the “B” side of the machine, because B was 1~2mm shorter than “A”, correct?

@andyn and @xxxolivierxxx thanks for your tips.

@andyn. The idea of creating more grub screws to align is very interesting. too bad I read this too late 🙂 we had already cut and lathed the axis coming from the engine to insert the jaw coupling.

we improved the machine in 2 ways, one of which might interest @xxxolivierxxx for your own machine.

SOLUTION 1 – using a jaw coupling
we did install an old Rotex jaw coupling that we had in the workshop and it started to work perfectly: the wobblyness was gone and it could shred better, although it would still get stuck from time to time. The problem was now that the spider was too soft for the force and after a few rotations it would twist and disappear inside of the body of the coupling.
in fact the rigidity of the spider material an its nominal Torsional resistance in Nm is one of the most important parameters when choosing the coupling. The one we had in our workshop is declared to withstand 10 Nm, while for the declared output of our gearbox we need at least 70 Nm, and the manufacturer suggests at least double that, to 140-160 Nm, to withstand also the initial torsional shock.
Now the problem is that to fit a spider with those characteristics we have to buy a bigger/ newer jaw coupling = spending a lot of money 🙁

It got so better that I think it should be declared in the precious plastic bill of Materials: it is highly unlikely that whoever is going to build those DIY machines will manage to get them so perfectly aligned not to need a coupling.

– at a certain moment we thought of building our own coupling with 2 cardan joints / universal joints. if compared to Jaw or oldham couplings, these are way cheaper to find and can accommodate a larger degree of movement between the 2 axis. What do you think about this? do you see any problems with this? is the “variables speed of the output shaft” a problem?

2-SOLUTION 2 – making the shredder’s size right
this might interest @xxxolivierxxx
As pointed out earlier, one of the problems we had was that the laser cut parts where not exactly 6mm, and they where ≈ 6,1mm. That meant that adding all the elements together we had 1 or 2 mm extra pushing outside the box.
because of this, if we tightened too much the four top and bottom bolts, opposite to where the cutting parts are, those would block the shaft from rotating as they where bending the 2 side plates to one side.
The solution is to either reduce all the elements to the exact expected size of 6mm (which is a lot of work and we didn’t want to do 😉 ) or better, to add the extra thickness on the missing side. If you look at the picture we’ve added a 1.5 or 2 mm plate on one side of the machine (opposite to the cutting parts that are held by the rods) and now it works perfectly: it’s super easy to turn by hand and it doesn’t block the rod no matter how much you tight those bolts!

we still need to solve the joint, so please tell me what you think of those

@andyn Correct. An OldHam coupling will end up being similar to the ridig coupling he already has, since they are more sensitive to misalignment . I believe a Jaw Coupling is the best on this case.

Looking at your last video it looks like the problem is caused by the rigid coupling between the gearbox and shredder. This is attached by grub screws on only one side which push the coupling sideways so it is not concentric with the shafts at either end. You would do better by having 3 or 4 sets of grub screws around the coupling that you can adjust to centralise it, you will never get it perfect, but perhaps good enough.

A jaw coupling as suggested by olivier is the best thing, if you have access to a milling machine they are not that difficult to make. Oldham couplers are the simplest type (two sliding keys at 90°) and will accommodate more misalignment, but need to be bigger to withstand the torque.

@lyricalpolymath installing a Jaw Coupling does not require any professional knowledge, the only thing you need to know is the diameter of the 2 shafts, then you buy 2 jaw couplings (1 for each shaft) and a rubber spider that goes in between the 2 couplings. On the video, they make it look way more complicated than it actually is.

@xxxolivierxxx thanks for your tips.
installing the jaw couplings looks like a serious professional task to do, with a lot of specific tools :S

it seems also that the couplings are specific for the size of the shafts, so I’ll have to first adapt the shaft size and then buy the jaw couplings.

are you a mechanical engineer?
Due to the tools required, do you suggest to do it by ourselves or you think that it is best if we give it to a professional shop that could to it for us?

Also, do you really think that that might clog the machine?

After watching your video, it is now clear that your machine has an alignment problem. Either the bearings are not perfectly aligned (maybe the holes that hold the screws of the bearing were not cut correctly) or your hex bar is twisted

Ideally, the best would be to disassemble the whole machine and validate that the shaft and both bearings are perfectly aligned, but a Jaw Coupling should help you to correct that misalignment. I recommend LoveJoy Jaw Couplings,but any other brand should do it:

@andyn thanks for pointing that out. check the video with the wobbling.

are you saying that the rubber parts are not good and I should take them out?

are there “flexible couplings” between the shafts that I can implement without having to move too much the engine? the movement is very little. If I had to put chain and sprockets I’d have to completely change the configuration of the table.

Good to hear that your motor, which is less powerful, can still make it.
I’ll try to measure the current and go back to the Nm calculations

I thought it was a 3 phase motor because of the condenser, but now I’m starting to think it is simply single phase. The condense is 35 µF