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A low cost shredder made from scrap

This topic contains 4 replies, has 3 voices, and was last updated by  pporg 1 month ago.

Peter independent

A low cost shredder made from scrap

10/04/2019 at 15:43

<p align=”center”><b>A low cost shredder made entirely (well almost) from scrap
Here are some notes on my first and second attempt at making a shredder in the hope that they may inspire others, or at least steer them away from some pitfalls. First some background. I was in need of some solid rods of “engineering plastic” for bearing bushes on another project and found it remarkably expensive so I set about making my own from HDPE recovered from milk containers. The plastic rod was vacuum formed in a heated tube. A couple of dozen milk containers were chopped up with scissors and inserted into a heated tube which was compressed by a piston drawn into the tube by a vacuum. The effort of slicing up the milk containers left me with sore hands and a blister. There had to be a better way and so the shredder project came about. Not that I needed a lot of plastic bushes but the idea of a new tool in the arsenal was appealing. After all how difficult could it be. The answer was, as with many of my projects; harder than first envisaged even after applying Hofstadter’s law but far from impossible.

With only a few of these rods required it was not worth spending too much time, effort or money on an elaborate contraption. The KISS principle applied. The objective was to make a shredder from available materials, preferably scrap, make it with available tooling, make it cheap, and make it now.
<p align=”center”>
Mark I</p>
The first attempt met all the above criteria but failed miserably at shredding. It was made from junk lying about the workshop, a 500 watt 3000 RPM blower motor driving a bunch of bicycle cogs on a vertical shaft in a conical hopper; basically an oversized blender. My minds eye visualisation, as I was constructing it, was of milk containers getting smaller and smaller with bits being ripped from them as they descended into the depths of hell. The conical design meant that only the small pieces could exit the bottom. The reality was far from the what was envisioned. Either the milk container ascended to heaven by shooting across the workshop or jammed the machine suffering only minor abrasions, see Illustration 1 below. This approach was abandoned with a greater appreciation of how tough plastic is. I had visions of making a duel shaft version with intermeshed cogs but the magnitude of the failure called for a complete rethink.
<p align=”center”><i>Illustration 1: Do not try this at home
Thinking about it, if one slashes at a plastic bottle with a knife it is likely to bounce off and even if it cuts it does not remove any material. The bicycle cogs even though sharpened on a grinder were not very sharp and the milk containers just deflected the energy. Scissors do a better job through their shearing action so mark II with shearing scissor action came to be.
<p align=”center”>Mark II</p>
I will discuss the drive train after covering the construction of the shredder. The idea of using disk brake rotors as the cutters, came when looking for parts at a metal recyclers. There was a bin full of them and after a bit of searching I found four identical rotors.

The shredder ingredients are:

four identical disk brake rotors;

two axles from a rear wheel drive car with matching bolt pattern;

some heavy steel angle for the frame;

some Square Hollow Section (SHS) steel tube for the stand;

some sheet steel for the housing;

miscellaneous steel bar for the stationary cutters;

two pillow block bearings; and

nuts and bolts including 4 long 12mm bolts.

All but the last two items came from scrap yards or auto service centres at the cost of about $1.00 per kilogram. This seems to be the universal going rate for iron, steel and motors.

As the parts for this project will differ with every instance there are no plans. The machine was built bootstrap fashion, measuring and cutting later parts to fit earlier parts, starting with the rotors.

The process was:

machine mating surfaces in the rotors;

cut the teeth in the rotors;

prepare the shaft;

drill and bore the hubs;

assemble the rotating cutters;

turn the bearing surfaces in the shaft;

fabricate frame;

true the rotating cutters;

install the stationary cutters;

build the enclosure; and

build the hopper.

Each of these steps is expanded below
<p align=”center”>    Turn the mating surfaces in the rotors</p>
The shaft is machined from a rear wheel drive automotive axle that matches the bolt pattern on the rotors. Before being turned into a shaft the axle serves as a mount for the rotors on a lathe. The axle is mounted in a three jaw chuck and the rotor mounted on the hub with dense rubber packing behind. The packing compresses so the rotor can be trued using a dial indicator and adjusting the torque on the wheel nuts. Turn the mating surfaces; cut a few millimetre recess and matching shoulder to ensure the rotors are centred and parallel to each other when bolted together.
<p align=”center”>    Cut the teeth in the rotors</p>
The teeth were cut with a 38 mm bi-metal hole saw on a drill press and the tangent cut on a band saw. Six teeth are cut into each rotor, all are identical. The staggered offset is because there are 5 stud holes and each is rotated 1/5 of a revolution.
<p align=”center”>    Prepare the shaft</p>
A few passes with emery paper to clean the axles up and ensure the diameter is constant. In my case they measured just over 33 mm diameter.
<p align=”center”>    Drill and bore the hubs</p>
The outer two rotors are located on the shaft by the hubs and the inner two are located by the mating surfaces. Both hubs are drilled and bored to fit onto the 33 mm shaft cut from the axles. The axles are made from high carbon steel, most of it is annealed but through some witchcraft (localised induction heat treating me thinks) the bearing surfaces are hardened and can be a challenge to bore out.
<p align=”center”>    Assemble the rotating cutters</p>
The wheel of doom, see Illustration 2 below, was assembled onto the shaft with the rotors sandwiched between the two hubs. All are facing the same way and held together with long 12 mm bolts. The shaft should be a tight fit but able to be slid into and out of the cutter assembly.
<p align=”center”><i>Illustration 2: Ready to weld the hubs to the shaft
    Turn the bearing surfaces in the shaft</p>
Now that the cutters are assembled the overall length can be measured and the shaft turned at each end to suit the pillow block bearings which act to prevent movement in the thrust direction. In my case 30 mm diameter shoulders. Finally the hubs are welded to the shaft.
<p align=”center”>    Fabricate frame</p>
The frame was cut from scrap angle to suit the bearing spacing and the legs added.
<p align=”center”></p>
<p align=”center”><i>Illustration 3: Its starting to come together
    True the rotating cutters</p>
In my case, despite all precautions there was some wobble in the cutters. They were not orthogonal to the shaft, about 0.2 mm run-out. This meant that the stationary cutters had gaps large enough to fit slithers of plastic through. These were ground true by fabricating a mount for an angle grinder and rotating the cutters until the volume of sparks was constant through a rotation. I am not sure where this crept in, possibly due to distortion in the flange and/or shaft caused by the welding. I may want to rethink the design, which was chosen because I did not want to machine four hubs and true each rotor individually. It may be unavoidable but grinding was a pain, still worth the effort given the performance improvement it gives. The milk jugs are around 0.2 mm thick and gum up the works if they get caught between the rotating and stationary cutters.
<p align=”center”>    Install the stationary cutters</p>
The stationary cutters were cut from a piece of scrap mild steel bar custom filed to fit. At this point I discovered two of my rotors were tapered, I assumed they wore evenly. The stationary cutters and frame were drilled and the frame tapped to suit. I have been experimenting with the shape and angle of the stationary cutters in a bid to control the size of the product (shredfetti, shredules, suggestions) but have not drawn any conclusions yet.

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10/04/2019 at 15:48

    Build the enclosureI lucked out and found a bunch of scrapped 1.2 mm thick sheet steel which looked like it came from a large roll. Otherwise I would have cut up an old washing machine or microwave oven, see

Illustration 5 below.

    Build the hopper

This is the only part that has not been fabricated but I plan to make a hopper that sits in the enclosure.

A few words on motors

The first motor tried was a 1 hp motor with a built in 5:1 reduction gear box. The reduction is not enough but it was on hand and came at no cost having been in the “may be useful some day bin” for a long time waiting for its time to shine. It spun the cutters with frightening speed, around 600 RPM. Hopes of their momentum carrying them through were dashed when it choked on the very first milk container (and on all other tests). My appreciation of just how tough plastic is notched up a bunch. The motor is back in the bin and still waiting.

Motor 2, a garage door opener opener, bought for $5 at a metal recyclers. The motor is under powered, only 375 Watt (½ hp) but heavily reduced through a worm gear with even more reduction through a 10:32 bicycle gear final drive. The final speed was a promising 30 RPM but given the worm gear is plastic and the drive through a flimsy bicycle chain hopes were not high for this one and it met expectations. It had one advantage of self reversing when jammed. The large diameter cutters of my machine need more torque than this provided. I don’t think this would cut it with just about any diameter cutter but may just work for very small machines.

Motor 3, a 1 hp 3 phase industrial roller door opener with built in worm gear reduction, see Illustration 6 below. I bought two of these for $30 from a junk yard at the going rate of $1 per kg. It spins too fast, around 120 RPM, but does the job, though only just. It occasionally jams so it is not a case of fill the hopper and forget. A reversing switch is a must. The containers tend to roll into a tight ball and jam the machine when it tries to cut through multiple layers simultaneously. It works fine if fed containers in half’s, half a container at a time. Possible improvements may be had by changing the cutter angle or position but there is no substitute for torque. I now stand in awe of the toughness of plastic.

I am on the prowl for a 2 kw conveyor motor and regret turning one down because it was too heavy to fish out of the bin when I found motor 3. By the time I realised I needed it and returned a week later it had been munched, now that’s an impressive shredder. Another possibility, but it will require some work, is to use the gear reduction units from starter motors. Most heavy vehicles have a gear reduction unit built into their starter motor and it may be possible to gang these together.


I met three of the design criteria: made from scrap, made it with available tooling, made it cheap; less than $100 but it was not as simple as first thought and took some time. The HDPE shredfetti it produces is quite bulky at 15 litres per kg its approximately 1/15th that of the solid. I don’t know what the density of commercially bought HDPE granules is but it has to be better than that. I am yet to try shredding other plastics. The shredder requires an estimated 400 Nm torque and in its current state with less than 100 Nm it is a long way from chowing down on ABS car bumpers.

A word of caution, I don’t know how durable the cast iron cutters will be. One does not see any cast iron drill bits, chisels, knifes etc. So far, it has only munched through a few kilograms of milk containers and is showing no signs ware. It may also rust with time, coating it with WD40 or the like will only contaminate the plastic. It may be possible to de-rust it by sending a sacrificial load through it. I should know by the end of the upcoming wet season.

Oh, the engineering plastic bush, I found a substitute brass bush while searching for parts for the shredder and the blister healed a few days later.

19/04/2019 at 04:37

Just wondering… if you would have had access to a brake lathe, designed specifically for turning rotors, would the machining have been easier.

Seems like you had a difficult time getting the rotors ready to turn. A brake lathe is designed to mount up rotors, and turn them.  I don’t have a good enough understanding of what you did to the rotors, to know if a brake lathe would have helped.

Using the rotors is a good idea. The cast iron should hold up well. The slow speed, and non-impact shredding is the kind of process cast iron is good at.  If you were crushing glass, or anything, at high speeds, creating rapid high impact collisions with the rotor, it would tend to fracture.

19/04/2019 at 07:27

I have never seen a brake lathe, I just know of their existence. The machining was not that difficult but I only turned mating surfaces. It did not occur to me that the rotors were not true. In hindsight it should have given their source, a skip bin. There was a reason they were there, I just assumed they wore evenly and had exceeded their minimum thickness and never checked.

I have have heard of people turning their rotors on hobby lathes but of course it would be easier on a purpose built machine. My problem was I had welded the rotors to the shaft before I discovered their imperfections and the entire assembly would not fit into my lathe. Actually it did but was a terrifying thing to look at when spinning and difficult to get the tool in between the cutters.

Even then I still had the problem of the entire assembly being slightly skewed on the shaft which I ground out. I don’t know how the skew crept in but will be on the lookout for it next time, that is if there is a next time. I am working on an extruder, again made from scrap, among other things now. If I revisit it and make mark III I have some ideas about how to work with vented rotors for a finer cut.

19/04/2019 at 14:54

thanks for sharing, honestly I am getting a little heart attack imagining this build shredding PET bottles at 400 Nm. As thumb of rule, I’d always size driveshaft and bearings and shredder mounts according to blade diameter/thickness and torque. Interesting exercise to figure those ratios in holy bible called ‘Machinery Handbook‘. As little tip already ahead, the 20mm driveshaft and 27 mm hexbar of the PP v3 design is obviously undersized, see broken drive shaft reports on the forum and in the internet.

on doing the extrusion with ‘commonly available tools and materials’ we post here a soon an article as well, same for the shredder. neat exercise though. we will limit it to lathe, mill and welder as well power tools.

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