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Things I have learned

This topic contains 7 replies, has 3 voices, and was last updated by  Melodie Murphy 12 months ago.

Melodie Murphy morethantencents

Things I have learned

25/03/2019 at 06:57

So… we are about to order the parts for another shredder, (this one is for our local plastic makers space, so excited to see them get up and running!) and there are a few things I have learned along the way that might help others who are not engineers. (I am told all engineers already know this stuff)


I have been meaning to write this post for a while but this is the first chance I have had.


I will list what I have learned below 🙂


For the shredder boxes:


1) Plasma cutting is not in any way as good as laser cutting (and I dont care what the ads say!) Unless you are prepared to do a lot of hand filing and cleaning slag off each part, pay the extra for laser cutting. You will be glad you did (here, laser cutting was actually $80 cheaper than plasma cutting so it also pays to shop around, we used a plasma cutter only once in our endeavour to keep everything local then spent almost a week cleaning everything up for assembly)


2) Not all fabricators are created equal so finding a good one who will weld up your assembled shredder box, is sometimes like finding hens teeth – shop around, visit them and have a chat face to face. The guy who takes an interest in what you are doing is the one you want and if they care enough to make suggestions for improvements, hold onto them like gold, pay them quickly and be nice to them!


3) Even laser cut parts need love… It still takes me around 3 hours to clean up the parts and assemble a box by hand, even with good laser cutting and a well machined axle.

4) even on a perfectly machined axle, the bearings will not fit without some extra attention – they are designed this way so they fit snugly on the shaft. – you may need to freeze the axle and heat the bearing in order to fit them but once they are fitted this way, they will be very, very hard to remove. Take your time, show some love and if you need to, employ a fine grade wet and dry sandpaper (this can take AGES! Be prepared)


5) Couplings and shafts need love too, see instructions for bearings above ^ there are also different types of couplings you can use, go have a chat with your local supplier and check them out. SKF do a great range that works well for us

6) when buying stainless bolts, ask for 304 or you will be sold 316 at a much higher price for very little difference in performance and a great difference in price.

7) sanding and filing parts is a careful process, going at it too hard will make your machine sloppy so do everything with love <3 it is the only way to get a good, silent result (the real goal…silence!)


For the motors and gearboxes

If you do not have your head around how these work, reclaiming a washing machine motor and old gearbox is actually quite complex, no matter Dave makes it looks so easy (we are not all legends like Dave but we can try to be!)


We supply new motors and gearboxes with our machines and employ an electrician to fit the switches but for those hoping to get parts from the scrap yard, do you homework, understand the ratios and torque before hand and check you can get the right shafts and couplings to make it all function properly. (It is a lot of work to assemble your machine and start using it only to find it doesnt have the torque you need and burns out or jams quickly)


These are just the things I can remember right now and I have to go get other things done but I hope it helps someone. I will add more when I remember, it has been such a HUGE learning curve, it will take me days to write.

Happy Plastic Processing folks!








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In reply to: Things I have learned

25/03/2019 at 10:37

your blade tips seem to sharp, attention there. if you don’t file them to a more rigid shape they will bend to a curl. about plasma: i can’t confirm that, with a proper setup it’s good enough to make a shredder. i hope that the shredder box will be actually re-designed to be made by a plasma cutter since it’s then easier and cheaper to make by any machine shop ..

In reply to: Things I have learned

25/03/2019 at 13:36

The blades are not ready yet, I just put all the parts together when I took a break and liked how they looked so I took a pic… Will upload another pic when this one is finished for you to have a look and approve the blades 😉


As for plasma being cheaper, as I said in my post, for us here, plasma was $80 more expensive so it may be worth shopping around your area?


As for a “proper setup” yep, my fabricator has all that and I have access to all the workshop tools I need but for the for average joe, making these shredders in their shed -a drill press and lathe for example are not something they might have access to.


This post is for them.



In reply to: Things I have learned

25/03/2019 at 14:59

lol, don’t worry, just want to point out an issue about the tips, we nearly ruined the blades by not treating them, actually making a twist along the edges preserves also a little power and overall performance.

plasma, again, most shops do only roughing, anything above the 60 A cutters will end in pretty non smooth edges but if you use a superior cutter like the hypertherm 45 xp  the results are pretty good. we have 2 shops around using them, i am quite excited.. in sometime we have one too, I hope i can rework the shredder in way that the blades can cut with the plasma, and more important, being able to harden them, another story 😉

if you could add the total prices and build times to your post that would be great !

In reply to: Things I have learned

25/04/2019 at 04:45

Hi Pporg,


I have been a bit flat out, sorry not to respond quicker.

We had a talk with our metal supplier about hardened steel, there is no problem to make this and it is actually a little cheaper but he didnt think it would make much difference to the wear? We are going to build one in hardened steel and see how it goes and we can make them if someone asks but for now all our orders are for stainless.

My build times rely on a team of folks who have different timelines 🙂 not sure it would be helpful for anyone else but for the shredder box, once everything is in one place (laser cut parts, axle, sieve, bearings, bolts, anlges) then it only takes a couple of hours to clean up the parts and assemble and then under an hour to weld the end boxes and seive.

With everything in place for a complete machine, (shredder box already assembled) assembling everything else only takes a few hours, the longest (timewise) part of the machine build is cutting and welding/bolting the frame and waiting for the electrician to wire it all up.


Testing and adjusting – It is great when you flick the switch and everything just works (surprisingly, most of the time!)- when this doesn’t happen as it should, adjusting and getting it right can take some time. I spent around 16hours on the box I mentioned above when it was plasma cut, just sanding and fitting.


My partner has been exploring laser cutters so we can do this ourselves but I think it will be a long time before we can afford this. He is enjoying looking, I don’t want to slow his excitement 😉 The guys we use now are brilliant and happy to work with us on customisation or changes to the plans so it works this way for us, for now.





In reply to: Things I have learned

25/04/2019 at 10:54

in either way, the margins in tolerances build up so badly that you can only put 14 blades in the shredder ( i can’t remember a single time anything fitted ). so if you don’t have a lathe around which can make special sleeves, then you have a problem ! also if you can’t file the blades inner hex profile to the hexbar : you have a massive problem as well.

So, if you can’t apply the sheets exact thickness to the drawings in fusion360 and then generate new laser files, then better take stainless.

and no, hardened steel doesn’t wear out as quickly as the stainless variant that’s why it’s widely adopted in shredders …

In reply to: Things I have learned

25/05/2019 at 22:44

Do you have a total tally of the bolts and nuts used?

In reply to: Things I have learned

27/05/2019 at 08:18

Hi Sharma

For the whole shredder it depends how you assemble it (We mount ours differently with the motor beneath the bench so its not the same) but for the shredder box Z(blade assembly) we use:

4 x 10mmx25mm (countersunk for bearings)
8 x 8mm x25mm + Nuts (Box sides)
4 x 6mm x 25mm (sieve)

You also need 4 x 10mm nuts for the threaded rod.

Hope that helps


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