Imperial Compost Shredder

I stumbled upon the metric version of this and really wanted to print one to speed up my home composting process. Sadly, 30x30x2 mm tube is hard to find at a reasonable price in the US. Even sadder, the original post had an imperial version at one point, but that appears to have been lost to time.

I spent... too much... time trolling the internet trying to find STLs for the imperial version. I was finally able to track some down, but they needed modification before I was happy with the result. I've included .f3d (Fusion 360) files for the things I needed to modify. In addition to getting the design printable, I made a few additional modifications that I feel result in a more user friendly implementation (the sides have thicker diverters, there's less tolerance between the shredder wheels and spacers which helps with dry and/or flexible things).

This shredder is pretty serious. I was initially impressed when it ate some avocado pits, but what really did it for me was eating through a mango pit. Granted, it went through the mango pit length wise and not width wise. Either way, 9/10 for strength.

During two months of regular use, I've broken two printed parts: a stick made its way into the hopper and I broke the input shaft after applying way too much force and one of the grinding wheels split along the layer axis while grinding up 1.5 year old, dried out canna lily rhizomes. As long as you stick to wet organic matter you're unlikely to break anything.

It can take a while for it to bite into larger objects. Large and/or thick watermelon rinds, large avocado pits, etc can tumble on the shredder wheels for a while before it takes a nibble out of them. You will learn to work around this by either cutting them into (still large) chunks, or starting to feed them into the shredder by hand.

Don't expect great results trying to mulching up dry plant matter (corn stalks, leftover annual/perennial foliage from the previous season, etc). They will pass through the grinder, and they will get split up some, but they will not be broken into pieces. I'm hoping to make some tweaks in the future to address this, but time will tell if that ever happens.

I'm a big fan and suspect that the time required to complete composting will fall, and the end result will be more uniform (ie no chunks of anything left over)..

To use this shredder you will need to drive a 24mm hexagon. I use a 24mm 6 point socket, along with a 12" long 1/2" drive ratchet

Hardware list (around $48 USD when I assembled mine)

• 2x 1 1/4" square tube sections that are 122mm(ish) long. There is some tolerance in the design if you're over/under in length by a small amount, so don't sweat hitting 122 on the nose. But get as close as you can.
• 4x R20-2RS bearings
• 4x 3/8" x 1" bolts
• 8x 5/16 x 1" bolts
• 12x #10 (3/16) x 1/2" screws

Print list (I used the remnants of three PETG roles, but there's probably around 1 kg of PETG in this shredder)

I used 0.32 mm layers for all the parts along with 1.6 mm skin (walls, top, and bottom) on a 0.4 mm nozzle. Note that I sliced mine with KISSlicer, which doesn't accommodate for infill extrusion thickness when pathing. My parts printed with well over their stated infill. The original design calls for less infill than what I printed so if you stick with that design, or what's listed below, you should be fine.

• 3x cup.stl (0.32 mm layers. These wind up being solid parts)
• 2x spur gear inch.stl (0.32 mm layers, 50% infill)
• 2x spur gear 15mm inch.stl (0.32 mm layers, 50% infill)
• 2x bearing holder inch.stl (0.32 mm layers, 33% infill)
• 2x bearing holder thin.stl (0.32 mm layers, 50% infill)
• 2x side.stl (0.32 mm layers, 33% infill)
• 1x for shaft long inch fixed.stl (0.32 mm layers, 100% infill). This is the part that is the most likely to fail, so it's important that you print this part solid. If you're planning on using PLA for this entire print, consider printing this part in PLA+ or PETG. The parts are less brittle and will experience some elastic deformation when you approach the torque limit. This will help prevent you from breaking anything
• 2x for shaft1 inch fixed.stl (0.32 mm layers, 50% infill)
• 1x for shaft1+5 inch fixed.stl (0.32 mm layers, 50% infill)
• 8x bioshredder spacer inch.stl (0.32 mm layers, 50% infill)
• 8x bioshredder inchv2.stl (0.32 mm layers, 50% infill)
• 4x print st inch.stl (0.32 mm layers, 33% infill)

Assembly see the design this was remixed from since it has assembly videos :)

Hopper link