Precious Plastic Shredder
Check out comments 11-13 for detail of build procedure and challenges in the imperial steel version.
Torque and RPM
- The torque of the Precious Plastic shredder is recommended at 70 rpm with a 2hp hp or higher motor. Using the Motor Torque Calculator, that comes out to about 200 Nm for full operation. Otherwise, motor will stall periodically or will be reduced to slower feedrate.
- A 1/4 hp motor can be used to obtain the required torque of 200 Nm - but it would be slow at 10 RPM. Note - there is lots of confusion on torque at the Precious Plastic forums. The fact is - a very slow motor can achieve the desired torque - it would just have to have a required geardown. For a super-low cost, high torque, but slow solution at about $30 in materials - one can consider gearing down a NEMA 23 stepper motor. 3D printed pulleys, 3D printed rubber belts, and Nylon bushings could be used - for a fully 3D printed geardown. This could recduce build cost from $1000 for the correct motor/geardown by a factor of 30.
- 50-100Nm was an educated guess, but I’ve since got my shredder working and taken some measurements. To shred bottle tops takes a maximum 20Nm, to shred the thickest plastic I could find (front panel of a domestic appliance) took 60Nm. Note that my machine is half the size of DH’s, so for that I would expect the figures to be roughly double, perhaps slightly less as the teeth are more pointed and will probably pierce with less force." - from 
- Imperial version of knives for shredder - for 1" hex shaft - https://davehakkens.nl/community/forums/topic/shredder-modifications/
- Imperial steel overall - https://davehakkens.nl/community/forums/topic/shredder-converted-to-imperial-system-inches/
Dimensions of screen are found in blueprints.
Shredder screen width is verified here based on angle - uses PDF of fabrication drawings imported into FreeCAD for tracing over
- 160 / 360 * 2 pi * r = 178 mm = 7 inches
- Actual screen - 149x180x1.5 mm = 5.9 x 7.1 x 0.06 in
- 1.5 mm is 16 ga = 0.06 inch. Use 1/4" holes.
- McMaster Carr - 
See working team meeting -
The Precious Plastic shredder is great. By using 1" hex shaft and 1" hex bearings, we avoid machining the shaft ends. Using 3D printing, a hex coupler is obtained readily, printed at 100% infill.
The advantage of using hex for the coupler is not needing keyways or set screws, though these can be used for additional holding force.
If PLA is 7250 PSI, that is 1/5 of mild steel. Thus, a plastic coupler can readily do 1000s of lbs of shaft holding force. For example, on a 1" shaft, one can likely attain 15000 in lb of hold using a coupler with 3/4" walls. It is useful to use a coupler with a metal sleeve, using 2" schedule 40 pipe, with the coupler press fit inside of the sleeve for a metal-plastic composite coupler. The plastic walls in this case would be around .5".
The entire shredder is relatively easy to build, but it is not particularly efficient for production. While parts are all cut to size, there is a sufficient number of them such that keeping track of everything requires some study. Once held together, in the imperial version, one still has to mind careful fine adjustments so when the entire sandwich is in place, there are no misaligned pieces that can cause a jam. This kind of attention to detail should be eliminated by design to allow for an unskilled build.
- 1" hex shaft - 
- 1" hex bearings only - . More specs at 1" Hex Bearing.
- 1" hex bearing block $20 - 
- 1" hex bearing block $11 -  - Your order number is 1021 - If you have any questions about your purchase, email us at email@example.com or call us at 641-572-0194.
- Motor + Geardown - 1" output shaft on geardown handles 1/2 hp at 29 RPM and 100 Nm
- 5/8" coupler - 
- Precious Plastic Imperial CAM Files - Imperial Version with 1" Hex Shaft - File:Ppshredder.zip. Source - https://davehakkens.nl/community/forums/topic/shredder-converted-to-imperial-system-inches/ . Note that the original file from Precious Plastic has hex shaft, but it is lathed on the ends to make it round for standard bearings. An easy solution to simplify coupling is to use a hex bearing, which is easy to source if you can make a 3D printed housing. That's what we did for OSE.
- Overall File in FreeCAD - export of Precious Plastic - File:Pp shredder.fcstd - shredder + hopper only.
- Bearing holder for 2.44" OD hex bearing (see BOM) - File:Hexholder.fcstd. STL - File:Hexholder.stl
- Bearing holder with motor mount - offset from body of the shredder as a 1-piece 3D printed Motor Mount with integrated bearing - File:Hexholdermotor.fcstd. STL - File:Hexholdermotor.stl
- M from 1" hex shaft to 1/2" round with keyway, - File:1hextohalf.fcstd. File:1Hextohalf.stl