Lyman Filament Maker Calculations

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  • The filament extruder motor of the Lyman Filament Maker has a 168 fold geardown. It has a 173 kg-cm (1730 N cm) torque! [1]. This is about half of what Precious Plastic recommends for their shredder.
  • 3000 N cm torque stepper used in the Recyclebot - $60 - [2]. That is about 300 kg cm.
  • Point: Stepper has over 2x performance to cost ratio

The torque of the Precious Plastic shredder is recommended at 70 rpm with a 1/4 hp or higher motor. Using the Motor Torque Calculator, that comes out to 2800 N cm. The stepper motor above meets this specification (for torque, but not speed).

  • Point - cheapest and quickest route to a functional but slow Precious Plastic Shredder is a geared down Nema 23 stepper above for $60, putting out as much torque as a $325 motor/geardown combo of the Precious Plastic Shredder. The $60 route is functional but slower. It does allow for industrial productivity on the scale of 100 lb/day regrind very easily - enough for 50 rolls of filament daily. This is enough for 20 pieces of 2x4 lumber

The Recyclebot motor above has 15 geardown. Say steppers go up to 1000 RPM - that is exactly about 70 RPM if there is a 15 fold geardown.

Take a regular Nema 17 with 72 oz in torque. That is about 5 kg cm. To get to 300 kg cm - need about 60 geardown.

As a first try, it is useful to go with the 15 geardown stepper above for an initial version of the shredder.

Higher Torque

  • Think of desirable torque as a human cranking on a 1 foot crank with all their weight (200 lb)
  • Thus - 200 ft lb is a desirable high torque for shredding plastic. That is about 270 Nm (27,000 N cm) - or about 10x the torque recommended above for robust operation
  • To get there, need a 9x geardown from the stepper motor above. It appears stock geardowns on nema stepper motors do not go this high.

High Torque 3D Printed Couplers

Hex shaft used in the Precious Plastic Shredder is a good idea. We can make a heavy duty plastic coupler as a plastic/metal composite structure:

  1. 3D print a 100% infill Hex Coupler with 1.5" walls, and about 4" long
  2. Embed 6 nut catchers, either 3/4" or 1/2" into the coupler to strengthen the hex-on-shaft force handling ability.
  3. Clamp down with bolts through the nut catcher to hold the shaft firmly in place

If 5000 PSI is the Tensile Strength of Plastic, ABS or PLA plastic, we can get to about 4,000 in lb (500 Nm, 50,000 Ncm) (or about 500 kg force at the 4" radius of shredder teeth) with a safety factor of about 10 using the plastic metal composite:

Other Options

  • What about using a much higher geardown Nema 23 stepper, such as 100x? This one has 25 Nm [3]. Doesn't work better than the 15x geardown - the geardown is not strong enough as the torque rating is actually slightly less for the 100x geardown.
  • Geardown for What has the answer - one spool of filament gets a 160:1 gearbox - see 3D Printed Gearbox