Gearing down the granular extruder.

It was recently questioned by Giles Bathgate as to why I don’t use the AC motor to drive the auger in the granular extruder. My first response was that I couldn’t adjust its speed and it would be difficult to gear down. However after some more time to think about the problem it became clear that the constant speed induction motor may be perfect for producing constant diameter filament. So it was decided that I would try to gear down the motor. Wanting to use as much of the existing drill press parts as possible I opted for a single, second stage reduction, located between its original two belt pulleys. However, the first task was to change the direction of the AC motor on the drill press. As it was, it would drive the plastic granules up out of the extruder rather than down. Unfortunately, as its an AC motor it wasn’t as simple as swapping the polarity of the input leads. The motor casing had to be physically opened and the start winding found. This is the winding that connects to the star capacitor and to the common. The polarity of these was then swapped before being reassembled.

The AC induction motor pulled apart to be reversed.

Next I cut a section of plywood into a rough circular shape before smoothing it down on a sander. This would become the large pulley wheel on the first stage reduction. The finishing touch was added by mounting it in the drill press before pressing a rasp against it to round it out. Then the same process was used but with a round file to add a groove where the pulley would sit.

Using a 'vertical lathe' (AKA a drill press) to rasp out a pulley wheel.

A second smaller wheel was also made in the same way. This was all then forced onto a onto a tube which held tight.

The finished pulley assembly.

Next I added a plate onto the top of the drill press that had previously had its cover removed from. Onto this plate I added two bearings sandwiched into a drilled groove between two pieces of the same plywood used for the pulley wheels. The shaft with the pulleys on them then slid into the bearings finishing the second stage reduction.

The finished product geared down by around 8.5:1.

It doesn’t look very attractive but it gets the job done. For those interested, the ‘belts’ are are just plastic cord which was joined by melting the two ends with a lighter and pressing them together. I estimate the total reduction to be around 8.5:1. So this reduces the 1400RPM 250watt AC motor down to around 165RPM. Still a little higher than I would prefer, but it will have to do. When I get some more time I will hook it up to the extruder and test it out. Then onto an automated collection spindle to coil the filament as its extruded unattended.

About Richard

I am a Materials Engineering working in the field of Magnetic Materials in Melbourne, Australia. This blog covers my personal interest in all things CNC.
This entry was posted in DIY Granular extruding and tagged , , , . Bookmark the permalink.

5 Responses to Gearing down the granular extruder.

  1. Laszlo says:


    You should encapsulate the belts, to prevent injury when it breaks. Because it will break away for sure as it wears.

    Best regards,

    • Richard says:

      I have found the plastic rope ”belt” is not particularly dangerous when it breaks, as has happened a half dozen times now. However getting part of my clothing caught in a pulley could indeed end badly. With a slight modification to the original belt cover I may be able to get it fit back on.

      Thanks for the input and for the link to your blog, it makes for an interesting read.

  2. James says:

    I would use a 3 phase induction motor with an inverter drive. Small ones can be had quite cheaply now, if you look you should be able to find a VFD for under $50 and a small 3 phase motor for less than that since there is not much demand for 3 phase stuff. Most inverters can run on single phase, though if not designed that way they have to be derated some.

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