Sunday, December 21, 2008


Non-captive Gearmotor

There was some discussion in this post regarding hacking a GM3 into a non-captive motor. Partly out of curiosity but mostly out of bloody-minded stubbornness, I decided to give it a shot. I couldn't find a spare GM3 but I had a GM17 lying around. It, like the GM3, has a coaxial pinion on its shaft.

Turns out it's possible to drill right through both. A 1/8" bit makes a hole big enough for an M3 screw but small enough not to ruin the pinion. I took apart the motor, removed the steel axle and drilled out the gears.

I put an M3 screw in place of the axle, glued an M3 nut onto the shaft and reassembled the motor.

I doubt the glue would take much torque. Need a much more solid connection. The M3 screw ends up acting as an axle for two gears rotating at different speeds. No idea what kind of wear that will produce.

On the other hand, the screw does move back and forth just like you'd expect, as long as you keep it from rotating.

UPDATE: Here's a shot of the same motor pulling 3mm ABS directly. The GM17 doesn't seem all that powerful but it was able to pull the filament fairly well. Oddly enough, it was relatively easy to keep the filament from turning, just by holding it in a gentle 90 degree bend (say, 15cm radius).

Ah cool! I was wondering if there were any Solarbotic equivalents which would accomodate the hack. Nice. So d'you think it would be able to survive the trials and tribulations of an extruder?
Hard to say. I tried feeding 3mm filament into it directly and it did fairly well. I've updated the blog entry with a new image.

Also, I found a spare GM3 lying around. Tried hacking it but it doesn't work nearly as well.
For completeness, from Steve:

> I measured the tangential force on the filament and got 25g at 20cm from the shaft, so 500g-cm. That's just to overcome friction and cut the thread. I'd say a gear reduction would be necessary for this to do actual extrusion.
For completeness, from Steve:

> I'm using 3mm ABS from RRRF. Not sure where it came from prior to that. It's about 2.99mm, give or take. The output of the hacked motor is a threaded piece that's consistently 2.90mm O.D.
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