Sunday 6 April 2008

Wear and tear

Half way thorough my evaluation of PCL the extruder's flexible drive coupling started to break up again. When I moved to ABS that was the final straw: -

The first one I made was only 2.5mm cable. This was a 3mm one from BitsFromBytes. I replaced it with some 3.2mm cable from B&Q. I drilled the hole out to 3.3mm so it is a snug fit. I also soldered it while it was held in alignment by my lathe so it is very straight.

I think the force required to bend a cable goes it with the fourth power of its diameter so this one is considerably stiffer. Possibly some of the motor torque is wasted in flexing it.

The good thing about the shaft I got from BitsFromBytes is that it solderable, so it makes it easy to replace. My original shaft was stainless steel so I had to glue the cable in with JB Weld, making it harder to replace.

My next extruder will be direct drive!

I also wore out the brushes on a second GM3 gearmotor. I replaced it with a 12V version which has to be ordered by phone from Solarbotics in Canada. It looks the same except that it has a black end cap instead of a white one. It runs a bit quieter but I don't know if it will last any longer. As you would expect the coil resistance is higher so the current through the brushes will be lower.


  1. Is there a good reason why the motor has to be on a flexible drive shaft, and not aligned with the axis of the threaded rod? The plastic being fed in is bent anyway, so a little more bend might be OK. I suppose this is what you intend for your next extruder.

  2. Yes, the original design is direct coupled and the filament bends. The offset drive was added to allow solid rods to be extruded, Wood's and Field's low melting point metals for example. I am not really interested in those at the moment. I have an idea to do the offset with a shaft that has a tapered thread on it but that makes it difficult to make without a thread cutting lathe.

  3. Hell! You've got the machining capability. Why don't you just make a universal joint and dump that cable nonsense?

  4. Mind, now that I think about it a CV joint might be better.

  5. Yes but can you get them 5mm outside diameter?

    A small pinion and a worm drive at right angles might be a simpler option.