Hobbed stepper shaft filament drive
May 25, 2011 09:17AM
I did a little experiment hobbing the shaft of a stepper motor for driving filament.

The stepper is the not so great NEMA17 motor that Sparkfun sells and the shaft is hobbed with a M4 tap from its 5m size down to 2.75mm. The basic idea is to give the filament the least amount of lever arm as possible against the shaft. It seems to work at least in concept. I will stick it in my cupcake and see how it holds up.

The filament tensioner is a Wade's style 4 bolt + springs pushing a bearing against the hobbed shaft. Instead of mounting the other side statically to the motor I clamped it with bearings to the shaft itself. The tensioner is prevented from rotating by the filament and the top of the hotend.

The hotend is a nophead style resistor heater block with brass block and a brass nozzle made from rod. The filament tube is 1/8 galvanized pipe nipple with 5/16 PTFE rod inserted into it. The PTFE rod is tapered in two steps. At the nozzle down to 1/4" and again at the tip so a piece of tiny (1/8" long) copper tubing can be placed over the tip. The idea is to reduce the insulating value of the PTFE right at the tip. This could be done with drilling the nozzle, but was easier for me to get the effect this way.

btw. What exactly is the point of the common use of brass instead of aluminum for heater blocks and nozzle? Brass really is not that great a heat conductor.

Any comments are much appreciated.
Re: Hobbed stepper shaft filament drive
May 25, 2011 11:19AM
I don't know why people use brass instead of AL. It is more expensive, a lot heaver, has a higher specific heat capacity and much lower conductivity.

I helped a friend commission his machine this weekend. Spent 8 hours testing and making measurements. The first print was perfect, no trial an error at all, just measurements and maths.

Anyway he used an extruder from Stoffel15, which worked very well, but I did notice that the temperature swing with bang-bang control was about 5C, with a brass heater block, whereas with exactly the same electronics and firmware I get about 2C swing with aluminium.

Re: Hobbed stepper shaft filament drive
May 26, 2011 02:25AM
the only reason is, brass is mutch easier to work with then with Alu. 3C diffrent... hmm... then i must try to make them in ALU winking smiley

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