No luck – … yet

As you might know, I’m in the process of building a Mendel.

There are two reasons why it’s not printing yet:

On the software side, I still didn’t manage to bring the Reprap software to life on my Mac. After tinkering around with some Java frameworks, settings and class paths, I was eventually able to launch the host software. But somehow I wasn’t able to establish a connection between the host software and the RepRap firmware on the Mendel.

Right now, I switched back to the MakerBot firmware, in order to be able to use ReplicatorG to test drive Mendel’s mechanics. When I solved the remaining problems on the hardware side (see below), I’ll probably try again to use Mendel’s native RepRap firmware/software.

On the hardware side, I still don’t have a high torque NEMA 17 motor on my hands. However, I have a NEMA 17 stepper laying around. It’s the same cheap, low torque type, I’m using to drive the X,Y and Z axis.

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This stepper is rated to provide a holding torque of only 0.23Nm, which is obviously too low for using it in a RepRap Thermoplast Extruder v2.0.

I tried it anyway: :)

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To grip the filament, I put a treaded pulley on the motor’s shaft. I built it on my lathe and tried keep the shaft’s diameter as low as possible: the pulley only adds 1mm to the motor’s shaft diameter, so it now has effectively 6mm diameter.

According to the rated torque, theoretically this motor should be able to pull the filament with about 76N (i.e. around 7.5kg on the balance).

But when I tried this setup, the motor started to skip steps at about 2kg on the balance (i.e. ~ 20N). That’s way to low, even for PCL filament.

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I guess, the main problem is, that I don’t use a real stepper driver to power the motor, but the RepRap/MakerBot extruder controller with the “2x DC motor drivers  -> stepper motor hack”. So the stepper motor probably doesn’t get enough current to produce it’s rated torque.

The obvious solution for this problem would be to buy a high torque stepper motor and/or use a well dimensioned stepper driver. Well, maybe I’ll eventually end up doing exactly this. But then again, I had this idea some time ago, when I lathed my first threaded pulley, to use one of these threaded pulleys as part of a worm gear.

I decided to first give this idea a shot…

Here’s the design, I came up with:

I drilled a 5mm hole in a piece of M8 threaded rod and added a set-screw, so it can be attached to the shaft of the stepper motor:

The central part of the gear box is the following part. It’s made out of brass and has two threaded grooves, one M8 (for the worm gear) and another one (M3) to grip the 3mm filament:

6 comments

  1. Hello. Great to see you making something I’ve been thinking about for a while: a machine screw based worm gearbox. That is disappointing perfromance.

    Are you using any grease on the worm? If not, then that would probably help a lot.

    Good luck with it.

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