Toothless

Irritatingly, whenever I try to make a new extruder using my existing one, it always breaks down forcing me to have to repair it, even though it is about to be made obsolete. It is as if they know!

The GM3 motor on my extruder started making a noise like a machine gun. On opening it up I found it has stripped a tooth of the final gear.



Since I moved from the 6V version to the 12V version I have been getting pretty good motor life. I do have to lock the clutch and sometimes glue the splined shaft into the last gear, but so far the gearbox has lasted well.

It is just as well my next extruder uses a stepper motor and an all metal drive chain: -



More details soon ...

Fantastic PLAstic

I have managed to get the GM3 to extrude PLA reliably with the following tweaks: -

• I locked the clutch as per Solarbotics instructions.
• I lubricated the GM3 with silicon grease as per Adrian Bowers suggestion.
• I sharpened the thread a bit with a half round file following Vik Olliver's instructions.
• I spaced the top half of the pump slightly further apart with some thicker washers. That gives the thread a gentle lead in.
• Plenty of oil on the filament.
• I throttled back the flow rate to 3/4 of the rate I normally use (π mm³/s).

The motor runs warm, but not alarmingly so. For some reason the filament is no longer rotating in the extruder.

My first attempt curled away from the MDF bed so I used 2mm balsa wood as Adrian has been using for PCL. That worked well so it's good that we can use it for both. I have yet to try it with HDPE and ABS.

I made my standard test block with 0.5mm filament extruded at 180°C (at the nozzle), layer height 0.4mm, pitch 0.6mm, fan on constantly. The results are excellent: very good filament compliance, i.e. sharp corners and flat sides, excellent layer bonding.

It is slightly more warped than my first test. That is probably because balsa is softer than MDF.



No warts on this one!

Locking the Solarbotics GM3 clutch

All the adverts for the Solarbotics GM3 gearmotor say the clutch can easily be locked but don't say how. I emailed them today and got a quick reply from Dan: -

Locking the clutch is actually very easy... we should make an effort to get the instructions on-line. All you have to do is glue the little plastic clutching mechanism in its cavity, but to ensure a good contact you should first wipe it out with rubbing alcohol to get the grease out, then score the surface up a bit with the tip of an Exacto knife and then you can use either super glue, model cement, or epoxy to lock it.

So that is what I shall try next to see if i can get it to extrude PLA without warts.

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.

Running repairs

No sooner than I had fixed my heater, the extruder motor failed!

I bodged the heater connection by putting some more solder on it. It's not a permanent solution because the solder is molten while the heater is on so it slowly oxidizes away. The last time bodged it that way it lasted six months though. It really needs a crimped connection.

The GM3 motor failed by running slowly, getting very hot and drawing lots of current. It eventually caused the protected MOSFET that is driving it to shut down. Opening it up soon revealed how it had failed :-



It has two pairs of copper brushes. Three of them have holes worn right through and the fourth has broken off. Its stub was touching the wrong side of the commutator, causing a short.

More expensive motors have carbon blocks on the end of arms which can wear down a lot further before they fail. Bigger motors have spring loaded carbon rods. The gearbox shows no sign of wear so it is let down by the cheap motor.

This motor is not really up to the job of driving the extruder. It is being severely abused by running it from 12V PWM when it is only rated at 6V. I anticipated it would not last long and ordered a spare when I bought it. I fitted that and HydraRaptor is up and running again. Curiously the second motor seems a lot quieter than the first.

At some point I think I will upgrade to a stepper motor. They are more expensive but, as long as you don't load the bearings, they last virtually forever. In the long run they probably work out cheaper and I can also dispense with the shaft encoder and the interference suppressor.

GM3 motor suppressor

I have also been asked for more details on my motor suppression circuit that I first blogged in dc-to-daylight, so here goes :-



The Solarbotics GM3 generates large amounts of RF noise from 20MHz up to at least the TV band, which is 470- 850MHz in the UK. I know this because I can see the 20 MHz on my scope and it was also affecting our TV reception.

This is the circuit I used :-



The 1nF capacitors were axial ceramics and the 10nF was a radial ceramic, mainly because that is what I had to hand. I don't know the spec of the ferrite beads because I salvaged them from an old disc drive. Here is what they look like though :-



They should be a low Q type rated for at least 1A. The current rating is not so much about how much current they can carry but about the point where the magnetic field saturates the ferrite and the inductance disappears.

We want them to have a high impedance from 20 MHz to 800 MHz. I don't have much knowledge in this area but think this is quite a big ask for a ferrite and that I fell lucky with these. To get more impedance at the low frequency end it is normal to increase the number of turns to increase the inductance which is proportional to their square. The problem with that is that it increases the capacitance, reducing the attenuation at the high frequency end.

These beads are a good compromise: they have nearly a whole turn compared to a straight through bead which is half a turn, hence four times the inductance, but the wires maintain 0.1" separation so minimizing the capacitance.

The first two 1nF capacitors are soldered to the motor case. This is easier than you might imagine because steel is such a poor conductor of heat compared to copper, although it has to be said I am using a 50W temperature controller soldering iron. I cleaned the area first with a PCB cleaning block.



This is the rest of the circuit before it was soldered on top of the two capacitor leads. Spot my mistake!



Ignore the back emf diode, it is specific to my controller and should really be part of it. I used twin screened cable with the braid grounded at the controller end and left unconnected at the filter end.