Sunday 26 April 2009

Tiny stepper torques big

Having calculated that the tiny stepper and GM17 gearbox combination should be able to drive a pinch wheel, I made a lash up to test the theory.

When you have a 3D printer "lash up" is probably not the right term as quite sophisticated parts can be made easily.

Here it is pulling a spring balance with a piece of HDPE filament.

It got to 10 Kg and then the coupling from the GM17 to the 4mm shaft of the pinch wheel let go.

Not surprising given the torque involved and the fact that it was made with 25% fill. I made it again with 100% fill. I can't remember the last time I made a solid part.

It is coupled to the shaft with a hexagonal steel insert drilled out to 4mm and tapped M3 for a set screw onto a flat on the shaft.

With the 100% fill coupler it easily pulled the scale to the end, i.e 12.5Kg. The motor was powered from 8V (to stop it getting too hot) and stepped at 200pps. With a step angle of 15°, the GM17 default gear ratio of 228:1 and a 13mm pinch wheel that gives a feed rate of: -
200 × 15 / 360 × 1 / 228 × 13 × π = 1.5 mm/s.
That would give an output rate of 54mm of 0.5mm filament per second. I think that is comparable to the rates Adrian Bowyer has reported from a NEMA17, but it only weighs about 60g whereas a NEMA17 is about 200g. There are a lot more parts to wear out though, so a NEMA17 may be a better option. Darwin can easily throw 200g about and HydraRaptor is moving table, so the head weight has little relevance.

I have some NEMA17's arriving this week. I tried one from an old disc drive but it didn't have much torque. I don't know if that was because it had aged in the 20+ years I have had it or whether modern motors are much better.

Wednesday 22 April 2009

GM17 stepper hack

I have thought for some time that the best thing to drive an extruder with would be a small stepper with a gearbox. The reason being is that a stepper motor has close to zero efficiency when moving slowly. Power is speed multiplied by torque, so as speed increases the efficiency increases until the torque falls away due to inductance. A gearbox allows a much smaller stepper to be used because it can be run faster producing more power.

I had a look for steppers with gearboxes, but they seem to be ridiculously expensive. An alternative idea was to replace the DC motor in a gear motor with a small stepper. I couldn't find one with the correct ratio though until Solarbotics started selling replacement gears for the GM17. They allow the standard ratio of 1:228 to be changed to 1:104 or 1:51.

That makes the GM17 very flexible as they also do a magnetic shaft encoder with an integral H-bridge driver. Great for robotics, but it seems a bit under powered for an extruder.

The motor is about the same size, and has the same shaft, as the tiny steppers I got from Jameco for my first attempt at an alternative Z-axis.

I cut away the plastic cylinder that holds the motor and RepRapped an adapter flange to mount the stepper.

Here it is assembled: -

The small pinion gear is a push fit on the motor shaft, but I found that with the higher torque from the stepper I had to glue it on.

I can run the stepper up to 1000 steps / second in full step mode, with a 12V constant voltage bipolar drive. The step angle is 15° so that is 2500 RPM! It has very little torque at that speed, but it gets multiplied by the gear ratio of course.

At lower speeds the current increases and the motor gets way too hot at 12V, so it needs to be driven from a constant current drive. That is what I was intending to use anyway.

Jameco state the holding torque as 140, so I have calculated the torque after the gearbox, assuming no losses as: -
Ratio Max Speed Max Torque
51 49 RPM 0.7 Nm
104 24 RPM 1.4 Nm
228 11 RPM 3.1 Nm

It seems remarkably high as NEMA23 steppers are only about 1 Nm. Note that the max speed is for about zero torque and the max torque is for about zero speed.

I attached it to a screw drive extruder and managed to extrude ABS at a rate equivalent to 0.5mm @ 19 mm/s with a step rate of 800 pps using the 1:51 gears.

So similar performance to a GM3 with these advantages: -
  • No brushes to wear out.
  • No shaft encoder and PID software.
  • No RFI suppressor.
  • Only needs step and direction pins on the controlling micro rather than two or three H-bridge controls and two quadrature inputs.
  • The output shaft and final gear are one piece, whereas on the GM3 the plastic shaft is on a metal splined shaft that can slip.
  • The clutch is one gear back from the output, so gives higher torque before slipping.
The interesting thing is that the projected torque figures indicate that it would be able to do a pinch wheel extruder with its original gear set. I will give that a go next.

I think the cost is about the same as a NEMA17. The advantage is it is smaller and lighter, the disadvantage is it would need separate bearings and a coupler. The NEMA17 will go a lot faster, but has less torque.

Tuesday 14 April 2009


Khiraly asked me to explain how I manage to put a thread on stainless steel, so here goes.

Aluminium and brass are fairly easy to thread, but stainless steel is very tough. In order to make it easier you need to use a split die and a holder designed for one.

By tightening the middle pointed screw you can force the die to spread and increase the diameter of the thread a little. That allows you to make a first pass that doesn't cut as deep, so does not require as much force. By loosening the middle screw and tightening the outer ones you can reduce the thread diameter and make a second pass.

Another thing that makes it easier is to use cutting compound to lubricate it. I use Trefolex on Adrian Bowyer's recommendation. It is a sort of green lardy gunk.

To start off you need to align the rod or tube that you are threading orthogonally to the plane of the die. The easiest way to do this is with a lathe. You put the work piece in the headstock chuck and mount the die in a die holder that slides along a bar held in the tailstock.

You then turn the chuck with one hand and the die holder with the other. I use the handy chuck grip that I RepRapped, but a chuck key can be used to turn the chuck in 1/3 turn increments.

You need to go about half a turn forward and then one third of a turn backwards to break the chips off. If you don't it may jam.

When you start you need to feed the die against the workpiece with some force, but once the thread is started it feeds itself.

It is unlikely the chuck will have enough grip for cutting a stainless steel thread from scratch. You may have to file some flats on the stock.

If you don't have a lathe, the next best thing is to put the workpiece in the chuck of a drill press and put the hand die holder flat on the bed. Let the weight of the head press the work into the die and turn the chuck by hand. Once started you can put the work in a vice and spin the die holder.

Using a die to extend the thread on a hex head bolt is much easier because you start on the existing thread and you can hold the head in a chuck or a vice.