Wobbly Stepper Debug

Just got this back from Adrian:

The ones we specify have 4 coils each with a resistance of about 5 ohms
(which makes 10 ohms when you wire them in series in pairs to set them
up as bipolar motors as per the instructions). They do 400 steps per
revolution. They are expensive, but they are good. In particular they
can be driven fast at high torque without losing steps. The default
setting on the sliders drives them fast.

Wobbling/weak motors is usually a sign of the wires being connected in
the wrong order, or not pushing the motor power up high enough (the %
figures in the preferences), or trying to drive them too fast.

Sequence of experiments to do:

1. Slow the speed down with the slider.
2. Put the power up to 100% (make sure there's a heatsink on the driver chip).
3. Swap wires:

Assuming the motor you're using has two 10 ohm (or more) coils, you
can't break anything by swapping the four wires around to get the order
right by experimentation - slow them right down with the Java window
slider, and also use the single-step buttons to really see what's going
on.

If people use 200-steps-per-rev bipolar motors with two coils of about
10 ohms each they should be a lot cheaper and should still work. But
the precision of the machine will only be 0.2 mm not 0.1 mm.

If people can post pix of their setups and e-mail me the URL (no deluge
of attached jpgs please...) I'll see if I can tell what's wrong.
Colours of all the wires will help me see.

Since it wasn't 100% obvious to at least one Reprapper I helped with this on IRC very recently:

The Preferences dialog calls the power settings XAxisTorque, YAxisTorque and ZAxisTorque. These are the "power" settings Adrian is referring to, so do not look for the word "power" in the Preferences dialog! BTW, increasing this apparently did the trick. One formerly wobbly stepper now turns :-)

And one followup question: if a set of Reprap RP-able parts are made on a Repstrap with "only" 0.2mm precision, will the resulting Reprap built from those parts still be a "real Reprap Darwin", and still be capable of the original 0.1mm precision if built with better (but expensive) 400step/rev motors?

In other words, does using cheaper 200step/rev motors lose *your* one robot some precision, or does it lose "your robot and all robots and extruders it makes the parts for" that precision? I'm guessing the former, but would appreciate confirmation from an expert.

Finally, if someone knows of a good source of the official Reprap steppers in the USA, please speak up -- that would be good info for the Wiki.

Thanks,

Jonathan

In the Darwin, how much of the precision is dependent on the motors, and how much is dependent on the gears/rods, etc? If we wanted more precision, could we get it by using a fancier driver to run our stepper motor at 1/8 step, or more? It might not matter how precise the motor is, if the gears and rods to which it is attached do not have the same precision.

Could a cheap 200 step/revolution motor be driven at 1/2 or more steps and equal or exceed the performance/accuracy of the more expensive motors, and save money?

Lastly, what are the required specifications of the motors such as holding torque, shaft diameter, etc, so that we can pick alternate motors that will work with the Darwin design?

Thanks,

-Samuel