cacked steppers? Z axis working erratically

Hi folks,
the printer, a rework using a ramps 1.4, is assembled and I'm at the fine tuning the
steppers stage. My steppers are 0.9^/2.4a per coil and they are being driven by DRV8825.

I however are having problems with the z axis. I have run the DRV8825 in full step, 1/4 and 1/32.
I have varied the DRV8825 to output between 1A to 1.8A.

Multiple problems tend to occur. The most common when homing the Z is one of the motor
will slow down compared to the other motor then catch up, leaving the X axis non-level.
Also when homing again, both motors will work at a good speed and then one motor
will stop completely.

Below is the code from my marlin build

#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0} // set the homing speeds (mm/min) normally 50*60,50*60,4*60
// default settings
#define DEFAULT_AXIS_STEPS_PER_UNIT {40,40,2000,107} // Z set to 1/4 uStep
#define DEFAULT_MAX_FEEDRATE {200, 200, 10, 25} // (mm/sec) // default 500,500,10,25
#define DEFAULT_MAX_ACCELERATION {9000,9000,50,10000}

Can anyone suggest a cause for my problems? Do I unfortunately have a couple of dodgy steppers?

many thanks,
Andrew
Oh, I bought the motors before I read the FAQs on what not to get in a stepper.

Edited 1 time(s). Last edit at 09/27/2015 09:43AM by friarfish.

If the motors are wired together and one is slowing down then it sounds like a wiring issue. Check the wires are right against the ramps board and the connectors are correct. I would also try slowing down your speeds until the problem is gone and then working your way back up to a better speed

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

You should check your x-axis belt tension. It might be too tight, causing z-axis binding at some place.
Also reduce homing feedrate to 3*60 until you´ve found the cause.
-Olaf

Hi folks,
I hope it isn't a wiring problem. Nightmare locating intermittent wiring issues.

Olaf, not sure about the x belt binding the carriage on the z shaft. When I built
the beast I replaced the x axis carriage parts with a design where the tension of
the belt is braced against x axis shaft ends. I think it was mean't to address the
very problem you raise. I don't know how good at this the parts are and I can't
remember the designer or part name.

But will give everyones suggestions a go.
Many thanks,
Andrew

Hi

Sounds indeed like something a bit strange is going on. Assuming you have the basics set up correctly I would try swapping around cable looms then motors , stepper drivers and see if you can get the problem to move as well so you can isolate what exactly is causing your problems.

It is also worth investigating the stepper moors them selves. As mass produced items they are not all "made equal" . Try measuring the coils with a multi meter. I always do this for the Z axis where I try to match 2 stepper motors that have similar resistances. This may be completely unnecessary but considering we , in the case of the ramps 1.4, are driving 2 steppers from 1 stepper driver it can only be an advantage. I also always make sure the connection cable looms are the same length despite the motors being physically at different distances from the driver.

good luck

You might also consider wiring the two Z-motors in series instead of parallel, this way you are sure that both motors will always get exactly the same current.