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Marlin + RAMBo: Unpredictable moves of the Z carriage;

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Hi folks!
I'm trying to commission a new Delta printer and am struggling with a problem wherein whatever motor attached to either of RAMBo's Z dual drivers will move the carriage at a faster rate than X,Y carriages. The printer will home normally most of the time on the G28 command. However, issuing a relative by 100 units command (G91 G1 Z-100 F1000) will result in X and Y travelling the correct distance (as measured by a ruler) with Z intermittently accelerating during the move, thus travelling more than 100 mm. From what i can tell running multiple trials is that the distance traveled every time is different on Z. Swapping and connecting a different stepper motor to either of the Z drivers moves the problem to that column.

So far I've tried different microstepping and SPU settings with very conservative homing and feed rates and reduced XY and Z jerk to 10 but was unable to solve this yet by just tweaking the settings. I tried both 1/16th and 1/4th microstepping with the same symptoms. 1/8th doesn't seem to be a configurable option with Marlin and my Rambo as Marlin seems to represent MICROSTEP8 and MICROSTEP16 using the same HIGH,HIGH combination of the pins.

The current settings I'm on are:

#define MICROSTEP_MODES {4,4,4,4,4} // [1,2,4,8,16]
#define DEFAULT_AXIS_STEPS_PER_UNIT {20, 20, 20, 760*1.1}
#define HOMING_FEEDRATE {200*3, 200*3, 200*3, 0} // set the homing speeds (mm/min)
#define DEFAULT_MAX_FEEDRATE {1500, 1500, 1500, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,9000,9000,10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.
#define DEFAULT_XYJERK 10.0 // (mm/sec)
#define DEFAULT_ZJERK 10.0 // (mm/sec) Must be same as XY for delta

Is there anything else I'm missing?

Thanks for all your help!
Val

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