Strange Z axis Error

I have a used Prusa Mendel (BotMill), that I changed over to marlin, from the unknown fiware that was not working well. I've worked out quite a few issues with your help, but I have one that is a real stumper. The Z axis only moves 80% of what is sent to it during prints. I've done the calibration several times and double checked the movement in 10mm steps from 10 to 110mm, and back, using pronter face. Each interval is within .03 of what is sent(10.01, 20.01, 30.02, etc).

When I do an actual print (slic3r-9.1), the Z axis only moves 80% of what it should. X and Y seem fine.

I even made a part that looks like stairsteps, each step being 2mm tall. Each step comes out 80% of what it should be in the Z axis. The top step ended at 7.78mm instead of the 10mm it should have been. I measured the part and the Z carriage off the platform to verify, both were right about 7.78mm. The g-code has the last Z height at 9.900.

I'm stumped where to look for the problem.... If pronterface does it right manually, and the g-code has the right height, how can the print be wrong? Seems like if the steps were wrong in the firmware, then the manual would be wrong as well......

One note: I notice that the Z axis rods spin considerably faster during prints than during manual operation. Maybe the stepper can't keep up with the command? Why would that speed be different.

Thanks for the help....

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in beautiful southern, Maryland...

Since this was purchased used, is it possible that your presumption about the thread pitch is wrong? I'd use empirical measurements to at least get you to the ballpark of "correct" operation, then back calculate the pitch. Maybe they used 1/4-16 threaded rod instead of 8mm/1.25mm pitch (fwiw, 1.25mm translates to roughly equivalent to 20 turns per inch)?

16/20 = 0.80

Did all that. Wrong step factor would be my target, except the machine works fine (100%), when you manually ask it to move using pronterface. It only does the 80% thing during prints....

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in beautiful southern, Maryland...

Pronterface uses a different Z speed than your slicer does, and the stepper motors can keep up with the pronterface speed but not with the slicer speed. They are loosing steps. You should change the max allowed Z speed in the firmware to something smaller, such that the printer can actually keep up with it.

it's this line in the config:
#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 10} // (mm/sec)

"Did all that. Wrong step factor would be my target, except the machine works fine (100%), when you manually ask it to move using pronterface. It only does the 80% thing during prints...."

Can you post your firmware settings for x, y, and z translation? Something sounds fishy if it's deriving 80% all the time.

#define MAX_STEP_FREQUENCY 40000
#define DEFAULT_AXIS_STEPS_PER_UNIT {40.254662,40.02193,3360,257.0736293}
#define DEFAULT_MAX_FEEDRATE {250, 250, 5, 45}
#define DEFAULT_MAX_ACCELERATION {4000,4000,100,10000}
#define DEFAULT_ACCELERATION 3000
#define DEFAULT_RETRACT_ACCELERATION 3000
#define DEFAULT_MINIMUMFEEDRATE 0.0
#define DEFAULT_MINTRAVELFEEDRATE 0.0
#define DEFAULT_MINSEGMENTTIME 20000
#define DEFAULT_XYJERK 20.0
#define DEFAULT_ZJERK 0.4
#define DEFAULT_EJERK 5.0

note: I have a Gen6 board, so only 1/8 steppers. makes things look too small sometimes...I think

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in beautiful southern, Maryland...

Weird, when I push all the standards through the prusa calculator, I don't get anywhere near your z-axis value.

DEFAULT_AXIS_STEPS_PER_UNIT {40.254662,40.02193,3360,257.0736293}

presuming that the last value in the triplet is your z-axis and not the extruder, for 1.25 pitch @1/8 stepping, 1.8 deg motors, (and no additional gearing), the number it churns out is 1280, which is way off what you have.

However, to be clear, these are the numbers I was interested in seeing from the marlin configuration.h file, at the beginning of the calibration section values shown presume 1/16 stepping, 1.8deg,1.25 pitch)

/** \brief Number of steps for a 1mm move in x direction. Overridden if EEPROM activated. */
#define XAXIS_STEPS_PER_MM 80
/** \brief Number of steps for a 1mm move in y direction Overridden if EEPROM activated.*/
#define YAXIS_STEPS_PER_MM 80
/** \brief Number of steps for a 1mm move in z direction Overridden if EEPROM activated.*/
#define ZAXIS_STEPS_PER_MM 2560

what's the feedrate that pronterface uses to move the z axis that you have checked to be correct? That should go into the config as the max z-feedrate

Xiando: That config started out from that calculator, then I made changes. Of the (40...,40...,3360, 257...) all but the 3360 are my adjustments from doing calibrations, more than once. I didn't mess with the max accel, or the other setting much yet. Though I did change the max feed rate to (250, 250, 5, 45), instead of (500,500,5,45). The X and Y were just going too fast.


Alagi: Didn't see the rate in pronter face before. It's set to 70 mm/min. The config file seems to be set to 5 mm/sec, right? Which would be 300 per min. So, I should change the feedrate to (70/60=1.666, seems too low) maybe 2, and see what happens.

Do you think the Zjerk and max acceleration of 100 are playing into this?

I do truly appreciate all the help. I've been to the point of standinig over the machince, ballbat in hand over head, ready to make that "final adjustment" several times....

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in beautiful southern, Maryland...

I'll admit I'm at a loss to understand. But 3360 seems a bit high a number of steps and doesn't match with any of the prusa calculator values or by hand (which honestly is pretty easy too.) even for 1/16 stepping. (~3.2K for a 1mm pitch which is the shallowest - highest resolution - of the bunch, but only at 1/16 stepping...half that at 1/8.)

It worked ! Slowing the to 2 did the trick. Part was within .03ish on each level.

Xiando;
These are the calc used to set that value
equation: ( (motor_steps_per_revolution * (1/driver_microstepping)) / leadscrew_pitch ) * (gear_ratio)

My machine looks like this;
Motor step angle: 1.8 degree
Driver microstepping: 1/8 - UStep (Gen6 board)
Leadscrew pitch: M8 - metric (1.25mm per rotation)
Gear ratio : 21 tooth (Z shaft pulleys) : 8 tooth (Motor pulley)
T5 Belts

so for my machine ==> ( ( (360/1.8) * (1/ (1/8)) )/1.25) * (21 / 8) = ??
( ( (200) * (6.4) * (2.625) = 3360 so Steps per mm value is 3360

and like I was saying, it works perect when you use the manual mode in pronterface, which led me to believe 3360 was right.

Thanks a bunch....onto the next issue....

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in beautiful southern, Maryland...

Everyone has the odd bad print or failure that results in something interesting or frustrating depending on how urgently you needed the part...

Ah...gear ratio...understood. I didn't realize you were running your screws with a geared drive.

"and like I was saying, it works perfectly when you use the manual mode in pronterface, which led me to believe 3360 was right. "

Sorry for the blather. and thanks for your patience