I3 variant incorrect Z height on prints

I built an i3 variant a little over a year ago and have been using it ever since. I've noticed that my prints are now not coming out at the proper Z height. It was losing about .25mm per 10mm of height meaning I print something 10mm high and it comes out about 9.75mm. I was printing at .25mm layer height and switched to the more exact (per the calculator) .2480, but that didn't help.

The printer uses the standard M5 threaded rod for the z-axis. The motors are coupled using aluminum couplers.

Yesterday, I ran the z-axis up and down and cleaned the threaded rods, then re-oiled them and the problem actually got worse. Now I appear to be losing about .5mm per 10mm of height.

As near as I can tell, moving the carriage manually using Repetier moves the proper amount. It's kind of hard to find a consistent place where I can put my calipers and try and get measurments, but moving 10-20mm seems to be giving me accurate movement +/- about .03mm.

The real weird thing is that I still have calibration objects I printed months ago and every one I measured averages the exact height they are supposed to be or very close to it.

The prints I'm getting now are very good. I really have no complaints about the quality, it's just the height is off. I'm building a new printer and I need to print parts for it that are pretty precise but this is a problem now.

My relevent firmware settings are below. I'm running Marlin, but probably a version more than a year old.

The things I've tried so far:

• Oiled the Z threaded rods
• Since I use ABL, checked to make sure the Z probe offset hadn't drifted. Seems dead on.
• Changed the feed rate from 2 to 1 and also tried 3, but the Z motors made a lot of noise at that setting.
• Increased the stepper voltage from .41 to .55v
• Double checked the e-steps and they seem almost perfect.

//// MOVEMENT SETTINGS
#define NUM_AXIS 4 // The axis order in all axis related arrays is X, Y, Z, E
#define HOMING_FEEDRATE {50*60, 50*60, 2*60, 0}  // set the homing speeds (mm/min)

#define DEFAULT_AXIS_STEPS_PER_UNIT   {80,80,4000,724.394}
#define DEFAULT_MAX_FEEDRATE          {1000, 1000, 2, 1500}    // (mm/sec)
#define DEFAULT_MAX_ACCELERATION      {600,600,20,333}    // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, 
#define DEFAULT_ACCELERATION          600    // X, Y, Z and E max acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION  333   // X, Y, Z and E max acceleration in mm/s^2 for retracts

// The speed change that does not require acceleration (i.e. the software might assume it can be done instantaneously)
#define DEFAULT_XYJERK                15.0    // (mm/sec)
#define DEFAULT_ZJERK                 0.4     // (mm/sec)
#define DEFAULT_EJERK                 5.0    // (mm/sec)

Any ideas as to what to try next would be appreciated.

Bruce

Forgot to mention that both Cura 15.04 and Slic3r 1.2.9a give me this issue. I also upgraded to the latest Marlin hoping it might make a difference, but it didn't.

Bruce

I was loosing movement on my prusa i3v because the z couplers were some cheap plastic tube squeezing the motor shafts and the z rod. Fixed it with some printed clamps, but I can't think of why you are loosing steps, and why manual movement works correctly.. Is there some setting in firmware that is affecting the gcode (aka why all your slicers are goofing up), but not repetier because direct commands ignore it? I really have no idea, but you may want to double check all your settings, everywhere.

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Master Tinkerer

It appears the issue did have to do with the Z offset for the ABL probe. While I checked it a few days ago and it seemed to be right on, when I went to do a manual bed leveling, I found the Z offset to be off by about .42mm. It's possible my micro switch may be a little wonky because it does seem to fluctuate a little bit, but now my prints are back to the normal height.