Z-axis isn't to scale even after calibration

Hello everyone,

I'm stoked as I got my reprap i3 (graber) up and running. The quality was beyond my expectations, but I noticed that holes were squished in the z-direction, causing oval shapes.
So I calibrated the z-axis: I printed a 25mm-cube and it was 19mm in the z-axis. This told me I should multiply the steps per unit x1.34 --> 4000*1.34.

I did this and started a second print of the cube, now it was 1.34*25 high (and breakable)!! So this tells me that it was previously correct. I'm baffled, because I'm back at 4000 and it's still too short but calibration don' help.

picture: [drive.google.com]

I've got a 0.5 nozzle and layer height is set at 0.4mm

Hopefully someone can help me because I need to get the baby up and reproducing haha

EDIT: I'm using repetierhost, because pronterface keeps giving me the 'w' error

Kind regards

Edited 1 time(s). Last edit at 01/30/2015 01:12PM by Llama16.

Check the motor current driving the Z axis. Also check the alignment of the mechanism- it may be binding near the bottom and causing the stepper(s) to slip. Also check connection between the drive screw(s) and the stepper(s) in the Z axis.

What error did pronterface throw?

The couplers! That I didn't think of it (probably I've been up to 4 yesterday getting it to run :p)

But yes they slip, I'll try and fix them and see what happens

Ok second question: when I home the axis, it works fine in repetier host. But when I start a print, it rams into and past the z-home axis (zmin), this was I think the reason why the flexible couplings wore out and started slipping. It also causes the calibration of the bed height to be useless. Any thoughts?

Which is strange because when I give it G28 homing manually it goes to home nicely

Edited 1 time(s). Last edit at 01/31/2015 08:33AM by Llama16.

Open the gcode and see if there is a G28 command at the beginning of the file or near it.

When you sliced the object, was it sitting at z=0 in the slicer?

Double check the Z-zero limit switch and mount and wiring to the controller board. Make sure nothing has moved.

What error was pronterface reporting? Was it related to the Z-zero limit switch?

I don't understand all your questions: Yep the G-Code has a G28 homing in it, and I first thought that was the fault, but when I manually send it G28 it goes nicely to the endstops and not ramming in them like when I start the print. WHen I start the print it just rams into the z-min endstop (and also the x-min I think but that's not so bad. The z-min is worse because it prohobits me to level the bed correctly

What do you mean slicing z=0? I already went through the config of slicer and can't really find what you mean.

There is also no error that pops up.

The z-axis is completely secure and never moves, neither does the cable.

PS: To get back to my initial problem: I haven't yet installed new couplings. But when I measure a distance on the z-axis by (say 30mm) and then measure the same distance by going to 30mm with increments of 0.1/0.01 (to simulate any slipping) It's spot on the same. So now I'm confused. When I go manually to the z-axis (with controls) it's correct, but when it prints and says 'z 30mm' (of the that layer) it's actually shorter

PICTURE: [drive.google.com]

Above cubes are identical and should be cubic. Below you can see oval shapes. lengths in x and y axis are spot on.

MY CONCLUSION: it's almost as if he prints the layers too thin. the extruder is really smearing, (making holes a bit smaller) even after the first layers (so not bed levelling), causing the extruder too skip and jam sometimes. yet I have it set to 0.5mm as extruder and 0.4mm layer height

Edited 3 time(s). Last edit at 01/31/2015 11:59AM by Llama16.

Ok I think I've completely solved the problem. Seems like both problems were connected. The homing wasn't ramming, it was skipping, but it sounded like it was ramming against the side.
I lowered the Z acceleration and put zjerk to 0, now I don't hear it anymore, the z height is correct and the layers aren't smeared anymore!

Thanks for helping me out here dentist, this is still siome virgin territory area this 3d printing

Lower your max feedrate in marlin for your z-axs.
Also lower your homing speed in marlin.

Haha, yeah I did exactly just that and it worked. Glad I found it.

Btw: another small question: With the glass on the heatbed my z endstop puts the nozzle below the heatbed. Any way to compensate that? I've heard I can just put a G1 command for the z axis in the beginning of the print in slicer, but I want to avoid switching programs and having it crash into the heatbed. Anything firmware related to help with this?