Serious Z axis problem

Hi folks,

I have just completed the build of a standard Prusa and I am trying to calibrate it.

Although I quote one example below the same has applied to every print I have tried.

Using the 0.5mm thin wall example file everything starts out wonderfully while printing the raft and the first couple of layers. However after a couple of layers it becomes apparent that the extruder is starting to catch on the previous layer. Leaving it to run for a few more layers it is then obvious that that the extruder is just ploughing through the existing print.

The X and Y dimensions are 23mm x 24mm but this is unreliable because of the bulging caused by the extruder.

I decided to check the movement of the Z axis by using a clock micrometer. Asking Prontertface to move the Z axis by 1mm (+ or -, the result is the same) results in a movement of about 0.48mm. The backlash is <= 0.05mm so for the sake of argument I will call the actual movement 0.5mm or exactly half of that requested. Trying a 10mm move results in an actual 5mm movement. Moving half the amount it should between layers would seem to be consistent with what I am seeing.

The Z axis is level and its movement appears to be smooth with no slipping. I have checked the M8 thread on the bar using a thread gauge and this is correct. The bars run true and are not warped.

The Electronics are pre-assembled Sanguinololu with the Sprinter firmware as loaded by the seller - The firmware has not been altered in any way. I am trying to print through Pronterface with Skienforge download 44_reprap_python_beanshell.zip (the latest as far as I can tell) and the only alterations I have made to this are some small tweaks to temperature and extruder feedrate.

As a very new newbie it seems to me that there should be a parameter I can change in either Sprinter or Skienforge to correct this but despite many happy hours reading wikis, forums and blogs I can't see anything that jumps out. I am a bit nervous about hacking Sprinter as I don't really know what I am doing but accept that might be the only answer. I'm a bit worried about bricking the whole lot.

I haven't tried the reprap host software as it fails to connect to the printer on my setup (64bit Win 7).

I hope there is a simple solution somebody can point me towards.

You probably have to adjust your Z_STEPS_PER_MM firmware value.

Or you are trying to move the Z-axis too quickly and it is losing steps.
What speed to you have set in Pronterface?
I have XY = 3000 mm/min, Z = 50 mm/min.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

DeuxVis Wrote:
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> You probably have to adjust your Z_STEPS_PER_MM
> firmware value.


From the Configuration.h file

//// Calibration variables
// X, Y, Z, E steps per unit - Metric Prusa Mendel with Wade extruder:
float axis_steps_per_unit[] = {80, 80, 3200/1.25,700};

Am I correct in thinking this is the line that needs changing and that the values should be {80, 80, 6400/1.25,700} to double the Z axis movement? I am using a wades extruder.

z steps per unit of 6400/1.25 means that you have a 1/32 step motor on your z-axis with a lead screw pitch of 1.25 mm per thread. I'm sure that the Pololu can't do 1/32 steps and can only go to as small as 1/16 steps. As Bob suggested, check to make sure that you're not missing steps. You might need to increase the the trimpot settings on the Pololu.

You also need to calibrate your extruder by measuring the actual length of feed that it pulls and then changing your e steps per mm in the firmware or entering the ratio of actual/entered length into SF 40+ parameter called "Filament Packing Density Ratio". If you calibrate by changing your firmware, them set the FPDR in SF to 1.

Quote

z steps per unit of 6400/1.25 means that you have a 1/32 step motor on your z-axis

Or more likely a 0.9 degree per step motor.

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[www.hydraraptor.blogspot.com]

Hmm, I just assumed that he had the same motors on the z, y an z axes.