<p>I have a Mini Kossel and I am going through calibration.</p>

<p>I can home carriages and find the bed with paper-test getting some Z value with <code>M114</code>. Then I run the effector almost full height <strong>up and then down</strong> - and now <strong>Z value for the bed is greater</strong>!</p>

<p>If I repeat the process I get greater and greater values in paper-test. It seems that travel per step is different moving in different directions.</p>

<p>How can I fix that? I am using RAMPS 1.4 with Marlin firmware.</p>

<p>UPDATE:</p>

<p>Z values near bed after subsequent runs of five passes of <code>G1 X100 G1 X10</code></p>

<pre><code>100% speed: 0.1 0.3 0.5 0.7 1.0

 20% speed: 0.1 0.4 0.9 1.4 4.6 6.6

300% speed: 0.0 0.7 1.0 1.3
</code></pre>

According to discussions in comments, I'm pretty sure the problem lies in too low current.
Please review
this answer
to
How do you make sure you have the right voltage on the trimpots on a A4988 stepper driver?
.
Why it happens?
If there is not enough current then motors can omit some steps as the stress is not equal while going up and down. Sometimes inertia can have higher influence than friction.
Additionally because the resistance/stress/friction on towers is different for sure then there are some issues in centering hotend.
So friction/inertia/assembly inaccuracy and current settings inaccuracy can cause such effects. If you are "on the edge" your printer may work well one day but the other day it can fail positioning.
Please refer to post mentioned above. Perform calibration and temperature test (finger test could be ok). I hope you'll manage the issue.