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Posted · SSH & Developer Mode

We have been using the U5 for a while now.  I recently got around to printing out a 20mm calibration cube to check the accuracy.  I found that it was off slightly my cube turned out to be:

20.34mm x 20.25mm x 19.93mm.  I simply wanted a way to tweak the steps/mm instead of using the fancy offset calibration grid (which is highly subjective - if leaving the ladder grid on the glass bed, there is parallax viewing error).  What I did was measure the actual cube with calipers and wanted to change the steps/mm value to correct.


I found that the firmware can be unpacked easily with 7zip so that I could look around (it is a squashfs unix filesystem).  I eventually found the steps/mm values in a file here:




New X value = 144*20/20.34 = 141.6

New Y value = 144*20/20.25 = 142.2

New Z value = 400*20/19.93 = 401.4 (although I may leave this at 400 because I can't remember at what layer height I printed the cube)


After a day of playing around with it, I enabled SSH in developer mode and found how to log in (there are 2 accounts which have very different capabilities).  I used the "ultimaker" account and the list/set commands to alter these 3 parameters:



set steps_per_mm_x = 141.6
set steps_per_mm_y = 142.2
set steps_per_mm_z = 401.4


You have to be careful where you measure a part that you aren't measuring on any highpoints because of printing artifacts (ie. over extrusion, ringing near sharp corners, squish adhesion to the bed on the first layer).


I will poke around with the "root" account where I can make a copy of the original file and alter the defaults directly.  I haven't yet found all the linux commands that are available because this OS is slimmed down a lot.  It looks like vi is an available editor.

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Posted · SSH & Developer Mode

Good stuff!


1) Be very careful when modifying json files.  If you get one semicolon out of place the printer can fail during boot and never get far enough to launch sshd.  So it's basically bricked.  But you can still get into the root account through a serial terminal using the olimex cable.  So I strongly recommend you spend the $10/10 euros or so and just get the thing so you have it just in case.  It would suck having to wait a few days for it.  In europe you can get it directly from them.  In USA I sell it in my store:



2) Did you print in high accuracy mode when you did the test?  I forget what the profile is called excactly but Cura has these high accuracy profiles now.


3) I wouldn't print a cube because of exactly what you said - the corners could be larger - so print an octogon or a cube with beveled corners.


4) It's probably best not to mess with steps/mm as these are probably much more accurate than you think.  Any error probably has more to do with things like material, temperature, nozzle diameter, and a few dozen other cura settings.

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Posted (edited) · SSH & Developer Mode

I redesigned the calibration cube and reset the step/mm parameters back to defaults and am printing another cube as a baseline.  Here is the cube I made which added features to mitigate the printing artifacts from getting in the way of making measurements.  The axis indicators I shrunk and put them in the corners so that there is plenty of space for calipers.  Also I added a chamfer to the edges on the build plate to prevent squish errors from adhesion and added a fillet to the vertical edges to prevent inertial overshoot.


BTW, that serial cable seems like cheap insurance in case something goes wrong.  Do you have any details on how it is used?  I poked around www.olimex.com/ but didn't really know what to look for.  Also, where does this attach to on the U5?


The STL is attached and should orient itself correctly in all slicers (I verified with Cura 4.4 and Makerbot Print)

Calibration Cube Orientation.png

MC Calibration Cube.STL

Edited by sparkix
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    Posted · SSH & Developer Mode

    Another thing to note... when using the ulitmaker account to login and change values.  The changes are temporary and do not survive a power cycle.  To make permanent changes, the values must be written to actual .json files.

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    • 1 year later...
    Posted (edited) · SSH & Developer Mode

    Hello, i've read your topic with high interest. Steps/mm are effectively editable in um3.json, but do you know where are the Z speed limit and the jerk/acceleration ? I have done a lot of research and nobody talk about this very important topic. It could probably solve problems like this

    Sorry for my bad English.. 


    Edited by CyrilAC
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