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How to calibrate X,Y and Z-STEPS?

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Hi,

I am trying to get dimensionally accurate printouts with my factory-fresh UM3. During set-up, I went through the usual calibration procedures - build plate levelling with the "calibration card", and running the "Z and XY offset calibration" with the supplied "XY calibration sheet".

My prints look visually good. Print bed adhesion is excellent. I thought everything would be fine.

But then I noticed that my UM had severe issues reproducing technical models accurately. This affects dozends of pre-designed models from the InMoov project like

 

 

These parts need to be interlocked after printing, but they do not fit. The models should be perfeclty fine as they have been printed and built dozends of times in the past years. Some sanding and deburring is required, obviously, but I would have to grind down almost half a millimeter of material to make these parts fit. Gael (the InMoov designer) recommends to calibrate the printer.

To do so I downloaded a "STEP Calibration Piece (X, Y and Z axis)" from Thingiverse (https://www.thingiverse.com/thing:195604)

The calibration object is 100mm on X and Y axis and 50mm on the Z-Axis.

Measured with calipers, my test piece is 100,38 × 100,33 × 50,16 mm, so there actually is an offset.

The instructions say:

"You can then calibrate your STEPS using this formula:

X,Y-Axis: 100 / [measured length in mm] [current STEPS]

Z-Axis: X,Y-Axis: 50 / [measured height in mm] [current STEPS]"

Where do I find the "current STEPS" value, and where do I enter the corrected values?

Thanks!

Edited by Guest

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You cannot set "current STEPS" without compiling your own firmware. But the good news is: you don't need to. What this person has done is very strange IMHO. The XY values should be 80. He is adjusting an offset with a gain. I bet that when he prints a calibration unit half the size, he would arrive at different values.

The XY resolution is determined by the stepper motors and the belts/pulleys. They are exactly 12.5microns/step (thus 80 steps/mm).

The 0.3-0.4mm offset that you measure comes from inaccuracies in the printing process. One example is the "elephant foot".

Another example is that holes are always printed a little smaller than designed in CAD. Due to various causes (search the forum).

Do you use Ultimaker filament? If not, you might need to tweak the print settings to get optimal results.

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Other source of imprecision while using default UM3 profiles is that because it uses so low jerk, it creates extra amounts of extrusion due the slow speed when changing directions.

You can edit that on advanced mode if you unhide the options bu default.

More about this

https://ultimaker.com/en/community/50090-overextrusion-on-edges

Also to change steps values on UM3 you don't need to build your firmware. You would need to activate Dev Mode, logging by ssh to your machine IP (ssh root@ipaddress password ultimakwr) and then find the jedi.json on the share/usr/griffin/griffin/machines) and then edit the text file using vi editor). Ofc all this if done wrong can brick your machine (not super dangerous but just a warning about changing values).

Edited by Guest

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Also, a better way to calibrate stuff is to check the final siE, then use a rule of three and apply a negative or positive Horizontal Expansion on cura (hidden option). That should give something more easier to control.

Edited by Guest
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Hi,

thanks for your reply. Sorry, I have no idea what "the final siE" or "a rule of three" is.

If my UM3+ prints with an offset of approximately 0.3 mm on the X and Y axis, can I just enter "-0.3 mm" in the "Horizontal Expansion" setting to get accurately sized prints?

I really do not understand why this problem even happens with such an expensive device like the UM3. InMoov is designed to be printed even on cheap 3D printers, and nobody else seems to have to do these hacks, except for very few people with totally uncalibrated DIY 3D printer kits…

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Hi,

thanks for your reply. Sorry, I have no idea what "the final siE" or "a rule of three" is.

If my UM3+ prints with an offset of approximately 0.3 mm on the X and Y axis, can I just enter "-0.3 mm" in the "Horizontal Expansion" setting to get accurately sized prints?

I really do not understand why this problem even happens with such an expensive device like the UM3. InMoov is designed to be printed even on cheap 3D printers, and nobody else seems to have to do these hacks, except for very few people with totally uncalibrated DIY 3D printer kits…

The issue, except the corners where slow yerk affects how filament is printed (low yerk low ringing, high yerk more accurate but ringing effect on print), the issue is how the filament cools down/expands while being printed.

About Horizontal expansion, afaik works exactly like that. More info, and also a very good guide about hidden options in cura and how to optimize stuff:

https://ultimaker.com/en/resources/21932-mastering-cura

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I tried the Cura setting for “Shell - Horizontal Expansion”. For a 0.4 mm nozzle size and a layer height of 0.2 mm ("Fast" profile) I guesstimated an offset of -0.2 mm.

With this compensation, the InMoov parts now come just right out of the printer; I do not even have to sand them down anymore.

Now I just have to grind down all the parts I already have printed in the past weeks, *sigh*.

Thanks for pointing me to the "Hoizontal Expansion" setting!

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