Help with calibration steps per mm x y z on kossel mini

I'm hoping someone can advise me here.

My kossel is printing well for the most part but I'm trying to figure out how to fix one last problem.

If I print a 20mm cube as a test the end result measures

x 20.01mm
y 20.02mm
z 21.75mm

Steps per mm is currently set at 100 calculated based on T2 belt and 16 tooth

So my question is:

1. Do I adjust steps per mm to correct this Z error? ( bonus points if you can explain why the calculated number would be off by this much )
2. If so, do I adjust x y and z steps per mm to be the same?
3. If 2 is correct, I presume once the z size is printing correctly I'll need to adjust rod length settings to bring x and y back into the correct dimension?

Based on the Z error I calculate I should change my steps per mm from 100 ( calculated based on t2 and 16 tooth ) to 91.95 based on the measured final print size.
I'm just looking for confirmation on my thinking here before I start going through re calibrating the delta using the new steps per mm. Still not sure I understand why the calculated value would be off by such a big amount.

Anyway, any help or comments would be greatly appreciated.

1. Yes. Could it be that the belts are a little stretched, or out of tolerance for some other reason? Try measuring the distance over 100 teeth.

2. Yes.

3. Yes, also adjust delta radius by the same proportion as you adjust rod length in order to keep the printing plane flat.

Edited 2 time(s). Last edit at 02/13/2015 02:56PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks DC42, I'll give it a shot this evening and post the results here.

Well, I adjusted the steps per mm to the calculated value and printed a 20mm test cube.

x = 20.02 mm
y = 20.03 mm
z - 20.01 mm


So I guess it worked as planned. Also, no adjustment of rod length was needed. The X and Y dimensions remained perfect, only the z dimensions changed with the change in steps per mm.


Thanks DC42.