Hi all,

Once our printer is calibrated (M665 and M666), is it possible to adjust scaling it without modify calibration ?
in other words, my printer is very well calibrated (less than 0.05 after autolevel) but a piece of 5cm is printing 4.87

There are some M codes defined for X and Y scaling. I plan to implement these in RepRapFirmware soon. However, if your print is that far out then I suspect you have a bulld problem that is likely to cause distortion as well. Check that your rod bearing spacing is the same at both ends of each pair of rods.

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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].

A 6 point calibration doesn't include delta rod length. That's where the scale factor comes from. Reduce rod length and calibrate again.
-Olaf