Undersized prints, non round circles

Hi,

I try to set my printer properly, but I still have one big problem. Everything I print is undersized. As you can see in the attached picture, it makes something about 0.2 mm in X-axis on 20 mm (1%) and 0.5 mm (2.5%) in Y-axis! The undersize also causes that all printed circles are not round and some lines don't touch perimeter (in the first picture are gaps between the hexagon and infill and in the other picture next to the left perimeter).





I thought it is typical backslash. I checked pulleys (sharpened a place for bolt), tightened belt (I thing that it is not possible to tighten belts more, I use simple clothespin belt tensioner), calibrated axises (It could be not perpendicular).

Then, I printed a simple test angle (the second picture). I noticed, that the size error is multiplying. I got error 0.65 mm in X on 50 mm (1.3%) and 1.1 mm in Y-axis (2.2%). I have made myself sure that the X and Y axises are really perpendicular.

(Prusa Mendel i2, Sprinter, Slic3r)

Any ideas?

Calibrated your steps per mm properly for all axis and extruder?

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Realizer- One who realizes dreams by making them a reality either by possibility or by completion. Also creating or renewing hopes of dreams.
"keep in mind, even the best printer can not print with the best filament if the user is the problem." -Ohmarinus