This simple test helps to diagnose slack in the x and y axis. The circular holes in the middle will show deformation if there is slack in the horizontal axis. In the arcs at the wall the imperfections will show up as less smooth transition between the straight lines and the arcs.

The small arrow in the bottom left indicates the origin of the xy coordinates. This helps to find out if both directions are affected in the same manner (witch is usually not the case).

If your belt tension is ok, the slack are caused by flexing of the frame or stretching of the belt. Since the tensile modulus is constant over an wide range, higher tension of the belt will not reduce the slack. It will only increase the frequency of the ringing (witch will improve dampening, because with higher frequency the given dampening is applied more often, witch than improves the visual appearance of the problem, witch is at least nice).

To obtain not only less ringing, but also better accuracy (for exact screw holes) you wont either lighten the moving parts, enforcing the drive system or lowering print speed, acceleration and jerk.

If the stacked lines are not aligned perfectly your printer either suffer from z wobble (if there is an repeating pattern) or any part is slipping randomly around (witch I found very hard to determine).

To reveal even smaller inaccuracy it helps to scale the part down.

To diagnose ringing see my Test Cube to Analyze Ringing