Calibration Cubes are boring!

One can not remove the ball inside. People love to puzzle how you made it.

You should be able to test small bridges on top of the cube, overhangs on bottom and top of the ball and check your x, y and z dimensions. The ball may roll on top of the print if you have bed adhesion problems. 

I print that without any support, brim, …

I'll go tension my belt now, because my cube is too small.