[amadeusw]

I think of rotation about any axis as if I inserted into the object a skewer along that axis, and rotated the skewer. For example, rotating an object laying on the desk is achieved by rotating about the Z axis, since the "skewer" is pointing up along Z axis.

This works well in OpenSCAD and I just saw that Slic3r displays a handy arrow in the 3D preview when you focus the text box that controls the rotation