How The Ball Bar Checks the Squareness of a C.M.M.

The first step is to incline the ball Bar at about 45 degrees.

Place the Heavy Duty Stand holding the Ball Bar on the C.M.M. table, with the Ball Bar pointing at right angles to the X axis.
Measure the Ball Bar. Reverse the Ball Bar about 180 degrees and measure it again.
The difference in the two measurements is the squareness error of the Z axis to the X axis.
Place the inclined Ball Bar at right angles to the Y axis.
Measure the Ball Bar.
Reverse the position of the Ball Bar about 180 degrees and measure it again.
The difference in these two measurements is the squareness error of the Z axis to the Y axis.
Place the Ball Bar horizontal and point it along one of the body diagonals. Measure the Ball Bar.
Point the Horizontal Ball Bar along the other body diagonal and measure it.
You have just measured the squareness of the X axis to the Y axis.

A full Ball Bar setup, for a medium C.M.M. costs less than one thousand dollars!