Ballbar is the fastest way to get a machine graded end to end. It is also the fastest way to talk yourself into work that does not need doing. Squareness is where most of the over-diagnosis happens.
What the plot really means
A squareness reading of, say, 30 µm/m looks alarming until you rerun the test at half feed rate and it drops to 12 µm/m. The delta is not squareness. The delta is servo lag showing up as a geometry error on the plot.
Before we quote for a scraper and a levelling job, we always run:
- Two feed rates at the same radius. If the number moves with feed, it is not geometry.
- Two radii at the same feed. If reversal spikes get worse at a smaller radius, backlash and stick-slip are in play.
- A quick axis-by-axis linearity check. Cheap to do, saves the customer the price of a full geometric alignment when the ballscrew nut is the culprit.
Ballbar reports are diagnostic. They are not a work order.
The rule we live by: no squareness correction is signed off until the number is stable across at least two feed rates and two radii. Otherwise you are chasing an artefact and the customer is paying for the chase.