Test theory and practice
The popularity of ballbar testing has been built on the basic simplicity of the test, simplicity of set up, quickness of use and the large amount of useful information that the test reports give the operator.
How does the test work?
In theory if you program a CNC machine to trace out a circular path and the positioning performance of the machine was perfect then the actual circle would exactly match the programmed circle. In practice many factors in the machine geometry, control system and wear can cause the radius of the test circle and its shape to deviate from the programmed circle.
If you could accurately measure the actual circular path and compare it with the programmed path you would have a measure of the machine's accuracy. This is the basis of all telescopic ballbar testing and of the Renishaw QC20-W ballbar system.
See a complete QC20-W ballbar test animation here; including the new partial arc feature:
The QC20-W ballbar test sequence
Renishaw ballbar testing consists of 3 simple stages, Set-up, Data capture and Analysis.
Data capture: 360° testing
Data capture: 220° 'partial arc' testing
Before the launch of QC20-W, testing in planes perpendicular to the standard X-Y test plane meant using special test mounts and repositioning of the centre mount. Now you can carry out tests covering 3 orthogonal planes without moving the centre pivot.
The secret to this is the ability of the QC20-W system to carry out a restricted arc (220°) in two of the planes. This produces a modified test analysis for that arc but still produces an overall circularity value for that test.
With all three tests carried out around a single point it allows the use of the (new for Ballbar 20) volumetric diagnostics report, giving you more information and quicker than with previous systems.