Ballbar testing explained
Ballbar testing popularity has been built on the simplicity of the test, speed of use and the large amount of quantitative data generated.
How does the ballbar 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 QC20 ballbar system.
QC20 ballbar testing consists of 3 simple stages
- Connecting to your QC20 ballbar is simple, thanks to its Bluetooth connectivity. Test set-up is quick and easy with Ballbar 20 software guiding the operator through each step. The 'part programme generator' will help you set up the corresponding programme on your machine tool.
- The powerful file administration feature lets you search and access existing test templates quickly.
- The centre pivot is positioned on the machine table. Using a setting ball provided in the QC20 ballbar kit, the spindle is moved to a reference point, and the test 'zero' coordinates set.
- The spindle is moved to the test start position and the QC20 ballbar is mounted between two kinematic magnetic joints.
- A simple G02 and G03 command program is all that's required to start the test.
Data capture: 360° testing
- The 'classic' test calls for the machine tool to perform two consecutive circles; one in a clockwise direction, the other counter-clockwise.
- In practice there is an extra arc added before and after the test circle to allow for the machine accelerating and then slowing down.
- With the use of extension bars the test radius can be selected to reflect the size of the machine and the sensitivity to particular issues (e.g. large radius circles are better at highlighting machine geometry errors, smaller circles are more sensitive to servo mismatch or lag).
- Data capture is shown live on screen, so any errors or problems can be detected as the test progresses and the test stopped without wasting additional time (important if you are carrying out a large radius test with a slow feed rate).
Data capture: 220° 'partial arc' testing
Before the launch of QC20 ballbar, 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 ballbar 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 volumetric diagnostics report in Ballbar 20 software, giving you more information and quicker than with previous systems.
Data analysis and diagnosis
- The user has a choice of several report formats according to International standards (e.g. ISO, ASME) and the comprehensive Renishaw diagnostics (including volumetric analysis) with a number of different screens views and links to the help manual.
- These reports give a value for a single overall indicator of machine positioning performance, e.g. circular deviation.
- Many reports can be customised and can deliver an in depth diagnosis of a machines errors; all from a single test.
Find out more about Ballbar 20 software
- Brochure: QC20-W wireless ballbar
- Brochure: QC20-W wireless ballbar system description and specifications
- Application note: QC20-W volumetric testing using a single program
- Application note: Errors diagnosed using partial arc analysis
- Application note: Risers for partial arc tool paths
- Data sheet: Traceability chart: Ballbar - UK, USA, Japan and Germany