Ballbar testing explained
The popularity of ballbar testing has been built on the basic simplicity of the test, quickness of use and the large amount of quantative information generated.
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.
- Connecting to your QC20-W is simple, thanks to its Bluetooth connectivity. Test set-up is quick and easy with Windows® based software guiding the operator through each step. There is even a 'part programme generator' to help you set up the corresponding programme on your machine tool.
- In many cases you will be using existing test templates. The powerful file administration features let you search and access these quickly.
- The centre pivot is positioned on the machine table and (using a setting ball provided in the QC20-W 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-W 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).
- As you can see the basic test is very quick once set up!
- 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-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.
- 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.
- Many reports can be customised and the final result used for written reports using the inbuilt 'cut and paste' facility. For further information see Ballbar 20 software.