Circularity test
The circularity test refers to a method for checking the positioning and to assess the control settings of a CNC - machine tool . The accuracy of the machine is assessed by measuring the position of the tool zero point when moving a circular contour . The various deviations of the actual contour from the target contour provide information about the errors and incorrect settings of the machine.
Measurement method
A simple method of the circularity test is the direct recording of the trajectory with the machine's own measuring systems. However, not all influences can be determined in this way. The method is mainly used when setting the K v factors (measure for the speed gain ).
Most common today is the double ballbar process (English double - double; ball - ball; bar - rod). The measuring device, which can be changed in length, is mounted on the spindle in the middle and in the center of the circle to be driven with a ball joint each and records the change in distance between the center of the spindle and the center of the circle - i.e. the radius of the circle being driven.
In both cases, the circularity test is run once with and once anti- clockwise .
The measuring method is standardized in ISO 230-4: 2005 Test rules for machine tools - Part 4: Circular shape tests for numerically controlled machine tools .
Deviations and causes
| measured circularity deviation | root cause |
|---|---|
| Peaks / deflections at quadrant transitions | Static friction (and subsequent tearing away); Backlash; Controller setting |
| Oval , inclined 45 °, in UZS the same as against UZS | the axes involved in the movement are not perpendicular to each other |
| Oval, inclined at 45 °, unequal in UZS as against UZS | different K v factors in both axes |
| Noise in opposite quadrant transitions | Stick-slip effect (e.g. due to insufficient lubrication ) in the axis where the quadrant transition is with noise; defective guides |
| complete circle noisy | Vibrations in the machine |
| complete circle wavy (cyclic) | Spindle pitch error |
| Outward hysteresis (positive radial jump at the quadrant transition) | Play in the drive train or guides |
| Inward hysteresis (negative radial jump at the quadrant transition) | Game in the guides; Hysteresis in the position measuring system ; Overcompensation of the reversal margin |
literature
- Manfred Weck , Christian Brecher : Machine tools Vol. 5: Metrological investigation and assessment . Issue 7. Springer, 2006, ISBN 3-540-32951-X . Book page on Google Books
- International standard ISO 230-4: 2005 Test rules for machine tools - Part 4: Circularity tests for numerically controlled machine tools
Web links
- Description of the measuring device with illustration
- Illustration of the measuring device with measurement curve
- Circular shape measurement in theory and practice
