High-precision rotary table measurement with TriAngle electronic autocollimators
Rotary tables are used in the machine tooling industry for the precise positioning of tools and work pieces. In doing so, the production accuracy achieved by the machine tools depends directly on the positioning accuracy of the rotary tables. Rotary table and machine tool manufacturers are accordingly reliant on highly precise testing methods for production and quality assurance in order to verify the quality of their products and to offer their customers the greatest possible accuracy.
Calibration of rotary tables with conventional measurement methods (e.g. interferometrically) requires a long and complex setup period. By contrast, rotary table calibration with electronic autocollimators from the TriAngle series is quick to learn and practiced users need fewer than five minutes for setup: the actual measurement can proceed automatically without monitoring by the user.
To perform the measurement, a certified reference polygon is fastened to the rotary table that is to be measured and the electronic autocollimator is positioned. The Laser Alignment Tool performs the alignment.
Following the calibration process, the measurement is performed with software that intuitively guides the operator with its user-friendly interface. Three measurement modes are available: "manual," "time-based" and "automatic measurement." Manual mode is suitable for rotary table calibration, if it is non-motorized, or for manual preliminary characterization of a rotary table. The user has full control over the measurement procedure in this mode. However, automatic measurement offers the greatest advantage, particularly in terms of time, as the software performs fully automatic measurements. A measurement procedure is stored in the software just one time and can then be used for every rotary table to be measured. The user simply starts the measurement and can then attend to other tasks. A crucial factor is that the software can operate independently of the motor control. The program recognizes when the polygon has approached a new position, saves the associated measurement value and waits for another movement by the polygon. Complex coupling of motor control and measurement device is thereby entirely eliminated.
After completing the saved measurement plan, the software automatically displays all important parameters on a results page. For a quick overview, the customer receives a chart of the positioning accuracy as a function of the rotation angle and a table with measurement values and indicators. The output of these measurement values can be adapted to various standards, such as ISO 230 or VDI 3441. Naturally, the measurement values can also be saved for later analysis or exported in a measurement certificate.
The underlying measurement principle is simple, but robust and reliable in its informative value. Measurement of positioning accuracy is based on the difference between the target and actual angle of the rotary table. The actual positioning angle is given by the rotation and simultaneously transferred to the polygon, as this is permanently connected to the rotary table. The autocollimator's software analyzes the angle differences of the successive polygon surfaces and can calculate the positioning accuracy. In order to compensate for the influence of the polygon, its angle deviation is certified in the factory and taken into consideration in the software. At the end of a measurement, the user receives an overview of the deviation of the actual values from their specifications, which serves as a measure of the rotary table's accuracy.
The limiting factors for measurement accuracy are the accuracy of the electronic autocollimator, the accuracy at which the polygon was measured and the grid point resolution of the polygon. The grid point resolution is limited by the angular division of the polygon. In practice, this means that the number of measured values per rotary table rotation corresponds to the face count of the polygon. This limitation can be handled in several ways. First of all, it is possible to use polygons with smaller angular division. Secondly, it is also possible to begin multiple measurement series at different starting points (offset). As a third option, the software offers interpolation between grid points by means of a Fourier series.
The results of the measurement can not only be used for qualification of the rotary table, however, but also to improve positioning accuracy: the measured data can be used to determine correction factors with which a positioning error in the rotary table can be compensated for by adapting the motor control. Users thus receive a tool not only for ensuring their quality, but also for increasing it.
The screenshot above shows the measurement results of a rotary table calibration with a TriAngle UltraSpec 300-57 with a measurement accuracy of +/- 0.25 arc seconds and a 12-sided polygon with 5 arc seconds' accuracy. A high-precision rotary table with a positioning accuracy of +/- 1 arc seconds was measured. The blue lines in the image show the positioning errors on the outward and return path and the resulting average value. The yellow lines show the standard deviation at every measurement position. The rotary table had a maximum deviation of +/- 0.7 arcsec and is therefore within specifications.