QMT Features: November 2007
Outsourcing calibration
Many production companies and smaller machine manufacturers find it easier and more cost effective to have their machines measured and tested by calibration service providers.

He’s not an athlete, but Jürgen Emslander has, in one move, considerably improved his performance, to be way ahead of the rest of his field. He runs a calibration service company called Geo Tec Messtechnik, and has recently upgraded his laser calibration system to the new Renishaw XL-80. The new system’s portability and ease of use minimises transportation and set-up times, allowing more jobs each month.

Many production companies and smaller machine manufacturers find it easier and more cost effective to have their machines measured and tested by service providers like Geo Tec Messtechnik, based near Stuttgart in southern Germany, using laser interferometer systems.

Geo Tec engineers visit manufacturers such as Edel, who make large multi-axis CNC machining centres, to check machines after final assembly. Jürgen Emslander says, “When calibrating machines, such as Edel machining centres, the new XL-80 is small enough to use a magnetic base,which allows us to get inside the machine. This makes the job much quicker and easier, and allows measurements where they were previously a problem.

LEDs on the top of the XL-80 make set up very straightforward, we can monitor the beam strength without going back and forth to the PC.”

The laser interferometer has a certified linear measurement accuracy of ±0,5 ppm, regardless of the ambient conditions. The measuring results can be referred directly to the German calibration standard of the PTB (Physikalisch-Technische Bundesanstalt, in Braunschweig).

Test procedure

During a typical test on a machine tool, actual machine positions are compared to where the machine ‘thinks’ it is, at numerous positions over the full travel of the machine’s slides. Correction data is then calculated by the Renishaw software to compensate for inaccuracies. As well as linear position, the system can also measure the straightness of the slide guides, the flatness of the tables and the angular difference in the movement of axes in relation to each other.

The PC-based software offers the operator many advantages. The measuring cycle is largely pre-programmed with the operator merely deciding on the necessary parameters. These parameters include axes to be tested, the number of intermediate positions to be measured and the number of measuring passes. The measuring process then runs automatically: the control system moves the machine slides to the relevant positions, e.g. at 50 mm intervals, the laser interferometer measures the exact position with an accuracy of less than 1.1micron/m and stores the data.

Once data has been captured, the evaluation software included with the system allows compensation of the position differences. It shows the measured data in the form of a table and a graph for simple analysis. The software evaluates the measured data statistically, either according to its own criteria or according to the national or international standards (e.g. VDI/DGQ 3441, VDI/VDE 2617, NMTBA, and BS4556). The graphs clearly show the permitted limit values and differences, allowing the operator to assess the measured data quickly and reliably. In many cases, compensation values needed for correction or repair can be derived directly from this information, which, when fed back into the machine control, correct some or all of the inaccuracy before mechanical adjustments are made.

The evaluation software also reduces the work on compensating mechanical faults through the control system. It automatically provides the correction data required in the necessary data formats for practically every numeric control system in use today. A simple press of a button transfers the data to the compensation computer or memory of the CNC control system, greatly reducing the time needed for the entire measuring and compensation process. Following the compensation process, the operator can start another measuring cycle to check that the compensation was successful.

In one simple step, the laser interferometer software will enable calibration certificates and measurement reports with graphs and tables to be printed out, showing the state of the machines before and after compensation or repair. When regularly performing measurements and tests, the comprehensive documentation can cover the whole history of the machine. This in turn means that the client can meet demands for complete and transparent quality assurance.l


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