The Swiss company Polymeca AG enjoys an excellent reputation in its sector for its precision of machining. Headquartered in Heerbrugg, Switzerland, the company manufactures, among other things, axis systems for theodolites with tight adjustments of up to 0.001 millimetres. "To achieve such low tolerances in automated rectification centres, we need to use advanced process metrology," explains Markus Petschnig, polymeca's director.In cooperation with the Swiss workshop of Jenoptik, Industrial Metrology Division, which is located in Peseux, Polymeca now manages to combine exceptional machining accuracy with high-efficiency series production.
Correcting axis systems for theodolites is one of the most demanding rectification tasks available. Finally, the accuracy of measuring GPS angle measurement instruments depends directly on the accuracy of the pivoting and tipping axes. Polymeca AG has been producing this type of high-precision axis systems using conventional grinding machines for more than 20 years. The traditional company has now entered a new era in terms of the accuracy and reliability of these processes. An innovative production, consisting of four fully automated precision grinding machines for indoor and outdoor machining, with the ability to rectify up to three sets per pairing operation, has paved the way for incredibly efficient and, therefore, more cost-effective production. "With our investment of four million Swiss francs in advanced production and logistics, we have been able to halve turnaround times, increase capacity by 35 per cent and reduce stock value by 20 per cent,"Petschnig said of the results of the upgrade. The most significant improvement made by the new production line is the reliability of the actual processes, obtained in mass production. This ensures a machining accuracy of up to 0.001 millimetres, even during working periods without human presence, especially on weekends. This was made possible by the new generation of MOVOLINE dimensional measurement instruments, proposed by Jenoptik's Industrial Metrology division. Directly integrated into the production facility, this technology automatically corrects trends and thus controls the process of correcting the axes, which guarantees the high level of automation.
The highest Swiss accuracy, in partnership
The pivoting and tipping axes get their final shape, at the near micrometer, in the four precision grinding centers. In order to achieve this result even during periods of work without human intervention, the machines are loaded with 960 raw parts on transport pallets. These pallets are designed to store parts after pairing. Specially developed gripping clamps capture the parts to be read and transport them to the three pre-process measuring devices specifically installed on a rotating tray. Parallel to measuring the inner diameter of the female piece, the axis groups are transferred to the grinding machine and corrected. Control during the machining operation is provided by Hommel-Movomatic measurement systems. The results of measuring each inner diameter are transmitted directly to the milling centres of the axes. This unique example of three-level rectification ensures that the axes fit exactly the three scopes of the piece. The axes are rectified in a single pass with an accuracy of up to ±0,0005 millimetres. Then the pieces are laser-marked and sent as a set.
A complete process metrology technique provided by a single source
This exceptional precision in a fully automated mass production environment is ensured by the principle of measurement before (pre-process), during (in-process) and after machining (post-process).
First, pre-process measurements determine the exact theoretical values for the machining of axes. These measurements are made using MANL-MOVOMATIC high-precision pneumatic measuring rings provided by Jenoptik's Industrial Metrology division. They make it possible to measure flexible and economical interior diameters, even at high frequency.
For in-process measurements, MANL-MOVOMATIC DM200 diameter measuring heads and MANL-MOVOMATIC DP200 positioning measuring heads are integrated into the machining medium. These instruments allow reliable measurement of external diameters, as well as passive axial positioning, even in the most difficult machining conditions. This minimizes external influences on the rectification process and ensures high accuracy of execution.
Immediately after the rectification operation, post-process measurements are made in the grinding machine using the same measuring devices. This allows for oval checks and new length measurements. This operation is used for quality control, statistical process control and "essentially for readjustment in relation to trend correction, in order to compensate for thermal influences, abrasion of grinding, etc.,"adds Petschnig. He also sees another advantage: "The combination of in- and post-process measurements using the same measurement device avoids the use of an additional final control station and increases efficiency, as no manipulation of parts is necessary."
THE HOMMEL-MOVOMATIC SZ400 control units are modern solutions that meet the highest requirements for the use and visualization of all measurement results.
A digital technique for high process reliability
"In fact, we have always used the dimensional industrial measurement of HOMMEL-ETAMIC or HOMMEL-MOVOMATIC, proposed today by Jenoptik, under the MOVOLINE product range,"says Petschnig. "With the new generation of in-process measurement systems, which we use in our production network, we have also been seduced by the modern digital technology, which is not offered by any other manufacturer." Petschnig considers the digital transmission of data to be a significant advantage, compared to analog systems, since the results of the measurement are transferred without interference, regardless of the length of the cable, even under the most difficult conditions. In addition, Fielbus field bus technology reduces the number of connecting cables required, reducing investment in the installation and maintenance of measuring components, and thus reducing the overall cost of production. In addition to digital transmission of measurement data, the new generation of measuring heads also offers a much more compact design. It is thus possible to carry out measurement configurations such as those set up in Polymeca's correction centres. Depending on the axis dimensions to be machined, it is possible to install up to five in-process measuring heads for measuring diameters and a positioning measuring head, all in a very narrow space.
"At first, no one believed that, thanks to this Jenoptik MANL-MOVOMATIC measurement principle, we were able to achieve this unmatched machining accuracy up to one micrometer in a fully automated standard production environment. However, we prove it every day, 24 hours a day, 7 days a week," Petschnig says with satisfaction.