Precision check

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Precision check – The truth lies in the Chemnitz measuring centre

When it comes to high-precision parts, quality assurance is the moment of truth – it depends crucially on meticulously planned measurement with seamless documentation. For six years, the Chemnitz measuring centre, which works not only internally for the Starrag Group but also for external customers, has played an important role in terms of μm precision. A highlight is a simultaneous precision check of several components in the ghost shift.

”Measurement is knowledge,” Werner von Siemens recognised as early as the 19th century. This truism has not changed over 100 years later. Today, components can be calculated and designed very reliably and accurately with the help of software, and their manufacturing process can be simulated. But at the end of the day, high-precision workpieces still have to undergo final inspection on a measuring machine.

Measuring centre: High precision even for XXL formats

This task has been taken over by the Chemnitz measuring centre since May 2016. Experienced experts not only check the dimensional accuracy of small and medium-sized workpieces with four 3D measuring machines and programming workstations from ZEISS, but they also measure gigantic major parts weighing up to 12 tons.

”Our precision measuring room meets the strict provisions of German VDI regulation 2627,” proudly explains Jens Knöfel, Head of Quality Management (QM). ”This is the highest standardised level there is. Our trademark is the accuracy we achieve with this measuring centre.” And they are impressive – for example, the ZEISS MICURA measures components with an edge length of 400 mm to an accuracy of 0.7 μm.

High-precision measurement thanks to flow optimisation and vibration decoupling

Chemnitz also owes this high precision to the holistic interaction of many measures. When ultra-high precision is required, the workpiece goes into the separate, fully air-conditioned area, which, thanks to a sophisticated ventilation concept in maximum climate class 1, keeps the temperature constant at 20 °C – with a maximum difference of 0.2 K. To ensure that passing trucks or vibrating machines from neighbouring halls do not falsify the measuring result, the measuring machines stand on a vibration-decoupled foundation and base plate ensemble.

The selection of the measuring quartet in Chemnitz was targeted. ”The MMZ major-part portal coordinate measuring centre, the MICURA high-accuracy measuring machine and the two PRISMO 3D coordinate measuring machines are graduated in relation to each other in terms of the size of the parts so that we get redundancies,” says the Head of QM. ”I can therefore also keep a machine completely free for a customer project.” The four machines can accommodate small components with an edge length of 500 mm, medium-sized workpieces (900 mm × 1,800 mm × 700 mm) and XXL components with a maximum measuring volume of 27 m³. ”Even a small car or a complete Heckert machine bed fits in the MMZ,” says the expert. ”I’m often at customers’ and suppliers’ sites, but at no company – including numerous wellknown companies – did I see a comparably equipped centre.”

The latest investment is a ZEISS PRISMO with a turntable for continuous four-axis measurement. The measuring centre thus responds to the trend at Starrag and its customers towards four-axis and five-axis machining of precision components. ”With a three-axis measuring machine, a cylinder with many bores cannot be measured completely because the sensor cannot get into every bore,” says Knöfel, citing a typical example from practice. ”The turntable allows us to position the cylinder so that the measurement succeeds. Thanks to the turntable, the PRISMO is now also a four-axis 3D coordinate measuring machine – and not everyone has that.”

High level of expertise: Three quality engineers for programming and operation

The measuring centre also differs from others in another respect. Instead of just one station, there are four ZEISS programming workstations where experts create measuring programmes using the ZEISS CALYPSO programme. The advantage is that, because programming is decoupled from the measuring process, no measuring machine is blocked. ”The expertise is also remarkable,” emphasises the Head of QM. ”Our three quality engineers programme and operate the four measuring machines – supported by a skilled worker as a stand-in. These experts are particularly distinguished by their many years of experience, which helps them to interpret measuring results correctly.” This is a demanding task for contract measuring jobs, which are often in the narrow tolerance range of four to five micrometres – far outside the usual limit of machining centres.

”Even a small car or a complete Heckert machine bed fits in the MMZ.”

As the Group’s centre of excellence, the experts primarily look after the Starrag plants in Bielefeld, Rorschacherberg and Chemnitz. Measuring expertise is particularly in demand when Starrag sells a Heckert machine with complete technology. Centres for machining engine blocks are a typical example. The measuring centre then proves that the machine tool has been run in correctly and that the engine blocks comply exactly with the strict tolerance provisions in the specifications.

Saxony’s measuring expertise also came externally from the very beginning. For example, Chemnitz took over the calibration of wheel axles on behalf of the German Railway until the state-owned company acquired its own measuring technology. Starrag now also wants to increasingly offer its measuring services to its typical clientele. This includes, for example, manufacturers of compressors, electric motors and combustion engines for the automotive industry, drives for trucks, construction machinery, agricultural machinery or ships, as well as industrial components of all kinds.

Secure measuring work even for development orders

But how does Chemnitz ensure confidentiality – for example in the quality check of prototypes? ”Only authorised persons can enter the 250-square-metre measuring room, which is strictly separated from the rest of production through a secured roller shutter,” Knöfel explains. ”We always know who comes in and out of here and when, thanks to the use of a card reader.”

Chemnitz even feels equipped to handle several projects at the same time. ”We often have two or three projects at short notice, but sometimes even up to eight longer projects at the same time,” reports the Head of QM. ”Then we approach the work in two to three shifts. That already requires a very high degree of flexibility on the part of the staff.”

”Even a small car or a complete Heckert machine bed fits in the MMZ.”

But the Saxons are also flexible when it comes to measuring technology – they recently tackled a specialty that only very few have mastered so far. ”Due to our machine size, we can measure parts with multiple clamping.” Knöfel gives details: ”For example, we recently performed an unmanned run-in and measurement of eight components with multiple clamping. With such a late ghost shift, the flexibility and performance of machining processes can be increased enormously.”