Safeguarding precision: Ensuring machine tool performance in automotive manufacturing

At the Volkswagen plant in Wolfsburg, Hexagon’s ETALON LASERTRACER-NG has been used for machine tool maintenance since 2016 to maintain the performance of the machines on site through regular optimisation and calibration.

Precision is paramount in the bustling Volkswagen automotive factory in Wolfsburg, Germany. When an unexpected snag occurs in the tool shop – a machine collision with a workpiece that could potentially disrupt production – Hexagon’s technology plays the central role in swiftly diagnosing the machine’s condition to allow continuous production. Since 2016, the ETALON LASERTRACER-NG has been an indispensable ally to VW’s maintenance team in confirming the accuracy and performance of their milling machines. 

Wolfsburg, Germany, 08:20: The tool shop of the prototyping department reports a machine collision with a workpiece. That shouldn’t happen because it endangers the whole production process, but the feed rate was just a little too fast. Material variation can sometimes lead to excessive milling forces, but this shouldn’t happen. The team now needs good advice: Is the machine warped? Can work continue?

Established in 1938, the Wolfsburg automotive factory in Germany is the Volkswagen Group’s main manufacturing plant. Covering an area of 6.5 km², with a production capacity of up to 800,000 vehicles a year, it is the world’s largest single car manufacturing complex, employing around 70,000 people.

About a dozen machine tools operate at the plant in the tool shop of the prototyping department alone and the adjacent test shop. These machines serve one purpose: to mill out the desired shapes or components from a cuboid of raw material based on 3D data. This is done by 3-, 4- or 5-axis milling, sometimes combined with rotary and swivel tables and swivel milling heads. The required tolerances of components vary depending on the application and size, but a few micrometres are not uncommon. Only some machines achieve this accuracy, so it takes some planning to select the suitable machines to perform the desired work step and ensure the correct result.

To determine the achievable accuracy of their milling machines, the maintenance team relies on modern technology, namely Hexagon’s ETALON LASERTRACER-NG. Using this laser-based measuring system, they can comprehensively assess a machine’s condition within half an hour, with all individual linear axes of the milling machine measured and analysed according to necessary standards.

At 08:30, the emergency call from the tool shop reaches the maintenance department. The component being milled is an integral part of a new tool to replace one already at its wear limit. But first, the effects of the collision have to be investigated. Externally, the milling machine appears unchanged, yet it is uncertain whether it still meets the high accuracy requirements.

Now it’s time for Jens Melahn and Heiko Tomala, maintenance technician at the plant,and his team. They quickly install the LASERTRACER-NG on the machine table, and after only 30 minutes, Melahn has the answer: Everything is fine; the machine geometry has not suffered, and work can continue. At 09:00, the machine operators clamp the new blank and correct the feed speed to avoid another crash. This time, everything goes well, and the new 
tool is finished in time. 

Everyone who plans and manufactures with machine tools knows incidents like this. Questions about the condition or potential of machining centres are part of daily work. And as the tolerances of the workpieces become tighter and tighter, it is often no longer sufficient to carry out a quick check with a dial gauge and ruler.

In addition, it is part of the scope of maintenance work to correct any deviations and errors that might exist in the machine tool. In the past, straightness standards, dial gauges and linear lasers were used for this purpose. Deviations of all axes were measured individually and then, at great time and expense, mechanically and electronically adjusted. For larger machines with axes several metres long, this could easily take several days. “A compensation measurement on our largest machines used to take up to two weeks,” explains Melahn. “It simply couldn’t be done quickly. And it was not possible to machine during this time, which meant standstill.”

In addition, the achieved quality of the workpieces is of paramount importance. “In the past, we always had the problem that a workpiece had contours, burrs and cracks that we had to laboriously rework,” says Melahn. “Fortunately, a few years ago, we were introduced to the LASERTRACER-NG.”

Accurate inspection and compensation over the entire working area of a machine tool

After some initial scepticism about the novel process, the advantages were quickly apparent. The system was totally convincing in the two essential tasks their maintenance team has to perform: the accurate inspection of machine tools and their compensation and improvement. “Right from the start, we could lift machine performance to a dimension they could not achieve before,” he says.

The LASERTRACER-NG eliminates the need for any manual alignment. User errors, previously caused by an insufficient alignment process using classic measuring tools, cannot occur with the system, which automatically aligns itself to the axes in a standard-compliant manner during verification measurements.

Compensation measurements and more in-depth axis analysis require no system alignment at all. A technician must only set up the laser head in a machine corner and clamp a reflector in the spindle. Then, the machine moves along a predefined path within the entire machine volume, with the laser following the reflector and permanently measuring the distance between the LASERTRACER-NG and the reflector in the spindle with an accuracy of less than one micrometre. After about 20 minutes, the measurement is finished, and the laser can be placed in the next corner of the machine. The process is repeated until the same movement path of the spindle has been measured with high precision from all machine corners.

All error parameters of the linear axes can be derived from the difference between the approached nominal coordinate and the measured values. The results are easily interpreted and, if necessary, assigned to mechanical causes. If the repeatability of the axes is high enough, a compensation file can be written at the end to correct the machine controller, resulting in more precise movement paths of the spindle and better-milled parts.

The tolerances of machine tools migrate over time due to various causes, such as temperature fluctuations and wear during operation.

“Our role is to ensure that a machine manufactures with high precision all year round,” says Melahn. His team, therefore, need to periodically compensate them to ensure they are always in the best possible condition. A sporadic check is enough for some machines, but for others, they have to compensate regularly.

“Compensation can often reduce machine deviations by up to 90%,” he says. “Depending on the machine class, we reduce the error on our large machines from a few tenths of a millimetre to a few hundredths. With our high-precision milling centres, we can improve from 80 µm to 5 µm.”

It is also critical for the maintenance engineer to be able to measure the machine completely and apply compensation throughout its entire working volume. “With conventional measuring equipment, we could only record the deviations of the machine on one measuring line,” says Melahn.

Thanks to the measuring principle of the LASERTRACERNG, they see the entire machine – the total volume of a machine is recorded, and compensation can be applied to the whole machine. “This is hugely beneficial because it means we do not merely have the highest accuracy at only one point in the machine but across the full machine volume. This system gives our operators the freedom to use the entire working area.”

And its fast. “We are using the fastest measuring system available for machine compensation and thus help keep the processes in the factory running,” says Melahn. “Today, when we measure and compensate the entire volume of a machine, we are done after about two days for the complete service including all necessary adjustments. Before, we needed at least a week, and even then, had only measured the machine on one axis.”

Department foreman Kevin Piotrowski sums up: “Using the LASERTRACER-NG gives us several major benefits: First, we are finished faster and leave behind better machines with higher machine availability. Second, we can better understand the machines and prepare ourselves preventively for necessary maintenance. And third, we save ourselves from the coordination and costs of an external service provider. Given the number of machines we have, this is an excellent investment that has brought us tremendous value. We’re convinced by the technology!”

“Furthermore, the service is top-notch, adds Melahn. “We always feel that all Hexagon employees are eager to help us as quickly as possible. And that’s something you don’t often hear about these days.”