Good impression

A sensor system that monitors sheet metal forming processes is claimed to detect faults at an early stage, thereby reducing the amount of rejected parts at the end of the production line.

Developed at the Fraunhofer Institute for Machine Tools and Forming Technology in Germany, the system's sensors monitor the shaping of metal material for parts ranging from car body components to kitchen sinks.

The collected data is transmitted online to a computer, where a software program analyses the information and determines whether there are any process fluctuations that need correcting.

The program does this by comparing the values recorded online with previously specified target data.

If any deviations are discovered, operators can intervene during the forming process and fix the problem.

For instance, it could be used to control deep drawing, a metal forming process that pushes a sheet metal blank into a forming die by punching it in mechanically. Pressure is applied to the sheet with a blank holder tool, which controls the flow of metal into the die.

'By monitoring the sheet metal flowing inside the tool we could monitor if a crack was developing,' said Fraunhofer researcher Sören Scheffler. 'We could then stop the process and change the parameters like the blank holder force before we unveil a part.'

Scheffler said that blank holder tools could be made of elastomeric material, which could be actively controlled with piezo-electric actuators. The right change in blank holder force will in turn correct the process in the desired way.

Scheffler said his group recently displayed a working demonstration model at Hanover's Euroblech trade fair, but there are still changes that need to be made before it goes into full commercial use.

For instance, he said, the current model uses laser sensors to monitor forming processes, but those sensors have certain limitations.

'First, laser sensors are quite expensive — about €3,000 (just over £2,000)' he said. 'The second problem is that a laser shoots straight and normally a tool surface isn't even.'

Also, he said, the sensor currently sits outside the metal forming tool and they would prefer one that could be integrated within the tool itself. So although the sensor works, Scheffler said they are seeking better options.

Sheet metal forming processes are becoming an increasingly delicate procedure as the need for thinner sheets increases. Car manufacturers, for example, want to build cars that are lighter and more fuel-efficient.

But while these sheets need to be thinner, they also need to be safe and strong. If the process used to draw, cut, perforate and calibrate the sheets is not set correctly, the resulting components will not meet the specified quality standards and have to be thrown away.

The Fraunhofer team believes car manufacturers will be able to limit the number of rejected parts and keep costs down if they use methods that make their processes more reliable.

'As we are able to detect process fluctuations at an early stage, we can significantly reduce the reject rate,' claimed Scheffler. 'What's more, we can recognise faulty parts and separate them out immediately. This fully automatic process supervision perfectly complements the subsequent manual quality inspection.'