QMT Features: September 2013
CT x-ray for production lines
The Zeiss VoluMax computer tomograph  generates 3D volume data in seconds and is unaffected by temperature fluctuations, allowing 100 percent inspection checks in production.

A car races by, the brakes lock and then – CRASH. Microsensors transmit a signal to an airbag hidden in the steering wheel: a small explosion causes the airbag to inflate within a few milliseconds. The airbag cushions the forward movement of the driver and prevents more serious injuries. Microsensors in airbags can save lives – but only if they are triggered at just the right time with just the right force.

One requirement here is that an electrical sensor embedded in a plastic case is positioned in exactly the right spot. This is something that cannot be easily inspected; after all, it would require destroying the part. The alternative: a METROTOM computer tomograph from ZEISS X-rays the part to create a 3D image of the sensor, a volume model, that displays all internal and external dimensions. This takes a few minutes and takes place in a measuring lab where the sensitive machine is protected against temperature fluctuations.

However, the trend with such safety-relevant parts like the airbag sensor is moving towards 100 percent inspection of a batch. In that case the time-consuming measurement in the lab is less ideal. For this reason, 2D radiography is often used for inspections on the shop floor. With this method, the X-rays also illuminate the workpiece. However, the results only state if a part is good or not. Information about the size and location of defects is limited.

The new VoluMax computer tomograph now combines the benefits of computed tomography and 2D radiography: it generates 3D volume like a computer tomograph in the measuring lab. Nonetheless, it is suitable for the shop floor like 2D radiography: VoluMax can be used between temperatures of 15 and 40 degrees Celsius and delivers the measuring results in just a few seconds.

Short cycle times

In order to achieve such a short measuring time with VoluMax, ZEISS increased the X-ray power compared to its existing computer tomographs. This can be compared to a camera: if a photographer selects a shorter exposure time, the camera opens the aperture to increase the amount of light that falls on the image per time unit. With photos, this entails a lower depth of field. With VoluMax, this means sacrifices in resolution.

On the other hand, the measuring time is reduced considerably: while the measuring software calculates the 3D volume model from several hundred images, the machine is already busy scanning the next workpiece. VoluMax needs only 10-50 seconds per workpiece – depending on the part and if it is loaded manually or with a robot. The cycle time can even be lowered to less than one second if the user loads the system with several parts at once – with a pallet of 100 airbag sensors for example.

Due to the short measuring time and the insensitivity to temperature fluctuations, VoluMax is ideal for the inspection of parts directly on the shop floor. Unlike METROTOM, this makes it usable for inspections of entire batches. Compared to 2D radiography, it is able able to forecast on the raw part which workpieces will meet the specifications after processing.

Inspecting part by part
100 percent inspections directly on the shop floor are attractive for more than just the manufacturers of safety-relevant parts. They enable medical technology manufacturers to forego extensive process validation procedures: instead of checking the production process with expenditure of 100 to 1000 man days, i.e. for validation, manufacturers have to measure every single part and document the results. In other words, they verify their processes.

The detailed measuring results of the computer tomograph can also help enhance processes: software tools such as PiWeb identify trends – the link between production batch sizes and certain quality criteria, for example. It is also possible to compare the performance of various production lines.

Another reason for 100 percent inspections in production: car makers, for example, are increasingly demanding extensive documentation for every single part from their suppliers. In a worst case scenario, this allows them to limit recalls to specific batches because the quality of each component has been documented.

However, computer tomographs can do more than inspect single parts, they are also ideal for entire assemblies. Looking inside an assembly is essential to recognizing defects without having to destroy the parts, e.g. during the quality inspection of complex plastic parts such as medical syringes. This is where computer tomographs flex their muscles over coordinate measuring machines on the shop floor.

Analyzing defects

VoluMax can also differentiate the quality of workpieces instead of simply rejecting them. With 2D radiography, cast aluminum parts are often labelled for rejection even if the defective area would be ground away in downstream production. Furthermore, many defects are only identified after a part has been reworked. Parts that are rejected later thus take up valuable machine time. The 3D volume model from VoluMax enables operators to predict on the basis of the raw part which defective workpieces will comply with the specifications later in the process and which supposedly good parts will end up as scrap. Because the system avoids wastage and saves unnecessary processing time, the investment costs can be quickly recovered within a short time. As a result, companies not only profit with reliability regarding the quality of their parts and production processes through VoluMax, they also save time and avoidable rejects, thus lowering their production costs. l
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Rob Tremain Photographer
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