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X-rays reveal the full inside story
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01/02/2007
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Roger Bishop reports on a remarkable advance in magnetic resonance tomography that allows engineers to reverse engineer competitive products and directly compare new developments against CAD data
Any company using advanced X-ray technologies to look inside components for, say, casting faults in cylinder heads and engine blocks is working at the leading edge of technology.
Now a team of engineers in Germany has taken magnetic resonance (MR) imagery – or computer tomography – a significant stage further. They have discovered how to adapt and use the equipment not only to see but also to measure the inner structures scanned components.
This breakthrough allows manufactured or prototype components to be directly compared with computer models and can be used in studies as new products are being developed. A competitive product, for example, can be scanned in precise detail – internally and externally – and given back to the research and development team as CAD data.
Engineers can also investigate internal workings of a component. And, of course, it is the ultimate quality control tool, allowing actual components to be dimensionally compared, externally and internally, with the CAE models from which the tooling was made.
Behind the development is a team from Delphi’s Customer Technology Centre in Wuppertal led by Peter Knauff, project manager of the Continuous Improvement Group.
Like so many good ideas, the new machine resulted from the engineer’s insatiable desire for complete understanding, in this case when Knauff found himself needing an MR scan for medical reasons a few years ago. He questioned the radiologists about its capabilities and potential and then, with his Wuppertal team, set to work with the equipment manufacturers, Xylon International of Hamburg, on an industrial machine that would add that crucial third dimension – precision measurement. Now Delphi is working with Bergisch University in Wuppertal to explore and investigate further opportunities for industrial applications.
Compared with equipment used in medical computer tomography, the substantial difference is that the machine remains motionless while the object being analysed is rotated. However, it is also built along the lines of a coordinate measurement machine (CMM) with a massive 6 tonne granite table – largely isolated from its environment – that minimises environmental influences and ensures the precision of movements between the individual system axes. The whole machine is encased in 8 to 12mm thick lead weighing more than 10 tonnes with the access door on its own weighing 400kg.
The computer tomograph has four basic elements: an X-ray source, the specimen stand, detector and manipulator. The component being examined is mounted on the stand and precisely position between the X-ray source and detector. Source energy levels are set to provide the ideal level of penetration for the component’s material. The stand is then rotated through 360deg over ±20min during which time 1,440 images are taken of the part.
These data are then transferred to the machine’s data processing computer which uses special software to reconstruct the information, creating a cloud of “many millions” of data points aligned to a coordinate system from which a three-dimensional scatter plot is generated. Precision is up to 7µm. The current computer being used by Delphi is a 1,600MHz machine with 5Mbytes of RAM and a 200Gbyte capacity for generating images of the tomographic data but, according to Knauff this will soon need upgrading. A 4,800MHz machine with 50Mbytes of RAM is being considered.
Components made from many different materials can be examined using the Delphi equipment simply by adjusting the X-ray source energy level. However, required energy levels increase with material density leading to more ‘noise’ and a resulting loss of accuracy. For practical purposes 6mm steel is thought of as being the threshold.
When the machine was demonstrated to European Automotive Design a four-way automotive connector of the type manufactured in Wuppertal was used as an example. For quality control purposes, 684 parameters are measured on this component and its tooling. And since each mould injection moulding tool produces four components, 2,736 details per tool are required to be measured. This used to take 75 man hours; now using computer tomography the work takes just 24 man hours. This is the level of testing required for, say, airbag and ECU connectors which have to be 100% reliable.
Computer tomography is used, above all, for the structural analysis of components during the development and construction of moulded parts. In frequently used objects – such as a switch – a series of X-rays can even map its function, so that the observer can view a motion sequence to check whether small springs for triggering the switch are correctly mounted, for example. These analyses enable moulds or the injection process to be optimised rapidly and at an early stage.
Exact measurement of dimensions
A second area of application is digital linear measurement. From the three-dimensional scattering of the X-rays, special software generates data records with the exact dimensions of the objects under examination. In several steps, the corresponding CAD data records from the design are then superimposed over the above data. Each measured detail of the component is compared with the CAD model as a numerical value. The greater the difference in measurement, the more intensely the software visualises this, showing the resultant deviation by means of a colour scale.
The graduated colours of this three-dimensional comparison enable the observer to recognise immediately the degree to which the real component matches the intended design. Previously, to achieve this the component would have had to be dismantled into individual pieces and measured manually. With complicated parts, this procedure could take several weeks. Furthermore, the data model can be rotated as desired in real time and sections cut and analysed in any plane. In future just a few days will be required for this work. The examined components are not modified or damaged in any way.
By screening tests for standardised computer tomograph settings, Delphi can demonstrate the repetitive accuracy and reproducibility of the results of dimensional measurements. Delphi says these will enable the company to demonstrate the capability of the computer tomograph as a measuring instrument, particularly for measuring tasks in the field of joining technology.
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Author
Roger Bishop
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