QMT Features: August 2014
Body-in-white scanning
Lasers have replaced tactile probing at the Fiat- Tofas plant in Turkey

Turkish automotive manufacturer FIAT-Tofas is implementing new inspection methodologies for diagnostic measurement of sheet metal components and body-in-white (BIW) assemblies

A key element involves the installation of Nikon Metrology XC65Dx-LS cross scanners and CAMIO multi-sensor metrology software, which are retrofitted on an existing Hexagon double-arm CMM. Inspection is now twice as fast compared to touch probes and provides better insight, reducing the time needed to diagnose problems and raising efficiency.

Scanners on CMMsSince 2013, the cross scanners have been installed on two Hexagon DEA BRAVO horizontal arm CMMs that are equipped with a continuous wrist CW43 interface. Incorporating three lasers in a cross pattern, the XC65Dx-LS captures full 3D details of features and surfaces in a single scan. By digitising complex features from three sides, the cross scanner acquires the complete 3D geometry of the features, driving the accurate extraction of positions and dimensions.

Smart laser intensity adaptation allows any surface, such as those with varying colour or high reflectivity, to be scanned without the use of a matt spray or other user interaction. This is enabled by automatic, real-time adjustment of sensor settings between successive laser stripes and for each individual point along the laser stripe. The XC65Dx-LS installed at Tofas has a longer stand-off distance that offers distinct advantages. By capturing geometry from up to 170 mm away, the scanner gains optimum access to BIW structures and can scan over the clamps that hold components in position.

The scanners can be used in combination with tactile probes for alignment of a part or for a mixed measuring routine.

During vehicle development, the scanners are used instead of touch probes for inspecting individual car panels as well as for complete diagnosis of the BIW, after the car’s sheet metal panels have been welded together but before the bonnet, doors and boot lid have been added. In this way, the vehicle’s entire sheet metal structure is inspected to very close tolerances, showing the interaction between the panels and allowing parts issues to be separated from process issues. Completed vehicles are also inspected, mainly for gap and flush spacing between different car panels.

Laser advantagesÖzgür Ogur in the quality department at Bursa commented, “The savings are significant when using scanners instead of tactile probes for feature and surface inspection of BIW on our CMM.

“Laser scanning fulfills our accuracy requirements and there are no significant differences in accuracy between tactile and scanning. We require 0.2mm uncertainty on a 5.5m diagonal and as the inspection device has to resolve to one-tenth of the tolerance band, the scanners need to measure down to 20 microns. Measurements have proven that the scanners achieve this requirement in line with their listed specifications. “Inspection is now twice as fast as when we used touch probes, reducing lead-time between design iterations and raising efficiency.”

Diagnostic measurement of anything from individual components to finished vehicles is carried out on the Hexagon horizontal arm CMM using the Nikon Metrology laser scanning heads, which check for gap and flush as well as inspecting features and surfaces. The equipment is used continuously over two shifts every day and it is rare to find touch probes at the end of the two arms. The data generated by the scanners gives full geometrical detail by fully digitising the parts. The large amount of part-to-CAD comparison information provides greater insight when analysing the inspection results, ultimately leading to faster decision making.Scanning on the CMM has eliminated the need for dedicated, stand-alone measurement systems and supersedes the use of laser scanners on portable measuring arms. Compared with the latter procedure, which requires two operators to measure both sides of a car, the CMM measures automatically and is inherently more accurate and repeatable. With portable scanning, an operator needs to be told which features to inspect, whereas on a CMM it is simply necessary to call up a program from a library and start the cycle.

Further advantages of scanning are that it greatly broadens the application scope of horizontal-arm CMMs, allowing better use to be made of the capital investment. Moreover, the controlled, process-oriented measurements are ideal for SPC. The technology leads to faster detection of problems and more prompt corrective actions, as well as greater flexibility and economy in the quality control laboratory due to the ability to run inspection cycles unattended for long periods.

Virtual assemblyIn such a dynamic production facility, the company needs a modern quality control function to support the development phase of new vehicle models. All body parts constantly undergo comprehensive checks, but the individual pieces of data collected do not show how well adjacent parts fit together. It is therefore necessary to assemble mating panels on high precision fixtures, called ‘meisterbocks’, that replicate the body shop’s assembly tooling and process. In this way, design, fit, function, gaps and flushness of sheet metal elements can be visually assessed and measured dimensions can be compared with the CAD model of the car.

The ultimate goal of Tofas is moving towards a situation where it will scan completely all prototype sheet metal panels and parts and create a virtual assembly using the 3D digital copies for geometry evaluation in software. This new, geometric verification approach is already revealing potential part fitting issues and aims to reduce the duration of individual prototype cycles as well as the number of prototype iterations needed to produce different car panels, saving time and cost. The 3D measurement data of initial prototype parts will also serve as a reference to analyse dimensional changes during serial production. Fiat is leading the study project and deployment is expected in one to two years.

Comparative accuracy testsNone of the above would be possible if the Nikon Metrology laser scanners were not able to capture the BIW data sufficiently accurately and repeatably. So Tofas engineers Özgür Ogur, Diagnosis Measurement Lab Leader, and Rahim Kosar, Aesthetics Diagnosis Expert, went to considerable lengths to test the performance of the XC65Dx-LS against the traditional touch probe.

Comparative benchmarks were performed in accordance with ISO 10360 acceptance and re-verification tests for CMMs and three criteria were investigated – probing error (MPEP), length measurement error (MPEE) and tactile scanning error (MPETHP).

In the worst case, MPEP was 7 microns after five scans on a ceramic sphere and in one of the zones, 1 micron error was recorded. The catalogue value was given as 15 microns, so all results were acceptable. Five MPEE measurements of a ball bar in seven diagonal positions yielded sigma values under 7 microns. As scanning is done from different angles, the 25mm sphere needs to be measured at five angular positions. Deviation was found to be 11 microns, within the 12 microns stated in the catalogue. All three error values were at least equivalent to those achieved with tactile probing.

Successful benchmarks on real partsThe Tofas team took the trials a step further to reality by laser scanning actual car parts. A Doblò side panel was selected and its contours and geometrical features were inspected first by touch probing and then by laser scanning. The former measurements were taken as the nominal and the latter values used to calculate the difference.

Again this showed that laser scanning is an acceptable procedure for quality control of BIW vehicles where as an example a tolerance band of < ±0.2mm is required for surface measurement.
Spheres were then attached to an assembled Doblò car to review diagonal length scanning accuracy. An uncertainty of 0.2mm was observed over a length of 5.5m, so scanning is considered acceptable for diagnostic measurements on a CMM with 45µm + 25L / 1000 uncertainty.

Overall Tofas found that the Nikon Metrology laser scanners on CMMs gave a number of benefits for sheet metal, body-in-white or full vehicle inspection. Complete parts can be digitised quickly and the data compared to the original CAD model or to a reference part. Features are scanned much faster than by tactile probing and with high accuracy, while gap and flush between components can be analysed seamlessly.

The Nikon Metrology laser scanners also easily measure shiny, uncoated sheet metal or painted components without the need for any special pre-treatments. Moreover, the resulting easy-to-interpret colour reports provide better insights for faster decision-making. This all makes laser scanning a leading technology to help manufacturers improve productivity and shorten their product cycles.www.nikonmetrology.com
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