As a result, the inspection of sheet metal components and body-in-white (BIW) assemblies is now twice as fast as it was when using touch probes. The results also provide a better insight into the production processes, reducing the time needed to diagnose problems and thereby raising efficiency.
Founded in 1968 and headquartered in Istanbul, Tofas manufactures cars, taxis and vans in Turkey and sells them internationally. Employing over 6,000 people, the company is one of the top two automotive manufacturers in the region, with a production capacity of 400,000 units a year.
Products are offered under several brands including Fiat (which jointly owns the company), Peugeot, Citroen and Opel. The Tofas-built Fiat Linea is Turkey’s best-selling passenger car, while the Fiat Doblo (also produced as the Opel Combo) claims first place in the ‘light commercial vehicle’ category.
In 2013, Tofas installed XC65Dx-LS cross scanners from Nikon Metrology (
www.nikonmetrology.com) — along with Camio multi-sensor metrology software — on two of its Hexagon DEA Bravo double-arm CMMs, which are equipped with a continuous-wrist (CW43) interface.
Incorporating three lasers in a cross pattern, each XC65Dx-LS captures full 3-D details of features and surfaces in a single scan; and by digitising complex features from three sides, the cross scanner acquires the complete 3-D geometry of the features, allowing the accurate extraction of positions and dimensions.
Moreover, ‘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 action (this capability is facilitated by automatic, real-time adjustment of sensor settings between successive laser stripes and for each individual point along the laser stripe).
The XC65Dx-LSs installed at Tofas have a long stand-off distance (170mm), allowing the scanner to gain optimum access to BIW structures; it can also scan over the clamps that hold components in position.
Moreover, the scanners can be used in combination with tactile probes for alignment of a part or for a mixed measuring routine. Meanwhile, DMIS-based Camio8 software from Nikon Metrology accommodates both tactile and laser scanning applications; it also provides a wide variety of inspection tools, including full part-to-CAD comparison, ‘intelligent’ feature extraction with GD&T and profile analysis.
Furthermore, Camio is optimised for processing large point clouds, making it a suitable solution for measuring sheet metal panels and assembled car bodies.
Speed and accuracy
During vehicle development, the scanners are used 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 panels.
Özgür Ogur, who works in the quality department at Bursa, says: “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 20µm. Measurements have shown that they achieve this.
Indeed, the repeatability of Nikon Metrology’s laser scanners is down to 1µm. Inspection times are now half what they were using touch probes, reducing the lead time between design iterations and raising efficiency.”
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. Moreover, the controlled, process-oriented measurements are ideal for SPC and lead to the faster detection of problems and prompt corrective action — plus there are the cost advantages of being able to run inspection cycles unattended for long periods.
Virtual assembly
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 fully scan all prototype sheet metal panels and parts to create a virtual assembly, using the 3-D 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.
The 3-D measurement data of initial prototype parts will also serve as a reference to analyse dimensional changes during serial production. Fiat is leading this ‘virtual assembly’ study project, and deployment is expected in one to two years.
None of the above would be possible if the Nikon Metrology laser scanners were not able to capture the BIW data accurately and repeatably enough. Tofas engineers 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 7µm after five scans on a ceramic sphere; in one of the zones, 1µm error was recorded. Five MPEE measurements of a ball-bar in seven diagonal positions yielded sigma values of less than 7µm.
Finally, as scanning is done from different angles, the 25mm sphere used needs to be measured at five angular positions. Deviation was found to be 11µm. All three error values were at least equivalent to those achieved with tactile probing.
The Tofas team took the trials a step closer to reality by laser scann-ing actual car parts. A Doblo 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 were 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 bands of <±0.2 mm is required for surface measurement.
Spheres were then attached to an assembled Doblo 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.