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Faurecia Torsas in Sweden designs and develops various types of exhausts and manifolds for different OEM car manufacturers such as PSA, Renault, Volkswagen, Ford, Volvo etc. As the OEM’s only accept top quality products, Faurecia decided to invest in a CMM based laser scanner to improve the inspection and reverse engineering of the manufacturing tools and to verify the crucial properties of manifolds such as wall thickness and weld quality.
Manifolds are manufactured by welding two stamped sheet metal plates together. A first challenge is to achieve a better tool design for the stamping process. Due to the spring back effect after stamping the plates,
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the stamping tool often needs to be adapted. When a tool is finally optimized to have the correct stamping, the tool differs from the initial CAD model.
A second challenge is to gain control on the wall thickness that is originally modeled to be 2mm. Due to the stamping process the final wall thickness can vary from 1.8mm to 2.2mm. As this has an important effect on the lifespan and sound quality, it is crucial to gain control on the final wall thickness of the manifold.
Traditonal methodology
The optimization of the tool design was mainly done in an iterative manner by trial and error. The original tool was milled from the CAD model and then adapted manually by grinding the tool. This is necessary due to the stamping process where effects such as wall thickness variations, excessive spring back play an important role. This iterative tool design causes a lot of time and material loss and finally there remains a difference between the CAD model and the adapted tool. In case you need to make a spare tool you need to restart from the original model and re-run the complete optimization cycle again.
To inspect the wall thickness, Faurecia was previously using a traditional tactile probe mounted on a CMM. This is a high accurate technique but very time consuming. Full analysis of a single part often takes several hours and since you can only inspect a limited number of points, you don’t’ have a full 3D dimensional analysis of the part.
Reverse engineering reduces tool design time
To gain more insight and control over the quality of the product, Faurecia invested in a Metris scanning solution that addresses both challenges. The optimized tools are scanned and the resulting pointcloud can now be reverse engineered into a CAD model. This CAD model can be directly used for CNC milling. As such the tool can be finalized in one step for the definitive production and spare tool can be easily produced.
Also the first produced article is scanned and the pointcloud is compared to the original CAD design. The full 3D comparison gives you immediate feedback on the wall thickness at every point.
“Thanks to the scanning technology our tool design is rapidly optimized.” comments Mr. Hammer, Quality Manager at Faurecia Torsas, “We can save a huge amount of time and money when we optimize our tool design. As we can closely analyze the first produced articles by the full 3D color deviation maps, we immediately have feedback on the quality of our tool design.”
The Metris scan process
Laser based inspection has many advantages over traditional touch probe measurements. Scanning results in a very fast way in a full 3D pointcloud, ready for comparison with the original CAD file. The particular benefit of the Metris scanner is that it is highly accurate, thus perfectly suited for integration with a high accurate CMM.
Faurecia uses a Wenzel CMM that smoothly interfaces with the Metris LC50 laser scanner. The LC probes are designed to operate attached to Renishaw PH10M, using the PH10 multiwire, which results in a plug and play integration without additional wiring. Additionally the probe is compatible with the Renishaw ACR auto exchange rack making it suitable for easy interchange between the optical and contact probes.
The LC probe projects a line of red laser light onto the part while a CCD camera monitors the profile of the laser line. Using the process of triangulation the image of the line is analyzed and the 3D coordinates of the line hitting the surface are calculated. Data points are acquired at the rate of 19,200 points per second, producing a dense high accuracy point cloud covering the majority of the surface being measured. As such the time needed to measure complex parts is reduced from days and hours to minutes.
The first step in the scan process is the preparation of the scan. The system is able to scan most materials and surface colors by varying the laser intensity. The part can be positioned randomly on the CMM table, but generally a dedicated clamping tool is used. A major advantage is that the clamping doesn’t require accurate positioning since the alignment to the reference system of the CAD model is afterwards performed in the software.
It is possible that different scans are necessary to view all details of the part. The scanning software allows the verification engineer to automate the calibration of the PH10M orientations that correspond to these views. The calibration ensures that all measured points are captured in the same co-ordinate system. For every orientation, the operator specifies the area to be scanned. The start of the scan, the end of the scan, the width of the area, and the minimum overlap between different passes determine the scanning area. Additionally, the operator specifies the interscanline distance. At Faurecia, the operators use an interscanline distance between 0.07mm and 1.15mm, depending on the size of the part to be verified. These parameters result in a CMM scanning speed between 1.75mm and 28.75 mm per second.
Recording the different steps into a macro and running it on similar parts automate the complete scanning flow. The recording of a macro to scan a typical manifold using 5 different views is finished in less than 30 minutes.
Verification of the digital model against the reference CAD model
The result of the scanning process is a dense digital copy of the physical part, represented by a pointcloud. The CADcompare software, a complete toolbox especially designed to easily handle large pointclouds is used to process the data before comparison. At first, data points that are not part of the model, such as the clamp or CMM table are cut from the pointcloud. Next, the pointcloud is filtered using 3D curvature dependent filtering. Mostly a data reduction factor of 10 is obtained.
The processed digital model is checked against the nominal CAD model. The pointcloud has to be aligned to this model. The chosen QC software offers a Reference Positioning System (RPS) for feature based registration as well as free form alignment. By using only a part of the pointcloud, it is possible to further refine the alignment. One can e.g. disregard those parts of the pointcloud that represent known defects.
Next, the pointcloud is compared to the CAD model. The operator can select to compare the complete pointcloud, or make a comparison at section lines.
The verification engineer can input the tolerances specified by the designer. At Faurecia these tolerances are typically in the range of –0.1 to 0.1 mm. for global verification and smaller for detail verification. The result is a full deviation analysis color plot of the part.
The verification results can finally automatically be written into reports. The QC software offers additional tools such as the creation of deviation fly-outs at critical points and section reports. The reports are sent electronically to verification engineers who accept or reject the incoming product batch and liaise with the suppliers, providing them with a detailed analysis of the problem.
Similar to the scanning software, the chosen QC software enables the automation of the comparison of similar parts with Visual Basic Automation.
Conclusions
Faurecia is optimizing their tool design using new technology using CMM based laser scanning. The combined scanning with reverse engineering workflow enables Faurecia to produce spare tools at a fraction of the cost of the traditional way.
The first produced parts are scanned using a laser probe that captures about 20000points per second. Setting up the laser and defining the scanning patches is straightforward. The dense digital copy of the physical part is aligned and compared to the reference CAD model to detect and analyze the problem areas in a very short time. As such a manifold is now inspected in less than one hour.
Download: Faurecia uses LC50 scanner for improving the manifold inspection process (PDF file)
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