HANDS-OFF Inspection

Manufacturing Engineering, Sep 2005 by Tolinski, Michael

Measuring equipment for automotive components and assemblies offers portability, noncontact inspection, and high data volumes

Automotive manufacturing is a world of unexpected delays caused (despite best efforts) when parts simply don't fit together properly. With multiple product variations and changeovers compounding the issue, fit and finish problems must be identified and corrected quickly to avoid production bottlenecks.

To this end, inspection and measurement practices in automotive production face sometimes competing interests: the need for quick measurement methods (and automated inspection when practical), for large volumes of accurate measurement data in useful form, and for movable shop-floor systems that can be taken to the part or area of interest.

Noncontact measurement techniques are helping manufacturing personnel find answers to these needs. Laser and vision-based inspection systems can supply huge amounts of data about form, fit, and contour in only a few minutes. The systems are being integrated with touch-probe measuring systems-or replacing them. Meanwhile, improved CMMs and software have been developed in response to the need for speed.

One approach to spotting problems early-on is to acquire more useful measurement data during inspection. Touch-probe methods are limited to discrete measurements, from which it's sometimes difficult to see the big picture about what went wrong. One solution is noncontact, laser-based surface scanning, which acquires millions of points to define a surface.

Various systems for real-time laser-probe surface inspection (RTLI) are being tweaked by vendors for use by major automakers. Essentially, these systems consist of three components. The first is a laser scanner that can gather over 15,000 points/sec with an accuracy of less than 100 µm. This scanner is usually attached to a multiaxis portable CMM (PCMM) to locate the scanned points in space. Data points are then sent to software and compared with the geometric model or CAD data using PCs, which are now powerful enough to handle these millions of points.

Providers of RTLI technology claim other benefits from the large numbers of points generated during scanning. Points can be displayed as color-coded, archivable "weather maps" of a part's surface, showing the position and degree of deviation from tolerance. These maps allow trends to be analyzed-such as areas on a molded part that indicate tool wear. They also allow "what if" analyses that show which tolerances are critical and which are excessive. And RTLI is said to be taking over the conventional CMM's role for making crucial firstarticle inspections.

Romer CimCore (Wixom, MI) has provided RTLI systems for production environments. Offering an example of RTLI's usefulness, the company points to an application of laser-scanning for a General Motors SUV assembly line. GM had found a problem with the fit between the back rear-window glass (which is slightly curved) and the center high-mount stop lamp. Laser scanning and a Romer 3000i PCMM gathered surface points and fed the data to Powerlnspect software from Delcam Inc. (Windsor, Ontario). The software converted the data to a mesh of triangles (as in STL data files) to compare with CAD data to show deviations from nominal in the mating surfaces between the two parts.

For applications like this, RTLI is said to be good for measuring continuous surfaces-and particularly useful for sculpted and contoured parts. And providers argue that the larger the inspected object, the more cost-effective RTLI becomes.

But this doesn't mean laser scanning eliminates the need for touch-probe inspection. Some companies, such as Leica Geosystems (Lawrenceville, GA), provide systems that combine the two. The company's laser trackers work together with its T-Probe and T-Scan products for measuring sheetmetal, tooling, or fixtures, says Leica's Metrology Division regional manager for automotive, Joel Martin. "The T-Probe gives you the functionality to do touchprobe measurements on hard fixtures, while the T-Scan gives you the ability to do high-density scanning over a very large volume."

The company estimates that the laser tracker and T-Probe working in tandem provide 50% inspection-time savings compared to using the laser tracker alone. This is because the TProbe can operate without CMM arms or wires as a "walkaround CMM" for measuring hard-to-access holes, slots, and features on complex tooling and large fixtures. "With the T-probe, we can set up a single station and measure the complete tool, or sometimes multiple tools from one set of locations, from the measurement station,"Martin says.

The probe's reported accuracy is 0.0023" (60 μm) over a measurement space of 56' (17 m). The T-Scan has also been shown to be accurate for acquiring surface data; benchmark studies by DaimlerChrysler and Peugeot resulted in measurements that deviated only 20 μm from standard CMM measurements, says Martin.

The combined systems align their measurement data with the vehicle's coordinate system, allowing sheetmetalstyle applications. "We actually probe the vehicle to take the four-way and two-way setup holes and fit into the coordinate system for the vehicle the same way you would with a CMM, yet still are able to add all the high-definition scanning data on top of it in one system and one piece of software."