O’Fallon Casting of St. Louis, Missouri, is a premier nonferrous investment casting company that has built its reputation on making high-quality, competitively priced castings to meet demanding end-user applications. One area of growth for the company has been in its rapid prototyping operation. It uses expendable patterns generated from a 3D model from a CAD file to quickly and cost effectively produce a part and verify its design prior to building production tooling.
“In recent years, there have been many improvements in the quality of pattern surface finishes, pattern accuracy, and pattern material alternatives, and today’s patterns are more user friendly to the near net shapes offered by the investment casting process,” said Ben Galmiche, O’Fallon Casting quality engineer. Because of these improvements, rapid prototyping of cast parts is good value for customers who want to reduce overall tooling costs and introduce products into the marketplace quickly. Quality inspection is a crucial aspect of the casting business.
Complex freeform part surfaces
For decades, the use of traditional touch probes on CMMs has been the gold standard. However, the time-consuming process becomes an even bigger drain on a quality department’s valuable resources. Metrology engineers are charged with increasingly complex freeform part surfaces that take an exceptionally longer time to thoroughly inspect.
To help enhance the turnaround time of its rapid prototype inspection process, O’Fallon Casting turned to 3D scanning and point-cloud-based inspection. O’Fallon Casting replaced its older CMM with a Global CMM from Hexagon Metrology and equipped it with a Nikon Metrology Cross Scanner, a high-speed, multi-stripe 3D laser scanner designed for inspecting part features that provides optimal point distribution in all directions. This new 3D measurement system was delivered with the PolyWorks software suite from InnovMetric Software Inc.
This investment proved to be the right move, one that paid off quickly. Particularly when O’Fallon Casting received a request in 2008 to inspect the housing for an integrated wing tip light of the Boeing 787 Dreamliner. Here’s how O’Fallon successfully integrated the point cloud based analysis in all phases of its rapid-prototyping process:
1. A pattern of the wing tip light housing was created from the customer’s 3D CAD model of the part using stereolithography (SLA) and selective laser sintering (SLS).
2. The pattern was then visually and manually inspected to determine if it conformed to the reference CAD model of the part and if it was properly proportioned to account for shrinkage.
3. Once the pattern was verified and approved through Nikon Metrology 3D scanning and PolyWorks inspection, O’Fallon Casting built a ceramic shell around the rapid prototype pattern.
4. The ceramic shell with the pattern went through a burn out process, where the pattern was vaporized to leave only the ceramic mold. The part was cast from A356 aluminum artificially aged to the T6 condition
5. The part was then measured using the Global CMM and the laser scanner. The point cloud data gathered during this routine was transferred to the PolyWorks software electronically via the company’s internal network.
In the overall process, O’Fallon Casting used 3D digitizing and PolyWorks to inspect both the foam pattern and its inherent aluminum part. When gathering dimensional data on these prototype parts, O’Fallon Casting uses a very slow scanning speed to assure that a high level of feature detail is captured. In the process, excess data is collected to capture freeform surfaces and accurately characterize 3D features, both being essential in increasing the quality of the final product.
As the Nikon Metrology scanner sweeps lasers in 3 directions, the entire geometry of features is captured consistently, regardless of the scanning direction. This allows PolyWorks software to extract features through hundreds of points, rather that relying on a handful of tactile inspection points. Simplified scanner motion paths also means more straightforward off-line CMM programming. This also compares favourably with tactile inspection where elaborate programming effort is required to define a rather lengthy sequence of touch sensor movements and measurements.
The Cross Scanner and Focus Inspection software from Nikon Metrology is a powerful package,” marked Galmiche. “In less than one week we were up and running to use 3D scanning in real customer projects. The non-contact scanning solution offers the tremendous advantage of gathering complete data sets, better than what we have ever been able to accomplish through tactile inspection. Likewise, we now perform a laser scanning job in less than a day whereas it previously took us nearly one week. We have reached the point where laser scanning is used in virtually all our rapid prototyping projects, paying off big time both for inspection and troubleshooting purposes.”
Powerful engineering tools
“We import the dimensional data into PolyWorks software’s IMAlign module where the partial scans are automatically aligned using a best-fit algorithm to create a point cloud model of the part,” Galmiche said. “At this point, we study the model carefully to determine if we have all of the data we need to make an accurate comparison between it and the customer’s CAD model. If necessary, we will reorient the part on the CMM and gather dimensional data from another view. We use the IMAlign reduce overlap function to automatically remove the excess in areas of scan overlap and create a single set of data points that precisely defines the dimensions of the part in these areas,” he said. Next, the aligned scans were merged using PolyWorks’ IMMerge module to create a highly accurate finely detailed polygonal mesh model of the digitized parts.
The polygonal mesh model was then exported to PolyWorks IMInspect module for the actual dimensional inspection process. This package uses data-to-CAD alignment techniques including constrained best-fit, and techniques based on features and reference points. For this specific project, a global comparison was performed by calculating the deviation between each digitized point and its corresponding CAD reference. A colour map was displayed according to the tolerances set by O’Fallon Casting. IMInspect includes comparison and measurement tools for a complete analysis of surface and cross-sectional deviations, feature dimensions and GD&T controls, clearance, and thickness.
O’Fallon Casting measured the clearance of the part by comparing 3D distances between two planes to ensure that the ray of light is projected without interference. “Using simple IMInspect tools, we can quickly see where a part is out of tolerance relative to the customer’s CAD model,” Galmiche said. “Because of the flexibility of the software, we can compare the models using different alignment methods, something that many of our customers find very helpful.”
Graphic geometry analysis
Using the report generation function of the PolyWorks software, Galmiche prepared an inspection report using the AS9102 format. “In addition to the data, we include snapshots of the model comparison so that customers can quickly see where the part is out of tolerance. We can also illustrate where the part is just a small amount out of tolerance or greatly so by using different colours to indicate different tolerance ranges. Finally, thanks to the PolyWorks IMView free viewer software, our customers can visualize their inspection projects in 3D from their own workstation.”
O’Fallon Casting also uses the CMM/laser scanner/PolyWorks software combination to inspect its own molds and compression-straightening tooling to ensure that it conforms to design. The PolyWorks/Inspector™ package quickly shows precisely where tooling may be worn or slightly out of tolerance.
The combination of the Global CMM, Nikon Metrology laser scanner, and PolyWorks software has significantly reduced the time it takes to turn around a rapid prototype part at O’Fallon Casting. “Globally, we are very satisfied with this new point-cloud-based inspection process,” said Galmiche. “For a typical part, we can gather the dimensional data we need in about one day and we can perform the data analysis using the PolyWorks software in less than half a day.
That is a reduction of about 50% in the amount of time the process took in the past. The visualization of the data also makes it faster for customers to review the inspection report and understand the implications of the dimensional data analysis. Also on the expenses side we benefit a great deal. By systematically gaining more profound insight, we are able to reduce the number of prototyping rounds, which saves tremendously on resources and throughput time. And if our old CMM was not to be replaced, our capital investment would have been limited to installing the scanner on any existing CMM system, regardless of the brand.”