Production Uses of Computed Tomography - Samples in an Aluminum Foundry 2006-01-0510
Cast parts are traditionally inspected prior to initial production runs and subsequently in support of high volume production to ensure consistent quality and accurate dimensions that match the “as designed” part within specified tolerances. Classical methods for dimensional measurements are CMM systems using touch probes, laser sensors, or optical techniques. Flaw conditions such as cracks, porosity and inclusions can be detected with “real-time” x-ray inspection. These techniques are quite effective on simple parts with two dimensional geometry and non-complicated structures. Specialized x-ray inspection systems for alloy wheel production are examples of such systems. Complex three dimensional castings such as cylinder heads and engine blocks have functional internal structures with close tolerances and morphology that cannot be verified by CMM systems externally or by real-time x-ray.
Computed Tomography (CT) has developed as an industrial measurement and quality assurance tool that simultaneously captures surface information and flaw conditions in complex three dimensional castings. The CT process is described for a typical industrial part examination in support of First Article Inspection. A stack of slices is collected in a process that requires 4 to 15 hours using automated system operation. The resulting stack of hundreds of grey level images is converted to an STL surface model of the part containing all dimensions and flaw information. The STL model of the actual part is then registered with the CAD model of the same part to produce a variance map showing the difference between the two as a colour coded model that can be viewed at any angle or sectioned to see internal details. The total analysis time ranges between one to three hours, depending on the type of part, but can be reduced for repetitive inspection of production parts.
The CT technology described here is used successfully in Volkswagen foundry Hanover for inspection of prototypes, modified ingot molds, cores and modification of shooting tools in core shooting machines.