Edited by Leslie Gordon, [email protected]
When you think of computed tomography (CT) do you think of the medical industry and capturing images of bones and organs inside the human body? Thanks to improvements in computer hardware, CT now also works well for industrial nondestructive testing. In fact, industrial CT is fast enough to be used for in-line 100% quality inspection and 3D metrology control.
Industrial CT machines consist of an X-ray tube and detector and a rotational stage housed in a radiation-shielded enclosure that usually ranges in size from 4 to 10 ft3. Adjacent to this is a console for setting up the part to be rotated and X-rayed. There is also a supercomputer workstation that transforms the collected 2D X-rays into a 3D CT volume and then a point cloud model or a polygon mesh, represented as a 3D grid of volume elements or voxels. This transformation process is called “reconstruction.” The resolution of the 3D CT model depends on the number of voxels generated during reconstruction.
Part of CT involves setting radiodensity values from which to reconstruct models. Users can program CT machines by radiodensity thresholds so different colors represent different model components. Models comprise from thousands to as many as 50 million polygons.
3D CT output formats include STL, WRL, and point cloud, among others, and they are used for analysis, rapid prototyping, and reverse engineering. To make the CT model into a surface or parametric model for editing in CAD, technicians use software such as Geomagic, Rapidform, or Polyworks. The softwares’ semiautomated tools transform the CT polygon mesh to Nurbs surfaces or even parametric 3D CAD models. In most cases, though, engineers can use the polygon mesh without modification for tasks such as measuring features.
In a recent project, Twin City Die Castings Co., Minneapolis, a company that makes precision aluminum, magnesium, and zinc die castings, tasked North Star Imaging’s Inspection Services Group with evaluating 3D CT as a way to inspect cast parts. The sample part under consideration was an exhaust-gas housing for diesel engines. The part weighed less than a pound and combined an aluminum casting with two integrally cast stainless-steel inserts. One project objective was to compare the manufactured casting’s dimensions with those of the original CAD model.
In this case, it was unnecessary to postprocess the polygon mesh. To compare the CT file with the original CAD file, the two models first had to be aligned. Geomagic Qualify is efficient software for this kind of work. It includes different alignment tools, ranging from fast and automatic Best Fit, to manual alignment. After the technician aligned the models, a 3D comparison option quickly created colored views showing dimensional differences between the CAD model and the CT surface (polygon mesh). Another option showed numerical values instead of colors.