The process is called, in a coinage from Carl Zeiss IMT Corp. (www.zeiss.com/imt), “metrotomography.” That’s one part “metrology” and one part “tomography.” Essentially what this comes down to is a system that can perform non-destructive testing (NDT) through the use of X-rays...and a means by which the results can be measured. The first part, tomography, is performed via the 225-kV X-ray system. A part is fixtured on the table within the METROTOM machine, and then exposed to the electromagnetic radiation. The X-ray beams go through the part and onto a detector surface, where they are recorded. The result of this is a two-dimensional gray-scale image (e.g., depending on the density of sections of the part and the geometry, there is varying intensity of the image), which is certainly helpful in terms of seeing what’s there from that particular point of view, but not as fully informative as it might be. That’s why there is the additional step: rotating the object 360° around its axis on the rotary table (which has a resolution of 0.36 arc seconds). The result of this is data that can be used to produce a 3D model of the part—a model that allows you to see whether there are any internal cavities or voids. Or, in the case that the part has deep recesses or other parts that can’t otherwise be assessed via the use of coordinate measuring machines (CMMs) or non-contact optical scanners, the METROTOM can obtain the information.
The system can use Zeiss CALYPSO metrology software that permits comparing the model generated via the X-ray analysis with a CAD model.
While all of this sounds as though it might be something that would be used only in labs, it is worth noting that Zeiss, which has significant experience in building CMMs that have shop-floor applications, uses guideways, drives, controller, rotary table, and software that are used by the CMM equipment. (Also, it has opened its North American Computed Tomography Technology Center in Brighton, MI, which offers a metrotomography service so one interested in the technology doesn’t need to secure a machine—although Zeiss certainly offers them, as well.)
The METROTOM system has an optimum measuring range of 300 x 300 x 300 mm. While there are some design and materials factors that need to be taken into account, generally speaking it can handle plastics up to 250-mm thick, light metal alloys such as aluminum and magnesium up to 120-mm thick, model construction materials (plaster, wood, resin) up to 200-mm thick, and steel up to 10-mm thick (although it can check defects when the thickness is up to 18 mm).