August 2006 Vol. 137 No. 2

Quality Scan

Computed Tomography and Quality Control

Today's advanced quality initiatives dictate process control, not just part inspection. Simply detecting and characterizing flaws isn't always enough, especially in parts that contain complex internal features. OEMs and suppliers must work closely together to identify defects and take corrective action.To do that, they need quantifiable data.

X-ray computed tomography (CT), a technology that not only detects but quantifies certain defects, is gaining acceptance as a powerful non-destructive (NDT) tool in addition to traditional real-time and film radiographic inspection. What's more, CT is the only technology that nondestructively measures internal features. This makes it an excellent complement to CMMs and noncontact optical scanners that cannot readily capture data from internal cavities, undercuts, or deep recessed portions of certain parts.

Before CT imaging was introduced, internal data were obtained by sectioning—a destructive, time-consuming technique of cutting, polishing, and measuring a part's internal features. Quite often, the reference points for the part alignment are lost in sectioning, having an adverse affect on the measurements. Depending on the materials used, forces may change as material is removed causing the internal measurements of the part to change radically. CT images of a part can quickly provide visual indications of internal surfaces even in the slice plane.

As an NDT tool, CT is used to see inside a part to identify cracks, porosity, blockage, delaminations, inclusions, voids, and mechanical fit. In many cases, CT can quantify these flaws and features by size and location. As a measurement tool, it offers nondestructive access to internal features like out-of-tolerance wall thickness caused by core shift in complex aerospace castings, or porosity in automotive castings.

Computed tomography uses X-rays to nondestructively produce cross-sectional views of a part. These grayscale slices reveal both internal and external features. Unlike 2-D film or digital radiographs, the features in a CT image are not superimposed on one another.With CT data, a 3-D volume image of the part can be created showing X, Y, and Z coordinates. Volume CT data are used for first-article inspection of entire parts. A series of CT slices is generated from the part. Then a volume stereo lithography model (STL file) of the internal and external features is reconstructed from the CT slices. The STL model can be merged with the CAD model in a variety of software applications, which generate a variance map of the two data sets. STL files generated from CT data can also be used in finite element analysis.

CT data are used in medical-implant device applications to benchmark processes, and to provide important data for refining manufacturing techniques. This technique has also gained acceptance in the aerospace industry, where parts must meet stringent quality and regulatory requirements. Because CT file formats can be imported into reverse engineering, solid modeling, and finite element software applications, design modifications can be made directly from the scan data. Another area where CT plays an important measurement role is on parts made of soft flexible materials. Conventional measuring systems can cause the part to flex when touched; CT captures the internal and external features simultaneously, without ever touching the part.

In CT technology, as the part gets larger, the resolution will become coarser. A simple way to calculate the pixel resolution of an industrial CT system is to take the diameter of the part and divide it by 1000. Even though variations of this general rule exist, over the years it has proven to be an effective calculation. By contrast, measurement accuracy, which is sampled at the sub-pixel level, can be three to five times better than the pixel resolution. Typically a 450-kV system utilizing linear array detectors achieves measurement accuracy in the range of 0.003 to 0.006" (75–150 µm). Microfocus CT systems in the 90–225-kV energy range can achieve measurement accuracies of 0.0015–0.0025" (38–64 µm).

The visual nature of CT data makes it a very powerful tool for quality managers, design engineers, and manufacturing engineers. CT data offer a clear picture of the internal state of many parts that previously had to be destroyed. CT collects quantifiable information that produces 3-D models of the complete part to help multiple departments of an organization reach their design and quality objectives.