GlossaryKind of term: Manufacturing termIndustrial CT
What is industrial CT?
Industrial CT uses X-rays to build a 3D volume of the whole part, inside and out, for internal dimensions and for finding voids without cutting it open.
Industrial computed tomography (CT) is a non-destructive measurement and inspection method. The part usually turns on a rotary table between X-ray source and detector, and a volume model is reconstructed from a large number of projections. The volume shows the outside and the inside: internal channels, voids, porosity, inclusions and the components of an assembly. You can align the volume with the CAD model and show the deviation at each point as a nominal-to-actual comparison, or take dimensions no probe can reach.
What does the accuracy of a CT measurement depend on?
What you can see and how accurately you can measure depends on voxel size; in a full scan it grows with the size of the part, so a large part is scanned more coarsely than a small one. Voxel size is not the same as resolution: the focal spot of the source and the detector also decide which details can still be separated. Material and wall thickness decide whether enough radiation gets through; aluminium and plastics are easier than steel, and thick steel sections or mixed materials can cause artefacts. A defect only a few voxels across, or a crack in an unfavourable orientation, can stay invisible. For dimensional work the measurement uncertainty also depends on how the surface is determined in the volume and on calibration with reference objects. The German guideline series VDI/VDE 2630 covers CT in dimensional metrology.
How does industrial CT work in practice?
Many machine shops have no CT system of their own and subcontract the scan, priced per scan or per batch and with its own lead time. If the drawing or the customer calls for a CT check, a line goes into the estimate and a wait into the plan. Ask exactly what is wanted: a search for voids above an agreed size, a full dimensional check on the volume, or a comparison with the CAD model. A full dimensional check usually takes more scan and analysis time. For printed metal parts with internal channels, and for castings, CT is one of the few ways to check the inside without sectioning the part.
What is industrial CT often confused with?
Industrial CT is not medical CT: there is no patient dose limit, energy and resolution are matched to metal and plastic, and usually the object rotates rather than source and detector. Nor does CT automatically replace the coordinate measuring machine: on accessible faces with tight tolerances a tactile probe often gives a lower measurement uncertainty. And CT is not 2D radiography: a radiograph projects the whole part onto one image without depth, while CT reconstructs the volume. For finding voids in castings a 2D radiograph is sometimes enough.
Frequently asked questions
When do you need a CT scan instead of a CMM?
For dimensions and defects a probe cannot reach: internal channels, voids, porosity, parts inside an assembly. For external dimensions and geometric tolerances on accessible faces, a coordinate measuring machine often gives a lower measurement uncertainty.
Can CT find porosity in a casting or printed part?
Yes, within the limits of its resolution. The scan shows voids and where they sit in the volume; voids only a few voxels across can be missed. The acceptable size follows from the customer's requirement or the standard the drawing refers to.
How do I price a CT scan in a quote?
As a subcontracted measurement at your inspection supplier's price, per scan or per batch, with the wait added to your lead time. Agree with the customer whether every part is scanned or a sample will do, and what has to be evaluated.