CT Dimensional Metrology: Measuring the Internal Geometry No Other Instrument Reaches

A hydraulic manifold can pass every external check and still leak on the test bench, because an internal bore, the one no gauge reaches, is two hundredths of a millimetre out of tolerance. While the part stays whole, that dimension can't be measured. You estimate it, or you cut the part open to read it in section and scrap it in the process. A tactile CMM and an optical scanner stop at the same place, for the same reason: they measure where the stylus or the light can reach, and inside a closed cavity neither one reaches. On the metrology side, industrial computed tomography exists to close that gap. It reconstructs the part as a volume and measures what sits inside, without opening it.

The stylus touches, CT passes through

The distance between CT and conventional measuring instruments is all in the gesture. A tactile coordinate measuring machine (CMM) touches the part with a stylus and records the coordinates of the points it can physically reach. An optical scanner projects structured light or laser and captures the surface it can see. Both work well as long as the geometry to be measured is exposed. CT instead acquires hundreds of radiographs as the part rotates, and from those projections it reconstructs a volume made of voxels, the three-dimensional pixel of the scan. In that volume every surface, external and internal, becomes a measurable point, because the X-ray beam has passed through the full thickness. It's the same technology as CT for defect analysis in castings, with a different goal: not to find a pore, but to measure a dimension. The two uses even sit in separate standards worlds, as we'll see.

From voxel to CAD comparison

The reconstructed volume yields the same quantities you would measure on the bench: bore diameters, centre distances, wall thicknesses, flatness, GD&T features. The difference is that they come out together, inside and out, from a single acquisition. The operation that changes first-article work the most, though, is the comparison against the nominal model. The CT volume is aligned to the part's CAD, and the deviation of every point of the real surface from the theoretical one is shown as a false-colour map: where the part carries excess material, where it falls short. On a die casting, on an additive component or on a plastic moulding, that map shows at a glance where shrinkage, distortion or mould wear are moving the geometry, long before a set of point measurements would raise the suspicion.

That's why CT metrology took hold where the critical dimensions sit inside the part. In aerospace they're the cooling channels of a blade or the thin walls of an additive bracket, geometry no stylus travels; in automotive they're die-cast housings and e-mobility components, where hole spacing and wall thickness decide the fit; in medical they're internal lumens and the porosity of implants, to be measured without cutting a part destined for the human body. In all three, the first article is when internal measurement is worth the most, because that's where you decide whether the mould, the build program or the process is on target, before the series starts.

How far you can trust it: uncertainty is task-specific

Here is where the question the quality department asks first lands, and the honest answer isn't a single number. CT accuracy depends on the part, the material, the geometry and the scan parameters, and it has to be stated for that specific task. The survey by Kruth and colleagues in CIRP Annals (2011), the founding text of the field, frames the whole subject around traceability to the metre and measurement uncertainty, not around image resolution. The German guideline VDI/VDE 2630, Part 2.1 (2015 edition), sets out how task-specific uncertainty is determined, along with the suitability of the measuring process against the tolerance to be held; Part 1.2 lists the variables that influence the result.

The practical route to a number is the substitution method, borrowed from ISO 15530-3 for tactile CMMs and adapted to CT: you measure a calibrated reference part, geometrically similar to the object, and anchor the CT measurement to that traceable value. Because both measurements happen under the same conditions, the method captures machine errors, environmental effects, evaluation strategy and operator in one pass. It has a cost. The procedure calls for at least twenty repetitions, which against CT scan times is no detail. Stating the voxel size, by contrast, doesn't prove the uncertainty achieved, and that's the point most specifications confuse.

CT, tactile CMM or optical scanner: what each measures

The three technologies don't compete on the same ground, and choosing well means knowing where each one gives way. On an accessible external feature at a tight tolerance, the tactile CMM stays the reference: it touches a few points at the lowest uncertainty there is. The optical scanner is unbeatable for speed on free-form external surfaces, where a dense cloud is needed in seconds, but on closed geometry and on shiny or deep surfaces it's as blind as the CMM. CT covers the whole volume, internal included, in a single scan, and it doesn't deform compliant parts because it never touches them.

The head-to-head on internal geometry is documented. A study in Precision Engineering (Villarraga-Gómez, Lee and Smith, 2018) measured the same artefacts with CT and with a tactile CMM: on internal features the deviation between the two was below 5 µm in most cases, with CT expanded uncertainties between 1 and 20 µm; on compliant parts, where the stylus applies a force that deforms, CT and a zero-force CMM converged to within ±2 µm.

The criterion then writes itself. If the dimension that decides the part is external, accessible and must be certified at the lowest uncertainty, the tactile CMM is the choice; if it's a free-form external surface to capture fast, the optical scanner; if it sits inside the part, if the component is too complex or too compliant to be touched, or if internal and external have to be measured in the same run, CT is the only one of the three that answers.

When CT isn't the answer

Being honest about this is worth more than any list of advantages. CT costs and takes time, and on large or highly absorbing parts, a thick steel block, beam absorption drives uncertainty up until the measurement becomes less reliable than a good tactile pass. There's a standards picture to know as well: the standard dedicated to CT as a coordinate measuring system, ISO 10360-11, is still at draft stage, so system acceptance today leans on VDI/VDE 2630 (Part 1.3, which applies ISO 10360 to CT sensors) and on ASME B89.4.23-2020, which a buyer can call up in the order to verify the stated accuracy specifications, at acceptance and at periodic reverification. There's no third way here that specifications keep looking for: either you qualify task uncertainty with a calibrated part similar to the object, accepting the substitution method's time, or the value CT returns stays an indication, not a certifiable dimension.

Start with the dimension you can't measure today

Anyone producing castings, additive parts or mouldings with critical internal geometry, and today estimating those dimensions or reading them by sectioning a part, has a direct way to see what they're missing: have a real component measured by CT, ideally one already disputed or already scrapped, and compare the deviation map against the CAD with what the current control actually saw. This is how PITECH sets up the assessment: analysis of the critical dimension and the drawing, a demonstrative scan on the customer's own part with CAD comparison and a task uncertainty estimate, and a reasoned proposal between a recurring service and an in-house system, including the honesty to say when a CMM or an optical scanner already suffice. An internal dimension exists even when no instrument reaches it; CT metrology is what stops it being treated as an unknown. To start the assessment with a sample part, describe the component and the critical dimension from the contact page.