Probe hits versus a mesh
A CMM samples a feature with a handful of probe points — typically 4 to 12 for a plane, 3 to 8 for a circle. A 3D scan represents the same feature with thousands of triangles. The tolerance definitions in ASME Y14.5 and ISO 1101 do not change, but the fitting problem does: with dense data the choice of fit (least squares, minimum zone, maximum inscribed) and the handling of outliers decides the result far more than with sparse probing.
This is why a scan-based inspection tool must state which fit it uses for every characteristic and must let you see the points that went into it. A number without the fit behind it is not a measurement.
Datums come first
Every position, profile or runout callout is meaningless until the datum reference frame is established. On a scan this means fitting the datum features (planes, cylinders, slots) with the same constraints the drawing implies — a primary plane constrains three degrees of freedom, a secondary cylinder two more, a tertiary feature the last one.
Ai-Inspect builds the datum frame explicitly from measured elements, shows the residual of each datum fit and refuses to evaluate a tolerance when a datum could not be verified on the scan (for example when the datum face is not covered by scan data). The refusal is reported with its reason rather than replaced by an estimate.
Form tolerances: flatness, roundness, cylindricity
Form tolerances describe how far a real surface departs from its ideal shape, independent of datums. On dense scan data the result is dominated by scan noise, so the honest answer always includes the scanner's stated accuracy as a lower bound: a flatness of 0.015 mm reported on a scan from a device whose stated accuracy is 0.02 mm is a statement about the scanner, not the part.
Good practice is to compare the form result with the scanner specification and to use the color deviation map to confirm that the pattern is real geometry (a bow, a step) rather than uniform noise.
Position, profile and the material condition modifiers
Position of a hole on a scan is computed from the fitted axis of the hole cylinder (or the fitted circle on a section plane for thin material) relative to the datum frame. Bonus tolerance under MMC or LMC is derived from the measured size exactly as on a CMM — the software needs the measured diameter, the nominal and the drawing's size tolerance.
Profile of a surface is the natural strength of scanning: the whole surface is compared with the CAD nominal, and the result is the width of the zone that contains every deviation. Ai-Inspect reports the profile result together with the point count, the coverage of the surface and the maximum deviation location.
- Position — fitted axis vs. datum frame, bonus from measured size
- Profile of a surface — full-surface deviation zone against CAD
- Profile of a line — evaluated on cross-sections, with per-section reports
- Runout — fitted datum axis plus radial deviation of the measured surface
What the report must contain
A GD&T report from a scan should be reproducible by another operator with the same scan. That requires, for each characteristic: the feature(s) it was computed from, the fit method, the datum frame, the number of points, the coverage and the result with its tolerance. Ai-Inspect prints these in the ballooned report and keeps the inspection as a recorded program so the same evaluation can be replayed on the next part.