39 terms

Scan-to-CAD glossary

Every term you'll hear in a reverse engineering project, defined the way a working engineer would explain it across a bench — not the way a marketing page would.

Structured light scanning: Projecting known fringe patterns onto a part and triangulating the distortion.; A projector throws a sequence of stripe patterns; two cameras record how the stripes deform over the surface and triangulate 3D points. Fast, dense and accurate on matte surfaces, but it struggles with shiny, dark or translucent parts without a scanning spray.; Find the right scanner
Laser line scanning: Sweeping one or more laser lines across a part, usually on a measuring arm.; Handles shiny and dark surfaces better than structured light and works well on large parts when mounted on a portable arm. Point density is lower per pass, so blade edges and small radii may need slower sweeps.
Photogrammetry: Building 3D geometry from many overlapping photographs.; Used both as a standalone capture method for very large objects and as a global reference framework: photogrammetry targets fix the overall scale so that a handheld scanner's local accuracy does not accumulate into global drift.
Volumetric accuracy: How much error accumulates across the full measuring volume, not per point.; Point accuracy tells you how good a single measurement is; volumetric accuracy tells you how much the far end of a 2 m part drifts. For reverse engineering large assemblies, volumetric accuracy is the number that matters.
Scanning spray: A thin matte coating that makes reflective or transparent surfaces scannable.; Adds a real thickness — typically 3–15 µm depending on the product — which must be accounted for on tight-tolerance features. Sublimating sprays evaporate within hours and leave no residue.
Registration targets: Reflective stickers used to stitch scan patches together.; Place them on the fixture or a surrounding frame rather than on critical surfaces, because the area under a target has no data and must be patched later.
Drift: Slow accumulation of alignment error across a long scan.; Common when tracking on geometry alone over a featureless surface. Symptom: a part that closes 0.3 mm out of round even though every local patch looks clean. Fix it with targets or a photogrammetry frame.
Point cloud: The raw set of XYZ measurements before triangulation.; Carries noise, outliers and overlapping passes. Most decisions about noise filtering are best made here, before the surface is fixed by triangulation.
Mesh (STL / OBJ / PLY): A triangulated surface built from the point cloud.; STL stores triangles only; OBJ and PLY can carry colour and texture. A mesh is a faceted approximation — it has no notion of a hole, a plane or a fillet, which is why it cannot be machined from directly with any reliability.
Decimation: Reducing triangle count while preserving shape.; Curvature-aware decimation keeps density on radii and edges and thins flat regions. Uniform decimation is faster but rounds off exactly the features you care about.
Watertight mesh: A closed mesh with no holes, no boundary edges and consistent normals.; Required for volume calculation, 3D printing and most automatic surfacing. A mesh can be watertight and still be geometrically wrong, so check deviation, not just closure.
Non-manifold edge: An edge shared by more than two triangles.; Usually created by overlapping scan passes or aggressive hole filling. Breaks Boolean operations, slicing and surfacing, and must be cleaned before modeling.
Normal: The outward direction of a triangle or surface.; Flipped normals make a mesh render inside-out and make deviation signs meaningless — a +0.2 mm error reads as −0.2 mm.
Noise floor: The scatter of the scanner's own measurements on a perfectly flat reference.; You cannot hold a tolerance tighter than a few times the noise floor. Measure it on a granite flat before promising an accuracy figure to a client.
Region segmentation: Automatically splitting a mesh into planes, cylinders, cones, spheres and freeform patches.; The starting point of parametric reverse engineering in Geomagic Design X. Good segmentation means the software has already found most of your sketch planes and axes.
Datum lock: Fixing the model's coordinate system to the part's functional datums before any modeling.; The single most important step in a reverse-engineering job. If the datum reference frame is set after modeling, every downstream dimension is arbitrary and the inspection report will not match the drawing.; Free RE workflow PDF
Parametric reverse engineering: Rebuilding scanned geometry as a feature tree of sketches, extrudes and revolves.; Produces editable, design-intent CAD. Slower than auto-surfacing but the only route to a model an engineer can change later.
Auto-surfacing: Wrapping a mesh in NURBS patches automatically.; Fast and faithful to the scan, but produces hundreds of patches with no design intent. Right for organic shapes and shape archives, wrong for parts that will be revised.
NURBS patch: A mathematically defined freeform surface panel.; Patch layout — how many, how aligned to curvature — determines whether the resulting surface is smooth in a real CAD system or shows visible ripples under a zebra map.
Hybrid modeling: Parametric features for prismatic regions, freeform surfaces for organic ones.; The pragmatic default for most industrial parts: mounting faces and holes stay editable, the aerodynamic or ergonomic region follows the scan exactly.; Reverse engineering roadmap
Design intent: The original engineer's reasoning: what is nominal, what is symmetric, what is a manufacturing artefact.; A worn, cast, hand-finished part is never nominal. Recovering intent — rounding 9.97 to 10, restoring symmetry, snapping to a standard module — is what separates reverse engineering from mesh copying.
Golden tooth: The least-worn reference feature on a repeating pattern, used to define the nominal.; On a worn gear or spline, every tooth differs. Rather than averaging the damage, you pick the best-preserved feature as the master and pattern it.; Golden tooth case study
STEP / IGES: Neutral CAD exchange formats.; STEP (AP214/AP242) carries solids and is the standard deliverable for manufacturing. IGES carries surfaces only and is legacy — use it only when the receiving system demands it.
Deviation map: A colour plot of the distance between measured geometry and nominal CAD.; Green is in tolerance, red is high material, blue is low. Always state the alignment used — the same part looks completely different best-fit versus datum-aligned.
Best-fit alignment: Minimising overall deviation between scan and CAD with no datum constraint.; Great for judging shape and manufacturing quality. Invalid for accept/reject decisions against a drawing, because it hides location error by design.
Datum reference frame (DRF): The primary-secondary-tertiary coordinate system that constrains all six degrees of freedom.; Primary takes three points (or a plane), secondary two, tertiary one. Every dimension on the drawing is measured from this frame — get it wrong and the whole report is wrong.
GD&T: Geometric Dimensioning and Tolerancing — ASME Y14.5 / ISO 1101.; A symbolic language for stating what a part must actually do, rather than only how big it is. Replaces ambiguous ± dimensioning with unambiguous tolerance zones.; All 14 GD&T symbols
Bonus tolerance: Extra positional tolerance earned when a feature departs from maximum material condition.; A hole toleranced at position 0.2 Ⓜ that is produced 0.1 mm larger than MMC gains 0.1 mm of positional tolerance. Free manufacturing headroom that many drawings leave unused.
Ra / Rz: Surface roughness parameters: arithmetic average versus peak-to-valley.; Ra averages the profile and hides isolated defects; Rz captures the worst peaks and valleys. Sealing and fatigue surfaces should usually be specified with Rz.; Surface finish converter
Gauge R&R: A study of how much of your measured variation comes from the measuring system itself.; If the measurement system consumes more than about 10% of the tolerance band, you are measuring your process and your scanner in equal parts.
Traceability: An unbroken chain of calibration back to a national standard.; Required for any inspection report that carries contractual weight. Keep artefact certificates and calibration dates with the report.
Draft angle: The taper that lets a cast or moulded part release from its tool.; When reverse engineering a casting, draft is a manufacturing artefact, not design intent — but removing it changes the part. Decide explicitly and document the decision.
Parting line: The seam where two halves of a mould or die meet.; Shows on the scan as a small ridge or step. Usually modelled out, but it tells you the tooling orientation and therefore where draft runs.
Machining allowance: Extra material left on a casting or forging for later machining.; Reverse engineering a raw casting without adding allowance back gives an unmachinable model.
Shrinkage compensation: Scaling a model up to offset material contraction during casting or moulding.; Typically 0.5–2.5% depending on material. A scanned production part is already shrunk — model the part, apply shrink to the tool.
Interference fit: A shaft deliberately larger than its hole, held by elastic deformation.; H7/p6 for a light press, H7/s6 for a heavy one. Assembly usually needs a press or thermal shrink.; ISO 286 fits calculator
Clearance fit: A shaft always smaller than its hole so the parts move or assemble freely.; H7/g6 for a precision sliding fit, H7/f7 for a running fit with lubrication, H11/c11 for loose assembly.; ISO 286 fits calculator
Module (gears): Pitch diameter divided by tooth count — the metric measure of gear tooth size.; Reverse-engineering a gear means recovering a standard module, not the measured one: 1.9873 is a 2 module gear with wear.; Gear calculator
Tolerance stack-up: The accumulated effect of individual tolerances across an assembly.; Worst-case stacking is safe but expensive; statistical (RSS) stacking is realistic for high volumes. Both need a datum scheme that reflects how the parts actually locate.

Related on CADfinity

GD&T Symbols

All 14 controls with zones and inspection notes.

Fits & Tolerances

ISO 286 hole-basis fit calculator.

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Reverse Engineering Advisor

Upload a part or scan and get a full RE plan.

Field Notes

Case studies and workflow write-ups from the studio.

Training Diplomas

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