Scan to CAD

Scan to CAD is the process of creating a CAD model with exact mathematical geometry from 3D scanning data, that is, from a point cloud or a polygon mesh. It is the core design step of reverse engineering: a scan describes the surface, while a CAD model describes the part so that it can be modified, dimensioned on a drawing and manufactured. The work is done in specialized software (such as Geomagic Design X) or directly in a CAD system, where the mesh is inserted as a reference.

The main approaches:

Automatic surfacing (auto surfacing) covers the mesh with a patchwork of NURBS patches and creates a closed solid that faithfully follows the scan, including its defects, wear and deformation. It is fast and suits organic and freeform shapes, but the result has no parameters or features, and modifying it is laborious.

Parametric modeling from a scan reconstructs the design intent. Planes, cylinders and cones are fitted to selected regions of the mesh, sketches are derived from cross sections and the part is rebuilt from features such as extrudes, revolves and fillets. Dimensions are rounded to nominal values, and parallelism, perpendicularity and symmetry are restored. The result is a parametric model with a feature tree. Parts that combine both characters get a hybrid approach: prismatic features are modeled parametrically, freeform surfaces are fitted to the mesh.

You need scan to CAD when the output is not just a mesh for printing or visualization but a basis for manufacturing and further design: a drawing with tolerances, a CNC program, a mold, a design change or a STEP file for a supplier. Typical cases are replacement parts, upgrades of older machines, digitizing handmade models and prototypes, or bringing a clay or foam design model into CAD.

Choose automatic surfacing for freeform parts where a faithful copy of the surface is enough (sculptures, ergonomic shapes, a reference for fit checks or simulation). Choose a parametric model for machine parts that will be manufactured, toleranced and changed. It usually takes several times more work.

The most common mistake is to take over the scan without engineering judgment. Worn and deformed surfaces must be replaced with nominal geometry, and the dimensions of fits, threads and standard features have to be looked up in standards or derived from the mating parts. For fits toleranced in hundredths of a millimeter, scanner accuracy is not enough and the dimension must be measured by contact, for example on a CMM or with a micrometer. A scanner also cannot see inside: internal channels and hidden features have to be measured another way.

The second mistake is passing off an auto-surfaced model as a full CAD model. In STEP format both look the same, but only the parametric model can be edited sensibly.

In your brief, state whether you need a native model with a feature tree or whether a STEP file is enough, what deviation of the model from the scan is acceptable on freeform surfaces, which datums the model should be aligned to and whether you want a drawing. As proof of quality, ask for a comparison of the finished model with the scan.