Scan to CAD, explained

What a scanner gives you

A 3D scanner measures where a part’s surface is. Laser and structured-light scanners sample it optically, CT scanners image the whole part, inside and out, and a CMM or hand probe measures chosen points. Whichever you use, the result describes the part’s shape, not the decisions behind it.

The data usually arrives in one of two forms:

• Point clouds: individual measured points, each with X, Y and Z coordinates and sometimes a colour or intensity. Common formats are E57, a vendor-neutral exchange format, and PTX or PTS.
• Meshes: the points joined into small triangles that form a continuous surface. STL and OBJ are the usual formats. An STL file holds triangles only; it doesn’t even record units, so agree on millimetres or inches before anyone measures from it.

CT data starts as a volume and is normally turned into a surface mesh before anyone models from it.

The example part used across this site is a 120 mm stepped shaft, and its example scan has 8,000 points. Every figure in this post shows that example geometry, not a customer part.

Three kinds of “3D model”

“Send me the 3D model” can mean three very different files. Knowing which one you have, and which one you need, clears up most of the confusion around scan to CAD.

A mesh only approximates curved surfaces. A bore is a ring of flat facets, and there is no cylinder in the file to measure or resize. A B-rep model, short for boundary representation, stores the true surfaces, which is why STEP (ISO 10303) and IGES are the standard ways to move geometry between CAD systems.

A parametric model goes one step further: it keeps the recipe. That means a sketch of the profile, the feature that turns it into a solid, and the dimensions that drive both. In practice a STEP or IGES file carries the finished geometry, not the recipe, so the feature tree only survives in a native CAD file.

Why converting a mesh isn’t rebuilding it

There are three common ways to get from a scan to something a CAD system can open, and they give very different results.

1. Mesh to solid. Many CAD tools can wrap a mesh into a solid by turning every triangle into a flat face. The file opens as a solid, but it is still the scan: no cylinder to select and no diameter to change.
2. Automatic surfacing. Reverse-engineering software can lay a patchwork of smooth freeform surfaces over the mesh and export it as STEP or IGES. It looks clean and suits freeform shapes, but it follows the scanned surface exactly, dents, wear and noise included, and still has no features to edit.
3. Feature-based rebuild. An engineer, or software that works the way an engineer does, works out how the part was made and models it again with CAD features: a revolved profile, a hole, a fillet. The scan becomes the reference you check against, not the model itself.

Only the third gives you CAD you can edit. It is also the only one that has to decide what the part is meant to be, and that is where the real work lies.

Rebuilding the way an engineer does

An engineer rebuilding a part from a scan doesn’t trace the surface. They set up references first and add detail in order. OriginShape’s AI agents follow the same four steps:

1. Datums: the axes and planes the part was made around, such as the axis of a turned part or the base of a bracket.
2. Design intent: the shape it was meant to be. Is that face flat, or worn? Is this diameter a standard size?
3. Features: the operations a designer would use, such as sketch, revolve, extrude and hole.
4. Dimensions: a name and a value for every size, so it can be changed later.

For the example shaft, that means one axis and a single profile revolved through 360°. The profile carries three diameters, Ø24, Ø36 and Ø28 mm, an overall length of 120 mm, and a linear taper that steps the collar down from Ø36 to Ø28 mm over 10 mm, which works out at 21.80°.

Each of those values is a parameter. Change the collar diameter and the taper updates with it, along with everything else that depends on it. That is the difference between a model and a measurement.

Where engineering judgment comes in

A scan records the part as it is today, not as it was drawn. Several things get in the way of reading the original design straight off the data:

• Wear and damage: bearing seats wear, edges round over and surfaces corrode. The scan measures what is left.
• Manufacturing variation: a part is made to a tolerance, not to an exact size, and one scan can’t tell you what that tolerance was.
• Gaps: deep bores, undercuts and shiny or dark surfaces can leave holes in optical scan data.
• Noise: every scan carries some measurement noise, and a straight conversion keeps it in the model.

So a good rebuild separates what the scan can settle from what it can’t. Clear, well-supported dimensions are rebuilt directly. Doubtful ones are flagged with the evidence behind them, then closed with a drawing, an extra measurement, or the engineer’s call. Nothing is quietly averaged into the model.

In the example, the shoulder position at z −35.00 mm is the dimension Oscar flags for review. It stays marked as a proposal until an engineer confirms it.

What to ask for

Whoever rebuilds your part, you’ll get a better result if you agree on these up front:

• The scan, or the part: point clouds (E57, PTX/PTS) and meshes (STL, OBJ) both work. No scan yet? Describe the part and agree on how it will be captured.
• What the part does: which faces locate it, what it mates with, and which dimensions are critical. Those are the ones worth confirming.
• What you already know: old drawings, a few calliper or micrometer readings, or the mating part can settle questions a scan can’t.
• The output: STEP for a solid that opens in most CAD, CAM and simulation tools, IGES if the receiving system is older or works mainly with surfaces, and a native feature tree if you’ll keep editing the design.
• How you’ll check it: for example, a deviation map of the model against the scan, and a list of the dimensions someone confirmed, with how they confirmed them.

If you get in touch with us, describe the part rather than sending confidential drawings. We’ll ask for what we need.

How OriginShape approaches it

OriginShape is building Oscar, an AI copilot for reverse engineering. Its AI agents read a scan the way an engineer does, rebuild the part feature by feature, and flag anything uncertain for review. The engineer makes the final call.

Oscar keeps the construction tree, not just the shape. The tree is native in FreeCAD today, with more CAD tools coming. For everything else, the model goes out as STEP or IGES.

Have a part without usable CAD? Tell us about it.

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