Mesh, solid, model, three things people mean by scan to CAD
A watertight STEP file is not a CAD model. Sorting out the three levels of scan to CAD, what each one is good for, and which one your shop actually needs.
August 27, 20265 min read
Ask five vendors for scan to CAD and you will get three different products. All of them are being honest. The confusion is that the phrase covers three levels of output, and the distance between the cheapest and the most useful is roughly the distance between a photograph of a house and the blueprints.
Here is how to tell them apart before you pay for one.
Level one, a cleaned mesh
Input a scan, get back a better scan. Holes filled, noise smoothed, decimated to something your machine can open without the fans spinning up, maybe remeshed to even triangles.
This is genuinely useful, and it is not CAD. The output is still a triangle mesh. You can 3D print it, drop it in a render, or check it against a nominal part for inspection. You cannot open it in SolidWorks and change a hole diameter, because there is no hole. There are four hundred triangles standing in a circle, doing an impression of one.
Most of the well known consumer tools live here. If somebody advertises AI 3D model generation and the output is an OBJ or a GLB, this is what you are buying.
Level two, a solid body
Input a scan, get back a watertight solid, usually as a STEP or Parasolid file. The surfaces have been fitted, so the flat is a real plane and the bore is a real cylindrical face. It opens in any CAD package as one honest lump of material.
This is a real step up. You can machine from it, put it in an assembly, run FEA on it. Plenty of traditional reverse engineering software does exactly this and does it well.
What you cannot do is edit it in any meaningful sense. There is no history. There is no parameter called plate thickness. If you want the wall 2 mm thicker you are dragging faces around and hoping the fillets survive, which for anything with blends they usually do not. Engineers call this a dumb solid, which is not an insult so much as a job description. The intent is gone. What survives is the result.
For a one off replacement part that nobody will ever modify, a dumb solid is completely fine, and you should not pay a penny more for anything else.
Level three, a feature tree
Input a scan, get back the model an engineer would have drawn. A sketch on a datum plane, an extrude with a distance, a hole with a diameter, a fillet with a radius, in the order somebody would actually build them, with the important dimensions exposed as named parameters.
This is the level where the file stops being a record and becomes a design. Change the parameter and the model rebuilds. Add a feature and it lands in a tree that makes sense to the next person who opens it, including the version of you that has forgotten everything about this part.
It is also by far the hardest level to reach, which is why so few tools claim it. Producing a tree means deciding things the scan cannot tell you. Was that a pocket cut into a block, or a boss standing proud of a plate? Both produce an identical final shape and behave completely differently the moment you change a dimension. Was that 5.97 mm bore meant to be 6 mm, or meant to be a quarter inch at 6.35 mm on a part that has had a hard life? A tool that cannot ask you that question has to guess, and it will guess wrong on precisely the parts you care about most. Murphy works in reverse engineering too.
Which one you need
Work backwards from what happens after delivery.
If the model goes straight to a printer or a CAM seat and never changes, level two is enough and you should buy the cheapest reliable option. A watertight STEP inside a tenth of a millimetre is a finished job, and paying for editability you will never use is like buying a manual gearbox for a lift.
If somebody is going to modify the part, level two is a trap. It looks like a deliverable. It lands in the drawing pile. Six months later a different engineer inherits a solid with no history, mutters something unprintable, and quietly remodels the whole thing from scratch. You paid for the scan work twice and only counted once.
If you are building a product family, a size range, or a fixture that gets adapted for the next customer, only level three pays back. The value is not the first model. It is the fiftieth variant, which took twenty minutes because the tree held.
The honest catch
Level three is not fully solved by anybody, us included. Prismatic machined parts and turned parts come out well. Organic freeform surfaces, castings with soft blends everywhere, and heavily worn parts are harder, and the right answer there is often a hybrid: a tree for the features that matter and a fitted surface for the ones that do not.
The useful question to ask any vendor is not whether they do scan to CAD. Everyone does scan to CAD. The question is what happens when you change a dimension in their output. If the answer involves opening the file and having a look first, you already know which level you are standing on.