How reverse engineering works: a part, step by step

What reverse engineering is, for a mechanical part

Reverse engineering a mechanical part means working back from the physical part to the design data that should exist for it: a 3D CAD model, and usually a drawing, that someone can change and manufacture from.

It’s done when that data is missing or can’t be trusted:

• A spare or legacy part with no drawings, or drawings that no longer match the part.
• A supplier that has gone, or tooling that has to be remade.
• A part that has to be improved, fitted to a new mating part, or made by a different process.
• An older or competing product that you want to understand (benchmarking).

The goal isn’t a copy of the part’s surface. A copy reproduces the wear, the dents and the measuring noise. The goal is the part as it was meant to be: its datums, its nominal dimensions and its features, in a model you can edit.

The seven steps at a glance

Scan

Profile

Solid

Review

1. Define the job

Before anyone measures anything, decide what the model is for. A part that only has to be remade as it was needs different checks from one that’s being redesigned.

• What does the part do, and what does it mate with? Those faces and bores are the ones that have to be right.
• How accurate does it need to be? The tolerance comes from the function: a drawing if one survives, the mating parts, or a standard fit.
• How was it made, and from what? Turned, milled, cast, moulded or bent from sheet: the process explains the shapes, such as draft on a casting or a bend radius on a bracket. A scan can’t tell you the material.
• What do you need at the end? A STEP file for a supplier, a native CAD file to keep editing, a drawing for the workshop, or all three.

2. Capture the geometry

Choose the measuring method for the part, not out of habit:

• Hand tools such as calipers, micrometers and gauges suit simple turned and prismatic parts. They’re accurate where you measure and silent everywhere else.
• A CMM (coordinate-measuring machine) measures the dimensions that need the most accuracy: bearing bores, hole positions, datum faces.
• A 3D scanner, structured-light or laser, captures complex and freeform shapes and castings in one go.
• CT scanning reaches internal passages and features that a surface scanner can’t see.

A scanner gives you a point cloud, which is usually turned into a mesh of triangles: STL, OBJ, PLY or 3MF. Shiny, black and transparent surfaces scatter the light, so a thin coat of scanning spray and more viewing angles help. Many jobs combine methods: scan the whole part, then measure the critical dimensions by hand or on a CMM. (More in Scan to CAD, explained.)

3. Prepare and align the scan

A raw scan needs light cleaning: delete stray points and the fixture, merge the scans from each side, and fill small gaps. Don’t smooth it hard. Heavy smoothing rounds off the edges you need to find.

Then align the mesh to the part’s own references, its datums, not to wherever the scanner put it. For a turned part that’s the axis of rotation. For a bracket it’s usually the mounting face and a hole pattern. Every later dimension is measured from these, the same way a drawing dimensions a part from its datums.

4. Divide the scan into surfaces

A mesh is only triangles. To measure it the way an engineer would, divide it into regions that each belong to one surface: this patch is a plane, that one a cylinder, this a cone, that a fillet. Fit the ideal shape to each region and you get numbers: a plane’s orientation, a cylinder’s axis and diameter, a fillet’s radius.

This is where the scan becomes geometry you can reason about. It’s also where the first judgment calls appear. Is this patch one slightly curved face, or two flat faces meeting at a shallow angle? Is that edge a small fillet, or a sharp edge the scanner rounded off?

5. Decide the design intent

Fitted sizes are measured sizes. They include wear, manufacturing tolerance and scan error. Before you build anything, turn them into the values the designer meant:

• Standard sizes. A fitted diameter just under a standard size is usually that size, worn.
• Fits with mating parts. A bore that takes a bearing follows the bearing’s size and an ISO 286 fit.
• Symmetry and patterns. Holes sit on a pitch circle, faces square to a datum, features repeat at equal spacing.
• How it was made. A turned part is round about one axis. A casting has draft.

Where the evidence doesn’t settle a value, flag it with what you know and leave it for an engineer to decide. On the example shaft, the shoulder position at z −35.00 mm is one of those. (More in Design intent: why a worn part isn’t the drawing.)

6. Rebuild it as features

Now build the model the way a designer would have: sketch a profile on a datum plane, then extrude, revolve, cut, and add holes and fillets in a sensible order. Give every key size a name, so that changing it later updates the model the way the design intends.

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

The result is a parametric model with a feature tree. Converting the mesh straight to a solid skips steps 4 to 6. It gives you the scanned shape with one face per triangle and nothing to edit (see STL to STEP).

7. Verify, then deliver

1. Compare the model with the scan on a deviation map, aligned on the datums, and give every area outside the tolerance a reason: wear, damage, a gap in the scan, or a mistake.
2. Confirm every key dimension with independent evidence: the mating part, a measurement, a standard.
3. Change a parameter and check that the model follows the way the design intends.
4. Deliver. STEP for suppliers and other CAD tools, the native file if you’ll keep editing (STEP doesn’t carry the feature tree), and a drawing with tolerances for manufacturing.

More in Is your rebuilt CAD right? and STEP or IGES?

Where AI fits

Steps 3 to 6 are where the redrawing happens, and much of that work is repetitive interpretation. That’s the part AI can take on. Oscar, OriginShape’s AI copilot for reverse engineering, finds the datums, recovers the intended shape, builds sketches and features with named dimensions, and checks the result against the scan on a deviation map. It flags every dimension it can’t prove, with its evidence, and the engineer decides.

Steps 1 and 7 stay with people: what the part is for, and signing the model off. Try Oscar in your browser at app.originshape.design, or inside FreeCAD with Oscar for FreeCAD. Have a part with no drawings? Tell us about it.

Sources and further reading

• Reverse engineering. Wikipedia, on the general idea and its uses in mechanical engineering.
• 3D scanning. Wikipedia, on structured-light, laser and other scanners, and their limits.
• Coordinate-measuring machine. Wikipedia, on measuring the dimensions that need it most.
• Industrial computed tomography. Wikipedia, on scanning internal features.
• Geometric dimensioning and tolerancing. Wikipedia, on datums and tolerances (ASME Y14.5, ISO 1101).
• Engineering fit. Wikipedia, on hole and shaft fits and the ISO 286 system.
• Scan to CAD, explained. Our post on meshes, surfaces and features.
• Why reverse engineering is hard: 6 problems to solve. Our post on what goes wrong at each step, and what helps.

Questions