Reverse engineering a spare part means measuring the physical part, rebuilding it as a CAD model and issuing a drawing that a manufacturer can produce to. Measurement delivers geometry only: the material, heat treatment, tolerances and surface requirements of the original have to be established separately and written on the drawing.
What is reverse engineering of a spare part?
The task comes up when the original drawing is missing, the supplier of the part has gone or the part is obsolete, but the machine still runs and needs it. The input is the worn part, ideally with its mating parts. The output is a 3D model plus a drawing that states size, tolerance, material, treatment and surface in a form any machine shop can quote. Our design and engineering support takes the same input and delivers the same output.
A worn part is not the part that was designed. Wear removes material from sliding and bearing surfaces, corrosion and damage change others, and a measurement records only what exists today. Reverse engineering is therefore interpretation: the engineer decides what the original dimension was and what the function requires.
What are the steps from worn part to production drawing?
The work runs in six steps.
Table 1. Workflow from worn part to production drawing
| Step | What happens | Output |
|---|---|---|
| 1. Document | Photograph the part in place, note its position, load and medium, mark the wear zones, collect mating parts and any unworn sample | Condition record |
| 2. Measure | Hand tools and gauges for simple features, tactile CMM for datums and bores, optical 3D scanner for free-form surfaces | Measurement list, point cloud or mesh |
| 3. Rebuild | Fit planes, cylinders and cones to unworn regions and set nominal values in CAD | Parametric 3D model, STEP |
| 4. Establish material and treatment | Chemical analysis, hardness test, coating and surface check (see the next sections) | Material and treatment specification |
| 5. Draw | Views and dimensions to ISO 129-1, geometrical tolerances and datums to ISO 1101, general tolerance class | Production drawing |
| 6. Verify | Measure the first part made against the model and fit it in the machine | Inspection report, fit confirmation |
Source: working sequence; drawing rules per ISO 129-1:2018 and ISO 1101:2017.
How do you measure a worn part?
The method follows the features. Callipers, outside micrometers and thread and radius gauges are enough for prismatic parts with a few critical sizes. A tactile coordinate measuring machine (CMM) gives point accuracy on datums and bores. An optical 3D scanner captures castings and free-form contours as a mesh without contact but sees only what the camera sees: blind bores, internal threads and undercuts have to be measured by contact. 3D scanning versus manual measurement compares what each method can and cannot tell you.
Two rules apply whatever the instrument:
- Start from unworn surfaces. Wear concentrates on bearing seats, sliding faces, thread flanks, gear teeth and sealing lips. Faces that sit against a housing, outer edges and recesses without contact wear least, so build the datums from them.
- Measure in several places and orientations. Taper and ovality show wear and form error that a single reading hides.
When you compare measuring systems, ask which acceptance test the accuracy figure comes from. ISO 10360-2:2009 defines acceptance and reverification tests for CMMs, and applies only to Cartesian machines with contacting probing. ISO 10360-13:2021 defines them for optical 3D systems. As of 2026-10-30 both are published, and ISO 10360-13 is in its periodic review.
What can a scan not tell you?
A scan is a geometric record. Material, treatment, tolerance and fit lie outside it.
Table 2. What measurement establishes and what it does not
| Property | Given by a scan? | How it is established |
|---|---|---|
| Current shape and size | Yes, within the measurement uncertainty | CMM, scanner, hand tools |
| Original nominal size | No, wear has changed it | Unworn surfaces, standard sizes, mating part |
| Tolerance and fit | No | Function and the mating part, ISO 286 fits |
| Material grade | No | Chemical analysis in a laboratory |
| Heat treatment | No | Hardness test, ISO 6508-1 for Rockwell |
| Surface finish and coating | Partly | Roughness and coating measurement on an unworn area |
| Internal features | No, not with optical scanning | Sectioning or computed tomography |
Source: ISO 6508-1:2023; ISO 9556:2025; ISO 18265:2013; ISO 286-1:2010.
Hardness narrows the heat-treated condition but does not identify the steel. ISO 18265:2013 states that converted hardness values apply directly only to the exact material tested and that tensile strength estimates are its least reliable converted values. The composition therefore needs a laboratory analysis. For total carbon in steel and iron, ISO 9556:2025 specifies infrared absorption after combustion for contents from 0.003 % to 4.5 % (mass fraction).
Material and treatment end up on the drawing as a designation and a requirement, for example a grade and a hardness range, not as a measured value copied from one worn sample.
How do you derive tolerances from a worn part?
The scan reports what the part measures, not what it was allowed to measure. Tolerances come from function:
- Identify the functional features: bearing seats, seal bores, locating diameters, sliding fits, thread engagement.
- Take the fit from the mating component or its manufacturer's data and write it as an ISO 286 fit. The ISO 286 fits and IT grades article lists the values.
- Leave everything else to the general tolerance class.
An example. A bearing seat measures Ø24.99 mm on an unworn stretch, so the nominal size is Ø25 mm. For nominal sizes over 18 up to 30 mm, tolerance grade IT6 is 0.013 mm wide (ISO 286-1:2010). A drawing that gives Ø25 mm and relies on ISO 2768-m allows ±0.2 mm for sizes over 6 up to 30 mm, a band of 0.4 mm and about 30 times wider. The part would pass inspection and could still lose the fit.
Size is also not the only requirement. A scan merges size, form and orientation in one mesh, while the drawing has to state each one. ISO 8015:2011 sets the rules for interpreting these indications, including that requirements are independent of each other unless the drawing links them. If a bore must be round and square to a face, write a geometrical tolerance to ISO 1101:2017 with a datum. The technical drawing checklist covers the title block and the other entries a supplier needs.
How do you check the first replacement part?
Treat the first part as a trial part. Measure it against the CAD model with the same method used for the original, and compare the unworn features of both parts, since these should agree. Then fit it in the machine and run it under the original conditions. Feed what you learn back into the drawing: a corrected fit, an added geometrical tolerance, a changed treatment. Raise the revision and release series production only on the confirmed drawing.
Is it legal to reverse engineer a spare part?
This is not legal advice, and the position below is as of 2026-10-30. Four points apply.
- Trade secrets. Article 3(1)(b) of Directive (EU) 2016/943 makes it lawful to acquire a trade secret by observation, study, disassembly or testing of a product that is publicly available or lawfully in the possession of the acquirer, if the acquirer is free of any legally valid duty to limit the acquisition. A contract that bans reverse engineering is such a duty.
- Patents and copyright. Neither is addressed by that article. A patent on the function of a part is not avoided by redrawing it, and drawings and software can carry copyright.
- Design rights. Directive (EU) 2024/2823 adds a repair clause. According to the European Parliament's summary, spare parts used to repair complex products, such as car parts, are exempt from design protection, provided the maker or seller of the part clearly indicates its commercial origin. Member states must transpose the directive by 9 December 2027, and member states that already protected such designs may keep doing so until 9 December 2032 for designs filed before 8 December 2024. The clause concerns designs only.
- Contracts. Supply agreements, licences and machine terms of sale can restrict copying. Read them before you order.
Frequently asked questions
- Can a 3D scan replace a drawing?
- No. A scan records the geometry of the part in its worn state as points or a mesh. It carries no tolerances, material, heat treatment or surface requirements, and it does not say which of its deviations are wear and which are original shape.
- What does a supplier need to start reverse engineering a spare part?
- The worn part, ideally with the mating parts or an unworn sample, plus the operating conditions: load, medium, temperature and the function of each surface. Without the mating parts, fits and tolerances have to be assumed.
- How do I compare the accuracy of two measuring systems?
- Ask for the result of the acceptance test the figure comes from. ISO 10360-2:2009 defines these tests for coordinate measuring machines with contacting probes, and ISO 10360-13:2021 for optical 3D systems. A bare accuracy figure without a named test is not comparable.
- Can a hardness test identify the steel grade?
- No. It indicates the heat-treated condition, not the alloy. ISO 18265:2013 states that converted hardness values apply directly only to the exact material tested, and that tensile strength estimates are the least reliable converted values.
- Is it legal to reverse engineer a spare part in the EU?
- Studying and testing a product you lawfully possess is a lawful way to acquire a trade secret under Article 3(1)(b) of Directive (EU) 2016/943, unless a valid duty such as a contract limits it. Patents, copyright and design rights are separate, so check them before you order copies.
Sources
- ISO 129-1:2018, Presentation of dimensions and tolerances, general principles (ISO catalogue)
- ISO 1101:2017, Geometrical tolerancing: form, orientation, location and run-out (ISO catalogue)
- ISO 8015:2011, GPS fundamentals, concepts, principles and rules (ISO catalogue)
- ISO 286-1:2010, ISO code system for tolerances on linear sizes (ISO catalogue)
- ISO 286-1:2010 standard tolerance grades, IT6 for 18–30 mm (table reproduced on Wikipedia)
- ISO 10360-2:2009, Acceptance and reverification tests for CMMs, linear dimensions (ISO catalogue)
- ISO 10360-13:2021, Acceptance and reverification tests for optical 3D CMS (ISO catalogue)
- ISO 6508-1:2023, Rockwell hardness test, test method (ISO catalogue)
- ISO 18265:2013, Conversion of hardness values (ISO catalogue)
- ISO 9556:2025, Steel and iron, total carbon by infrared absorption after combustion (ISO catalogue)
- Directive (EU) 2016/943 on trade secrets, Article 3 (text as reproduced by legislation.gov.uk)
- Directive (EU) 2024/2823 on the legal protection of designs (recast), summary with repair clause and dates (European Parliament Legislative Observatory)
Questions on a drawing? sales@ertas.engineering