In reverse engineering, an as-designed model is the nominal, corrected parametric CAD model representing what a physical component was originally designed to be , before operational wear, corrosion, thermal cycling, and mechanical loading degraded its geometry. It is not a copy of what currently exists; it is an engineering reconstruction of what was originally intended to exist.
Producing an as-designed model requires significantly more engineering expertise than producing an as-scanned mesh. It demands that engineers understand not just what the scanner measured, but what the deviation between the measured value and the correct value means , and how to correct it.
Restoring What Was Originally Designed
Every manufactured part is produced to a nominal design with defined tolerances. A shaft, for example, might be specified at 50.00 mm diameter with an h6 tolerance (−0.000 / −0.016 mm). After years of service, that shaft may measure 49.51 mm , nearly 0.5 mm undersize due to abrasive wear. Scanning that shaft and modeling it at 49.51 mm produces a replacement that is undersize from the first day of operation, guaranteeing early bearing failure.
The as-designed model restores the shaft to its nominal 50.00 mm h6 dimension , the value it was manufactured to before wear , so the replacement part functions as originally engineered.
The Distinction Between As-Scanned and As-Designed
| Property | As-Scanned | As-Designed |
|---|---|---|
| Captures wear | Yes | No , corrects it |
| Captures corrosion | Yes | No , corrects it |
| Dimensional values | Measured (degraded) | Nominal (intended) |
| Output format | Mesh (STL, OBJ) | Parametric solid (STEP) |
| Suitable for CNC machining | No | Yes |
| Suitable for forensic analysis | Yes | No |
| Engineering judgment required | Minimal | Significant |
The Dimensional Rationalization Process
Dimensional rationalization is the systematic process by which GDS engineers convert measured scan values into correct, intended nominal values. It is the engineering core of the as-designed model workflow.
Bore and Shaft Restoration
A least-squares cylinder is fitted to the scan data to establish the measured center axis and diameter. The measured diameter is then analyzed against standard engineering fit systems (ISO 286 metric or ANSI B4.2 inch):
- If the bore measures 49.87 mm, it is rationalized to 50.00 mm H7 (a standard slip-fit bore)
- If the shaft measures 49.93 mm, it is rationalized to 50.00 mm h6 (a standard clearance-fit shaft)
The rationalized values are modeled as mathematically exact analytical cylinders in the STEP file.
Planar Surface Correction
Machined flat faces are fitted using a least-squares plane to establish their actual orientation. Perpendicularity, parallelism, and coincidence constraints are applied between mating faces using the measured surface normals as the reference. Warped or heat-distorted faces are corrected to nominal perpendicularity, not copied in their deformed state.
Standard Thread Application
Optical scanners cannot resolve internal thread geometry at the resolution required for manufacturing. GDS technical team can physically gauge internal and external threads with thread plug gauges or ring gauges when access allows, then model common standard thread profiles such as ISO 261 metric or ASME B1.1 unified inch when that approach matches the project requirements. The thread is modeled as a nominal standard feature, not as a scanned approximation.
Casting Wall Normalization
Cast components frequently exhibit wall thickness variation from shrinkage during solidification, heat distortion, and wear. GDS engineers calculate the average wall thickness across the scan data, apply symmetry analysis where the casting design allows, and model the wall at its nominal average thickness rather than at any single worn measurement.
How GDS Applies Engineering Judgment
Dimensional rationalization requires the application of mechanical engineering knowledge that the scan data itself cannot provide. GDS engineers bring:
- Knowledge of standard tolerance systems (ISO 286, ANSI B4.2) to rationalize measured values
- Understanding of manufacturing processes (casting, forging, machining) to identify which deviations are wear and which are original design features
- Failure analysis experience to identify features that must be corrected vs. features that should be preserved
- Mechanical assembly logic to ensure mating interfaces are dimensionally compatible
All rationalization decisions are documented in the project report. Every dimension that was corrected from the measured value is flagged explicitly, with the measured value, the rationalized nominal value, and the engineering basis for the correction.
Verification: Proving the Restoration Is Accurate
When verification reporting is included in scope, a GDS as-designed model can be checked using Computer-Aided Verification (CAV): the completed STEP model is aligned over the original raw scan mesh in metrology software, and the spatial deviation between the two is calculated and color-mapped. This heatmap confirms that:
- The as-designed model falls within the expected deviation range (rationalization corrections, not random errors)
- No surfaces were inadvertently modeled beyond the allowable tolerance band
- The model correctly represents the reconstructed design intent
The CAV report is delivered alongside the STEP file as a documented quality reference.
When an As-Designed Model Is Required
| Scenario | As-Designed Required? |
|---|---|
| CNC machining of replacement part | Yes , mandatory |
| Injection molding of replacement part | Yes , mandatory |
| FEA structural analysis | Yes , nominal geometry needed |
| Engineering drawing generation | Yes , nominal dimensions needed |
| 3D printing of forensic replica | No , as-scanned mesh is sufficient |
| Facility documentation | No , as-built BIM is appropriate |
Quick Facts
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FAQ
What is dimensional rationalization?
Dimensional rationalization is the engineering process of converting worn, measured scan values to their correct nominal design values. For example, a bore measured at 49.87 mm due to wear is rationalized to 50.00 mm H7 , the standard slip-fit dimension it was originally manufactured to. GDS engineers can reference standard tolerance systems such as ISO 286 or ANSI B4.2 when appropriate and document rationalized values when required by the project scope.
How does GDS decide what the nominal dimension should be when no drawings exist?
GDS engineers analyze the measured scan value in the context of the part's functional role (bearing bore, sealing face, mating flange), cross-reference against standard engineering tolerance systems, and apply manufacturing process knowledge to determine the most likely original nominal dimension. Rationalized dimensions should be documented and flagged in the project report when that reporting requirement is included in scope.
Is an as-designed model always better than an as-scanned model?
No , the correct choice depends on the project goal. For manufacturing a functional replacement part, as-designed is mandatory. For forensic failure analysis, wear tracking, or producing an exact replica of the existing component, as-scanned is the correct deliverable. GDS discusses both options during project scoping to ensure the right model type is specified.
Connect this article to the right GDS workflow
Most physical-to-digital projects touch more than one service. GDS can help determine whether the right starting point is 3D laser scanning, 3D modeling, reverse engineering, or consulting before scope, pricing, schedule, and deliverables are finalized.
GDS lists nationwide coverage from its locations page, including posted major metropolitan areas such as Houston, Dallas, San Antonio, Austin, Los Angeles, San Diego, San Jose, Long Beach, Fort Worth, Irvine, Riverside, New Orleans, Baton Rouge, Shreveport, Las Vegas, and Beverly Hills.
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GDS delivers verified STEP AP242 as-designed models from any worn physical part , complete with dimensional rationalization report and CAV deviation heatmap.