# Design intent: why a worn part isn’t the drawing

A scan measures a part as it is today. Recovering the design means deciding what it was meant to be: nominal sizes, mating parts, and what to flag.

- Published: 2026-10-05
- Author: OriginShape (https://originshape.design/about)
- Section: Explainers
- Topics: Design intent, Reverse engineering, Tolerances, Worn parts
- Web page: https://originshape.design/blog/design-intent-reverse-engineering

![The example shaft as a solid with Oscar’s yellow mark on the shoulder, beside a review list waiting for the engineer.](https://originshape.design/images/blog/design-intent-reverse-engineering.png)

_FIG. 1: The example shaft with its shoulder position flagged for review, the one dimension waiting for the engineer. Illustration with the example part’s values._

## In short

- A scan records a part as it is today: worn, and somewhere inside its manufacturing tolerance. The design behind it used deliberate, nominal sizes.
- Design intent is the set of decisions behind a shape: which sizes matter, how features relate, and how the part fits with others.
- Recovering it means weighing each measurement against evidence: mating parts, standard sizes, relationships, how the part was made, and where it doesn’t wear.
- Where the evidence agrees, the nominal value goes into the model with a note of why. Where it doesn’t, the dimension is flagged for an engineer.
- Averaging a worn scan into the model copies the damage. A flagged dimension with its evidence is worth more than a confident guess.

## What design intent means

A part on a drawing is a set of decisions. A diameter is the size it is because a bearing sits on it. A face is square to the axis because something seats against it. Holes share a pitch circle because they line up with another part. Design intent is that set of decisions: which sizes matter, how features relate to each other, and how the part fits with the parts around it.

In CAD, intent lives in the model as parameters and relationships. On the [example shaft](https://originshape.design/#scope) used across this site, the taper runs from Ø36 to Ø28 mm over 10 mm, and its 21.80° angle follows from those three values. Model it that way and the taper stays consistent when a diameter changes. Type the angle in as a measured number instead and the two can drift apart.

A scan records none of this. It records where the surface is, today.

## Why a scan can’t read it off

Even a perfect scan measures the part, not the drawing. Three things stand in the way:

- **Wear:** running surfaces and seats wear, edges round over, and faces erode. The scan measures what is left, not what was made.
- **Tolerance:** no machine holds a dimension exactly to its nominal value, so every part sits somewhere inside a tolerance band. One scan can’t tell you where the middle of that band was.
- **Damage and distortion:** a bent shaft or a warped casting scans perfectly well. The scan simply records the bend.

Noise and gaps in the data add to this (see [Scan to CAD, explained](https://originshape.design/blog/scan-to-cad-explained)), but even a clean scan of a worn part gives worn numbers. Average them into the model and the copy reproduces the damage.

_FIG. 2: Wear changes the measurement, not the intent. The scan follows a worn seat on the journal; the nominal Ø24.00 mm holds. Illustration on the example part, with the wear exaggerated._

## The evidence an engineer weighs

Recovering intent means checking each measurement against evidence that the scan doesn’t contain:

- **Mating parts:** a shaft and the hole it fits share one nominal size; only their tolerances differ. A bearing, seal, or bushing with a known part number gives you the nominal size of its seat directly.
- **Standard and preferred sizes:** designers choose round values, catalogue sizes, and preferred numbers such as the Renard series, standardised as ISO 3. A measured value just under a standard size is a strong hint.
- **Relationships:** diameters that share an axis, faces square to it, holes on one pitch circle, features mirrored about a centre plane. Intent often shows up as a relationship rather than a number.
- **How the part was made:** a turned part has its diameters on one axis, and a casting has draft and fillets. Machined faces carry the precise dimensions; as-cast surfaces don’t.
- **Where it doesn’t wear:** a surface no other part touches stays closer to its original size than a seat that has been running. Compare the two.
- **Documents:** an old drawing, even an incomplete one, a purchase specification, or a sister part from the same batch.

## From evidence to a dimension

With the evidence gathered, each dimension goes one of two ways:

1. **The evidence agrees.** One nominal value fits the measurement, the mating part, and the standard size. It goes into the model as a named dimension, with a note of why.
2. **The evidence doesn’t agree, or there isn’t enough of it.** The dimension is flagged with what was measured and what was checked, and an engineer decides with an extra measurement, a drawing, or their judgment.

The example shaft shows both. Its diameters rebuild as named dimensions; the Ø36 collar, for instance, comes from the cross-section at z −10.00. The shoulder position at z −35.00 mm is flagged instead, and waits for the engineer.

**Example decision record for the illustrative shaft**

| Dimension | Value | How it was established | Status |
| --- | --- | --- | --- |
| Collar diameter | Ø36.00 mm | Cross-section at z −10.00 | Confirmed |
| Taper | 21.80° | Follows from Ø36 to Ø28 mm over 10 mm | Derived |
| Shoulder position | z −35.00 mm | Proposed by Oscar | Waiting for the engineer |

## Write the decisions down

A rebuilt model should come with its reasons. For each key dimension, record the measured value, the nominal you chose, the evidence, and who confirmed it. It takes minutes, and it turns “why is this the size it is?” from an argument into a lookup.

It also protects the next person. When the part changes or fails, they can see which dimensions came from evidence and which were judgment calls.

## Oscar proposes, the engineer decides

This is how [Oscar](https://originshape.design/#oscar), OriginShape’s AI copilot for reverse engineering, works. Its AI agents propose nominal values where the evidence supports them and flag the rest with the evidence attached. In the example session on our home page, Oscar finds a worn bore and asks before it commits; the engineer sets the size. Every step stays visible and can be undone.

_FIG. 3: From the example session on our home page: Oscar flags a worn bore, and the engineer sets its nominal size. Illustration, not a screenshot of the app._

Have a worn part you need back in CAD? [Tell us about it](https://originshape.design/contact).

## Sources and further reading

- [Engineering tolerance](https://en.wikipedia.org/wiki/Engineering_tolerance). Wikipedia, on nominal (basic) sizes, tolerances, and fits.
- [Preferred number](https://en.wikipedia.org/wiki/Preferred_number). Wikipedia, on the Renard series, standardised as ISO 3.
- [Scan to CAD, explained](https://originshape.design/blog/scan-to-cad-explained). Our explainer on meshes, surfaces, and features.

## Questions

### What is design intent in reverse engineering?

The decisions behind a part’s shape: which sizes matter, how features relate to each other, and how the part fits with the parts around it. Recovering it means rebuilding those decisions as nominal dimensions and relationships, not copying the scanned surface.

### How do you choose a nominal dimension from a worn part?

Weigh the measurement against evidence the scan doesn’t contain: the mating parts, standard and catalogue sizes, relationships such as a shared axis, how the part was made, and surfaces that don’t wear. If one value fits all of it, use it and record why; if not, flag it for an engineer.

### Why not just average the scan?

Because the average of a worn surface is still worn. Averaging smooths noise, but it can’t undo wear or find the middle of the original tolerance, so the copy reproduces the damage.

### Can software recover design intent automatically?

Partly. Software can find axes, planes, and likely standard sizes, and propose nominal values. Where the evidence is thin, the dimension should be flagged with its evidence so an engineer makes the call.

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From the OriginShape blog: https://originshape.design/blog. Have a part without usable CAD? https://originshape.design/contact