A CAD file holds the shape of a part, and depending on the format it may also hold the steps used to build it, the dimensions that drive it, the material assigned to it and the structure of an assembly. What it usually does not hold is everything a manufacturer needs to make the part correctly: tolerances, surface finish, heat treatment, inspection requirements and the exact material grade normally live on the drawing or in the order.
Understanding this gap prevents a lot of expensive mistakes. This guide explains what is inside different kinds of CAD files, what gets lost when a file is exported to STEP or IGES, and what you should never assume a model contains.
Quick answer
- Native CAD files (SLDPRT, IPT, CATPart and similar) can hold geometry, the feature history, parameters, assembly mates, materials and properties.
- Neutral files (STEP, IGES) usually hold only the final geometry, sometimes with assembly structure, names and colours. History and parameters are almost always lost.
- Tolerances, finish, heat treatment and certification are typically not in the model. They sit on the 2D drawing, or in 3D annotations if the designer added them and the format carries them.
- Treat the model as nominal. A model shows the ideal shape, not the allowed variation.
What a CAD file can contain
1. Geometry
This is the core of every CAD file: the faces, edges and vertices that define the shape, plus the coordinate system and units. In a solid model the geometry is exact, with true cylinders, planes and curves, not triangles. Geometry is the one thing every CAD format carries.
2. Feature tree and history
A native CAD file records how the part was built: sketches, extrudes, cuts, holes, fillets, patterns and the order in which they were made. This feature tree lets a designer go back and change a dimension, and the model updates. It is also a record of design intent, showing why a part was built the way it was.
3. Parameters, equations and configurations
Dimensions in a native file can be linked by equations or driven from tables, so one file can describe a family of sizes. A single native file may contain several configurations of the same part, such as different lengths or hole patterns.
4. Assembly structure
An assembly file holds the list of components, their positions, the mates or constraints between them and the hierarchy that becomes the bill of materials. Moving one part can update the rest.
5. Appearance and metadata
Colours, textures and layers sit here, along with properties such as part number, description, revision, author and drawing number. These are only as accurate as the person who filled them in.
6. Material assignment
Most CAD systems let the designer assign a material to a part. That assignment gives the software a density so it can calculate mass and centre of gravity, and it may set an appearance. It is useful, but it is not a material specification, which we cover below.
7. Annotations and PMI
Some CAD files hold 3D annotations, often called product and manufacturing information (PMI). These can include dimensions, tolerances, datums, GD&T symbols, surface finish symbols and notes attached directly to the 3D model. This approach is often called model-based definition. It is only present if the designer added it.
What survives when a file is exported to STEP or IGES
Neutral formats exist so that different CAD systems can exchange parts. They do this by passing on the final shape, and most other information is dropped or reduced.
| Information | Native CAD file | STEP file | IGES file |
|---|---|---|---|
| Final solid or surface geometry | Yes | Yes | Yes (older versions may carry surfaces only) |
| Feature tree and history | Yes | No | No |
| Sketches and driving dimensions | Yes | No | No |
| Equations and configurations | Yes | No | No |
| Assembly structure | Yes | Usually | Limited |
| Colours and layers | Yes | Often | Often |
| Material | Yes | Usually not, or only a name | Usually not |
| 3D annotations (PMI) | If added | Only if exported (mainly newer versions such as AP242) | Rarely |
| Tolerances | Only as PMI | Only as PMI | Rarely |
The result is sometimes called a "dumb solid": the shape is accurate, but there are no features to edit. A hole is just a cylindrical face, not a hole feature with a diameter you can change. Editing it means using direct modelling tools instead of changing a parameter.
What a CAD file usually does not contain
Tolerances
A 3D model shows nominal dimensions, the ideal values. It does not tell you how much a dimension is allowed to vary, unless PMI is present. A hole modelled at 20.000 mm could be a loose 20 mm clearance hole or a 20 mm H7 bore with a 0.021 mm tolerance band. Only the drawing, or PMI, says which.
Designers also model to different conventions. Some model at nominal size, some at the middle of the tolerance band. You cannot tell which from the model alone, so never program or inspect a tight feature from the model dimension without checking the drawing.
Fits, surface finish and coatings
Fit codes such as H7/g6, roughness values (Ra), plating, anodising and paint specifications are drawing information. The model does not show them.
Material grade, condition and certification
The material in a CAD file is often a generic entry such as "Steel" or "Aluminium". A manufacturer needs the exact grade (for example 304 vs 316 stainless), the temper or condition, the standard and sometimes a mill test certificate. The density used by the CAD system may also differ from the real material, so mass figures from the file are estimates.
Heat treatment and hardness
Hardening, tempering, stress relieving and hardness targets are process requirements that belong on the drawing or purchase order.
Real threads
Many CAD systems model a tapped hole as a plain hole at the tap drill diameter, with the thread shown as a cosmetic annotation. When such a part is exported to STEP, the thread information may be gone and only a plain hole remains. For an M8 x 1.25 thread, that hole could be about 6.8 mm, not 8 mm. If you see a hole that does not match a standard size, check the drawing for thread callouts before assuming anything.
Quantities, cost, lead time and inspection requirements
How many parts are needed, when they are needed, how they will be inspected and how they should be packed are order details. They are not stored in the model.
The current revision
A file on your desktop may be an old revision. Check the revision number and date against the drawing and the order.
Why the drawing still matters
In many companies the 2D drawing remains the controlling document for tolerances and acceptance, even when a 3D model exists. The model describes the shape and the drawing describes what is acceptable. When they disagree, the drawing normally wins, but the rule should be agreed in writing, not assumed.
Model-based definition aims to put all of this inside the 3D model. It is growing, particularly with newer STEP versions that can carry PMI, but adoption is uneven. Many suppliers and customers still work from drawings, and a PMI-rich file only helps if the receiving software can read the annotations.
What each person should check
- Designers: Add tolerances, material grade, finish and notes to the drawing or PMI. Do not rely on the model to say it.
- Buyers: Make sure the request includes the model, the drawing, the material specification and the quantity.
- Estimators: Open the file, confirm units and size, and read the drawing for tolerances before pricing.
- Machinists and programmers: Verify critical dimensions against the drawing, check threads and confirm the revision.
What to send with a CAD file
- The 3D model in STEP (and the native file if appropriate)
- A 2D drawing in PDF with dimensions, tolerances, threads, finish and notes
- The exact material grade and condition
- Heat treatment or coating requirements
- Quantity and revision number
- Any inspection requirements
- A note saying which document controls if the model and drawing disagree
How to check what is inside a file you received
- Confirm the format. Is it solid geometry, or a mesh such as STL or 3DS? Meshes have no true features.
- Check units and size. Measure one known dimension and compare it to the drawing.
- Look for multiple bodies. Some files contain several parts, hidden bodies or loose surfaces.
- Inspect the holes. Check whether tapped holes appear as plain drill-size holes.
- Look for annotations. Open the file in a viewer that shows PMI if you expect it.
- Check the revision. Match the file name, metadata and drawing.
- Compare with the drawing. Look for any differences before you start.
Common mistakes
- Assuming a 3D model includes tolerances
- Expecting to edit features in a STEP file
- Trusting the material or mass in a file without a specification
- Machining a threaded hole at the modelled diameter
- Using an old revision
- Mixing up model and drawing authority
- Sending only a model when the order needs a drawing
FAQ
Does a STEP file contain tolerances? Usually not. A STEP file normally holds geometry only. Newer versions such as AP242 can carry tolerances as PMI, but only when the exporting software includes them and the receiving software reads them.
Does a STEP file contain feature history? No. History and parametric features are lost when a model is exported to a neutral format.
Can I edit a STEP file? You can modify the geometry using direct modelling tools, but you cannot change the original feature dimensions the way you could in the native file.
Does a CAD file contain the material? A native file may have a material assigned, which gives the software a density. A neutral file often does not carry it, and even when it does, the entry may not be a full specification.
Which controls, the model or the drawing? It depends on the contract or company practice. In many cases the drawing controls tolerances. Agree the rule in writing.
Is the weight from a CAD file accurate? It is an estimate. It depends on the density used, the quality of the geometry and whether features such as threads and finishing allowances are modelled.
Summary
A CAD file is a strong record of shape and, in native form, of how a part was built. A neutral file such as STEP keeps the shape but loses the history, and neither format reliably holds tolerances, finish, heat treatment or material certification unless PMI is added. Treat the model as nominal, rely on the drawing for acceptance limits, and check units, threads, material and revision before any work starts.
Check what you received
AlwaysReach's free CAD Viewer opens STEP, IGES, SLDPRT, 3DS and Rhino files in your browser, so you can inspect a model and measure features before you quote. The STEP File Weight Calculator can give a quick mass estimate from the file. As this guide explains, weight and dimensions from a model are nominal figures, so confirm anything critical against the drawing.