Preparing and optimizing STEP files for manufacturing
A STEP file is the 3D model your manufacturer actually machines from. A clean STEP file gets faster, cheaper and more accurate quotes. This guide covers what STEP is, how to export it properly and how to make the design inside it cheaper to make.
What a STEP file is
STEP (ISO 10303, file extension .step or .stp) is the neutral exchange format for 3D CAD. It stores exact boundary geometry: true planes, cylinders and spline surfaces, not triangles. That is why every CAM system can import it and why it is better than STL for manufacturing.
| Protocol | Contains | Use it when |
|---|---|---|
| AP203 | Geometry and basic structure | The receiver has old software |
| AP214 | Geometry, colours, layers | Default for most CAD systems, fine for quoting |
| AP242 | Geometry plus PMI: tolerances, GD&T, notes | You use model-based definition and the shop's CAM reads it |
STEP compared with other formats
| Format | Exact geometry | Good for |
|---|---|---|
| STEP | Yes | Machining, moulding, casting quotes; the default choice |
| Parasolid (.x_t) | Yes | Shops using SolidWorks, NX or Solid Edge |
| IGES | Surfaces, often not solids | Legacy only; avoid when STEP is available |
| STL / 3MF | No, triangles | 3D printing only |
| DXF | 2D only | Laser, waterjet and the flat pattern of sheet metal |
Export checklist
- One part per file. Export each part separately unless the shop asked for an assembly.
- Millimetres. Check the export units. A part that arrives 25.4 times too big or small was exported in inches.
- Solid body. Export a closed solid, not surfaces. If your CAD shows the part as a surface body, fix the gaps first.
- Final geometry. Hide or delete construction bodies, sketches and reference geometry.
- Threads as tap drill holes. Model threaded holes at tap drill size with a cosmetic thread. STEP does not carry thread data, so the thread must be on the drawing.
- Name with part number and revision, for example MP-1001_B.step, and use the same in the drawing title block.
- Sensible origin. Put the origin on a datum corner or the centre of a turned part. It helps the shop and your drawing.
Making the design in your STEP file cheaper
Most cost is decided in the model, long before anyone quotes it. The changes below rarely affect function and often save 10–40% on machined parts.
- Give inside corners a radius of at least a third of the pocket depth.
- Fit the part into standard stock: 19.5 mm instead of 20.5 mm can drop a plate size.
- Use standard hole sizes and reasonable thread depths.
- Put features on as few sides as possible.
- Avoid walls thinner than 1 mm in metal.
- Keep tolerances loose except where they are functional.
Once the model is right, the DrawMyPart tool turns the STEP file into a dimensioned drawing per body, so the shop can quote exactly what you designed.
Common STEP problems
- Tessellated STEP. A STEP made from a mesh (for example from a scan or an STL) contains thousands of tiny flat faces. CAM cannot machine it cleanly. Rebuild the model in CAD.
- Huge files. Embedded colours per face, text and fasteners from libraries bloat the file. Remove what is not part of the machined part.
- Missing or broken faces. Usually caused by export tolerances. Run your CAD's geometry check before export.
- Assemblies sent as one body. Parts that touch can get merged into one solid by some exporters. Check that each part is still its own body; DrawMyPart lists every body it finds.
STEP plus drawing
Send the STEP and a PDF drawing together. The STEP defines geometry; the drawing defines what the STEP cannot: tolerances, threads, fits, surface finish, material, coating and inspection dimensions. Read what a manufacturing drawing must contain.
Draw your own part
Drop a STEP file into DrawMyPart. It finds every body, asks you to confirm threads, fits and tolerances, and draws a sheet your manufacturer can build from.
Open the tool