You exported the model.
It looked fine.
You sent the STEP file to a vendor.
They replied:
"We can't manufacture this."
What happened?
Most vendor rejections aren't about bad design.
They're about file-level reliability issues that appear after export.
If your STEP file was rejected, here are the most common technical reasons why.
1. Open Shells (Non-Watertight Geometry)
Your model may appear solid in CAD.
After export to STEP, small surface stitching gaps can appear. These create open shells, meaning the body is no longer watertight.
CAM systems require closed, valid volumes to generate toolpaths. If a model is not watertight:
- Toolpaths may fail
- Simulations may crash
- Material boundaries become ambiguous
- Vendors lose confidence in machining reliability
Even tiny gaps can cause rejection.
If you're unfamiliar with this issue, read our full breakdown of what an open shell is in CAD.
For deeper technical background on the STEP format, see the official overview of the ISO 10303 standard on ISO.org.
2. Knife Edges and Zero-Thickness Geometry
A knife edge occurs when two surfaces meet at a mathematically sharp boundary with no material thickness.
While technically valid geometry, knife edges are:
- Difficult or impossible to machine
- Structurally weak
- Problematic for CAM toolpath generation
CAM software and CNC machining rely on realistic material thickness and manufacturable edge conditions. Zero-thickness geometry often causes toolpath instability or breakage risk.
Learn more in our guide to knife edges in CAD.
3. Non-Manifold or Ambiguous Topology
Non-manifold geometry occurs when:
- An edge is shared by more than two faces
- Internal surfaces exist unintentionally
- Bodies intersect improperly
- Surface bodies remain unstitched
These conditions may not produce visible warnings in CAD.
But when imported into CAM systems like those used in CNC machining workflows, ambiguity becomes failure.
For a technical explanation of non-manifold geometry, see this overview from Autodesk.
4. Wall Thickness Violations
Even if your geometry is technically valid, vendors may reject files when:
- Walls are too thin for machining
- Thin sections risk deflection
- Geometry violates minimum material constraints
Manufacturing processes have practical limits. If your file doesn't respect those constraints, vendors often reject rather than guess.
Wall thickness problems are especially common in early prototype cycles.
5. Units, Tolerances, and Translation Issues
Sometimes the geometry isn't broken.
It's ambiguous.
Common causes:
- Incorrect unit interpretation (mm vs inches)
- Overly tight tolerances
- Missing manufacturing intent
- Export tolerance thresholds too loose
STEP is a translation format.
Translation introduces variability.
Your CAD system may tolerate ambiguity. Vendors won't.
For background on how STEP functions as a neutral file format, see the overview from NIST.
Why These Issues Survive Export
Export is often treated as final.
It isn't.
STEP and IGES are translation formats. During translation:
- Surface definitions may change
- Stitching tolerances shift
- Geometry precision thresholds differ
- Internal topology may degrade
Your design intent remains intact.
But geometry integrity may not.
The Real Cost of Vendor Rejection
The biggest cost isn't the file error.
It's the loop:
- Vendor flags issue
- File sent back
- Engineering investigates
- Model updated
- Re-export
- Re-send
- Re-quote
That's days lost.
Multiply that across multiple prototype cycles, and hardware timelines quietly stretch.
Manufacturing reliability is directly tied to iteration speed.
How to Prevent STEP File Rejection
Before sending a file to a vendor, validate:
- Is the geometry watertight?
- Are there open shells?
- Any knife edges?
- Any non-manifold conditions?
- Are wall thicknesses realistic?
- Are units and tolerances clear?
This is what we call DFM pre-flight.
Not full manufacturability analysis.
Not design optimization.
Just file-level reliability validation before handoff.
Manufacturing Reliability Is a Speed Multiplier
The fastest hardware teams don't design faster.
They remove preventable rework.
Vendor rejection is usually a reliability gap upstream.
Close the gap before handoff.
If you want to validate STEP or IGES files before sending them to a vendor, try our geometry error checker.