STEP to STL Converter
Browser-local, high-precision conversion for ISO 10303-21 .step CAD files with live 3D mesh verification.
CAD Conversion and Mesh Inspection Workbench
Drop .STEP or .STP files here
Single or Batch • 100% In-Browser Memory • Zero Uploads
Large CAD files (>50MB) are recommended for desktop workstations due to mobile browser memory limits.
Tessellation Parameters
📐 Deflection Tolerances(Draft: 0.5mm • Std: 0.1mm • Fine: 0.01mm)▼
Converting CAD Model
Initializing tessellation...
model.stl
READYReady for 3D slicing and CAD export
Batch Conversion Queue
Model Statistics
Mesh Quality & Health
PASSTopological integrity and normal orientation verification completed.
100% Client-Side Privacy
Your CAD geometry never leaves your workstation. OpenCASCADE executes entirely inside a browser-local WebAssembly sandbox.
Model Inspection
Inspect the exact tessellated mesh buffer in 3D before exporting. Toggle wireframe rendering, coordinate axes, and reference grid.
Mesh Validation
Detect zero-area degenerate triangles, duplicate facets, and non-manifold edges to prevent 3D printer slicing failures.
Dedicated CAD Tools
ISO 10303 StandardsSlicer reporting non-manifold edges or inverted normals?
Bambu Studio, PrusaSlicer, and Cura reject open boundary shells. Use our in-browser mesh repair engine to heal broken CAD topology.
STEP to STL Engineering Guide & Conversion Parameters
Practical specifications for ISO 10303 B-Rep tessellation, chordal deflection, dimensional units, and multi-body assembly handling.
Tessellation Tolerance & Chordal Deflection
STEP files define exact mathematical curves and surfaces (NURBS, cylinders, spheres). STL files represent 3D geometry using planar triangular facets. Conversion requires tessellation controlled by two fundamental tolerances:
- Linear Deflection (Chordal Deviation): The maximum allowable distance between any point on a curved B-Rep surface and the approximating triangle facet.
- Angular Deflection: The maximum allowable angle between normal vectors of adjacent triangular facets along curved boundaries.
• Fine (0.01 mm / 0.20 rad): For SLA/DLP resin printing, molds, and tight mechanical bores.
• Standard (0.10 mm / 0.50 rad): Recommended default for FDM printing (Bambu, Prusa, Cura).
• Coarse (0.50 mm / 0.70 rad): For fast rendering and lightweight visualization.
Multi-Body Models & Assembly Translation
STEP files frequently contain multi-body parts or complex multi-component assemblies. The converter processes these structures based on how they are stored:
- Monolithic Assemblies: When all components are contained in a single STEP file (AP203, AP214, AP242), all solid bodies are tessellated and mapped into a single shared 3D coordinate space.
- External Reference Trees: Assemblies exported with external links (where the master file references separate
.stpparts viaDOCUMENT_FILEentities) cannot be converted from the master file alone. The browser sandbox cannot read un-uploaded files from disk.
.stp file instead? Use our dedicated STP to STL Converter for identical OpenCASCADE WebAssembly tessellation and troubleshooting guidance.Units & Dimensional Scale Preservation
ISO 10303-21 files declare explicit dimensional units via SI_UNIT or CONVERSION_BASED_UNIT entities (e.g., millimetres, metres, or inches).
In contrast, STL files are completely unitless. A 10×10×10 box in STL is stored merely as coordinates from 0 to 10. Slicers (Bambu Studio, PrusaSlicer, Cura) interpret STL values as millimeters by standard convention.
Distinguishing CAD Metrics vs. Mesh Metrics
To correctly interpret inspection data, understand the clear distinction between source CAD representation and tessellated mesh representation:
Empirical Engine Benchmarks (Standard 0.10mm Deflection)
Measured in Chromium WebAssembly (occt-import-js v0.0.23) on Linux x64 hardware. Zero cloud processing.
| Fixture / Model | Category | STEP Size | Triangles | STL Size | Cold Time | Warm Time | Bounding Box (X × Y × Z) |
|---|---|---|---|---|---|---|---|
| Simple Basic Cube | Simple Solid | 8.1 KB | 12 | 684 B | 509 ms | 129 ms | 300 × 300 × 300 mm |
| Cube 10x10 mm | Simple Solid | 9.2 KB | 12 | 684 B | 155 ms | 151 ms | 10 × 10 × 10 mm |
| Conical Surface | Curved Geometry | 9.4 KB | 286 | 14.1 KB | 131 ms | 147 ms | 7 × 7 × 7 mm |
| Rounded Cube (Fillet) | Curved Geometry | 20.6 KB | 40 | 2.0 KB | 161 ms | 126 ms | 10 × 10 × 10 mm |
| CAX-IF Tail Turbine | Curved Geometry | 39.5 KB | 202 | 10.0 KB | 185 ms | 156 ms | 120.8 × 120.8 × 48.0 mm |
| CAX-IF Multi-body Asm (AP203) | Assembly (3 bodies) | 136.5 KB | 3,384 | 165.3 KB | 260 ms | 248 ms | 5080 × 5080 × 300 mm |
| CAX-IF Mech Asm dm1-oc | Complex Assembly | 221.0 KB | 1,448 | 70.8 KB | 234 ms | 269 ms | 52.5 × 52.5 × 120.0 mm |
| CAX-IF Large Assembly as1-oc | Complex Assembly | 431.6 KB | 2,524 | 123.3 KB | 310 ms | 295 ms | 200 × 200 × 80.0 mm |
Benchmark Methodology: Cold time includes initial WebAssembly module instantiation and JIT compilation. Warm time reflects steady-state execution with compiled WASM modules. Timing represents total transfer, B-Rep parsing, tessellation, and binary STL serialization.
Client-Side Engine Limitations & Boundaries
Downstream CAD Workflows & Mesh Tools
Repair non-manifold meshes, inspect B-Rep dimensions, and prepare geometry for 3D slicing.Full support for aerospace & industrial AP203/AP214/AP242 STP assemblies.
Auto-repair inverted facet normals, cull degenerate slivers, and make STL watertight.
Extract volume, surface area, bounding box calipers, and open edge diagnostics.
Queue multiple CAD files and download converted STL models in a single ZIP archive.
Verify build plate envelope fits and optimize orientation for Bambu Studio, PrusaSlicer, and Cura.
Deep dive into mathematical B-Rep curves vs planar triangular facets for additive manufacturing.
Frequently Asked Questions
What tessellation tolerance should I choose for 3D printing?
For standard FDM 3D printing (0.4mm nozzle, 0.12–0.20mm layer height), the Standard preset (0.10mm linear deflection, 0.50 rad angular deflection) produces smooth cylindrical curves without excessive triangle counts. For high-resolution SLA/DLP resin printing or precision tooling, select the Fine preset (0.01mm linear deflection).
How are multi-body assemblies converted from STEP to STL?
Monolithic multi-body STEP files (AP203, AP214, AP242) are tessellated into a unified coordinate system. All solid bodies preserve their relative spatial positions in the exported binary STL. Note that external-reference assembly trees (where part geometries reside in separate files) must be exported as monolithic files before conversion.
How does StepSTLTools prevent 25.4× scale errors between inches and millimeters?
STEP files contain explicit ISO 10303-21 LENGTH_UNIT declarations (such as millimetre, metre, or inch). Binary STL files are unitless by design and standard slicers assume millimeter coordinates. StepSTLTools reads the declared CAD unit and normalizes all vertex coordinates to millimeters during tessellation.
What is the difference between source CAD metrics and output mesh metrics?
Source CAD metrics describe mathematical Boundary Representation (B-Rep) geometry: topological solids, analytical faces, shells, and trim curves. Output mesh metrics describe the discrete faceted approximation: triangle facets, vertex counts, binary STL file size, and facet topology checks (such as zero-area degenerates or non-manifold edges).
What are the client-side size and memory limits?
StepSTLTools operates 100% in browser WebAssembly memory without server uploads. Files up to 50 MB are supported. Processing complex CAD models with hundreds of bodies or thousands of curved faces is constrained by available browser tab memory (~2 GB WebAssembly heap).