CAD Metadata Preserved When Exporting Models

When exporting models from Computer Aided Design (CAD) systems, the preservation of metadata depends heavily on the destination file format, the export schema, and the translator used. While native CAD files store extensive parametric histories, manufacturing instructions, and administrative records, converting these models into intermediate or neutral formats selectively filters this information. This article details the specific categories of metadata that survive CAD file exports, explains how common formats handle non-geometric attributes, and outlines what data is typically lost in translation.

Categories of Preserved Metadata

1. Geometric and Topological Properties

Most CAD export formats retain the mathematical definition of shapes, which inherently includes geometric metadata:

  • Spatial Coordinates and Orientation: Origin points, coordinate systems, and orientation vectors.
  • Bounding Boxes and Extents: Dimensional limits (length, width, height) calculated from the geometry.
  • Topology: Surface curvature, edge connectivity, face definitions, and vertex data (in B-rep formats like STEP or Parasolid).
  • Tessellation Data: Vertex normals, polygon counts, and mesh coordinates (in mesh formats like STL or OBJ).

2. Assembly Structure and Hierarchy

When exporting assemblies, multi-body structures are typically preserved through relational metadata:

  • Parent-Child Relationships: The tree structure linking top-level assemblies, subassemblies, and individual parts.
  • Instance Transformations: Spatial transformation matrices defining the location, rotation, and scale of repeated components.
  • Component Identifiers: Part names, file names, and instance IDs corresponding to the assembly tree.

3. Visual and Material Attributes

Physical and appearance properties can be transferred depending on the format standard:

  • Color and Styling: Face-level, body-level, or part-level RGB values, transparency, and basic texture coordinates.
  • Physical Materials: Assigned material designations (e.g., "Steel, 1018") and associated density values.
  • Mass Properties: Center of gravity, mass, volume, and surface area (often recalculated upon import, but sometimes explicitly written as text attributes).

4. Product and Manufacturing Information (PMI)

Modern Model-Based Definition (MBD) workflows rely on geometric dimensioning and tolerancing (GD&T) annotations stored as metadata:

  • Semantic PMI: Computer-interpretable tolerances, datum reference frames, surface finish symbols, and thread specifications.
  • Graphical PMI: Visual representations (billboards/polylines) of dimensions and notes tied to specific model views.

5. Administrative and User-Defined Attributes

Custom metadata assigned in the CAD property manager can be mapped into neutral file schemas:

  • Project Records: Part number, revision level, author/creator, company name, and creation/modification timestamps.
  • Lifecycle Information: Release status, approval signatures, and export dates.
  • ERP/PLM Custom Fields: Supplier details, cost estimates, and standard internal identification tags.

Metadata Retention by Export Format

Different export formats offer varying capacities for metadata storage:

  • STEP (ISO 10303):
    • AP203: Preserves basic 3D geometry, topology, and assembly configuration.
    • AP214: Adds preservation of colors, layers, geometric tolerances, and design-intent metadata.
    • AP242: The benchmark for MBD; fully preserves semantic PMI, 3D tessellation, kinematic data, and rich custom PLM metadata.
  • JT (Jupiter Tessellation): Highly capable format used widely in automotive and aerospace. Preserves detailed assembly hierarchy, kinematic constraints, semantic PMI, attributes, and exact B-rep alongside lightweight visualization meshes.
  • Parasolid (.x_t) and ACIS (.sat): Geometric modeling kernel formats. They preserve precise boundary representation (B-rep), body names, and basic user attributes, but lack extensive support for manufacturing annotations (PMI) or administrative schemas.
  • IGES (.igs): An older standard that preserves basic surfaces, wireframes, and limited layer/color data. Most custom attributes, assembly relationships, and modern PMI are dropped during IGES export.
  • STL (.stl): Strictly preserves triangulated surface geometry. All assembly hierarchy, part names, color, material definitions, PMI, and custom metadata are completely stripped.
  • 3D PDF (PRC / U3D): Encapsulates assembly hierarchies, interactive visual PMI, dynamic views, and text attributes specifically for human-readable design review.

Data Commonly Lost During Export

Regardless of the target format, certain native CAD metadata is almost universally discarded:

  • Parametric Feature Trees: The chronological modeling steps (extrusions, fillets, cuts, drafts) are flattened into non-parametric bodies ("dumb solids").
  • Sketches and Constraints: 2D sketch dimensions, geometric relations (tangency, concentricity), and underlying construction geometry.
  • Internal CAD Formulas: Equations, variable tables, design tables, and internal software-specific behavioral links.