Tolerancing

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GD&T 3D annotations

Geometric dimensioning and tolerancing (GD&T) means the specification of permissible limits/deviation of dimensions. Here we're talking strictly about geometric tolerances: a real-world error of shape, size, location, orientation in relation to the designed ideal.

There's also a distinction between the terms "limits and fits" and "geometric tolerances and surface conditions".

Standards

There are 2 ANSI/ISO standards:

  • ISO 1101 / ASME Y14.5 Stating and interpreting Geometric Dimensions and Tolerances (Definitions and symbols) 1
  • ISO 16792 / ASME Y14.41 Model-based definitions (CAD data) 3D model presentation and geometric dimensioning and tolerancing 2

ISO 16792 is a part of "Geometrical Product Specification" (GPS), which defines dimensional and geometrical tolerancing, surface properties and the related verification principles, measuring equipment and calibration requirements, including the uncertainty of dimensional and geometrical measurement.

There's also ISO 10303 (informally "STEP"), which defines the file format for exchanging product and manufacturing information, which is sourced from model-based definitions.

Approaches

There are two approaches to specifying Product and Manufacturing Information (PMI).

  • The legacy approach (Traditional) of producing 2D drawings with GD&T symbols. In some industries, this approach is also referred to as Technical Product Documentation (TPD). How this is achieved with FreeCAD is described in TechDraw Geometric dimensioning and tolerancing.
  • The modern 3D approach Model-Based Definition (MBD) which embeds GD&T data into the model and produces drawings with GD&T symbols
Traditional MBD
3D models with 2D drawings containing GD&T/PMI 3D model with embedded GD&T/PMI
Human-readable Human- and machine-readable
Relies on human interpretation Relies on an automated process
Labour-intensive Automated
Multiple files Single data model
Manual synchronization between CMM/CAM/CAI software Single source of truth

Existing software

Implementation so far

Forum threads

Tutorials and learning materials on GD&T