Tolerancing

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.

• GPS Table of contents

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
• The modern 3D approach (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

• Adding geometric tolerance with feature control frame in AutoCAD
• Working with Drawing Symbols in Fusion 360
• Tech Tip Tuesday: Using the GD&T Quick Creation Menu in GOM Inspect
• CATIA V5 - FUNCTIONAL TOLERANCING & ANNOTATION
• SmartProfile GD&T Analysis Software

Implementation so far

• TechDraw symbols SVG files
• GDT workbench
• TechDraw balloon macro
• A branch that introduced a new type of DocumentObject: DrawViewGDTReference

Forum threads

• Enhancing TechDraw with geometric tolerances and surface finishings - Developer discussion (2019)

• Geometrical tolerances - User feature request (2016)
• GD&T Workbench for FreeCAD - A workbench for FreeCAD 0.16 (2016)
• TechDraw balloons - A general-purpose TechDraw Macro which can be used to place GD&T symbols on a TechDraw page (2019)
• ISO 1101: 2017 Geometric Tolerancing Standard - A link to the ISO 1101:2017 PDF file (2019)
• Geometric Dimensioning & Tolerancing - Developer proposing contributions (2019)

Tutorials and learning materials on GD&T

• Brief introduction (Video)
• How Precise Is That Part? Know Your GD&T
• GD&T for beginners | step by step approach to do GD&T for mechanical drawing (Video)
• | Straight To The Point Engineering (Video tutorials)