Size tolerances alone cannot say where a hole is, how flat a face must be or which surface to measure from. Geometrical tolerancing (GD&T) can, and this module teaches you to read and write it.
2 of 6 lessons published so far, about 30 minutes, each with a self-check quiz.
The six lessons build the language in the order you need it. The first shows why plus and minus tolerances fall short. The next four cover datums and the datum reference frame, form and orientation, position with maximum material condition and bonus tolerance, and profile and run-out. The last reads any feature control frame in full. ISO GPS practice (ISO 1101 and its companion standards, pulled into UK drawings by BS 8888) and ASME Y14.5-2018 practice are taught side by side. Where their default rules differ, the lessons compare them.
Start with Module 1 if you have not read a production drawing before, in particular the lesson on dimensioning and general tolerances. The lessons are released two a week; lesson 4, due on 6 October 2026, adds an interactive true position and bonus tolerance explorer. The geometrical tolerances you learn here feed the tolerance stack-up calculator.
GD&T basics: why plus/minus size tolerances cannot control form or location, the GD&T symbols grouped by type, and where ISO 1101 and ASME Y14.5 differ.
15 min · 5 outcomes · 5-question self-check
GD&T datums explained: datum features and datums, the 3-2-1 datum reference frame, why datum order matters, datum targets, and ISO 5459 vs ASME terms.
15 min · 5 outcomes · 5-question self-check
GD&T form and orientation controls: flatness, straightness, roundness, cylindricity, parallelism, perpendicularity and angularity, their zones and uses.
Coming 6 Oct 2026
True position explained: the cylindrical zone, the 2√(x² + y²) formula, maximum material condition, bonus tolerance and virtual condition, with an explorer.
Coming 6 Oct 2026
Profile of a line and of a surface, circular vs total run-out (US: runout), and why ASME dropped concentricity: zones, dial set-ups and a run-out budget.
Coming 13 Oct 2026
How to read a feature control frame cell by cell: symbol, zone, modifiers and datums, with the ISO 1101 and ASME Y14.5-2018 default rules compared side by side.
Coming 13 Oct 2026
| Standard | Edition | Title |
|---|---|---|
| ISO 1101 | 2017 | GPS, Geometrical tolerancing, Tolerances of form, orientation, location and run-out |
| ASME Y14.5 | 2018 (R2024) | Dimensioning and Tolerancing |
| ISO 8015 | 2011 | GPS, Fundamentals, Concepts, principles and rules |
| ISO 14405-1 | 2025 | GPS, Dimensional tolerancing, Part 1: Linear sizes |
| ISO 5459 | 2024 | GPS, Geometrical tolerancing, Datums and datum systems |
| ISO 2692 | 2021 | GPS, Geometrical tolerancing, Maximum material requirement (MMR), least material requirement (LMR) and reciprocity requirement (RPR) |
Editions were checked on the date each lesson was last updated. Standards are copyright documents: the lessons explain them, but buy or borrow the current edition before relying on a value.