Most drawings tell a workshop how big something is. Far fewer tell it how much the shape itself is allowed to vary — whether a face has to be flat, whether a hole has to be square to that face, whether two features have to be concentric. That second conversation is geometric dimensioning and tolerancing, and on Australian drawings it sits alongside the conventions in AS 1100 rather than replacing them.
This is a practical introduction for drafters who are comfortable with dimensions but have never had to defend a feature control frame.
The problem GD&T exists to solve
Consider a plate with two holes, dimensioned the ordinary way: a distance from the left edge, a distance from the bottom edge, each with a plus-or-minus tolerance. That looks unambiguous. It is not.
Plus-or-minus dimensioning creates a rectangular zone of acceptable positions. A hole sitting in the corner of that rectangle is further from nominal than one sitting on the edge, yet both pass. Worse, the inspector has to decide which edge to measure from, and two inspectors can reach different answers on the same part. The tolerance is honest about size and silent about everything else.
GD&T replaces that with a round tolerance zone of a stated diameter, measured from an explicitly nominated origin. Same intent, roughly 57% more usable area for the same worst-case error, and — more importantly — no ambiguity about where measurement starts.
Datums: the part everyone skips and shouldn’t
A datum is the surface, axis or plane everything else is measured from. Nominating datums is the single highest-value thing a drafter can add to a drawing, and it is the thing most often left out.
Datums are ordered — primary, secondary, tertiary — and the order is not decoration. It describes how the part is constrained for inspection: the primary datum establishes the plane the part sits on, the secondary stops it sliding, the tertiary stops it rotating. Change the order and you have described a different inspection setup and, in effect, a different part.
The rule of thumb that serves well: nominate as datums the surfaces that actually locate the part in service. If a bracket bolts to a machined face and is pinned on two holes, those are your datums, because that is what the assembly cares about.
Reading a feature control frame
The feature control frame is the boxed symbol that carries the whole statement. Read left to right, it answers three questions in order:
- What kind of control? The leading symbol — position, flatness, perpendicularity, concentricity, profile and so on.
- How much? The tolerance value, preceded by a diameter symbol where the zone is cylindrical.
- Relative to what? The datum references, in order of precedence.
A frame with no datum reference is making a claim about the feature in isolation — flatness and straightness are the common cases, because a surface can be flat without reference to anything. A frame controlling position or perpendicularity without datums is almost always an error, because those are relationships and a relationship needs two parties.
The controls worth learning first
You do not need the whole symbol set to add real value. Four controls cover the large majority of general engineering work:
- Position — where a feature sits relative to the datums. The workhorse, and the one that replaces plus-or-minus on hole patterns.
- Flatness — how much a surface may deviate from a true plane. Matters wherever two faces have to seal or bear evenly.
- Perpendicularity — squareness to a datum. The control that stops a bolted joint from being drawn into misalignment.
- Profile of a surface — a general-purpose envelope for contoured shapes where no simpler control fits.
Learn those four properly before reaching for runout, concentricity or the material condition modifiers. A drawing that applies position and datums correctly is already far clearer than one that scatters exotic symbols without a datum structure behind them.
Where GD&T goes wrong on real drawings
The failures we see in drawing review are consistent:
- Datums nominated on surfaces nobody can inspect. A datum on an as-cast face or an internal feature the inspector cannot reach makes the drawing unverifiable.
- Over-tolerancing. Tight geometric controls applied to features that do not need them, raising cost with no functional gain. Tolerance is a budget — spend it where the assembly actually cares.
- Double dimensioning. A position tolerance and a plus-or-minus dimension controlling the same thing, leaving the workshop to guess which governs.
- Symbols without a drawing standard called up. The title block has to state which standard applies, because the same symbol can carry different meaning between systems.
Which standard applies
Two systems dominate. The ASME Y14.5 system is published by the American Society of Mechanical Engineers and is widespread in manufacturing. The ISO system of geometrical product specification is adopted in Australia through standards available from Standards Australia, and sits naturally alongside the AS 1100 drawing conventions most Australian drawing offices already work to.
They are broadly similar in intent and differ in detail — notably around default conditions and some symbol interpretations. The practical rule is simply to call up one of them explicitly in the title block and apply it consistently. A drawing that mixes conventions silently is harder to make than one with looser tolerances honestly stated.
For the broader Australian drawing conventions that sit underneath all of this, see our guide to AS 1100 drawing conventions. If you have a drawing set that needs a tolerancing review before it goes out for quote, get in touch with our drafting team.
