Section 1 · Scope & framework
One language, many disciplines
A technical drawing is a document — one or more sheets — that conveys information pictorially, in text, or both, usually identified by a drawing number and title. Its purpose is singular: to communicate technical intent so precisely that two people, in two places, reading the same drawing, arrive at exactly the same object.
In Australia the foundations of that language are set out in AS 1100.101, Technical drawing — Part 101: General principles. Part 101 is deliberately discipline-neutral: it establishes the rules that apply everywhere — to engineering, architecture, surveying, drafting technology and technical education alike — while the sibling parts layer on the specialised conventions of each field. Together they form a single coherent system.
Part 101 works through nine themes in sequence: the materials and layout of drawing sheets, the types of line and their meanings, letters and symbols, scales, methods of projection, sectioning, dimensioning and tolerancing, and standardised conventional representations. The sections below follow that same arc.
Abbreviations and shorthand save space, but only where their meaning is beyond question to the intended reader. Where there is any doubt, the standard is blunt:
WHEN IN DOUBT, SPELL IT OUT.
Section 2 · Materials, sizes & layout
The sheet: a controlled canvas
Before a single line is drawn, the sheet itself is standardised — its size, its borders, and the fixed places where identifying information lives. A common sheet format means any drawing can be filed, folded, microfilmed or reprinted without surprises.
2.4 Preferred sheet sizes — the ISO-A series
The preferred sizes are the familiar ISO-A series, each derived by halving the one before across its longer dimension so proportions stay constant. Where preprinting demands slightly wider borders, an R-prefixed size is used (RA0…RA4); where extra length is needed, elongated sheets such as A3 × 3 are available.
| Designation | Size (mm) | Typical use |
|---|---|---|
| A0 | 841 × 1189 | Large assemblies, general arrangements |
| A1 | 594 × 841 | Assemblies, site plans |
| A2 | 420 × 594 | Sub-assemblies, layouts |
| A3 | 297 × 420 | Detail drawings, schematics |
| A4 | 210 × 297 | Single details, notes, indexes |
2.5 Laying out the sheet
Every sheet carries a consistent framework so information is always found in the same place. A ruled drawing frame sits inside a border (wider on the filing edge). A grid reference system — capital letters along one edge, numerals along the other — lets a reviewer point to any feature by cell, exactly as on a street map. Camera alignment marks centre each side for microfilming, and the title block anchors the bottom right-hand corner.
2.5.9 What the title block must carry
The title block is the drawing's identity card. At minimum it names the organisation, states the drawing's title, gives a unique drawing number located near the corner, and records the signatures and dates of those responsible. Scale, method of projection, dimensional units and the governing standard are shown alongside. Three short instructions typically sit near it and prevent a world of trouble: DO NOT SCALE (never measure the paper — trust the figures), DIMENSIONS IN MILLIMETRES, and the projection symbol.
Section 3 · Lines
A line is never just a line
On a technical drawing the character of a line — its weight, whether it is continuous, dashed or chained — carries meaning as surely as its position. The standard defines a compact alphabet of line types, each lettered A to K, so that a reader can tell a visible edge from a hidden one, a centre-line from a cutting plane, at a glance.
Two weights, no more
Restrict a drawing to two line thicknesses wherever practicable — one thick, one thin. Some disciplines add a medium weight for clarity, but discipline in weight keeps a drawing legible.
Survives reproduction
Lines must reproduce cleanly. After copying or microfilming, no line should be thinner than 0.18 mm, and originals should be matt, dense and high-contrast against the sheet.
Clear spacing
Parallel lines are spaced at least twice the thickest line's width apart, and never closer than 1 mm — so a group of lines never reads as a smudge.
Where several lines fall on the same path, only one can be drawn — so a priority applies. Visible outlines win first; then hidden outlines; then cutting planes; then centre-lines and symmetry; and finally projection lines. Draw the highest-priority line, and the reader still understands the rest from context.
Sections 4 & 5 · Characters & scale
Legible characters, honest scale
4 Letters, numerals & symbols
Lettering exists to be read without hesitation, so the standard calls for distinct, uniform characters of consistent height and style — traditionally upright and open, and increasingly the responsibility of CAD fonts rather than the hand. A small set of symbols and terminators does heavy lifting: item references (balloons on leader lines that key parts to a list), arrowheads and dots that terminate dimension and leader lines, and the size-modifier symbols placed in front of a value to declare what it measures.
5 Scales
Scale is the ratio of a length on the drawing to the true length of the object. It is declared explicitly — as a ratio prefixed by the word SCALE (e.g. SCALE 1:100), as a graduated block scale, or as NOT TO SCALE where the drawing is diagrammatic. Where several scales appear on one sheet, each is shown beside the view it governs and the title block notes scales as shown. Road and profile drawings routinely carry different horizontal and vertical scales, each stated in full.
| Category | Recommended ratios |
|---|---|
| Enlargement | 50:1 · 20:1 · 10:1 · 5:1 · 2:1 |
| Full size | 1:1 |
| Reduction | 1:2 · 1:5 · 1:10 · 1:20 · 1:50 · 1:100 · 1:200 · 1:500 · 1:1000 … |
The recommended range extends in either direction only by multiplying a listed ratio by a power of ten — so scales stay comparable across a whole document set. For a very small object drawn large, adding a full-size outline gives the reader a sense of true size at a glance.
Section 6 · Projection
Turning three dimensions into two
Projection is the heart of the discipline: the set of rules for representing a solid object on a flat sheet. Every method answers the same question — where does the observer stand, and how do the lines of sight reach the object? — and the answer determines what the finished drawing looks like.
6.3 Orthogonal projection — and the third-angle convention
Orthogonal (orthographic) projection is the workhorse of engineering drawing. The lines of sight are parallel and strike each viewing plane at right angles, so every view is dimensionally true. An object is unfolded onto a set of mutually perpendicular planes — the "glass box" — to give the front, top, side, bottom and rear views. Most objects are fully described by three well-chosen adjacent views; some need only one, suitably dimensioned.
Two conventions decide where those views are placed relative to one another. In third-angle projection the viewing plane sits between observer and object, so each view represents the side of the object nearest to it — the top view goes above the front view, the right view to the right. In first-angle projection the object sits between observer and plane, and the arrangement inverts. Australian practice — and every drawing in AS 1100.101 — uses third angle, and the chosen method is always declared by the projection symbol on the sheet.
6.3.4 Selecting views
Choose the fewest views that describe the object without ambiguity — enough to avoid hidden-line clutter and needless repetition. Any three adjacent views usually suffice.
6.5–6.6 Pictorial
Isometric, dimetric and oblique (cavalier, cabinet) views give a single three-dimensional picture — invaluable for assembly and explanation, though not for precise dimensioning.
6.7 Perspective
With converging sight lines and one, two or three vanishing points, perspective is the most lifelike but least measurable projection — reserved for illustration and presentation.
Section 7 · Sections
Cutting through to what's inside
When interior detail would otherwise vanish into a thicket of hidden lines, the drawing cuts the object open. A section is the view revealed at an imaginary cutting plane — the material the plane passes through is shown by hatching, and the plane itself is marked so the reader knows exactly where the slice was taken and from which direction it is viewed.
45° hatching
Cut surfaces carry evenly spaced thin lines, usually at about 45° to the sheet edge — angled differently if 45° would run parallel to an outline.
Adjacent parts
Where two parts meet in section, their hatching runs at different angles — commonly at right angles — so the join between them is unmistakable.
Consistency
All the cut areas of one part share the same angle and spacing across every view, so the eye reads them as one continuous body.
Thin material
Sheet, packing and gaskets — too thin to hatch — are filled solid, with a small clear gap kept between adjacent solid-filled areas.
Section 8 · Dimensioning & tolerancing
Size, and the honest limits of size
A drawing that shows shape but not size is only half a specification. Dimensioning states how big every feature is; tolerancing states how much each may vary and still be acceptable. Together they define the finished product completely — without dictating how it is to be made.
8.1.2 The fundamental rules
Complete, but not redundant
Every necessary dimension appears once; no more are given than needed. Dimensions are chosen to suit the part's function and how it mates with others, and must never be open to two interpretations.
Never measure the paper
Neither scaling a length off the drawing nor assuming a size is permitted. If a value matters, it is written down. Unless stated otherwise, all dimensions apply at 20 °C.
Define, don't manufacture
The drawing gives the diameter of a hole, not whether it is drilled or reamed. It specifies the what, leaving the how to the maker — unless process is essential to the requirement.
Right angles are implied
Where centre-lines or features are drawn at right angles with no angle stated, a 90° angle is understood. Every dimension carries a tolerance unless marked basic, auxiliary, maximum or minimum.
8.2 The anatomy of a dimension
A dimension is built from a small kit of elements, each a specific line type. Projection (extension) lines — thin — carry the feature's edges clear of the outline; a thin dimension line spans between them, terminated by arrowheads; and the value sits above or within it. A leader points from a note or symbol to the feature it describes.
8.4 Geometric tolerancing (GD&T)
Size tolerance alone cannot say whether a face is flat, an axis straight, or two holes truly aligned. Geometric tolerancing controls the form, orientation, location and runout of features against reference datums. Its requirements are written in a feature control frame — a compact box reading, left to right: the geometric characteristic symbol, the tolerance value (a diameter symbol denotes a cylindrical tolerance zone), and the datum reference letters that anchor it.
Two related ideas complete the picture. The maximum material condition (MMC) recognises that a feature at its heaviest — the largest pin, the smallest hole — is the worst case for fit, and lets a positional tolerance grow as a feature departs from it. Datums establish the frame of reference — the surfaces and axes from which everything else is measured — so that a measured part can be checked against precisely the same reference the designer intended.
Section 9 · Conventional representations
Agreed shorthand for common features
Some features recur so often that drawing them in full every time would waste effort and clutter the sheet. The standard therefore fixes conventional representations — an agreed shorthand for screw threads, springs, knurling, splines, repeated holes and the like. Because everyone reads the convention the same way, a simplified symbol conveys the full feature without ambiguity, keeping drawings quick to produce and easy to interpret.
In summary
Nine principles worth carrying
- Standardise the sheet first. ISO-A sizes, a consistent frame, grid references and a bottom-right title block make every drawing filable, foldable and reproducible.
- Let line character speak. The lettered A–K vocabulary distinguishes visible, hidden, centre and cutting lines — kept to two weights and a clear priority where lines coincide.
- Write to be read. Uniform, legible characters; symbols placed before their values; and when brevity risks confusion, spell it out.
- Declare the scale. State it explicitly, keep to the recommended ratios, and never let the reader measure the paper.
- Project by rule. Orthogonal projection gives dimensionally true views; Australian practice uses third angle, always identified by the projection symbol.
- Section to reveal. A marked cutting plane and disciplined 45° hatching expose interior detail without a tangle of hidden lines.
- Dimension completely, once. Every necessary size, chosen for function, stated a single time, each with a tolerance.
- Control geometry where it counts. Feature control frames and datums govern form, orientation, location and runout — the difference between "the right size" and "actually fits".
- Use the agreed shorthand. Conventional representations keep recurring features fast to draw and unambiguous to read.