The ability to read mechanical drawings — also called engineering drawings or technical drawings — is a foundational skill for mechanical engineers, machinists, fitters, welders, and anyone involved in manufacturing or fabrication. In Australia, mechanical drawings follow AS/NZS 1100 conventions with GD&T per AS 1101.101.
This guide explains every element of a mechanical drawing — from the title block and projection type through to tolerances, surface finish symbols, and GD&T — so you can confidently interpret any drawing you encounter on the job.
What Is a Mechanical Drawing?
A mechanical drawing is a technical document that fully defines a manufactured part or assembly. It communicates every dimension, tolerance, material specification, surface finish requirement, and manufacturing note needed to make the part — without ambiguity. A well-drawn mechanical drawing can be sent to any workshop in Australia and the part will come back exactly right.
Mechanical drawings typically include:
- Orthographic views (front, top, side)
- Section and detail views
- Dimensions and tolerances
- GD&T symbols
- Surface finish callouts
- Material and treatment specifications
- A title block and revision history
- A bill of materials (for assemblies)
Step 1: Read the Title Block First
The title block — bottom-right corner of every drawing sheet — contains everything you need to verify before reading geometry:
| Field | What It Tells You |
|---|---|
| Part Name / Drawing Title | What the drawing depicts |
| Drawing Number | Unique reference ID for document control |
| Revision | Current revision letter (A, B, C…) — always use the latest |
| Scale | e.g. 1:1, 1:2, 2:1 — ratio of drawn size to actual size |
| Material | e.g. 304 SS, 6061-T6 Aluminium, 1020 Steel, HDPE |
| Surface Treatment | e.g. Anodise, Zinc Plate, Powder Coat, Passivate |
| General Tolerances | Default tolerances when not specified individually (e.g. ±0.1 linear, ±0.5°) |
| Projection Symbol | First angle (ISO / AS) or third angle (US/ASME) — critical for reading views correctly |
| Drafter / Checker / Approved | Accountability chain |
Australia uses First Angle Projection (AS/NZS 1100). The projection symbol is a truncated cone — in first angle, the cone points left. In third angle (common in US drawings), the cone points right. Confusing the two means you’ll read the top view when you should read the bottom view.
Step 2: Understand the Views
Mechanical drawings use multiple 2D views to fully describe a 3D object:
| View Type | Description | When Used |
|---|---|---|
| Front View | Primary view — the most descriptive face of the part | Always |
| Top View | Looking straight down at the part | Most parts |
| Side View (Left or Right) | Looking from the left or right | When front + top don’t fully describe the part |
| Isometric View | 3D pictorial — for visualisation only, not dimensioning | Complex parts, assembly aids |
| Section View | Cut through the part to show internal features | Holes, bores, internal pockets |
| Detail View | Magnified area with its own scale callout (e.g. “DETAIL A — 2:1”) | Small features, thread details |
| Auxiliary View | View projected from an angled surface | Angled faces that aren’t captured in standard views |
Step 3: Read Line Types
Every line type in a mechanical drawing means something specific (AS/NZS 1100):
| Line Type | Appearance | Meaning |
|---|---|---|
| Visible outline | Thick solid line | Edge visible in this view |
| Hidden line | Thin dashed line | Edge behind the current view surface |
| Centre line | Thin long-short-long dash | Axis of symmetry, centre of holes/cylinders |
| Dimension line | Thin solid with arrowheads | Shows what is being measured |
| Extension line | Thin solid projecting from feature | Extends to meet dimension line |
| Leader line | Thin line with arrowhead and note | Points to a feature with a note or callout |
| Section line (hatch) | Thin diagonal lines at 45° | Cut surface in section view |
| Phantom line | Long-short-short dash | Alternate positions, adjacent parts, repeat features |
| Break line | Irregular freehand or straight with Z | Shows a portion is removed for space |
Step 4: Read Dimensions
Dimensions are the core of any mechanical drawing. Australian mechanical drawings use millimetres (mm) as the default unit (the “mm” is often omitted once stated in the title block). Key rules:
- Never scale the drawing — always use the stated dimensions
- Dimensions without a tolerance apply the title block’s general tolerance
- Chain dimensioning — dimensions in a chain; errors accumulate
- Datum dimensioning — all dimensions from a common reference; no error accumulation
- ⌀ prefix = diameter (cylindrical features)
- R prefix = radius
- □ prefix = square feature
- SR = spherical radius, S⌀ = spherical diameter
Step 5: Read Tolerances
Tolerances define how much a dimension can vary and still be acceptable. There are two main systems:
Linear Tolerances
- Bilateral: 25.00 ±0.05 = acceptable range 24.95–25.05
- Unilateral: 25.00 +0.10/−0.00 = acceptable range 25.00–25.10
- Limits: 24.95 / 25.10 (max and min written directly)
Geometric Dimensioning & Tolerancing (GD&T)
GD&T uses feature control frames to specify form, orientation, location, and runout. In Australia, GD&T follows AS 1101.101 (equivalent to ISO 1101). Common symbols:
| Symbol | Characteristic | Applies To |
|---|---|---|
| ⏤ (straight line) | Straightness | Axis or surface |
| □ | Flatness | Surface |
| ○ | Circularity (roundness) | Cross section |
| ⌭ | Cylindricity | Full cylinder |
| ∥ | Parallelism | Surface or axis |
| ⊥ | Perpendicularity | Surface or axis |
| ∠ | Angularity | Surface or axis |
| ⌖ | Position (true position) | Hole or feature location |
| ◎ | Concentricity / coaxiality | Axis to datum |
| ↗ (circular arrow) | Circular runout | Rotating surfaces |
A feature control frame reads left to right: geometric characteristic symbol | tolerance value | datum references. For example: ⌖ | ⌀0.05 | A | B means “true position within a diameter 0.05mm zone, relative to datums A and B.”
Step 6: Read Surface Finish Symbols
Surface finish symbols (per AS/NZS 1100.303) specify the required surface roughness. The value given is the Ra (arithmetic mean roughness) in micrometres (µm):
- Ra 0.8 µm — fine machined (ground, honed)
- Ra 1.6 µm — standard machined finish
- Ra 3.2 µm — general machined
- Ra 6.3 µm — rough machined
- The “tick” symbol (√) with a horizontal bar means the surface must be machined; with a circle in the tick means no machining allowed (as-cast/as-forged)
Step 7: Read Thread Callouts
Threads are called out with a standard notation. In Australia, most mechanical threads are ISO metric per AS 1275:
- M12 × 1.75 — 6H = Metric thread, 12mm nominal diameter, 1.75mm pitch, 6H tolerance class (internal thread)
- M12 × 1.75 — 6g = External thread (lowercase = external, uppercase = internal)
- M8 THRU = M8 thread through the full thickness
- M8 × 20 DEEP = M8 thread 20mm deep (blind hole)
Step 8: Read Welding Symbols (if applicable)
Welded assemblies include welding symbols per AS 2812 / AS/NZS 2980. The reference line runs horizontally. The arrow points to the joint. Symbols below the line indicate the arrow side; above the line indicates the other side:
- Fillet weld — right triangle symbol; size given as leg length (e.g. 6 ←▷)
- Full penetration butt weld — single or double bevel/V symbols
- All-around weld — circle at the arrow/reference line junction
- Field weld — flag at the junction
Step 9: Read Assembly Drawings
Assembly drawings show how individual parts fit together. Key features:
- Item balloons — circles with item numbers pointing to each part
- Bill of Materials (BOM) — table listing item number, part number, description, material, quantity
- Exploded view — parts shown separated along their assembly axes
- Assembly dimensions — overall envelope dimensions and critical fit dimensions
- Individual part drawings are referenced by part number — read both together
What is the difference between first angle and third angle projection?
First angle projection (used in Australia and Europe per AS/NZS 1100 and ISO) places views as if the object has been rolled away from the viewer — the right side view appears on the left, the top view appears below the front view. Third angle projection (used in the USA per ASME Y14.5) places views as if the object rolls towards the viewer — the right side view appears on the right, the top view appears above the front view. Always check the projection symbol in the title block. Misidentifying the projection type will cause you to read the wrong view.
What does ±0.1 mean on a mechanical drawing?
±0.1 is a bilateral tolerance. It means the dimension can be 0.1mm larger or 0.1mm smaller than the stated value. For example, if a hole is dimensioned as ⌀20.00 ±0.1, the acceptable range is ⌀19.90mm to ⌀20.10mm. This is different from a unilateral tolerance like +0.2/−0.0, which would mean 20.00mm to 20.20mm. The general tolerance in the title block applies to all dimensions that don’t have an individual tolerance callout.
What does Ra 1.6 mean on a mechanical drawing?
Ra 1.6 specifies a surface roughness of 1.6 micrometres arithmetic mean roughness (Ra). This is a standard machined finish achieved by turning, milling, or grinding. Smoother surfaces have lower Ra values (e.g. Ra 0.4 for ground or lapped surfaces). Rougher surfaces have higher values (e.g. Ra 6.3 for a rough milled surface). The surface finish symbol in Australia follows AS/NZS 1100.303. The value is always in micrometres (µm).
How do I read a feature control frame in GD&T?
A feature control frame is read left to right in three compartments: (1) the geometric characteristic symbol (e.g. ⌖ for position, ⊥ for perpendicularity), (2) the tolerance value — often preceded by ⌀ if it’s a diameter zone — e.g. ⌀0.05, and (3) the datum references (e.g. A, B, C). So ⌖ | ⌀0.05 | A | B means: the true position of this feature must fall within a cylindrical tolerance zone of diameter 0.05mm, located relative to datums A and B. In Australia, GD&T follows AS 1101.101, equivalent to ISO 1101.
Need mechanical drawings prepared to Australian standards? ASTCAD’s mechanical drafting team produces AS/NZS 1100-compliant drawings for manufacturing, fabrication, and engineering projects across Australia. Request a quote.