Drawing interpretationReading time 10 min
You have a drawing in front of you. Where do the first five seconds go?
Read the projection method wrong and the whole drawing is left-right reversed. That information hides in the little symbol beside the title block that looks like a decoration.
01First angle and third angle
The same object drawn in the two systems has its views arranged in exactly opposite positions. Europe, Taiwan and most of Japanese industry use first angle; North America and parts of the electronics industry use third angle.
| First angle (E) | Third angle (A) | |
|---|---|---|
| The right view goes | To the left of the front view | To the right of the front view |
| The top view goes | Below the front view | Above the front view |
| Concept | The object sits between the viewer and the plane | The plane sits between the viewer and the object |
| Common in | Europe, Taiwan, Japan, China | North America, Australia |
02What a title block must contain
ISO 7200 defines the data fields for a title block. Formats vary between companies, but the following cannot be missing.
| Field | Contents | What happens if it is missing |
|---|---|---|
| Drawing number | Unique identifier | No traceability, no filing |
| Revision | A / B / 01 / 02 | Parts made to an old revision |
| Share | 1:1、1:2、2:1 | Misjudging the real size |
| Units | mm or inch | The whole drawing out by a factor of 25.4 |
| Projection method | First or third angle symbol | Mirror-image parts |
| General tolerance | ISO 2768-mK, for example | Untoleranced dimensions cannot be accepted |
| Material | Grade and standard | Cannot be purchased or traced |
| Surface treatment | Anodising, zinc plating, blackening | No clear basis for dimensional acceptance |
| Designed / drawn / checked | Signatures | No traceability of responsibility |
The trap in the scale
“1:2” means the drawing is at half size, but the numbers written on it are always the real dimensions, never what you get by measuring the paper. That is why “DO NOT SCALE DRAWING” appears in most title blocks. Where a detail view is enlarged, its own scale is noted beside it.
03Sheet sizes and zones
| Sheet | Size (mm) | Typical use |
|---|---|---|
| A0 | 841 × 1189 | Large assemblies, layouts |
| A1 | 594 × 841 | Assemblies |
| A2 | 420 × 594 | Sub-assemblies |
| A3 | 297 × 420 | Part drawings (the most common) |
| A4 | 210 × 297 | Simple parts, specifications |
The zone references around the border, numbers along the top and bottom, letters down the sides, exist so that documents can point at a location: a revision history reading “zone C3”, or a parts list saying “item 12 at B2”. Larger sheets have more zones; A3 is usually 8 by 6.
04The first five seconds with an unfamiliar drawing
- Projection method. First angle or third?
- Units. mm or inch? Are they mixed?
- General tolerance. Which class of ISO 2768? Is a general geometric tolerance specified?
- Revision and date. Is this the latest? What does the revision history say?
- Material and surface treatment. Are dimensions before or after treatment?
All five live in the title block and around the border, not in the drawing itself. But they define how the whole drawing is to be read. Confirm those five, then start looking at dimensions, do it the other way round and you will often work for nothing.