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An engineering drawing is the contract between the designer and the workshop. It describes a part completely and without ambiguity: its shape, its size, how accurate each feature must be, what it is made of and how it is finished. Even with 3D models everywhere, the 2D drawing is still what a machinist works from, what an inspector checks against and what a supplier quotes on. Reading one well is a core engineering skill.
What’s on a drawing sheet
- Views of the part from several directions, plus section and detail views where needed.
- Dimensions with their tolerances, giving the size and position of every feature.
- Symbols for surface finish, geometric tolerances, welds, threads and hole types.
- Notes for anything the views can’t show: heat treatment, coatings, deburring, general tolerances.
- The title block, usually bottom right: part name and number, material, scale, units, projection method, general tolerance standard, revision and approvals.
Orthographic projection
A drawing shows the part as flat views looking straight at each face. Each view shows two of the three dimensions, so you need at least two (usually three) views to describe a solid. The views are arranged in a fixed pattern called the projection method, and the drawing must say which one it uses.
| First-angle projection | Third-angle projection | |
|---|---|---|
| Used mainly in | Europe, India and most of Asia (ISO, IS standards) | USA and Canada (ASME) |
| Top (plan) view | Placed below the front view | Placed above the front view |
| Right side view | Placed to the left of the front view | Placed to the right of the front view |
| Symbol | Cone’s circles on the left | Cone’s circles on the right |
The difference is easy to forget and causes real mistakes: a feature can end up on the wrong side of the part. Always check the projection symbol in the title block before you read the views.
Other views
- Section views show the part as if cut along a cutting-plane line (thick chain line with arrows and letters, such as A–A). Cut material is hatched with thin diagonal lines. Sections make internal features readable without a mass of hidden lines.
- Detail views enlarge a small area, marked with a circle and a letter, at a larger scale.
- Auxiliary views look square-on at an inclined face, so its true shape and size can be dimensioned.
- Isometric (pictorial) views help the reader understand the shape, but are not normally dimensioned.
Line types
| Line | Appearance | Meaning |
|---|---|---|
| Visible outline | Continuous thick | Edges you can see |
| Hidden line | Dashed thin | Edges behind the surface, such as the inside of a hole |
| Centre line | Thin chain (long and short dashes) | Axes of holes and cylinders, lines of symmetry |
| Dimension and leader lines | Continuous thin, with arrowheads | Show what a dimension or note applies to |
| Hatching | Thin diagonal lines | Cut surfaces in a section view |
| Cutting plane | Chain, thick at ends and corners, with arrows | Where a section is taken |
| Break line | Thin wavy or zigzag | A view shortened or partly removed |
Dimensions and symbols
| Symbol or format | Meaning |
|---|---|
| ⌀20 | Diameter 20 mm |
| R5 | Radius 5 mm |
| 2 × 45° or C2 | Chamfer 2 mm at 45° |
| 4 × ⌀8.5 THRU | Four holes of 8.5 mm diameter, through the part |
| ⌴ ⌀14 ↧ 9 | Counterbore 14 mm diameter, 9 mm deep (for a socket head screw) |
| ⌵ ⌀18 × 90° | Countersink 18 mm diameter at 90° |
| M10 × 1.5 – 6H | Metric internal thread, 10 mm, pitch 1.5 mm, tolerance class 6H |
| ⌀25 H7 | Hole with an ISO fit tolerance (see tolerances and fits) |
| 50 ±0.1 | Dimension with its tolerance |
| (75) | Reference dimension, for information only, not inspected |
| Ra 1.6 with a check-mark symbol | Surface roughness limit in micrometres |
| Rectangular frame, e.g. ⌖ ⌀0.1 A B | Geometric tolerance (GD&T) |
Dimensions on metric drawings are in millimetres unless stated. Any dimension without its own tolerance falls under the general tolerance in the title block, often ISO 2768-m (for example ±0.3 mm on a length between 30 and 120 mm).
A step-by-step way to read a drawing
- Read the title block first: part name, material, units, scale, projection method, general tolerances and revision.
- Get the overall shape. Look at the pictorial view if there is one; otherwise match the main views to build a mental 3D image. Find the largest features first.
- Relate the views. Features line up between views: a hole’s circle in the top view lines up with its hidden lines in the front view.
- Read the dimensions feature by feature: size, then position, then the datum each is measured from.
- Find the critical features: the tight tolerances, fits, GD&T frames and surface finish callouts. These decide how the part is made and checked.
- Read every note, including heat treatment, coating, deburring and marking requirements.
Worked example: reading the bracket
Read the bracket in the diagram and find its overall size, the hole details and its mass if it is made from steel (7.85 g/cm³).
Step 1: overall size. The front view gives the width (120 mm) and height (100 mm); the right side view gives the depth (60 mm).
Step 2: features. The base is 20 mm thick (dimensioned on the side view), and the upright scales to the same 20 mm, although that dimension is missing. The top view shows a ⌀20 hole through the base; its hidden lines in the front and side views confirm it goes all the way through.
Step 3: volume.
Step 4: mass.
A careful reader would also notice what’s missing: the hole’s position isn’t dimensioned, and neither is the upright’s thickness. On a real drawing those would be queries back to the designer before anything is made. A complete drawing never leaves the workshop to measure or guess.
Key points
- Start with the title block: material, units, projection, general tolerances, revision.
- Check first-angle or third-angle projection before reading the views.
- Hidden lines are dashed, centre lines are chain lines; sections replace hidden detail with hatching.
- Read size, position and tolerance for each feature, then all the notes; query anything that’s missing.