Drawing Conventions, Line Types and Dimensioning
Exam note, from the official syllabus: "No questions for the end semester examination" from this block. It is assessed in the laboratory and internal examinations only.
Do not skip it. Every drawing you produce for the rest of the course is marked against these conventions, and a correct construction presented with the wrong line types still loses marks.
Why technical drawing exists
A technical drawing is a contract. It must convey shape, size, material and finish to someone who has never spoken to the designer, possibly in another country and another language, without ambiguity.
That is why almost nothing about it is left to preference. The line types, the dimension placement, the projection method are all standardised — in India by the Bureau of Indian Standards, chiefly SP 46 and IS 10711. A drawing that follows the conventions is readable by any engineer; one that does not is a picture.
Types of lines
| Line | Appearance | Used for |
|---|---|---|
| Continuous thick | ▬▬▬▬ | visible outlines and edges |
| Continuous thin | ──── | dimension, extension, leader, hatching |
| Continuous thin freehand | ∿∿∿ | short break lines |
| Dashed thin (or medium) | – – – | hidden edges and outlines |
| Chain thin (long-dash dotted) | —·—·— | centre lines, axes, pitch circles |
| Chain thin, thick at ends | —·—·— | cutting plane |
| Chain thin double-dashed | —··—··— | adjacent parts, extreme positions |
Two line thicknesses are used, in the ratio 2 : 1 — for example 0.7 mm thick and 0.35 mm thin. Getting that contrast right is most of what makes a drawing look professional.
Precedence, when lines coincide. Draw in this order and let the higher one win:
- Visible outline
- Hidden line
- Centre line
So a hidden edge falling exactly on a visible edge is drawn as visible, not hidden. Students often draw both and produce a mess.
Dimensioning
The two systems:
- Aligned — dimension figures are placed above the dimension line and read from the bottom or the right-hand side of the sheet.
- Unidirectional — all figures are horizontal, read from the bottom only. Preferred in CAD.
Do not mix them within one drawing.
Elements: extension (projection) lines, dimension lines with arrowheads, leader lines, and the dimension figure.
Rules worth memorising, because each one corresponds to a mark:
- Every dimension appears once only. Repeating it invites contradiction after a revision.
- Dimension lines sit outside the view wherever possible, at least 10 mm clear of the outline and 8 mm apart from each other.
- Extension lines start about 1 mm clear of the outline and extend about 2 mm beyond the dimension line.
- Place the smallest dimension nearest the view and larger ones outside it, so extension lines do not cross dimension lines.
- Arrowheads are closed and filled, about three times as long as they are wide.
- Never use a centre line or an outline as a dimension line.
- Dimension visible outlines, not hidden ones. If you find yourself dimensioning a hidden line, the drawing needs a section instead.
- Units are millimetres and the unit symbol is omitted; state "All dimensions in mm" once on the sheet.
Common symbols: ⌀ diameter, R radius, □ square, SR spherical radius, × for "places" (as in 4× ⌀10).
First angle and third angle projection
Two conventions for arranging views, and the difference is a common exam question.
First angle — the object sits between the observer and the plane, so each view is projected through the object onto the far side. The view from the left is placed on the right, and the top view goes below the front view. Used in India and Europe.
Third angle — the plane sits between observer and object, so views appear on the near side. The view from the left is placed on the left, and the top view goes above. Used in the USA and Japan.
Each has a standard symbol — a truncated cone shown in two views — placed in the title block so that the reader knows immediately which convention applies. Reading a first-angle drawing as third-angle produces a mirrored part, which is exactly the kind of expensive error the conventions exist to prevent.
Unless told otherwise, use first angle.