Working Drawings & Conventions
What Is a Working Drawing?
A working drawing is the definitive document from which a component or assembly is manufactured. It must be complete, unambiguous and fully standardised so that any qualified engineer or machinist — anywhere in the world — can interpret it correctly. In the UK, working drawings follow the conventions set out in BS 8888 (Technical product documentation). The standard covers everything from the layout of the title block to the way a diameter dimension is written.
Third-Angle Orthographic Projection
This course uses third-angle orthographic projection. The object is imagined placed inside a transparent glass box. Each face of the box is a projection plane: the view from the front is projected onto the front plane, the view from the top is projected onto the top plane, and the view from the right is projected onto the right plane. When the box is unfolded, the standard arrangement places the front view (elevation) in the centre, the plan (top view) above it, and the right end view to its right. The internationally recognised symbol for third-angle projection — a truncated cone seen from the narrow end — must appear in the title block of every working drawing to confirm which standard is used.
In third-angle orthographic projection (standard in the UK), the plan view is placed above the front view and the right end view is placed to the right of the front view. The symbol is a truncated cone viewed from its narrow end.
The Title Block and Standard Conventions
Every engineering drawing has a title block, typically in the bottom-right corner. It contains the drawing title, drawing number, designer's name, date, scale, sheet size, material, finish, projection symbol and revision history. The title block ensures traceability — if a component is made incorrectly, the drawing can be traced back to the person who produced it and the revision that was current at the time. All measurements on a working drawing are in metric units (millimetres unless otherwise stated). The scale used must be stated clearly — for example, 1:1 (full size), 2:1 (twice full size), or 1:5 (one fifth full size). Drawing to scale allows the viewer to check proportions visually.
Dimensioning Conventions
Dimensions are added using dimension lines, projection lines and leader lines, all drawn as thin continuous lines. Dimension lines run parallel to the feature being measured, with arrowheads touching projection lines that extend just beyond the outline of the part. The dimension value is placed above or in a gap in the dimension line. Linear measurements give the straight-line distance between two points. Radii are written with the prefix R (e.g., R12); diameters are written with the ⌀ diameter symbol or the abbreviation DIA. Surface finish requirements — how smooth a surface must be — are shown using the surface texture symbol. A square cross-section is indicated by the abbreviation SQ or the □ symbol before the dimension value. Standard abbreviations used on drawings include: AF (across flats), CL or ℄ (centre line), DIA or ⌀ (diameter), DRG (drawing), MATL (material), SQ (square).
BS 8888 defines six line types that every drawing must use correctly. Outlines (continuous thick lines) show visible edges and contours of the object. Hidden detail lines (short dashes, thin) show edges and surfaces that are behind other features and cannot be seen in that view. Centre lines (thin chain — alternating long and short dashes) mark the axes of symmetry, circles and cylindrical features. Projection lines (thin continuous) extend from the outline to indicate where a dimension applies but do not touch the outline. Dimension lines (thin continuous with arrowheads) carry the dimension value. Leader lines (thin continuous with arrowhead or dot) point from a note or symbol to the feature it refers to. Confusing outline and hidden detail lines is one of the most common errors in exam drawing questions.
The six line types in BS 8888 are: outlines (thick continuous), hidden detail (thin dashes), centre lines (thin chain — long-short dash), projection lines (thin continuous), dimension lines (thin continuous with arrowheads), and leader lines (thin continuous with arrowhead or dot).
Tolerances
No machining process is perfectly precise. A tolerance specifies the permissible variation in a dimension — for example, a shaft might be dimensioned as ⌀25 +0.02/−0.01 mm, meaning the diameter must be between 24.99 mm and 25.02 mm. Tolerances are essential for ensuring interchangeability of parts in mass production: if every component is made within its tolerance, any shaft will fit any matching hole without selective fitting. Tighter tolerances require more precise (and more expensive) machining processes. The designer must balance functional requirements against manufacturing cost when specifying tolerances.
Representing Mechanical Features
Some features have standard drawing conventions because they cannot be drawn realistically at small scale. External and internal threads are shown using thick outlines for the crest of the thread and thin lines for the root, with a cross-hatching convention visible in section views. Holes are shown in plan as circles and in elevation with centre lines. Chamfers (angled cuts at the end of a shaft or hole edge) are shown with angled lines at the relevant corner. Countersinks (conical enlargements at the entrance of a hole) are shown with a cone symbol in the elevation view. Knurls (textured grip patterns on a cylindrical surface) are shown with a cross-hatched pattern on the surface.
The same mistakes come up again and again in working drawing questions. First, omitting the third-angle projection symbol from the title block. Second, placing the plan view below the front view instead of above it (that is first-angle, used in mainland Europe). Third, using a continuous thick line for hidden detail (it should be short dashes, thin). Fourth, writing dimensions in inches or mixing units. Fifth, omitting tolerances on a critical dimension when the question states that the component must fit a mating part. Practise producing complete, correctly labelled working drawings including the title block to avoid these errors.