First-Angle Orthographic Projection

Why we need orthographic projection

A photo or a 3D sketch of an object looks realistic, but it is almost useless for building something accurately — angles are distorted and hidden surfaces are hard to measure. First-angle orthographic projection solves this by drawing an object as several separate flat views, each looking straight along one axis, so that every line is drawn true to length and can be measured and scaled with confidence.

In first-angle projection (the system used in South Africa and most of the world outside the USA), the object is imagined sitting between the viewer and the plane it is projected onto. This fixes the arrangement of views on the page: the top view is drawn directly underneath the front view, and the side view is drawn to the side of the front view — always lined up so that features stay aligned across views.

Line types

A working drawing uses different line types so that a reader can instantly tell what each line means:

• Dark (outline) lines — thick, solid lines showing the visible edges and outline of the object.
• Feint (construction) lines — very thin, light lines used while setting out a drawing, or to project points between views. They are not part of the final outline.
• Dashed (hidden detail) lines — short dashes showing an edge that is hidden behind other material in that particular view.
• Wavy (break) lines — freehand wavy lines used to show that a long object has been shortened on the page to save space.
• Chain (centre) lines — alternating long and short dashes marking the centre of a symmetrical feature, such as the middle of a circular hole.

Scale and dimensioning

Very large or very small objects cannot be drawn at their real size on a sheet of paper, so drawings are made to a scale — a fixed ratio between the drawing and the real object, such as 1:20 (1 unit on paper represents 20 units in real life) or 2:1 for a small object drawn larger than life-size. The scale used must always be written clearly on the drawing.

Dimensioning follows conventions so measurements are unambiguous: dimension lines run parallel to the feature being measured, with arrowheads touching thin extension lines that project from the object, and the measurement itself is written above or within a break in the dimension line. Dimensions are usually given in millimetres, and each measurement should only appear once on a full set of drawings.

Worked example: a stair and ramp design

Imagine a design brief: "Design a flight of steps and a wheelchair ramp giving access to a community hall 600 mm above ground level." A brief like this specifies constraints the design must satisfy — for example, number of steps, riser height, going (tread depth), and for the ramp its width and gradient (slope), plus a handrail for safety.

• Steps: a total rise of 600 mm split into risers of about 150 mm gives 4 steps; a comfortable going (tread) is around 280–300 mm.
• Ramp: a safe gradient for a wheelchair ramp is roughly 1:12 to 1:15 (for every 1 unit of rise, 12–15 units of horizontal run) — so a 600 mm rise needs a ramp at least 7.2–9 m long, with a width of at least 900 mm–1 m and handrails on both sides.

Reading and building up complex objects

Once you can read a simple cube, more complex 3D objects — such as an L-shaped bracket, or a stepped block — can be broken down feature by feature. Each notch, step or hole in the 3D object should correspond to a visible or hidden line in each of the three 2D views. A useful check: every corner of the object should be traceable to a point in at least two of the three views, and points must line up (project) accurately between the front view and the view directly below or beside it.