Produce and interpret orthographic drawings using first-angle projection, with front, side and plan views, correct dimensioning and line conventions

What this dot point is asking

SEAB wants you to produce and interpret orthographic drawings using first-angle projection, with front, side and plan views, correct dimensioning and line conventions. Orthographic projection is the precise, dimensioned drawing used to make a product. You should know the three views, how first-angle projection arranges them, the main line types, and how to dimension a drawing.

The answer

What orthographic projection is for

An orthographic drawing shows an object as a set of flat, true-to-scale views looked at straight-on from different directions. Unlike a pictorial drawing, which gives the overall look but distorts dimensions, orthographic views are accurate and fully dimensioned, so they provide the exact sizes a maker needs to build the product. Orthographic projection is therefore the standard working drawing for manufacture; pictorial drawings are for presenting the look.

The three main views

An orthographic drawing usually has three views:

• Front view (front elevation). The object seen straight from the front; usually the most informative view.
• Side view (side elevation). The object seen from the side (end), showing the depth.
• Plan view. The object seen from directly above, showing the top.

Each view shows two of the three dimensions true to scale. Together they fully describe the object's shape from three directions.

First-angle projection

The views must be arranged by a convention so the drawing is read correctly. In first-angle projection (the convention commonly taught here):

• The front view is drawn first.
• The side view is placed on the opposite side to the direction of viewing: looking from the left, the left-side view is drawn on the right of the front view.
• The plan view is placed below the front view.

First-angle projection is identified by its symbol (a truncated cone shape). The arrangement matters because placing views wrongly makes the drawing ambiguous or misread.

Line conventions

Orthographic drawings use standard line types so the drawing is unambiguous:

• Thick continuous lines for visible outlines and edges.
• Dashed (hidden detail) lines for edges or features present but hidden from that view, such as a hole inside the object.
• Thin chain (centre) lines for the centres of circles and axes of symmetry.
• Thin continuous lines for dimension and projection lines.

Using the correct line type tells the reader exactly what each line means.

Dimensioning

A working drawing must carry the sizes needed to make the object. Dimension lines are thin lines with arrowheads spanning a feature, with the measurement written on them, kept off the object using projection lines. Good dimensioning gives every size once, clearly, without clutter, so the maker knows precisely how large each part is rather than measuring off the drawing.

Examples in context

Example 1. A working drawing for a workshop. A student hands a first-angle orthographic drawing of a pencil holder to be made: front, side and plan views, each dimensioned, with dashed lines showing the internal cavity and chain lines on the circular holes. The maker reads every size directly from the drawing and builds the holder accurately without guessing. The precise, dimensioned views are exactly what manufacture needs and a pictorial view could not provide.

Example 2. Reading a flat-pack instruction drawing. Flat-pack furniture instructions rely on clear views and dimensions so the buyer fits parts correctly. Orthographic-style views show where each hole and fitting goes, with sizes and positions marked, removing ambiguity. The convention-based, dimensioned drawing communicates precise assembly information that words alone would struggle to convey, showing orthographic projection used to instruct as well as to make.

Try this

• Cue. Name the three orthographic views and what each shows. Answer: front elevation (the front), side elevation (the side/end), and plan (from directly above).

• Cue. State the line type used for a hidden internal hole and the type used for visible edges. Answer: dashed (hidden detail) lines for the hidden hole; thick continuous lines for visible edges.

• Cue. Explain why orthographic, not pictorial, drawings are used to manufacture a product. Answer: orthographic views are true to scale and fully dimensioned, giving the exact sizes needed to make the product, whereas pictorial drawings distort dimensions and only show the overall look.