Revising the Projection Conventions

≈ 34 min

Revising the Projection Conventions

Grade 11 changes the problems, not the rules. Before assemblies begin, the conventions must be automatic.

Third angle for mechanical work: each view sits on the side you looked from — top view above, right side view right. First angle for civil and solid geometry: each view is pushed through to the far side — plan below, right side view left.

Line types still carry the meaning. Continuous thick is a visible edge; short dashes are a hidden edge; long-short-long chain is an axis or symmetry; thin continuous is construction, projection and dimension work.

Worked example. An assembly makes convention errors expensive rather than merely wrong. A single component drawn with its side view on the incorrect side produces a part that is the correct shape but the wrong hand — and in an assembly it will not mate with the parts around it, so the error surfaces as a fit failure rather than an obvious drawing mistake.

Core checkpoint: State the convention on every sheet, and check each view against it before adding detail.

Which convention applies to a mechanical assembly drawing, and which to a building floor plan?

Third angle for the assembly, first angle for the floor plan.

First angle for the assembly, third angle for the floor plan.

Third angle for both.

The draughtsperson chooses freely for each.

Why is a projection-convention error more expensive on an assembly drawing than on a single component drawing?

The part comes out the correct shape but the wrong hand, so the failure only appears when it will not mate with its neighbours.

Assembly drawings are printed at a larger scale.

Assemblies use more dimensions, so more figures are wrong.

It is not more expensive — the cost is the same.

Which line type marks an axis or a line of symmetry rather than an edge?

A portfolio contains an assembly sheet in third angle and a floor plan in first angle. What should a marker conclude?

Both are correct — convention is a property of the drawing type, and each sheet states its own.

One of the two sheets must be wrong, since a portfolio should use one convention throughout.

The floor plan should be redrawn in third angle to match.

The portfolio cannot be marked until both use the same convention.