Question 7 of 10: First-Angle vs. Third-Angle Projection

Question 7: First-Angle vs. Third-Angle Projection (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. No part is supplied — the question asks the candidate to choose one. A simple L-shaped bracket with one through-hole in its vertical leg is used here: simple enough to sketch quickly, but with enough asymmetry (the hole, the L-step) that its top view genuinely differs in appearance from its front view, so the projection-system choice is visibly meaningful.

Find. Two projection layouts — first-angle and third-angle — of the same L-bracket, arranged so the SAME two views (front and top) occupy visibly different relative POSITIONS on the sheet, which is exactly what distinguishes the two systems.

Approach. In both systems the object sits in one quadrant formed by intersecting projection planes; the difference is which side of the object the projection plane (and hence the "image") is imagined to lie. Third-angle: the plane lies BETWEEN the observer and the object, so each view is placed on the SAME side as the observer looked from (top view, seen by looking down, is placed ABOVE the front view). First-angle: the plane lies BEYOND the object, so each view is placed on the OPPOSITE side (top view is placed BELOW the front view). The part's own shape never changes — the SAME front view and the SAME top view are used in both — only their arrangement on the page changes.

1. Sketch the front view once. The L-bracket's silhouette (vertical leg with a hole, foot at the base) is identical in both layouts.
2. Sketch the top view once. The bracket's plan outline (the L-footprint) is likewise identical content in both layouts.
3. Third-angle placement. Place the top view directly ABOVE the front view, connected by vertical projection lines — this matches "you are looking down at the top, and you record what you see on a plane held up between your eye and the part."
4. First-angle placement. Place the same top view directly BELOW the front view — this matches the historical (European/older-Canadian) convention of projecting the image through the object onto a plane on the far side.
5. Add the projection symbol. The truncated-cone projection symbol (frustum with the narrow end toward the small circle for third-angle, the wide end toward the small circle for first-angle) is added to the title block to remove any ambiguity for a shop reader, per standard drafting practice whenever a drawing set does not use its country's assumed default.