Orthographic projection from an isometric pictorial: 04-BS-15, May 2015, Q2

Question 2: Orthographic projection from an isometric pictorial

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.

The pictorial shows a post-and-flange bracket: a circular base plate with a central bore, a square mounting flange (with two diagonally-placed bolt holes) rising from the base, and a quarter-round curved arm sweeping up and over from the flange to a circular pad. Because every surface on this part is either planar (base, flange) or a surface of revolution (bore, arm), the three PRINCIPAL views (front, top, right-side) fully describe it — no auxiliary view is needed since no surface is a true inclined (non-curved, non-principal) plane.

Approach. Use third-angle (CSA/ANSI) glass-box projection: the top view is placed directly above the front view, and the right-side view directly to the right of the front view, with widths/depths transferred by projection lines (or a 45° miter line) between views, never re-measured from the pictorial.

Front view. Shows the base and flange as rectangles in elevation and the curved arm as its true circular-arc profile — the view that best shows the arm’s curvature is selected as the front.
Top view. Shows the base and bore as concentric circles, the flange as a square with its two holes as true circles, and the arm’s cross-section as a small circle projected down from the front view.
Right-side view. Mirrors the front view’s arc (confirming the part is symmetric about the front-view plane) and is retained only because it clarifies the arm’s wall thickness — per CSA convention a view that adds no new information is normally dropped, but the rubric explicitly asks that views be "selected and positioned as necessary."

Fig. Q2 — third-angle three-view projection derived from the isometric pictorial.

Item	Result
Sketch scale	1:1 (full size) — the given pictorial was drawn at double size
Views selected	Front, top, right-side (third-angle); no auxiliary view required

The grading rubric marks view selection, projection technique and proportion — not an exact replica of the freehand original.

Topic: View selection and third-angle projection

Key relations: Third-angle (Canadian/US) glass-box: object sits in the box; the observer's side of each plane is unfolded outward, so TOP goes above FRONT and RIGHT-SIDE goes to the right of FRONT.

Why this works: Every part surface here is either a plane perpendicular to a principal axis or a surface of revolution about a principal axis, so three orthographic views are sufficient without an auxiliary view; an auxiliary view is only required for a TRUE-SHAPE view of an inclined (non-principal) planar surface, which this part does not have.

Common pitfall: A common error is drawing the arm’s curved surface as an ellipse in a view where it is actually a true circle (looking straight down its axis) — always identify which view is looking straight along a revolved surface's axis before sketching its silhouette.

Source: Giesecke et al., Technical Drawing / Engineering Graphics, ch. on the topic below.

Question 2 of 9: Orthographic projection from an isometric pictorial

Question 2: Orthographic projection from an isometric pictorial