Question 6 of 10: Section View Selection

Question 6: Section View Selection (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. A circular plate/hub with a central bore and three elongated, rounded-end bosses (ribs) arranged at 120° around the bore; the side view shows the plate's thin cross-section threaded with several internal hidden-line features corresponding to those ribs. None of the internal detail is visible from outside, so a plain front + side pair leaves the reader guessing at wall thickness and bore depth.

Find. The section type that best reveals the hidden internal geometry, and the correct hatching convention for it.

Approach. Because the internal detail is symmetric about a horizontal cutting plane that would pass squarely through the bore and two of the three ribs (the third rib sits off that plane at 120°), a straight full section along a single diametral plane would only cut cleanly through two of the three ribs; the appropriate choice is either (a) an offset section, whose cutting plane steps to pass through the bore and each rib in turn, or (b) recognizing that ribs cut lengthwise are conventionally left unsectioned regardless — in which case a simple full section on the horizontal plane is adequate and simpler. The simpler, and here the more "appropriate," choice is the full section, applying the rib-convention exception rather than a stepped cutting plane.

1. Choose the cutting plane. A single straight plane A-A through the bore's centreline, horizontal in the front view, is marked with the cutting-plane line (long-dash/two-short-dash, thick, with arrows showing the direction of sight) and labelled A at both ends.
2. Cut and remove material in front of the plane (conceptually) to reveal the bore and the two ribs the plane passes through directly.
3. Section-line the solid material that the cutting plane actually passes through — 45° hatching, evenly spaced, on the plate body and any rib the plane happens to cross squarely.
4. Apply the rib exception. Where the cutting plane runs the LENGTH of a rib/web (as it does for the bore-aligned ribs here), CSA/ASME convention leaves that rib UNHATCHED even though the plane technically passes through it — hatching it would visually suggest the plate is solid there, which misrepresents the thin-web design intent.
5. Show the bore as open (unhatched) space bounded by its true-size solid outline, and re-draw all edges visible beyond the cutting plane as ordinary solid lines.