The 30° Lug: Auxiliary View or Rotated View
A candidate is producing a third-angle orthographic set for a cast-aluminium mounting bracket on A3 at 1:1. The body is 96 mm long, 64 mm wide and 48 mm high. A lug projects from the top face at 30° to the horizontal and carries a Ø12 mm hole whose axis is normal to the lug face; in the front view the lug is foreshortened to a projected length of 41 mm and the hole appears as an ellipse. The candidate proposes to add a full section A–A through the lug centre and, beside it, a rotated view of the lug drawn in the same position and labelled R. Earlier feedback noted an auxiliary view projected from the wrong view and hidden lines drawn where a visible outline already existed. The sheet must be submitted in two days and the candidate wants to avoid another resubmission.
1. Advise which view should be added to show the lug face and its hole in true shape, and justify rejecting the rotated view.
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Use a partial auxiliary view projected at 90° to the inclined lug face, taken from the front view. Projection lines must run at right angles to the 30° face, so the auxiliary plane is parallel to the lug face and perpendicular to the front view. On it the face appears true shape and the Ø12 hole appears as a circle, which is the information the driller needs.
The rotated view is not a projection. It is drawn in place by rotating the feature about a point, so it cannot be checked with projection lines, its alignment with the rest of the set is undefined, and it contradicts the third-angle layout. BS 8888 / ISO 128 practice is a properly projected auxiliary view, or a removed view clearly labelled and connected by a viewing arrow. Because only the lug needs true shape, a partial auxiliary view is smaller and clearer than a full auxiliary view of the whole bracket.
2. The candidate also proposes a full section A–A through the whole bracket at the lug centre. Is this appropriate, and what would a section through the lug actually show?
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A full section is not appropriate here. Cutting away half the bracket removes the outline that makes the 96 × 64 × 48 body readable and adds hatching that carries no new information. A local section through the lug is enough: the cutting plane is perpendicular to the lug face and contains the hole axis, and the broken-out portion is bounded by a thin freehand or zig-zag break line.
In that local section the lug wall appears true thickness, the hole appears as an unhatched rectangular gap of true depth (12 mm diameter, not an ellipse), and the 30° face appears true length. Section hatching at 45° with about 3 mm pitch is applied only to the solid cut material. Hidden lines behind the cutting plane are omitted, since the section already defines the interior.
3. State the hidden-line conventions the candidate should apply, given that a visible outline already exists in the same view.
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Hidden detail is used only where it is needed to make the drawing understandable, and it must never compete with visible outlines.
- Do not duplicate. Where a visible outline already describes the feature, the hidden line is omitted; a dashed line lying along a continuous outline is a defect.
- Do not hide behind sections. In the view where a section is taken, hidden lines behind the cutting plane are normally omitted.
- Prioritise. Hidden lines give way to centre lines, and centre lines give way to visible outlines. Dashes should meet at corners and be about 3–4 mm long with 1 mm gaps at this scale.
- Use an auxiliary view instead. The Ø12 hole is fully defined by the partial auxiliary view, so no hidden ellipse is required in the plan. If the hole depth is needed, the local section supplies it.