Cast bracket section: ribs, an off-plane bolt hole and the half-section boundary
A foundry supplies a cast-iron support bracket for a maize-mill drive. Its base plate is 140 mm × 90 mm × 15 mm with two Ø14 mm bolt holes drilled 35 mm either side of the longitudinal centre line and 110 mm apart along it. A vertical web 12 mm thick rises from the base on that centre line and carries a boss with a Ø40 mm through-bore whose axis is 70 mm above the base. Two triangular stiffening ribs, 10 mm thick, lie in the same longitudinal centre plane and run from the web out to the ends of the base plate. A junior drafter is asked for a full section A-A taken on the vertical longitudinal centre plane, plus a half section. On the returned sheet the base plate, boss, web and both ribs are hatched in the same direction and at the same 2 mm pitch. A note reads 'section through centre, hidden detail omitted', yet no bolt holes appear anywhere in the section. The half section is bounded by a thick continuous line down the middle.
1. Decide which regions of section A-A may be hatched and which may not, and justify the treatment of the web and the ribs.
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Hatched: the base plate (15 mm thick) and the boss around the Ø40 bore. These are cut across their substance by the cutting plane, so section lining is correct.
Not hatched: the web and both ribs.
Reasoning:
- The cutting plane A-A lies in the longitudinal centre plane, and the web and the ribs lie in that same plane. The plane therefore slices them lengthwise, flatwise, not across their thickness.
- If they were hatched, a 10 mm rib would read as a solid mass of metal in the plane of the paper, and the foundry would misjudge both the mass and the shape of the casting. The convention exists precisely to stop that misreading: thin features cut longitudinally are drawn in outline, unhatched, with a visible edge line where the rib meets the boss and where it meets the base plate.
- The same features cut transversely (cutting plane across the rib thickness) are hatched normally - the convention depends on direction, not on the part name.
- Hatching itself: thin continuous lines at 45°, pitch 2–3 mm, consistent across the whole part, stopping at the visible outlines and never running through a bore or across a gap. The bore of Ø40 is a hole, so it stays empty.
- Add a one-word note such as 'ribs not sectioned' only if the client's office might otherwise query the sheet; the convention normally speaks for itself.
2. The drafter's note says 'hidden detail omitted'. Assess this, and say where and how the two Ø14 bolt holes should appear.
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Omitting hidden detail in a section is correct practice, but the note has been used to hide a genuine omission.
The problem: the bolt holes are 35 mm either side of the longitudinal centre line, so a true section A-A on that plane passes between them and cannot show them. Dropping hidden detail then removes the only chance of showing two functional holes, and the machinist drilling the base plate gets no guidance from this sheet.
Options, in order of preference:
- Offset (stepped) section. Run the cutting plane along the centre line, step it sideways through one bolt hole, back to the centre line, and out through the second hole on the opposite side. Draw the ends of the cutting plane thick, join the steps with a thin line, label both ends A-A, and add the viewing arrows. In the resulting section the holes appear as open, unhatched gaps with visible edges, exactly as if they were on one plane; the bend is not drawn inside the section itself. Because the base plate is flat and the holes are parallel, the stepped cut gives a truthful section.
- If the stepped cut is thought too fussy for a simple bracket, leave the holes to the plan view where they appear as true circles, but then the note must read 'hidden detail omitted - see plan for bolt holes', and the plan must carry the Ø14 callouts and the 110 mm spacing.
What is not acceptable is to draw the holes as dashed circles inside the section, or to leave them out silently.
3. Describe how the half section should be laid out and what should form the boundary between the sectioned and unsectioned halves.
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Layout
- Half sections suit this bracket because it is symmetrical about the vertical longitudinal plane. Remove one quarter of the object - conventionally the front quarter - so the view shows one half in section and the other half as an ordinary outside view. The two halves together still read as one complete component and no extra view is needed.
- The boundary is a thin chain centre line, about 0.25 mm, running where the material was cut away. A thick continuous line, as drawn, implies a real edge on the casting and is wrong.
- Hatching occupies only the sectioned half and stops cleanly at the centre line. Hidden lines are omitted from both halves, as in any section.
- Keep the outline weight thick and continuous around the outside of both halves so the component still reads as one silhouette.
- The cutting plane for a half section is usually indicated by a chain line with thick ends and one arrow showing the direction of viewing; arrows at both ends are unnecessary because symmetry supplies the rest. Label it A-A if a full section A-A also exists on the sheet, otherwise label it as the half section.
- If the bore and the boss are concentric, the sectioned half alone gives the bore diameter, wall thickness and boss projection; the unsectioned half gives the outside profile and the rib outline, which is why the rib should be left visible on that side.