Gearbox Housing: Sectioning a Rib and an Offset Hole
A cast-iron gearbox housing is to be drawn in third-angle projection at 1:2 on A2. The main section B–B runs longitudinally through the shaft centre line and through a 14 mm thick external stiffening rib on a 12 mm wall. A Ø60 mm bearing boss lies on the same centre line, but a Ø18 mm drain-plug hole is offset 35 mm to one side and does not lie on the cutting plane. The candidate has hatched the rib with the same 45° lines at 3 mm pitch as the wall, and has drawn the offset drain hole in the section as a full outline. A steel end cover sits against the housing in the same section and has also been hatched at 45° with the same pitch and direction. On the check plot the rib reads as solid mass and the cover cannot be distinguished from the casting.
1. State how the rib should be represented in section B–B and explain the reason for the convention.
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The rib is cut longitudinally by B–B, so it must be drawn in outline but not hatched. The section line should pass around the rib rather than through it.
The reason is communicative, not arbitrary: hatching a thin rib that lies flat in the cutting plane makes it read as a thick mass of material and gives a false impression of the casting's weight and stiffness. The convention (ISO 128 / BS 8888) is that ribs, webs and similar thin features are left unhatched when the cutting plane passes along their length, but are hatched normally when cut across, for example in a transverse section through the same rib. If the candidate wants the rib thickness dimensioned, a separate cross-section through the rib at 90° to B–B is the correct addition.
2. How should the Ø18 mm drain hole, offset 35 mm from the cutting plane, be treated in section B–B?
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It must not simply be drawn as a full outline as though it lay on the plane, because that would misrepresent its position. Two acceptable options exist.
- Aligned section (rotated into the plane). The hole is rotated about the shaft axis into the cutting plane and drawn as if it were on it, with no hatching through the hole. The view must be labelled to show that the feature has been rotated, or the offset must be dimensioned clearly in plan.
- Local section. Show the hole in a separate broken-out section or detail view at the 35 mm offset position, leaving B–B to deal with the main bore and boss.
If neither is used, the hole may be shown as hidden lines in the unsectioned view, but never hatched over: the hole is a void, so section lines stop at its outline.
3. Explain how the housing and the steel end cover should be hatched where they meet in the same section.
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Adjacent parts cut by the same plane must be hatched differently so the boundary between them is unmistakable. The usual treatment is to keep the same pitch and line weight but change the angle: housing at 45°, cover at 135° (or the reverse). Changing the pitch (for example 3 mm against 1.5 mm) is an acceptable alternative; using both a different angle and a different pitch is unnecessary and makes a busy drawing.
Hatching must stop at the joint line, and the joint line itself must be a continuous visible line, not a dashed one. The same component must keep the same hatch throughout the sheet, so the cover is hatched identically in every section where it appears. Thin parts such as gaskets and standard items such as bolts are normally left solid, not hatched.