Bearing bracket for a German customer: third-angle sheet offered against a first-angle requirement
At a small engineering works in Blantyre a trainee is producing a working drawing of a cast-iron bearing bracket: a base plate 120 mm × 80 mm × 15 mm, a vertical web 60 mm high and 15 mm thick on the transverse centre line, and a Ø30 mm through-bore whose axis lies 45 mm above the base. The customer's drawing office in Germany works only to first-angle projection. The trainee's sheet shows the front view at the top left, the plan directly above it, and a view of the left-hand face of the bracket placed to the left of the front view. The Ø30 bore appears only as a circle in the plan; the front and left-hand views carry no hidden lines. No projection symbol appears beside the title block. The checker also finds that the bore centre line stops at the outline of the boss, that the base-plate edges are drawn at 0.7 mm but the web edges at 0.25 mm, so the web reads as a construction line.
1. Which projection system has the trainee produced, and precisely what must change to meet the customer's requirement? Justify each move from the geometry of the two systems.
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System used: third angle. The giveaway is the pair of placements, not the shapes of the views:
- The plan sits above the front view.
- The view seen from the left is placed on the left of the front view.
That is exactly third-angle logic, where the projection plane lies between the observer and the object, so each view lands on the same side as the observer who sees it.
Changes needed for first angle (object between observer and plane, so each view is projected through the object to the far side):
- Move the plan to below the front view, keeping vertical projectors - every width on the plan must still line up with the front view.
- Move the view of the left-hand face to the right of the front view, keeping horizontal projectors so heights match the front view, and its depths must agree with the plan.
- Do not mirror or redraw any view. Only positions change; the left-hand-face view drawn on the right is still a view taken from the left. Depth is transferred with dividers or a 45° mitre line from the plan.
- Add the first-angle symbol beside the title block, since the sheet will travel to an office where either system may be expected.
A useful check: the front view is the principal view in both systems; only the secondary views migrate. If a colleague rotates a view through 90° to 'fix' the system, the drawing is wrong.
2. The Ø30 bore appears as a circle in the plan only. Decide whether hidden lines are required in the front and left-hand views, and answer the trainee's argument that 'the section on the other sheet shows it anyway'.
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Include the hidden detail in the front and left-hand views.
Reasoning:
- The bore is a single, simple, functional feature. In the front view it produces two short dashed lines 30 mm apart running through the 15 mm base; in the left-hand view it produces the same pair. It is not a case of clutter - one hole cannot swamp a view.
- Hidden lines are omitted only when they would confuse reading and the same information is reliably available on the same sheet, normally by a section. Here the section lives on a different sheet, so a fitter holding only this sheet would have no evidence of the bore in two of the three views.
- Sections themselves never carry hidden detail; that is a separate convention. Hidden detail is dropped in the section, not in the orthographic views beside it.
- Because the hole is drilled, the machinist also needs its centre line and, ideally, the note 'Ø30 THRO' with the depth clearly implied.
Draw the hidden lines as medium dashes (about 0.35 mm, dash roughly 3 mm with 1 mm gaps) and keep them clear of dimension lines. If the customer later insists on a fully sectioned sheet, the section replaces the hidden work in that view only - the remaining views stay complete.
3. Correct the line work on the sheet and state exactly what the first-angle projection symbol must contain.
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Line work corrections
- Bore centre line: redraw as a thin chain line (about 0.25 mm, long dash–short dash) and extend it 2–3 mm beyond the boss outline at both ends. A centre line that stops dead on the outline is not usable for setting out.
- Web edges: these are visible outlines and must be drawn at the same thick weight as the base-plate edges, 0.5–0.7 mm. Drawing them at 0.25 mm makes a real edge look like a construction line and hides the fact that the web is 15 mm thick.
- Hierarchy rule: the sheet carries only two or three weights. Thick continuous = visible outlines; medium dashed = hidden outlines; thin continuous = dimensions, extension lines, leaders, hatching; thin chain = centre lines and cutting planes. Construction lines must be erased before issue.
- Hidden lines for the bore added as medium dashes, already argued.
Projection symbol
The symbol is a truncated cone: a trapezium with its small end to the left and its large end to the right, with the two concentric circles of the end view drawn to the right of the trapezium. The circles on the right are the whole point - they show that the view of the left-hand end is placed on the right, i.e. first angle. Label it 'First angle projection'. The third-angle version is the same pair of figures with the circles on the left.