Datum dimensioning and section convention on a mounting plate drawing
A machine shop has paused a batch of 40 aluminium mounting plates because the drawing supplied by a transition-year student is ambiguous. The plate is 180 mm × 120 mm × 12 mm, with four Ø11 mm clearance holes, a central Ø40 mm bore and two 8 mm stiffening ribs on the underside. The shop works to ±0.2 mm and uses first-angle projection. The student dimensioned each hole from the nearest edge in a continuous chain. The overall length is given as 180 mm, but the chain of dimensions along that edge adds to 182 mm. In section view A–A, hidden lines are still shown, the hatching runs without a break across one rib, and the Ø40 bore is indicated by a leader line to the circle in plan rather than by a diameter dimension on the section. The machinist says the drawing is “crowded but probably fine”. The student’s teacher has asked for a written technical review before the drawing is returned for correction.
1. Identify which of the drawing’s problems are convention errors and which create genuine manufacturing ambiguity. Justify each classification.
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The chain dimension is not merely untidy: it is a real conflict. The overall length is 180 mm while the chain sums to 182 mm, so the machinist cannot know which feature is wrong. That is a manufacturing ambiguity, not a convention error. By contrast, the leader line to the Ø40 circle is a convention error: a full circle should carry a diameter dimension with the Ø symbol, preferably on the section where the bore is clear. Hidden lines in a section are also a convention error - cut views should omit hidden detail unless it is essential. Hatching continuously across a rib is a convention error because a rib cut longitudinally is normally left unhatched, with its outline shown, to avoid implying a solid mass. The use of first-angle projection is correct for the shop; the problem is not the projection angle. The machinist’s “probably fine” is unsafe because the 2 mm discrepancy could shift the hole pattern or the bore, so the drawing should be rejected until the chain is replaced.
2. Propose a dimensioning scheme that a machinist can set up from, naming datums and giving example dimensions.
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Use three datums: A = left machined edge, B = bottom machined edge, C = top face. Dimension overall size as reference in brackets: (180) and (120), not as part of a chain. Locate each hole from A and B as coordinate pairs, for example (25, 30), (155, 30), (25, 90), (155, 90), all with Ø11 THRU. Locate the Ø40 bore from A and B, for example 90 mm from A and 60 mm from B, and give it as Ø40 THRU. Put the two rib positions from datum A, with 8 mm thickness and note “2 off”. Keep dimensions outside the outline where possible, and put depths on the section. Add a note: “All dimensions in millimetres. Interpret to first-angle projection. Unless stated, tolerances ±0.2 mm.” The machinist can then set A and B on the machine and use one setup. This removes the cumulative error and makes inspection straightforward.
3. Describe how section A–A should be drawn and explain why the rib treatment matters.
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Section A–A should be a full section taken through the centre of the Ø40 bore and through one rib, with the cutting-plane line and arrows shown on the plan. In the section, omit hidden lines, draw the bore as a clear void with a centre line, and hatch only the material actually cut. The rib should be outlined but not hatched when the cutting plane runs along its length; that convention distinguishes a thin stiffening feature from a solid wall. If the rib were hatched continuously with the plate, the reader would infer a much heavier section and could misjudge weight and stiffness. Add the Ø40 dimension on this section, and use a separate detail or note for the 8 mm rib thickness. This treatment also makes the drawing easier to read at A3 size, which matters when the machinist is working from a printed sheet.