Dimensioning a Stepped Shaft for the Workshop
In a school workshop drawing exercise, a student prepares an A3 front view of a stepped shaft whose overall length is 180 mm. The shaft has diameters 40 mm, 30 mm and 20 mm and a 10 mm wide keyway. The student dimensions the length with five chain dimensions taken from the left end - 25, 35, 40, 45 and 35 mm - and then repeats 180 mm as an overall dimension above the view. A hidden bore is dimensioned directly to its dashed edges. Two leader lines cross near the keyway, one arrowhead is placed outside an 8 mm gap, and every extension line touches the object outline. The instructor writes “scheme and conventions need correction” on the sheet and asks for a written diagnosis before any redraw. The student insists that all the individual numbers add to 180 mm, so the drawing must be correct.
1. Diagnose the length-dimensioning scheme. Why is the chain plus duplicate overall risky, and what baseline scheme would you substitute? Use the given numbers.
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The five chain dimensions each carry a tolerance, say ±0.2 mm. Because each dimension is measured from the previous shoulder, the errors accumulate: the right-hand end can vary by about ±1.0 mm. Repeating 180 mm as an overall dimension adds a second, independent requirement, so the drawing contains a closed dimension loop; the inspector cannot tell whether the overall size or the chain controls. A baseline scheme removes the stack. Choose the left face as the datum and dimension each shoulder from it: 25, 60, 100 and 145 mm, with 180 mm to the right face. Locate the keyway from the same datum, for example 70 mm to its start plus 10 mm width. Each dimension is then independent and the tolerance at any feature is only its own ±0.2 mm. The overall 180 mm may be shown as a reference dimension in parentheses, or omitted if the baseline dimensions already define it. The student’s arithmetic is correct, but arithmetic correctness does not make a dimensioning scheme functional.
2. The bore depth is dimensioned to hidden lines and two leaders cross. State the convention-based fix and the exception.
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Hidden lines are not dimension lines. Dimensioning to dashed edges forces the reader to infer which hidden feature is intended and produces poor reproduction on copies. Replace the hidden-line dimension with a full section, an offset section or a revolved section through the bore, and dimension the visible bore depth in that view. Leaders should point to the visible outline, preferably at a curved surface, and should not cross each other or other dimension lines. If the 8 mm gap is too short for two arrowheads, place the arrowheads outside the extension lines and extend the dimension line beyond them; do not shrink the arrowheads. As an exception, a hidden feature may be dimensioned directly when no section is possible, but then use a clear note or an auxiliary view and keep such cases to a minimum. The fix is therefore a view change, not a line-style change.
3. Specify the line types and weights for this sheet, including the extension-line gaps.
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Use a clear hierarchy. Visible outlines are continuous thick lines, about 0.7 mm. Hidden outlines are dashed medium lines, about 0.35 mm, with dashes about three times the gap and dashes starting and ending on the outline. Centre lines are thin chain lines, about 0.25–0.35 mm, extending 2–3 mm beyond the feature. Dimension, extension and leader lines are thin continuous lines, about 0.25–0.35 mm. Extension lines should begin 1–2 mm away from the object outline and extend 2–3 mm beyond the arrowhead; they must not touch the outline. At corners, visible lines should meet without gaps, while hidden and centre lines should show their correct pattern. Arrowheads should be about 3:1 length-to-width and uniform. These conventions make the baseline scheme readable and prevent the heavy outline from being confused with dimension lines.