Over-dimensioning, half section and a rescaled print
A bracket is drawn at 1:2 on A3 and issued as a PDF; the drawing shows the symmetric part in half section. The dimension set includes an overall length of 120, a chain of 60 + 60 locating two M8-6H tapped holes, a Ø20H7 bore with a Ø32 counterbore 6 deep, and the general note ISO 2768-m. A checker also observes that the hidden lines of the bore are still shown in the unsectioned half, and that the hatching in the sectioned half runs at 30° with visibly uneven spacing. The PDF is then printed on A4 with "fit to page" (70.7 %), and a fitter measures 42.4 mm across the outside of the part on the paper and orders a fixture made to that size.
1. Identify the over-dimensioning and state what should be kept, with the reasoning.
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The hole positions are given twice: the chain 60 + 60 and the 120 overall, which closes the chain. Under ISO/UNE 129 each feature is dimensioned once, and a closed chain of dimensions is not permitted because the last value is already determined by the others - any small error in the two 60s contradicts the 120.
The choice is functional, not arbitrary. The 60 pitch is what the mating component needs, and the 120 sizes the blank or the overall envelope. A good solution keeps the 120 as the overall dimension and dimensions the tapped holes from a common datum (the left end face) as 30 and 90, or as 60 pitch plus one locating dimension. Baseline dimensioning from a datum is preferred here because it does not accumulate tolerances, whereas the chain does. What must not remain is 60 + 60 + 120 together.
2. State the half-section conventions that apply and say what is wrong with the hidden lines and the hatching.
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Because the part is symmetric and the cutting plane passes through the axis, half the view shows the interior and half shows the outside. The symmetry axis is a centre line (long dash, short dash) that must extend slightly beyond the outline, and the dividing line between the sectioned and unsectioned halves is that axis, not a continuous line.
In the sectioned half, hidden lines are omitted because the section already reveals the interior; they are kept in the unsectioned half only when they carry information available nowhere else. Here the Ø20H7 bore and the counterbore are fully legible in the sectioned half, so their hidden lines in the other half are redundant and should be deleted.
Hatching: thin continuous lines at 45° to the principal outline, with constant spacing of about 2–3 mm on the sheet, same direction and spacing throughout one part; adjacent parts get a different direction or spacing. A 30° hatch with uneven spacing reads as a different material or a different part and is a rejectable error.
3. Explain what went wrong with the fitter's 42.4 mm and give the correct reasoning chain.
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The stated scale is 1:2 on the A3 original, but the A4 print is 70.7 % of that original, so the effective scale on the paper is 1 : (2 / 0.707) = 1 : 2.83. A measured 42.4 mm therefore corresponds to 42.4 × 2.83 ≈ 120 mm, which is exactly the overall length already dimensioned on the drawing.
Two rules follow. First, never scale a drawing: reproduction, fit-to-page settings and paper shrinkage all change the paper size while the stated dimensions stay valid. Second, never derive a manufacturing size from a print - use the stated dimensions; if a dimension is missing, ask for it or compute it from the given geometry and mark it as a reference dimension in brackets.
The fixture should be made to the 60 mm pitch (and to the datum dimensions), not to 42.4 mm. Reproduced sheets also carry the note "do not scale" precisely to prevent this chain of errors.