Reading a reduced-print spindle drawing: scale, datums and broken-out section conventions
An A3 drawing of a stepped stainless spindle is issued to inspection at scale 1:2. It carries a full section through the axis and a broken-out section that exposes a Ø6 cross-hole sitting under a keyway. The inspector measures on the printed sheet: overall length 75.6 mm, large journal diameter 19.0 mm, and 31.5 mm from the left end face to the first shoulder. Unknown to him, the PDF was made by reducing the original A3 plot to 90% so it would fit the office printer. The sheet also shows a chain of dimensions taken from the left end, a shoulder dimensioned twice (once in the chain and once as 60 mm from the right end), a callout reading only "M20", a surface-texture symbol floating in the middle of a hatched face with its tail inside the hatching, and a dimension line that runs straight across the irregular break line of the broken-out section. No datum features are identified anywhere on the drawing.
1. Recover the real dimensions behind the inspector's three measurements, and state the rule about scaling a drawing that he has broken.
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Two independent reductions are stacked on that print, and they must be undone in the right order: the reproduction first, then the drawing scale.
- Overall length: 75.6 mm on paper ÷ 0.90 = 84.0 mm on the original A3 sheet; at 1:2 that is 84.0 × 2 = 168 mm.
- Journal diameter: 19.0 ÷ 0.90 = 21.11 mm; × 2 = 42.2 mm, consistent with a nominal Ø42.
- End face to first shoulder: 31.5 ÷ 0.90 = 35.0 mm; × 2 = 70 mm.
Order matters: dividing by 2 first and then by 0.9 gives 37.8, 9.5 and 15.75, all wrong. Note also that a scale factor applies to lengths only; an area would scale by the square.
The rule broken is fundamental: dimension text governs, never the scaled geometry. A drawing is not a measuring instrument - paper stretches, printers reduce, and scale notes are often stale. The inspector should read the stated dimensions and use measurement only as a rough cross-check, and any missing dimension should be requested from the designer rather than scaled off the sheet. A 90% PDF proves the point: nothing on that print can be trusted to a rule.
2. Redesign the dimensioning scheme for the spindle: datum choice, chain versus baseline dimensioning, and placement of dimensions relative to the section, the hatching and the break line.
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The present scheme is a tolerance stack waiting to happen.
Datums. Identify functional datums first: the two bearing journals (the surfaces that actually locate the shaft) and the shoulder face that seats the bearing axially. Mark them with datum triangles (A, B, C) and give the geometrically critical tolerances - coaxiality of the journals, runout of the shoulder - against them. With no datums, the ±0.1 on each step accumulates.
Chain versus baseline. A chain of dimensions from the left end adds every tolerance along the shaft; the shoulder position can drift by several tenths. Replace it with baseline (ordinate) dimensioning from one datum, or from two datums with the joint at a non-functional feature. Keep the overall length as a reference dimension in parentheses so the chain is not closed, and delete the duplicate 60 mm - one feature, one dimension. Dimension each feature on the view where it appears true shape (the section is correct for internal and stepped features), outside the hatching, never on a hidden line, and never across the irregular break line of the broken-out section: the dimension should be moved clear or the break line shifted. The "M20" must become a full designation, e.g. M20 × 1.5 – 6g, since pitch, tolerance class and hand all change the part.
3. Correct the broken-out section convention and the surface-texture symbol. State what a broken-out section must look like and where the texture symbol belongs.
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Broken-out section. Its purpose is to reveal one local feature without sectioning the whole part. The boundary must be a continuous thin irregular line (freehand) or a thin straight line with zig-zags - never a thick line, never a chain line, and never drawn to look like a real edge. Inside the broken-out area the material is hatched exactly as in a normal section, and hidden lines are omitted there, since the whole point is that the feature is now visible. The break line must not coincide with a visible edge, an extension line or a dimension line; here it does, so the dimension crossing it must be moved outside the broken-out area.
Surface-texture symbol. The apex of the symbol must touch the line that represents the surface, standing on the outside of the material, and be readable from the bottom or the right-hand edge. Floating it in the middle of the hatching, with the tail inside the material, is wrong on both counts: it does not identify which surface is meant, and the hatching is not a surface. Move it onto the shoulder outline in the section, or onto an extension line of that face. If every surface carries the same finish, one general note above the title block serves; otherwise one symbol per surface, each anchored on its own line.