Dimensioning a stepped shaft without duplication
A stepped shaft is 180 mm long. Its first 40 mm is Ø25, the next 55 mm is Ø35, and the remaining 85 mm is Ø20. A keyway 12 mm wide and 30 mm long is cut on the Ø35 portion, starting 60 mm from the left end. The candidate dimensions each step length in a chain (40, 55, 85), adds an overall 180 mm, and places the three diameters on the same side. Dimension text is 2 mm high, arrowheads are 1.5 mm long, extension lines touch the outline, and several dimension lines pass through the shaft. The keyway is dimensioned only by its width. The candidate writes 'NTS' under the drawing and claims that this removes the need for exact dimensions. The drawing is on A3 at 1:1.
1. Propose a datum-based dimension scheme. List the dimensions to keep, identify the redundant ones, and explain why the diameters are not redundant.
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Use the left end face as the datum. Length dimensions from that datum: 40 mm to the first shoulder, 95 mm to the second shoulder (40 + 55), and 180 mm overall. Keyway: 60 mm from the left end to its start and 30 mm length; width 12 mm; depth still missing. Diameters: Ø25, Ø35 and Ø20, one per step.
Do not also give 55 and 85 as chain dimensions, because with 40, 95 and 180 they are redundant. Redundant dimensions create conflicting tolerances: the shop cannot tell which dimension controls if the chain and the datum dimensions do not agree. The three diameters are not redundant because each defines a different cylindrical feature. The keyway width and length are separate features, so both are needed, but its depth or keyseat dimension must be added.
2. Correct the text height, arrowhead size, extension line gaps and dimension line placement. Explain the status of the 'NTS' note.
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Use 3.5 mm text height for A3; 2 mm is below the usual minimum of 2.5 mm and will be hard to read. Arrowheads about 3 mm long and 1 mm wide, roughly 3:1 length-to-width. Extension lines: 1-2 mm gap from the outline and 2-3 mm beyond the dimension line. Keep dimension lines outside the shaft where possible, group them by feature, and avoid crossing.
'NTS' is a warning that the drawing is not to scale; it does not permit missing or inexact dimensions. If the drawing is not to scale, still dimension every feature exactly. The note may be added, but it cannot replace a complete dimension scheme.
3. Identify the missing information and state how it should be annotated.
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Missing information includes: keyway depth or keyseat width, chamfers or radii at the shoulders, tolerance for the Ø35 bearing fit, surface finish, and whether the 60 mm keyway start is measured to the keyway edge or centre.
Annotate with a keyway note such as 'keyway 12 wide × depth ...', a tolerance callout such as Ø35 +0.02/0, and a finish symbol. If the shaft is symmetrical, state that; otherwise give a datum and avoid assuming. A note such as 'break sharp edges 0.5 × 45°' is also useful if chamfers are intended.