Shaft and Bush: Datum Choice, Fit and Text Height
A detail drawing of a Ø25 mm shaft and its mating bush is being checked before manufacture. The drawing is at 2:1 on A3. The shaft is dimensioned by chain from the left end as 18, 4, 8 and 17 mm to four shoulders, each with the general note tolerance of ±0.2 mm. The bush bore is marked Ø25 H7 and the shaft Ø25 g6, with no tolerance written against the chain dimensions. The drawing note reads: general tolerance ±0.2, dimensions in mm, do not scale drawing. The inspector rejects the drawing because the accumulated chain error could exceed the fit allowance, and because the 3.5 mm high dimension figures measure about 7 mm on the printed sheet. The designer argues that the drawing is at 2:1, so the text is correct at double size.
1. Explain the datum problem with the chain of dimensions and propose a dimensioning scheme for the shaft.
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Chain dimensioning makes every dimension depend on the one before it, so tolerances accumulate. With four ±0.2 mm links, the position of the far shoulder can vary by ±0.8 mm in the worst case, against a diametral fit clearance of only a few hundredths of a millimetre. The shoulder that locates the shaft in the bush therefore cannot be controlled at all by this scheme.
Use baseline (datum) dimensioning: choose the locating shoulder or the left end face as datum A, mark it with the datum symbol, and dimension every shoulder from that single datum. Each dimension then carries only its own tolerance, and the critical shoulder can be given a tighter tolerance such as ±0.05 mm. Keep dimensions off the views where possible, group them outside the outline, and do not repeat a dimension in two views.
2. Calculate the limits and clearance for the Ø25 H7/g6 fit and state what type of fit it is.
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For Ø25 mm, IT7 = 21 µm and IT6 = 13 µm.
- Hole H7: lower deviation 0, upper +0.021 mm, so limits 25.000 / 25.021 mm.
- Shaft g6: fundamental deviation for g is −7 µm, so limits 25 − 0.007 = 24.993 mm and 25 − 0.020 = 24.980 mm.
- Minimum clearance: 25.000 − 24.993 = 0.007 mm.
- Maximum clearance: 25.021 − 24.980 = 0.041 mm.
This is a running clearance fit, appropriate for a shaft that rotates or slides in the bush. The point for the drawing is that the total 0.041 mm allowance can be consumed several times over by the ±0.2 mm chain tolerance, which is exactly why the inspector is right.
3. Rule on the text-height dispute and state the correct practice at 2:1.
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The designer is wrong. Lettering and dimension-figure heights are specified on the sheet, independent of the drawing scale. A 3.5 mm text height means 3.5 mm high on the printed A3, so the figure should measure 3.5 mm on the paper, not 7 mm.
At 2:1 the model geometry is twice full size, but the text is still set at 3.5 mm on the sheet. If the candidate sets the text at 7 mm, the drawing breaks the house standard, the figures crowd the views and the sheet cannot be reduced to A4 without the text becoming unreadable. The rule to apply: scale applies to the object, never to the annotation. Check by measuring a known figure with a rule on the printed sheet, not in the CAD model space.