Dimensioning a Stepped Shaft for the Machine Shop
An ASSEB candidate is preparing a shop drawing of a stepped shaft at 1:2 on an A3 sheet. The shaft is 120 mm overall: Ø30 for the first 45 mm, Ø40 for the next 35 mm, and Ø25 for the last 40 mm. The candidate dimensions each step by chain dimensioning from the left end (45, 35, 40), repeats the overall 120 mm above the view, and places the diameter values on leaders that cross the hatching of a broken-out section. The dimension text is 3.5 mm high, sits 1 mm above the dimension line, and is oriented so that every value is read from the right-hand edge. Arrowheads are 3 mm long and 1 mm wide; where two extension lines are 6 mm apart, the arrows are drawn outside with the text between them. A note on the drawing says ALL DIMENSIONS IN mm. The drawing is about to be issued to the machine shop.
1. Identify the dimensioning choices that are non-conforming or ambiguous, and explain the effect of each on manufacture.
Read the worked answer
The main faults are:
Chain plus repeated overall. The chain 45, 35, 40 already fixes the 120; repeating 120 creates a redundant dimension. If the chain and the overall disagree after rounding or tolerance, the shop cannot tell which governs. Each feature should be dimensioned once.
Leaders crossing hatching or other lines. A leader must touch the feature with its arrow and run clear of other lines, with a short horizontal shoulder. Crossing the section hatching makes the diameter hard to associate with the correct cylinder.
Text orientation. All text should read from the bottom or from the right-hand edge; forcing every value to read from the right makes horizontal dimensions appear sideways and invites misreading.
Diameter placement. Diameters should be given on the view where the circle appears, or with the Ø symbol on a leader to the cylindrical surface, not scattered on leaders to the section.
Arrowheads outside the 6 mm gap. Two 3 mm arrows plus text cannot fit in 6 mm, so outside arrows are acceptable, but the extension lines must be extended beyond the arrows and the value placed between the arrowheads; the 1 mm gap and 3.5 mm text are otherwise acceptable.
2. Propose a corrected dimensioning scheme for the shaft, including the datum choice, and justify it for both manual and CNC use.
Read the worked answer
Use the left end face as the common datum, because it is the locating face that the shaft will be held against. Baseline-dimension the step positions from that face: 45 and 80. Omit the overall 120 as a working dimension; if it is useful for checking, show it as a reference dimension in brackets, (120). Give each diameter once, on the cylindrical view: Ø30, Ø40 and Ø25, each clearly associated with its length segment. Add the chamfer (for example 1 × 45°) and any surface-finish symbol outside the outline.
Baseline dimensioning avoids the tolerance stack of chain dimensioning: with ±0.2 on each chain link, the 80 mm step could drift by ±0.4, whereas each baseline dimension is independent. For CNC work the datum corresponds to the program zero, so the operator can enter the X values directly, and the reference overall dimension gives a quick check without becoming a conflicting instruction. Keep dimensions off the hatched section.
3. At 1:2, state the plotted sizes of the 3.5 mm text and the 3 mm arrow, and explain why the numerical dimension values do not change with scale.
Read the worked answer
At 1:2 the geometry is halved on the sheet, but the text is still drawn 3.5 mm high on the sheet, representing 7 mm of the real shaft; the arrow is 3 mm long on the sheet, representing 6 mm real. These are graphical conventions, not measured features, so they are chosen for legibility rather than scaled with the part. The 120 mm shaft is drawn 60 mm long but is still written 120; the Ø30 is still Ø30. Scale changes only the drawn length of the geometry, never the numerical value attached to a dimension. If the drawing is later plotted at a different size, the values remain correct but the annotation scale (DIMSCALE in CAD) must be changed so the text still prints at about 2.5–3.5 mm.