Runout vs. Profile for a Thin-Wall Cylinder - and Whether Total Runout Detects Taper
A thin-wall aluminum cylinder (Ø80 OD, 2 mm wall, 100 mm long) has its OD controlled by |⌖|... total runout 0.05|A| where A is the Ø80 bore. The part is machined in a single setup, then heat-treated, causing the bore to become slightly out-of-round (0.03 mm) and the OD to taper from Ø80.00 at one end to Ø79.94 at the other. The inspector mounts the part on a precision expanding mandrel simulating the bore's actual mating envelope, rotates it, and measures OD runout. The maximum indicator reading is 0.04 mm. The inspector accepts the part. The designer is concerned about the taper and asks whether total runout adequately controls it.
1. Evaluate whether total runout controls the taper described, and whether the acceptance is correct.
Read the worked answer
Total runout applied to a cylindrical surface controls: circularity, cylindricity, coaxiality, and taper (it is a composite control over the entire surface, unlike circular runout which controls only individual cross-sections). Total runout requires that, at every position along the surface, the indicator reading not exceed the tolerance while the part rotates and the indicator traverses axially.
A taper of 0.06 mm over the length means the OD decreases by 0.06 from one end to the other. Relative to the datum axis A (the bore's AME), this taper produces a diametrical variation of 0.06 mm across the length - which exceeds the 0.05 total runout tolerance if the taper is eccentric to the datum axis. However, if the taper is concentric (the OD is a cone centered on A), the indicator reading at any single cross-section could still be small, and total runout would detect the taper only as the indicator traverses axially.
A maximum reading of 0.04 mm is within 0.05, so on the measured data the part passes total runout. But the inspector must confirm the indicator was traversed over the full length (total runout requires full-surface traverse, not just circular readings at one plane). If only circular readings were taken, the taper was not evaluated and the acceptance is invalid.
2. If the designer's real concern is wall thickness uniformity and roundness of the bore, is total runout on the OD the right control? Recommend a better scheme.
Read the worked answer
Total runout on the OD relative to the bore controls the OD's relationship to the bore axis, but it does not directly control:
- Bore roundness (out-of-round of the datum feature itself), since the datum is the bore's AME - an out-of-round bore still establishes a single axis.
- Wall thickness uniformity, which depends on the relationship between OD and ID surfaces at each angular location, not just coaxiality of axes.
Better scheme:
- Control the bore with a circularity (roundness) tolerance, e.g., 0.02, to limit out-of-round.
- Control the OD with total runout to the bore (or profile of a surface if taper/form must be tightly bounded and the surface is basic-dimensioned).
- If wall thickness is functionally critical, apply a profile of a surface to the OD with the bore as datum, or specify a separate wall-thickness requirement - runout alone does not bound wall thickness when the bore is out-of-round, because the datum axis is derived from the AME and local wall variation can exceed the runout value.
The combination of circularity on the bore plus total runout (or profile) on the OD gives the designer the control actually needed.