Profile of a Surface Controlling Both Location and Size of a Slot - and the Effect on a Position Callout
A plate has an internal slot 20.0 ± 0.1 wide, 60 long, with a basic width of 20.0 and a profile of a surface callout |⌖|...| - specifically |⌖|2.0|A|B|C| applied to the slot surfaces, where the slot width is given as a basic dimension. In addition, a position callout |⌖|Ø0.2|A|B|C| is applied to the slot's center plane. The machinist produces the slot at 20.06 wide, with the slot's center plane offset 0.08 mm from true position. The inspector reports the slot passes position (0.08 < 0.10) but fails profile because the surface deviation reaches 1.15 mm on one wall. The designer asks whether profile or position governs the slot's size and location.
1. Explain the relationship between the profile callout and the position callout on this slot, and identify which controls size and which controls location.
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When a feature of size is dimensioned with a basic width and controlled by profile of a surface, profile controls both size and location of the surfaces. The profile tolerance zone is a 2.0 mm total band (i.e., ±1.0 about the true profile) within which both walls must lie. The slot's actual width and its location are therefore both governed by the profile zone.
The position callout on the center plane is redundant and potentially conflicting with profile for size control. Position of a feature of size controls the center plane location and orientation within a Ø0.2 zone, but does not control the size or form of the walls. Applying both to a basic-dimensioned slot is a drawing error: the basic width removes the size tolerance that position would otherwise work with, and profile already controls location.
Correct practice: use profile of a surface alone (with basic width) to control size, form, orientation, and location; or use a size dimension with tolerance plus position to control location while size is controlled by the limit dimension. Do not mix basic width + profile + position on the same feature.
2. Given the measured data (width 20.06, center offset 0.08, one wall deviation 1.15 mm), evaluate acceptance under the profile callout and explain why the position result is misleading.
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Under the profile callout, each wall must lie within a 2.0 mm band centered on the true profile (i.e., within ±1.0 mm of the basic surface location). A wall deviation of 1.15 mm exceeds the ±1.0 mm limit, so the part fails profile. Note that the slot width of 20.06 (0.06 over basic) is itself within a ±1.0 profile band on each wall only if each wall is within 1.0 of true - a 0.06 width increase is fine, but the 1.15 mm wall deviation is not.
The position result (0.08 < 0.10) is misleading because position evaluates only the center plane, not wall form or individual wall location. A slot can have its center plane well-located while one wall bulges or is displaced, producing a large surface deviation that position cannot detect. This is exactly why profile of a surface is the correct control for a basic-dimensioned slot: it evaluates the actual surfaces, not just the derived center plane. The part must be rejected on profile.