The zone is centred, and the value is the whole of it
An equally disposed profile tolerance puts half its width on each side of true profile. A frame stating 0.20 mm allows 0.10 mm outside and 0.10 mm inside, and the surface has to fit inside that single band everywhere.
That makes the binding number twice the worst single deviation, not the sum of the extremes. A surface 0.14 mm outside at one point and 0.05 mm inside at another needs a zone 0.28 mm wide, because the band has to be centred and the worst point sets its width.
Surface against line
Profile of a surface applies a three dimensional zone over the whole surface. Profile of a line applies a two dimensional zone in each cross section independently, so an extrusion can satisfy every section while the sections drift or twist relative to one another.
The two symbols differ by a single stroke and are drawn wrong regularly. If the relationship between sections matters, the surface version is the one to use, or a line profile refined by an orientation control.
Unequal disposition and datumless profile
An unequally disposed profile tolerance keeps the first value as the whole zone width and uses a second value to say how much of it sits in the added material direction. A 0.40 mm zone with 0.30 mm outward leaves 0.10 mm inward, which is how a coating thickness or a minimum wall is protected without changing the model.
A profile frame with no datum references is not an error. The zone is then free to be best fitted, so what survives is a statement about shape alone, which is a legitimate refinement when another frame handles location.
Composite profile and extent
A composite profile frame locates the surface loosely in the full datum reference frame in its upper tier and controls shape and orientation tightly in its lower tier, with the tighter zone free to move inside the looser one. Both apply at once.
All around, all over and between answer where the control applies rather than how much. They never change the tolerance value, and confusing all around with all over either leaves surfaces uncontrolled or imposes a requirement nobody costed.
A surface measured 0.14 mm out and 0.05 mm in
The frame states an equally disposed profile of a surface tolerance of 0.25 mm to A, B, C.
- Worst outward deviation
- 0.140 mm
- Worst inward deviation
- 0.050 mm
- Zone width the surface requires
- 0.280 mm
- Zone width allowed
- 0.250 mm
- Verdict
- Reject
- A tighter line profile case for comparison
- 0.120 mm
Note that the two deviations sum to 0.19 mm, which is inside the stated tolerance. Summing them is the mistake this example exists to break: the zone must be centred, so the worst point alone decides.