Ⓤ Unequally disposed profile
Unequally disposed profile · Modificadores de zona · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⌒ 0.2 Ⓤ |A|” states a unequally disposed profile requirement with a tolerance of 0.2 mm, measured from datum A.
What controls
By default, the zone of a profile tolerance is split equally on both sides of the true profile: half outward and half inward. Ⓤ breaks that split and shifts the zone towards one side without changing its total width. In ASME Y14.5, the value written after Ⓤ is how much of the zone lies in the direction that would add material: a profile of 0.4 with Ⓤ 0.1 leaves 0.1 outward and 0.3 inward; with Ⓤ 0 the zone is entirely unilateral inward. In ISO the same effect is written with the UZ indication and a signed value that offsets the center of the zone. What Ⓤ does NOT change is the size of the tolerance: the value in the frame is still the full width of the zone; it has only been moved.
How to verify
In the CMM or scanning software, the profile must be entered as an unequal or unilateral zone with the offset value and its direction loaded, not left at the default symmetrical split: with the same point cloud, the wrong split accepts and rejects different parts. Always check the software's sign convention (into the material or outward) against a master part or a known case before releasing a lot. The deviation report must show the direction of each deviation, because here +0.15 and -0.15 are not equivalent.
Common mistakes when interpreting it
- Reading the value after Ⓤ as an additional tolerance: it is a split within the zone already specified, not extra material.
- Getting the offset direction wrong and programming the stock allowance backwards, which uses up the margin on precisely the side you don't want.
- Programming the machining to the nominal CAD profile as if the zone were still symmetrical, wasting half of the tolerance.
How to get it in the workshop / if it comes out of tolerance
It is the way to say on which side you want material to be left over or missing. It shows up on castings or forgings where you want to leave stock outward so the surface can be cleaned up later, on blades and airfoil profiles, and on mounting surfaces where excess material outward prevents seating but a slight shortfall does no harm. For the machinist, the key thing is to aim at the center of the offset zone, not at the nominal CAD profile: if you program to nominal, you are using only half of the available tolerance, and the other half of your margin sits on the side that is no use to you.
If the profile consistently comes out shifted towards the narrow side of the zone, correct the offset in the program: move the tool offset or the finishing stock by half of the effective width available on that side. If there is excess material on a casting or forging, check the pattern and the parting line before the machining. And if the rejection only shows up in the report but the part assembles fine, check that the offset direction was loaded correctly in the evaluation.
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