⌓△ Dynamic profile tolerance
Dynamic profile tolerance · Perfil · ASME Y14.5
Example in plan
How to interpret it
The upper line (⌓ 0.5 relative to A and B) controls the size and location of the profile; the lower line with △ requires the form to fit a 0.1 mm band that may shift uniformly inwards or outwards from nominal. A slightly oversize but well-formed part meets the second line.
What controls
ASME Y14.5-2018 profile modifier, drawn as a triangle (△) after the tolerance value. It controls the form of the profile but not its size: the tolerance zone may shift uniformly inwards or outwards from the nominal profile, so the part conforms if its form fits a band of the stated width even when it comes out slightly over or under nominal size. It is used as a form refinement: a normal profile line sets size and location relative to the datums, and a second line with △ refines the form only.
How to verify
Measure the full profile (CMM or scanner) and compare it with nominal allowing only a uniform inward or outward offset of the whole surface, without rotating or translating it relative to the datums of its line: what remains is the form variation. Size and location are evaluated separately against the upper line of the frame. It needs the profile defined by basic dimensions or a CAD model and software that supports that offset fit.
tolerance zone
Profile band that may shift uniformly inwards or outwards from nominal: it controls form, not size.
Workshop guideline values
- General
- Uncommon outside advanced industries; when it appears, 0.2–0.5 mm on complex-shaped parts.
- Accuracy
- 0.1–0.3 mm on blades or tight aerodynamic surfaces.
- High precision
- 0.02–0.05 mm on precision aerospace components with an associated CAD model.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Evaluating it with a free best fit that rotates and translates the part: △ only allows a uniform offset of the profile; orientation and location stay tied to the datums of its line.
- Rejecting a uniformly oversize part on the △ line: size is judged by the upper line of the frame, not the dynamic one.
How to get it in the workshop / if it comes out of tolerance
It exists only in ASME Y14.5-2018, not in ISO. It usually appears on two lines: the upper one (e.g. ⌓ 0.5 |A|B|) governs size and location, and the lower one with △ (⌓ 0.1 △) limits the form. A part that is uniformly thick or thin but well formed meets the △ line; what it cannot have is waviness or changes of curvature larger than that value.
Because it depends on an associated 3D model, first verify that the CAM post-processor uses the SAME CAD revision being inspected: a model revision not synchronized between engineering and the shop generates deviations that look like machining errors but are not. With that ruled out, the causes are the same as for classic profile of a surface: stepover, ball end mill wear and chatter from tool overhang.
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