GlossaryKind of term: Manufacturing termProfile tolerance (line and surface)
What is a profile tolerance?
A profile tolerance limits how far a line or surface may deviate from its theoretically exact shape; with datums it also controls orientation and location.
There are two profile tolerances. Profile of a line ⌒ limits the contour in each cross-section, profile of a surface ⌓ the whole surface. The nominal shape is defined by theoretically exact dimensions or by the 3D model. By default the tolerance zone lies symmetrically about that shape: a band as wide as the value, bounded by the envelope of circles or spheres of that diameter centred on the nominal shape. Without a datum a profile controls the form and size of the contour, which may still shift and rotate as a whole. With datums, such as 0.1 A B, orientation and location are fixed too, as far as the datums constrain those degrees of freedom.
What does the standard say about a profile tolerance?
Symbols and zones are defined in ISO 1101. A zone that is not symmetrical about the nominal shape is indicated in ISO notation with the UZ modifier and an offset; ASME Y14.5 uses a circled U for the same purpose. Which datums apply, and in what order, is stated in the feature control frame. Without theoretically exact dimensions or a model as the source of the nominal shape, a profile requirement is ambiguous.
How does a profile tolerance work in practice?
A profile requirement on a curved surface mainly drives finishing and measurement. A freeform surface is machined with a ball-nose or bull-nose cutter in small stepovers. The stepover is set chiefly by the roughness and the scallop height; form accuracy depends more on the machine, tool wear and deflection. Whether a 3-axis machine will do, or a 5-axis machine with shorter tools works better, depends on the shape, access and datums. Measurement is against the nominal shape: on a coordinate measuring machine, with an optical scan or, for a line profile, with a contour tracer. Writing the measuring program takes time as well.
What is a profile tolerance often confused with?
A profile tolerance is not a size tolerance on a radius: R20 ±0.1 limits the radius; how the form may vary within it is ambiguous under ISO, while ASME Y14.5 defines a zone between two arcs. A profile limits the whole shape, including the blends. Nor is it a position tolerance: position limits the location of a centre point, axis or median plane, profile a line or surface.
Frequently asked questions
What does a profile tolerance without a datum mean?
The requirement then limits the form and size of the contour, but the contour may shift or rotate as a whole. With datums, orientation and location are fixed relative to the part as well, which is a stricter requirement.
How do you measure a freeform surface against the drawing?
On a coordinate measuring machine or with an optical scan, compared with the nominal shape from the 3D model. You need the model at the same revision as the drawing; ask for it if it is missing.
Can you hold a profile tolerance on a 3-axis machine?
On a contour in one plane or a simple blend, often yes. On a doubly curved surface with datums on several sides it becomes a trade-off between extra setups, longer tools and one setup on a 5-axis machine.