GlossaryKind of term: Manufacturing termPosition tolerance
What is a position tolerance?
A position tolerance limits how far a feature, such as the axis of a hole, may deviate from its theoretically exact location.
It is a location tolerance with the symbol ⌖, and besides an axis it can apply to a median plane, a point or a surface. That location follows from boxed theoretically exact dimensions and usually from the datums in the feature control frame, for example ⌖ ⌀0.1 A B C. With the ⌀ sign, the zone for an axis is a cylinder of that diameter over the full length of the feature: the axis of the hole must lie entirely within it. Without ⌀ the zone lies between two parallel planes. A hole drilled at a slant therefore uses up position tolerance too.
What does a position tolerance depend on?
A round zone allows the same deviation in every direction, which suits the function: a bolt in a clearance hole has the same play all round. A square zone from ± dimensions in X and Y allows more along the diagonal than straight along X or Y. With the maximum material requirement Ⓜ after the value, the permitted position deviation grows as the hole departs from its maximum material size. Whether a tight position is held depends on the machine, the workholding, drill wander and whether the features are made in one setup.
What does the standard say about a position tolerance?
ISO 1101 defines the position tolerance and its zone, ISO 5458 its application to patterns of features, and ISO 2692 the maximum and least material requirements. Under ISO, a tolerance on a thread applies by default to the axis of the pitch cylinder, unless the drawing states otherwise. ASME Y14.5 follows the same idea, with rules of its own for patterns and bonus tolerance, among other things.
How does a position tolerance work in practice?
Features with a tight position between them are preferably made in one setup, so that the repeatability of reclamping does not count. With larger values, or workholding that repeats closely, reclamping can work. It is usually measured on a coordinate measuring machine, from the datums in the order of the frame. With Ⓜ in the frame a functional gauge can check the requirement; without Ⓜ you measure the actual location of each axis.
What is a position tolerance often confused with?
A position tolerance is not the dimensional tolerance on the hole: ⌀8 H7 says how big, ⌖ ⌀0.1 says where. A boxed theoretically exact dimension is not a dimension with zero tolerance: it gives the ideal location, and the tolerance sits in the frame. And a position tolerance is not a ± dimension on the location: that gives a rectangular zone and depends on the dimension chain.
How do you see a position tolerance in Blake?
A position tolerance from the drawing sits with the part in Blake as a proposal for you to confirm or correct. Whether the features share one setup is something you record yourself in the route, and setup has its own line in the estimate. More on the factory.
Frequently asked questions
May a position tolerance grow when the hole is bigger?
Only when Ⓜ follows the value. The permitted deviation then grows as the hole comes out larger than its smallest permitted size. Without a modifier the value holds as written, whatever the size of the hole.
How do I measure position?
From the datums, in the order of the frame, not from the easiest edge. Usually on a coordinate measuring machine, which establishes the axis of each hole and compares it with the zone. With Ⓜ in the frame a functional gauge can do it too.