GlossaryKind of term: Manufacturing termPerpendicularity
What is perpendicularity?
Perpendicularity is the orientation tolerance for a surface, axis or line that must stand square to a datum, given as a zone width in millimetres.
Shop floors often call it squareness; in the feature control frame it appears as ⟂, with at least one datum. For a face square to a datum plane, the zone lies between two parallel planes the tolerance value apart, square to the datum. For a bore or pin with ⌀ in front of the value, the zone is a cylinder square to the datum plane. For a face square to a datum axis, the two bounding planes stand square to that axis. The value is a zone width in millimetres, not an angle in degrees.
What does the standard say about perpendicularity?
The symbol and zones are defined in ISO 1101, the construction of datums in ISO 5459. Without a datum, a perpendicularity requirement is incomplete. The zone also limits form: a face within its perpendicularity zone is flat within the same value. A right angle that is only drawn, with no requirement of its own, falls under general perpendicularity when the drawing invokes ISO 2768-2 with class H, K or L. Total axial runout relative to an axis gives the same zone as perpendicularity of that face to the axis.
How does perpendicularity work in practice?
Perpendicularity between a face and a bore is easiest to hold when both are made in the same setup: machine geometry and the stiffness of tool and part then set most of the error. When they are made on different sides, the accuracy of reclamping comes in, for example a vice that does not clamp square or an angle plate that has to be clocked in. On a lathe, the squareness of a faced end to a diameter from the same chucking mostly depends on how square the X-axis travel is to the spindle axis; if the datum diameter was made in an earlier chucking, how the part sits in the chuck counts as well. Checking is done with a dial indicator on a surface plate against a square reference, or on a coordinate measuring machine. Whether a dedicated fixture pays off depends on batch size, repeat orders and the value required.
What is perpendicularity often confused with?
Perpendicularity is not a 90° angle dimension with plus-minus: that has no datum and a tolerance in degrees or minutes. Nor is it position: a bore can stand perfectly square and still sit in the wrong place. A position tolerance with the square face as primary datum does limit perpendicularity as well.
Frequently asked questions
Is the perpendicularity value in degrees?
No, in millimetres. The value is the width or diameter of the zone. The longer the feature, the smaller the angular error a given value allows. The same value is therefore stricter on a long bore than on a short one.
Why is perpendicularity easier to hold in one setup?
Because face and bore then share the same machine orientation. After reclamping, the error of workholding and alignment comes on top, and it has to stay well inside the value. Aligning, probing or a dedicated fixture keeps that error down.
Do I need a dedicated fixture for perpendicularity?
Not necessarily. For a batch with tight squareness across two sides, a fixture can be faster and more repeatable than clocking in each time. For a one-off, aligning and probing is the usual route. Quantity, repeat orders and the value decide.