Perpendicularity

Controls

A surface, axis or centre plane must lie within a tolerance zone at exactly 90° to the datum reference. For a surface, the zone is two parallel planes perpendicular to the datum. For an axis, the zone is a cylinder of diameter t whose axis is perpendicular to the datum.

Perpendicularity is the most common orientation control — it governs upright walls, vertical bores, flange faces and pressed-in pins.

Example

A jig bushing pressed into a drill plate must have its bore axis perpendicular to the plate base face within Ø0.05mm. If the bushing is tilted, drilled holes will be angled — compounding position errors across the workpiece. Perpendicularity is also critical on O-ring grooves, sealing flanges and connector housings.

Inspection

• Precision engineer's square + dial gauge — sweep face against square referenced from datum surface
• CMM — measure surface normal vector and compare 90° to datum plane
• Sine bar — used when verifying axes at compound angles
• Functional go-gauge at MMC — verifies assembly fit without measurement

Datum & Modifiers