Perpendicularity
Perpendicularity is a GD&T orientation tolerance that requires a surface, axis, or center plane to be exactly 90° to a specified datum. The tolerance zone consists of two parallel planes for surfaces or a cylindrical zone for axes, controlling only angular orientation—not size. This ensures machined features are square to the datum within the given limit, preventing assembly issues like binding or misalignment.
On the shop floor, perpendicularity is how a drawing tells the machinist to hold a face or bore square to a datum, not just 'close enough.' In CNC machining, the datum is typically a ground face or fixture plane, and the controlled feature must stay inside the tolerance zone along its entire length or axis. For a vertical wall on a milled block, the wall must fit between two imaginary planes set at 90° to the datum; for a hole, the axis must remain within a cylindrical zone perpendicular to the datum. This is why machine tram, rigid fixturing, tool deflection control, and probing are critical. A part can be dimensionally correct in X/Y/Z yet fail perpendicularity if the face is tipped or the bore leans, affecting how parts seat, bearings load, and assemblies stack without forcing. Common real-world examples include keeping cabinet sides square for edgebanding, ensuring dowel holes align, and maintaining hinge cup bores perpendicular to the face.
- Machine or spindle tram error: An untrammed mill head or non-square gantry cuts faces that look flat but lean relative to the datum. The part may pass caliper checks yet fail CMM inspection or cause assembly issues.
- Fixture error or bad datum choice: Clamping on a warped, dirty, or poorly indicated setup face corrupts the datum itself. Even a perfect cutter path becomes meaningless, as all perpendicularity measurements are invalid.
- Drill or cutter deflection on deep features: Long drills, small end mills, or boring bars can walk under load, causing a hole axis or wall angle to drift off 90°. The error often appears after part release when the true axis is checked.
Does perpendicularity control angle directly?
No. Perpendicularity controls the feature’s allowable location inside an oriented tolerance zone; the tolerance value is expressed in distance units, not degrees.
Must perpendicularity always reference a datum?
Yes. Perpendicularity is a datum-based orientation control, meaning it always requires a specified datum to define the reference plane or axis.
What is the difference between flatness and perpendicularity?
Flatness controls a surface’s own form without referencing a datum, while perpendicularity controls that surface’s orientation relative to a datum.