Perpendicularity vs. Angularity: How to Read Datum References and Inspect with a CMM

On a precision part drawing, a nominal angle such as 90 or 30 degrees does not fully control the orientation of an actual surface or feature axis. An angular size tolerance limits an angle value, while a geometrical orientation tolerance constrains the actual feature within a tolerance zone established from a datum reference frame. Perpendicularity and angularity are two common controls used for this purpose. Correct interpretation supports functional design, stable machining setups, and repeatable inspection. A technically valid CMM report still may not represent assembly function if the datum reference frame is established incorrectly or the measured feature is under-sampled.

• Reading a 0.02 mm geometrical tolerance as plus or minus 0.02 degree.
• Applying angularity without a fully defined basic angle.
• Selecting a small or unstable datum that does not represent assembly function.
• Machining the datum and controlled feature in separate setups without a reliable transfer strategy.
• Distorting a thin component through excessive clamping.
• Using too few points or measuring only one bore section.
• Comparing two reports built from different alignment definitions.

An effective RFQ should include a legible 2D drawing with GD&T, a 3D model, material, quantity, heat-treatment or surface-finish requirements, the governing drawing standard, and the required inspection documentation. For critical characteristics, specify whether a CMM report is required, the sampling quantity, inspection frequency, and preferred report format. VietTech provides CNC milling, CNC turning, precision grinding, wire EDM, and suitable dimensional inspection, including CMM inspection where required by the feature. The final process and inspection plan is reviewed against part size, workholding, probe accessibility, tolerance, and documentation requirements.