True Position

True Position in CNC/millwork GD&T is the allowed variation of a feature’s location from its theoretically exact, basic-dimensioned location, usually measured relative to specified datums. ASME Y14.5 names the control Position and uses the crosshair symbol ⌖. In practice, it controls where holes, slots, pins, bores, and similar features must land so mating parts assemble correctly, replacing loose ± location tolerances with a defined geometric tolerance zone.

On the drawing, the engineer provides basic dimensions to the ideal location and attaches a position feature control frame tied to datums. The CNC programmer then targets those nominal coordinates, while the inspector evaluates the actual feature center or axis against the tolerance zone. For a hole pattern, the zone is typically cylindrical, making true position critical for dowel holes, fastener holes, locating pins, and precision assemblies. In millwork and cabinetry, the same principle applies to hinge cups, connector bores, shelf-pin rows, and dowel locations. Consistent location across parts is more important than hitting a nominal edge distance. The common shop-floor calculation for 2D positional error is 2 × √(ΔX² + ΔY²), converting linear offsets into a diametric error compared to the callout. This approach ensures assembly fit when pattern location, not just feature size, determines functionality.

• ⚠️Linear tolerance trap: When a hole is checked only by X/Y limits or edge distances, it can pass each linear check yet still exceed the circular position zone and fail assembly.
• ⚠️Datum reference error: If the part is flipped, clamped differently, or probed from the wrong reference frame, the measured position shifts relative to the drawing’s datum structure, producing false pass/fail results.
• ⚠️Size–location confusion: A feature can be perfectly in-size yet still fail because true position controls where the axis is, not just the diameter; this causes dowel-insertion, hardware alignment, and jigging problems.