Why is True Position better than rectangular coordinates?

True Position tolerances define circular tolerance zones (diameter = 2 × radial tolerance), which is more efficient than rectangular zones. A hole can deviate equally in all directions (360°) while staying within spec. This saves material in manufacturing while maintaining functionality.

What's the difference between TP and radial deviation?

Radial deviation is the measured distance from the nominal position to the actual position. True Position tolerance is 2 × radial deviation, defining the full tolerance diameter. For example, if max radial deviation is 0.010 in, the TP tolerance is 0.020 in diameter.

How do I measure true position on a part?

Use a CMM (Coordinate Measuring Machine), surface plate with dial indicators, or optical comparator to measure actual X/Y coordinates of the feature (e.g., hole center). Compare to nominal values, calculate radial deviation, then multiply by 2 to get TP. This calculator automates the computation.

Can true position be used for non-circular features?

Position (GD&T) can apply to rectangular slots, tabs, etc., but the tolerance zones change shape. For holes and circular features, circular zones are standard. For slots and rectangles, square or rectangular zones may be used with different formulas.

What datum planes are required for true position?", a: "True Position is typically controlled relative to three mutually perpendicular datum planes (A, B, C). Datum A is usually the back face, B and C are edges or holes. The three datums establish a coordinate system on the part." }, { q: "How tight should true position tolerances be?", a: "Depends on functional requirements. Typical tolerances: assembly holes (loose fit): ±0.015-0.020 in, precision alignment: ±0.005-0.010 in, high-precision (optics, aerospace): ±0.0005-0.002 in. Tighter tolerances cost more in manufacturing." }, { q: "What does 'bonus tolerance' mean in GD&T?", a: "When a feature (like a hole) has size variation, you may gain bonus tolerance on True Position. If a hole is 0.500 +0.005/-0.000, and the hole is at minimum size (0.500), the full TP tolerance applies. At maximum size (0.505), you get additional tolerance." }, { q: "Why multiply radial deviation by 2 for True Position?", a: "The 2× factor comes from the definition of a circle. A tolerance zone with radius R has diameter 2R. If a feature can deviate 0.010 in in any direction from nominal, the tolerance diameter is 0.020 in, and we use 2 × radial deviation in the formula."