Table of Contents
What Is True Position?
True position is a Geometric Dimensioning and Tolerancing (GD&T) concept defined by ASME Y14.5 that specifies the ideal exact location of a feature of size (typically a hole or pin) on a part. It replaces the outdated coordinate tolerancing method and provides a more accurate and functional way to specify location tolerances.
True position tolerance is expressed as a diametral zone (a circular or cylindrical tolerance zone) within which the axis of the feature must fall. This diametral zone is centered on the nominal (ideal) location defined by basic dimensions from specified datums. The advantage of diametral true position over coordinate tolerancing is that it provides 57% more usable tolerance area (a circle inscribed in a square vs. the square itself).
In manufacturing and quality inspection, true position is measured using Coordinate Measuring Machines (CMMs), optical comparators, or functional gauges. The actual deviation from the nominal position is calculated from the X and Y coordinate differences and converted to a diametral value for comparison against the tolerance specified on the engineering drawing.
True Position Formula
The factor of 2 converts the radial deviation to a diametral value, matching how true position tolerance is specified on drawings. The feature passes inspection if the calculated true position is less than or equal to the allowed tolerance.
MMC and Bonus Tolerance
When a true position tolerance is specified at Maximum Material Condition (MMC), a bonus tolerance is available. The bonus equals the difference between the actual feature size and its MMC limit. For a hole, MMC is the smallest allowable diameter; as the hole is made larger, more positional tolerance becomes available.
| Condition | Symbol | Bonus Tolerance | Application |
|---|---|---|---|
| RFS (Regardless of Feature Size) | None | 0 (no bonus) | Critical fits, precision assemblies |
| MMC (Maximum Material Condition) | Ⓜ | Actual Size - MMC | Clearance holes, pin fits |
| LMC (Least Material Condition) | Ⓛ | LMC - Actual Size | Minimum wall thickness |
Worked Examples
Example 1: Simple True Position
A hole is nominally located at X=2.000, Y=1.500. The measured location is X=2.003, Y=1.497. The true position tolerance is 0.010 dia at RFS.
dX = 2.003 - 2.000 = 0.003. dY = 1.497 - 1.500 = -0.003. True Position = 2 x sqrt(0.003^2 + 0.003^2) = 2 x 0.00424 = 0.00849. Since 0.00849 is less than 0.010, the feature PASSES.
Example 2: With MMC Bonus
Same hole, but tolerance is 0.010 dia at MMC. Hole MMC = 0.500, actual hole size = 0.504. Bonus = 0.504 - 0.500 = 0.004. Allowed tolerance = 0.010 + 0.004 = 0.014. The feature still passes (0.00849 < 0.014) with even more margin.
Frequently Asked Questions
Why is true position multiplied by 2?
The sqrt(dX^2 + dY^2) calculation gives the radial distance from the nominal location. Since true position tolerance zones are specified as diameters (not radii), you multiply by 2 to convert. This matches the callout format on engineering drawings where tolerance is given as a diametral zone.
What is the difference between true position and concentricity?
True position locates a feature relative to datums using basic dimensions. Concentricity controls the median points of a feature of size relative to a datum axis. Concentricity is much more expensive to inspect (requires multiple cross-sections) and is rarely used in modern practice. ASME Y14.5-2018 actually removed concentricity as a standalone control.
Can true position be applied to non-circular features?
Yes, but the tolerance zone shape changes. For round features, the zone is a cylinder. For non-circular features like slots, the tolerance is typically specified with two parallel planes (using coordinate-style position callouts). The ASME standard provides rules for each scenario.