DefinitionTrue position is the theoretically exact location of a feature, such as a hole or pin, defined by basic dimensions from a datum reference frame. A position tolerance then sets a zone, usually a cylinder of stated diameter, within which the feature's actual axis or center plane must fall.
Explained
What Does True Position Mean in GD&T?
Strictly, "true position" is the ideal location and "position tolerance" is the allowance around it, but on the shop floor both the callout and the measured result are usually called true position.
The location comes from basic dimensions: boxed values on the drawing that carry no ± tolerance of their own and are treated as theoretically exact. All the permitted variation lives in the feature control frame instead. A Ø symbol in front of the tolerance value makes the zone cylindrical; without it, the zone defaults to a total width between two parallel planes.
Position controls the location and orientation of a feature's center — its axis, center point or center plane. It does not control the feature's size or form, which are set by the size dimension.
How Is True Position Calculated?
After a CMM measures the actual center of a feature, it reports how far that center sits from the basic location in X (Δx) and Y (Δy). Because the tolerance zone is a diameter, the radial offset is doubled:
Position = 2 × √(Δx² + Δy²)
A hole 0.03 mm (0.0012 in) off in X and 0.04 mm (0.0016 in) off in Y is 0.05 mm from true position, so it measures Ø0.10 mm (Ø0.004 in). It passes a Ø0.10 callout, but only just.
What Does MMC Mean for a Position Tolerance?
Maximum material condition (MMC) is the size at which a feature holds the most material: the smallest allowed hole or the largest allowed pin. Adding the Ⓜ modifier after the position tolerance means the stated tolerance applies at MMC, and any departure from MMC adds bonus tolerance equal to that departure.
| Modifier | When the stated tolerance applies | Bonus tolerance | Where it fits |
|---|---|---|---|
| None (RFS, regardless of feature size) | At every produced size | None | Location matters at any size, such as bearing or balance-critical features |
| Ⓜ MMC | Smallest hole / largest pin | Grows as the hole gets larger or the pin smaller | Clearance assemblies; can be checked with a fixed functional gauge |
| Ⓛ LMC (least material condition) | Largest hole / smallest pin | Grows as the hole gets smaller or the pin larger | Protecting the material left around a feature |
Example: a hole toleranced Ø10.00–10.10 mm with position Ø0.10 Ⓜ is allowed Ø0.10 of position error at 10.00 mm, and Ø0.16 at 10.06 mm.
How to Specify and Check True Position
- Locate every feature of size with position from datums, and make its locating dimensions basic. Do not stack ± coordinate tolerances on the same feature.
- Use Ⓜ where the function is a clearance fit. It passes more good parts and lets a fixed gauge replace a CMM routine.
- Leave the modifier off where location must hold at every size.
- Name the datum features the part is actually located by in assembly, so the inspection setup matches the function.
Machined parts at NICE Rapid have critical dimensions verified by CMM or manual gauging, and CMM reports are available on request for CNC machining orders.
Key takeaways
- True position is the theoretically exact location of a feature defined by basic dimensions from a datum reference frame.
- Position tolerance defines a cylindrical or parallel-plane zone around true position that the feature's axis, center point or center plane must occupy.
- Position controls location and orientation of a feature's center, but not its size or form.
- True position is calculated as twice the radial offset from the basic location: Position = 2 × √(Δx² + Δy²).
- At MMC, the stated position tolerance applies when the feature has the most material, and bonus tolerance grows as the feature departs from MMC.
- Use MMC for clearance fits and fixed functional gauges; leave the modifier off when location must be maintained regardless of feature size.
Example
Related terms
References
- Dimensioning & Tolerancing — University of Portland ME481/482 course notes
- US Patent 7,194,387 B1, Detailed Description — USPTO
- GD&T Symbol: Position (ASME Y14.5-2018 §10.2) — Tec-Ease
- GD&T Tip: Feature Control Frame — Tec-Ease
- GD&T Tip: Bonus Tolerance — Tec-Ease
- GD&T Tip: Coaxiality (Part 1) — Tec-Ease
- KSC-GP-435 Vol. I Rev. H Chg. 1, Engineering Drawing Practices — NASA Kennedy Space Center