What Is a True Position?

True 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.

In short: true position tells the machinist where a feature belongs and how far it may wander, in one callout.

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.

Frequently Asked Questions

What is true position in GD&T?

It is the exact location of a feature, defined by basic dimensions from the datums. The position tolerance in the feature control frame sets the zone, usually a cylinder, that the feature's actual axis or center plane must stay inside.

How do you calculate true position?

Measure the actual center of the feature, find its offsets from the basic location in X and Y, and apply Position = 2 × √(Δx² + Δy²). Doubling the radial offset converts it to a diameter so it can be compared directly with the Ø tolerance on the drawing.

What does MMC mean in GD&T?

MMC means maximum material condition: the smallest hole or largest pin the size tolerance allows. With the Ⓜ modifier, the position tolerance applies at that size, and as the feature departs from MMC, the difference is added as bonus position tolerance.

Related Services at NICE Rapid

• CNC Machining — 3-, 4- and 5-axis milling and turning to ±0.01 mm, with critical dimensions verified by CMM or manual gauging.
• Rapid Prototyping — an engineering review with every quote, and inspection records on request for every order.
• Request a Quote — send a STEP file and a drawing with its GD&T callouts, and note which features need a CMM report.

Sources

1. Deborah Munro, Ph.D., "Dimensioning & Tolerancing," University of Portland ME481/482 course notes — positional tolerance defines a zone within which the axis, median plane or surface may vary from true position; true position is set by basic, theoretically exact dimensions from a datum reference frame; generally applied at MMC — faculty.up.edu
2. US Patent 7,194,387 B1, Detailed Description — "true position" is the theoretically exact location established by basic dimensions; position tolerance types RFS, MMC and LMC and the "bonus tolerance" from those modifiers — image-ppubs.uspto.gov
3. Tec-Ease, GD&T Symbol: Position (ASME Y14.5-2018 §10.2) — position controls the location and orientation of the feature's center, not its size or form — tec-ease.com
4. Tec-Ease, GD&T Tip: Feature Control Frame — the default zone is total width; the Ø symbol makes it cylindrical — tec-ease.com
5. Tec-Ease, GD&T Tip: Bonus Tolerance — at MMC, bonus tolerance is the difference between the MMC size and the actual mating envelope size — tec-ease.com
6. Tec-Ease, GD&T Tip: Coaxiality (Part 1) — with MMC a fixed gauge can check position and suits assembly; with no modifier, regardless of feature size applies, as for bearing and balance uses — tec-ease.com
7. NASA Kennedy Space Center, KSC-GP-435 Vol. I Rev. H Chg. 1, Engineering Drawing Practices — machined parts toleranced per ASME Y14.5 "using positional tolerances for all features of size"; centerline datums typically at MMC — standards.nasa.gov