Understanding True Position Calculations
True position is a Geometric Dimensioning and Tolerancing (GD&T) specification used to ensure machined features, like holes or bosses, are accurately positioned relative to a datum. This calculator checks if the measured deviations in X and Y directions fall within the specified tolerance zone.
The actual position deviation is calculated as 2 * sqrt(X_dev^2 + Y_dev^2). If this value is less than or equal to the total tolerance (specified tolerance + bonus tolerance), the feature passes inspection. Bonus tolerance may increase with departure from Maximum Material Condition (MMC).
True Position Tolerance Comparison
| X Deviation (in) | Y Deviation (in) | Tolerance (in) | Deviation (in) | Status | Typical Use |
|---|---|---|---|---|---|
| 0.005 | 0.005 | 0.014 | 0.0141 | Pass | Small precision hole |
| 0.010 | 0.010 | 0.014 | 0.0283 | Fail | Medium bolt hole |
| 0.015 | 0.015 | 0.030 | 0.0424 | Fail | Large mounting boss |
How to Use the True Position Calculator
- Measure the X deviation of the feature from the nominal position (in inches or millimeters).
- Measure the Y deviation of the feature from the nominal position (in inches or millimeters).
- Enter the specified true position tolerance from the engineering drawing (in inches or millimeters).
- Input any bonus tolerance (e.g., from MMC departure, typically 0 if not applicable).
- Select the unit (inches or millimeters).
- Click "Calculate True Position" to see the actual deviation and pass/fail status.
True Position Deviation Chart
Actual position deviation (in) for different X/Y deviations (0.014 in tolerance, no bonus).
Frequently Asked Questions
What is true position in GD&T?
True position defines the allowable deviation of a feature’s center from its theoretically exact position, typically specified as a cylindrical tolerance zone.
What is bonus tolerance?
Bonus tolerance is an additional tolerance allowed as a feature departs from its Maximum Material Condition (MMC), often increasing the tolerance zone.