How to Use True Position Calculator
The True Position Calculator matches the typical CMM workflow.
- Choose 2D or 3D — Most position callouts are 2D (X, Y). Use 3D for spherical zones.
- Enter basic location — The nominal coordinates from the drawing.
- Enter measured location — The actual coordinates from your inspection report.
- Enter tolerance — The position tolerance from the feature control frame.
- (Optional) Enter MMC bonus — Add bonus if a material condition modifier applies.
- Read the result — True position, deviation, allowed tolerance and pass / fail.
Formula & Theory — True Position Calculator
The True Position Calculator implements the standard ASME Y14.5 formula:
ΔX = X_actual - X_basic
ΔY = Y_actual - Y_basic
ΔZ = Z_actual - Z_basic (3D only)
TP = 2 × √(ΔX² + ΔY² + ΔZ²)
Pass if TP ≤ Tolerance + Bonus| Symbol | Meaning |
|---|---|
| ΔX/Y/Z | Deviation from basic dimension |
| TP | True position value (zone diameter) |
| Bonus | MMC / LMC bonus tolerance |
Position vs concentricity
True position is the modern, default replacement for older callouts such as concentricity and symmetry because it works with virtual condition and allows bonus tolerance.
Use Cases for True Position Calculator
- CMM inspection — Translate raw deviations into the GD&T-compliant zone value.
- First article inspection — Generate quick pass / fail decisions on AS9102 or PPAP reports.
- Engineering review — Verify whether a borderline feature actually passes after MMC bonus.
- Quality training — Teach the difference between deviation and zone diameter.
- Reverse engineering — Quantify how far a “good” sample drifts from nominal.
- Documentation — Capture the calculation in inspection reports without manual arithmetic errors.
The True Position Calculator removes a frequent source of inspection mistakes and ensures consistent reporting across teams.