Metrology & Calibration Knowledge Center

1. What is True Position?

True Position tells you how far a feature (like a hole) is from its ideal (theoretical) location.

* It controls location in both X and Y directions
* The tolerance zone is circular (not square)

👉 Think: “How far did my hole shift from where it should be?”

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🔹 2. Basic Idea (Visualization)

* The exact location = center of the circle
* The actual hole location = somewhere inside or outside
* Allowed area = circular tolerance zone

If the actual point lies inside the circle → OK (PASS)
Outside → NOT OK (FAIL)

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🔹 3. Formula Used

TP = \sqrt{(\Delta X)^2 + (\Delta Y)^2}

Where:

* ΔX = difference in X direction
* ΔY = difference in Y direction

👉 This is basically Pythagoras theorem (distance calculation)

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🔹 4. Example from Image

Nominal (Target):

* X = 10.00 mm
* Y = 25.00 mm

Measured (Actual):

* X = 10.08 mm
* Y = 25.13 mm

Deviation:

* ΔX = 0.08 mm
* ΔY = 0.13 mm

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🔹 5. Calculation

TP = \sqrt{(0.08)^2 + (0.13)^2}
= \sqrt{0.0064 + 0.0169}
= \sqrt{0.0233}
= 0.15 \text{ mm}

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🔹 6. Tolerance Check

* Given tolerance = Ø 0.5 mm
* Radius = 0.25 mm

👉 Condition:

* If TP ≤ 0.25 → PASS
* If TP > 0.25 → FAIL

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🔹 7. Final Result

* Calculated TP = 0.15 mm
* Allowed = 0.25 mm

✅ PASS (within tolerance zone)

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🔹 Key Takeaway (Very Important)

True Position is:

The straight-line (radial) distance between actual and theoretical position

This image explains the difference between accuracy and precision using a target (bullseye) example:

1. Accurate & Precise (Top Left)

* All shots are very close to the center.
* They are also close to each other.
* ✅ Best case: measurements are both correct and consistent.

2. Accurate but Not Precise (Top Right)

* Shots are around the center, but spread out.
* Average is correct, but individual readings vary.
* ✅ Correct on average, ❌ not consistent.

3. Precise but Not Accurate (Bottom Left)

* Shots are very close to each other, but far from the center.
* Consistent results, but all are wrong.
* ❌ Systematic error (like calibration issue).

4. Not Accurate & Not Precise (Bottom Right)

* Shots are spread out and far from the center.
* ❌ Neither correct nor consistent.

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Simple Definitions:

* Accuracy = How close measurement is to the true value
* Precision = How close measurements are to each other (repeatability)

📐 GD&T (Geometric Dimensioning & Tolerancing) – The Language of Precision!

If you work in design, manufacturing, or quality, understanding GD&T is a must. It helps communicate exact requirements for parts beyond just dimensions.

✔ Controls form, orientation, location & runout
✔ Ensures proper fit, function & performance
✔ Reduces ambiguity in engineering drawings

From Flatness to Position, Parallelism to Runout — each symbol has a purpose that directly impacts product quality.

👉 Remember: Without GD&T, dimensions are just numbers.