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

High-Performance Liquid Chromatography (HPLC) is an analytical technique used to separate, identify, and quantify components in a mixture. It operates on the principle of partitioning between a mobile phase (liquid solvent) and a stationary phase (solid adsorbent) packed inside a column. The sample is injected into the mobile phase, which is pumped through the column at high pressure, enabling rapid and efficient separation.

Different compounds interact differently with the stationary phase based on polarity, size, and chemical affinity, causing them to travel at different speeds. As a result, each component elutes at a specific retention time. A detector, commonly UV-visible, records the separated compounds as peaks in a chromatogram, where peak area corresponds to concentration. The most widely used mode is reverse-phase HPLC, where the stationary phase is non-polar and the mobile phase is polar. HPLC is valued for its high resolution, sensitivity, and accuracy in analyzing complex mixtures.

🔹 Validation vs Calibration

1. Validation

* Definition: Confirms that the equipment or process works as intended.
* Objective: To check whether measurements are accurate and reliable for the application.
* Focus: Whole system (machine + method + environment + operator).
* Why it matters:
Ensures the results you get are valid and fit for purpose.

👉 Example (your field):
Validating a CMM program means verifying that the inspection method gives correct results for a part, considering fixturing, probing strategy, etc.

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2. Calibration

* Definition: Comparing a device’s measurement with a known standard and adjusting it if needed.
* Objective: To correct measurement errors or drift.
* Focus: Instrument accuracy (sensor/gauge itself).
* Why it matters:
Ensures the instrument reads the true value consistently.

👉 Example:
Calibrating a micrometer using gauge blocks to remove zero error or drift.

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🔍 Key Difference (Simple Way)

* Calibration = “Is the instrument reading correctly?”
* Validation = “Is the overall process giving correct results?”

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🧠 Practical Understanding (Important for you)

* You can have a calibrated instrument but still wrong results if method/setup is poor → ❌ Not validated
* You must calibrate first, then validate the process → ✅ Best practice

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)

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📐 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.

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