1. What is Precision-to-Tolerance Ratio (PTR)?

Definition: PTR is a dimensionless ratio that compares the precision of a measurement system to the tolerance of the feature being measured.

Purpose: It helps quality engineers and metrology professionals determine if their measurement system is capable of adequately measuring a given tolerance.

2. How Does the PTR Calculator Work?

The calculator uses the formula:

Where:

• \( PTR \) — Precision-to-Tolerance Ratio (dimensionless)
• \( P \) — Precision of the measurement system (mm)
• \( T \) — Tolerance of the feature being measured (mm)

Explanation: The precision (often 6 times the standard deviation of repeated measurements) is divided by the total tolerance range.

3. Importance of PTR Calculation

Details: A lower PTR indicates better measurement capability. Industry standards often recommend PTR ≤ 0.1 (10%) for critical measurements.

4. Using the Calculator

Tips: Enter the measurement system precision and part tolerance in the same units (typically mm). Both values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's considered a good PTR value?
A: Generally, PTR ≤ 0.1 (10%) is excellent, 0.1-0.3 may be acceptable depending on application, and >0.3 indicates measurement system may be inadequate.

Q2: How is precision determined?
A: Precision is typically calculated as 6 times the standard deviation of repeated measurements of a reference standard.

Q3: Can PTR be greater than 1?
A: Yes, but this indicates the measurement system variation exceeds the tolerance, making it unsuitable for that measurement.

Q4: What's the difference between PTR and %GRR?
A: %GRR compares precision to total variation (not just tolerance), while PTR specifically evaluates measurement capability against requirements.

Q5: How does PTR relate to Cpk?
A: Both assess capability, but Cpk evaluates process capability while PTR evaluates measurement system capability relative to tolerance.