CPT Measurement & Calibration 3 — Questions and Answers
Question 1: A Gage R&R study is conducted and the %GRR result is 18%. According to AIAG guidelines, this result is:
- Unacceptable and the gage must be replaced immediately
- Marginal and may be acceptable depending on application (Correct answer)
- Acceptable with no action required
- Perfect with zero measurement variation
Correct answer: Marginal and may be acceptable depending on application
AIAG MSA guidelines classify 10–30% GRR as marginal; acceptability depends on the importance of the application, cost of the gage, and cost of repairs.
Question 2: What is the primary purpose of a calibration interval?
- To schedule preventive maintenance on the instrument
- To determine how frequently an instrument must be recalibrated to maintain confidence in its accuracy (Correct answer)
- To set the time between operator shift changes
- To document the production run schedule
Correct answer: To determine how frequently an instrument must be recalibrated to maintain confidence in its accuracy
Calibration intervals define the maximum time between calibrations to ensure the instrument remains accurate and fit for its intended use.
Question 3: A height gage is used on a granite surface plate. Why is a granite surface plate preferred over a steel surface plate for precision work?
- Granite is less expensive than steel
- Granite has a lower coefficient of thermal expansion and does not rust (Correct answer)
- Granite is heavier and more stable than steel
- Granite is easier to machine to flatness
Correct answer: Granite has a lower coefficient of thermal expansion and does not rust
Granite is preferred because it is dimensionally stable, has a very low thermal expansion coefficient, and does not rust or burr like metal plates.
Question 4: During a measurement process, a technician always gets a different reading each time they measure the same part with the same instrument. This variability is most likely caused by:
- Instrument bias
- Poor repeatability (Correct answer)
- Incorrect calibration due date
- A systematic offset in the standard
Correct answer: Poor repeatability
Repeatability is the variation in measurements obtained by one operator using the same instrument on the same part under the same conditions; high variation indicates poor repeatability.
Question 5: A vernier caliper has a main scale reading of 1.200 inches and the vernier scale indicates 0.013 inches. What is the correct reading?
- 1.187 inches
- 1.200 inches
- 1.213 inches (Correct answer)
- 1.013 inches
Correct answer: 1.213 inches
The vernier scale reading is added to the main scale reading: 1.200 + 0.013 = 1.213 inches.
Question 6: Which type of measurement error cannot be corrected by calibration because it occurs randomly during use?
- Bias error
- Zero error
- Random error (Correct answer)
- Linearity error
Correct answer: Random error
Random errors have no predictable pattern and can only be minimized by taking multiple readings and averaging; calibration corrects systematic errors, not random ones.
Question 7: An out-of-calibration instrument is discovered after it has been used to inspect parts for the past month. The FIRST action a technician should take is:
- Immediately recalibrate the instrument and continue production
- Notify the quality department and quarantine or re-inspect affected parts (Correct answer)
- Document the finding and wait for the next scheduled audit
- Replace the instrument and discard all previously inspected parts
Correct answer: Notify the quality department and quarantine or re-inspect affected parts
When an out-of-calibration condition is discovered, affected product must be quarantined and evaluated for disposition; the quality department must be notified per standard nonconformance procedures.
CPT Measurement & Calibration 3
A Gage R&R study is conducted and the %GRR result is 18%.
According to AIAG guidelines, this result is: