CMS CMS Metrology & Measurement Systems 2 — Questions and Answers
Question 1: What is the primary advantage of a Coordinate Measuring Machine (CMM) over manual gauging?
- Lower initial purchase cost
- Ability to measure complex 3D geometries with high accuracy and generate detailed reports (Correct answer)
- Faster measurement speed for simple go/no-go checks
- No requirement for operator training
Correct answer: Ability to measure complex 3D geometries with high accuracy and generate detailed reports
CMMs use a precision probe to capture 3D point coordinates, enabling measurement of complex geometries, GD&T features, and automated reporting not possible with manual gauges.
Question 2: Which GD&T (Geometric Dimensioning and Tolerancing) symbol controls the flatness of a surface?
- ⊙ (Position)
- ⏥ (Flatness) (Correct answer)
- ○ (Circularity)
- ⌭ (Cylindricity)
Correct answer: ⏥ (Flatness)
The flatness symbol (two parallel lines) specifies that all points on a surface must lie within a tolerance zone defined by two parallel planes.
Question 3: What is gauge R&R expressed as a percentage of tolerance considered 'acceptable' per AIAG MSA guidelines?
- Less than 10% (Correct answer)
- Between 10% and 30%
- Greater than 30%
- Between 30% and 50%
Correct answer: Less than 10%
Per AIAG MSA guidelines, a Gauge R&R less than 10% of the tolerance (or study variation) is acceptable; 10–30% may be acceptable; above 30% requires corrective action.
Question 4: A surface finish measurement is reported as Ra = 32 µin. What does Ra represent?
- Maximum peak-to-valley height
- Arithmetic mean roughness average of the surface profile (Correct answer)
- Root mean square roughness
- Waviness of the surface
Correct answer: Arithmetic mean roughness average of the surface profile
Ra is the arithmetic mean of the absolute deviations of the surface profile from the mean line, and is the most widely used surface roughness parameter in US manufacturing.
Question 5: What is the purpose of a 'gauge block' (slip gauge) in metrology?
- To prevent gauges from falling off workbenches
- To provide highly accurate reference length standards for calibration and setup (Correct answer)
- To measure surface roughness
- To check thread pitch diameter
Correct answer: To provide highly accurate reference length standards for calibration and setup
Gauge blocks are precision-ground steel or ceramic blocks with certified lengths used as reference standards to calibrate instruments and set comparators.
Question 6: Which statistical tool is used to determine whether a measurement process is capable of distinguishing between parts within the process tolerance?
- Control chart
- Gauge R&R study with number of distinct categories (ndc) (Correct answer)
- Histogram of part dimensions
- Failure mode and effects analysis (FMEA)
Correct answer: Gauge R&R study with number of distinct categories (ndc)
The number of distinct categories (ndc) from a Gauge R&R study indicates how many groups within the process variation the measurement system can reliably distinguish; ndc ≥ 5 is required.
CMS CMS Metrology & Measurement Systems 2
What is the primary advantage of a Coordinate Measuring Machine (CMM) over manual gauging?