Week 2 - Eng metrology and instrumentation Flashcards
Measurement Standard
- Inch, foot; based on human body
- 4000 B.C. Egypt; King’s Elbow=0.4633 m, 1.5 ft, 2 handspans, 6 hand-widths, 24 finger-thickness
- AD 1101 King Henry I ->yard (0.9144 m) from his nose to the tip of his thumb
- 1872, Meter (in Greek, metron to measure)- 1/10 of a millionth of the distance between the North Pole and the equator
- Platinum (90%)-iridium (10%) X-shaped bar kept in controlled condition in Paris->39.37 in
- In 1960, 1,650,763.73 wave length in vacuum of the orange light given off by electrically excited krypton 86.
MEASUREMENT AND INSPECTION
- Metrology
- Inspection Principles
- Conventional Measuring Instruments and Gages
- Measurement of Surfaces
- Advanced Measurement and Inspection Techniques
Measurement
Procedure (unknown quantity is compared to a known standard, using an accepted and consistent system of units
- May involve a simple linear rule to scale the length of a part
- Or it may require a sophisticated measurement of force vs deflection during a tension test
- Provides a numerical value of the quantity of interest, within certain limits of accuracy and precision
Metrology
-science of measurement 7 Quantities 1. Length(m) 2. Mass (kg) 3.Time(s) 4.Electric current(A) 5. Temperature(K) 6.Light intensity (candela) 7. Matter (mole)
Metrology (derived from basic quantities)
- Area
- Volume
- Velocity and acceleration
- Force
- Electric voltage
- Heat energy
Manufacturing metrology
Measures:
- Length and width
- Depth
- Diameter
- Straightness, flatness, and roundness, etc.
- Surface roughness
Selection of Measuring Instruments
- Accuracy
- Precision
- Magnification (amplification)
- Sensitivity (Resolution);
- Rules of 10
- Stability (drift);
- Standard measuring -temperature 20C
Accuracy
closeness of agreement between an observed value and a standard
Precision
closeness of agreement between randomly selected individual measurements
Sensitivity (Resolution)
the smallest difference in dimensions that the instrument can detect or distinguish
Rules of 10 (gage maker’s rule)
At least 10 times more accurate than the tolerance
Stability (drift)
capability to maintain calibrated status
Repeatability (equipment variation)
Variation in multiple measurements by an individual using the same instrument.
Reproducibility (operator variation)
Variation in the same measuring instrument used by different individuals
Calibration
Comparing a measurement device or system to one having a known relationship to national standards
Repeatability and Reproducibility Studies
Quantify and evaluate the capability of a measurement system
- Select m operators and n parts
- Calibrate the measuring instrument
- Randomly measure each part by each operator for r trials
- Compute key statistics to quantify repeatability and reproducibility
Accuracy
- Degree to which a measured value agrees with the true value of the quantity of interest
- A measurement procedure is accurate when it is absent of systematic errors
- Systematic errors: positive or negative deviations from true value that are consistent from one measurement to the next
Precision
- Degree of repeatability in the measurement process
- Good precision means that random errors in the measurement procedure are minimized
Accuracy vs precision
slide 15 pic
Dominant systems of units
- U.S. customary system (U.S.C.S.)
2. SI (for Systeme Internationale d’Unites) - the “metric system”
Product quality for manufacturing importance
- For interchangeable manufacture
- Standardization and mass production
- Product components must fit/assemble properly and be replaceable
- Prevention of defect
To verify quality-requires inspection
High product quality:
Reliable performance
Cost is high
Low quality product
Less reliable/fails after short time
Low cost
Difficult to use
Inspections
Inspection involves the use of measurement and gaging techniques to determine whether a product, its components, subassemblies, or materials conform to design specifications
Types of Inspections
- By variables: product or part dimensions of interest are measured by the appropriate measuring instruments
- By attributes:product or part dimensions are gaged to determine whether or not they are within tolerance limits