In order to ensure a measurement method is accurate and producing quality results, a method must be defined to test the measurement process as well as ensure that the process yields data that is statistically stable.
Measurement Methods
Various terms used in measurement systems are
- Measuring instruments – They are typically expensive and should be treated with care. Measuring tools must be calibrated on a scheduled basis as well as after any suspected damage.
- Reference/Measuring Surfaces – A reference surface is the surface of a measuring tool that is fixed. The measuring surface is movable.
- Transfer Tools – Transfer tools have no reading scale, an example, is spring calipers. The measurement is transferred to another measurement scale for direct reading.
Types of Gages used are
- Attribute Gages – Attribute gages are fixed gages which typically are used to make a go, no-go decision. Examples of attribute instruments are master gages, plug gages, contour gages, thread gages, limit length gages, assembly gages, etc. Attribute data indicates only whether a product is good or bad.
- Variable Gages – Variable measuring instruments provide a physical measured dimension. Examples of variable instruments are line rules, vernier calipers, micrometers, depth indicators, run out indicators, etc. Variable information provides a measure of the extent that a product is good or bad, relative to specifications. Variable data is often useful for process capability determination and may be monitored via control charts.
Attribute screens are screening tests performed on a sample with the results falling into one of two categories, such as acceptable or not acceptable. Because the screen tests are conducted on either the entire population of items or on a significantly large proportion of the population, the screen test must be of a nondestructive nature.
Measurement Systems Analysis
It refers to the analysis of precision and accuracy of measurement methods. It is an experimental and mathematical method of determining how much the variation within the measurement process contributes to overall process variability. Characteristics contribute to the effectiveness of a measurement method which is
- Accuracy – It is an unbiased true value which is normally reported and is the nearness of measured result and reference value. It has different components as
- Bias – It is the systematic difference between the average measured value and a reference value. The reference value is an agreed standard, such as a standard traceable to a national standards body. When applied to attribute inspection, bias refers to the ability of the attribute inspection system to produce agreement on inspection standards. Bias is controlled by calibration, which is the process of comparing measurements to standards.
- Linearity – It is the difference in bias through measurements. How does the size of the part affect the accuracy of the measurement method?
- Stability – It is the change of bias over time and usage. How accurately does the measurement method perform over time?
- Sensitivity – The gage should be sensitive enough to detect differences in measurement as slight as one-tenth of the total tolerance specification or process spread.
- Precision – It is the ability to repeat the same measurement by the same operator at or near the same time with nearness of measurement in any random measurement. Its components are
- Reproducibility – The reproducibility of a single gage is customarily checked by comparing the results of different operators taken at different times. It is the variation in the average of the measurements made by different appraisers using the same measuring instrument when measuring the identical characteristic on the same part.
- Repeatability – It is the variation in measurements obtained with one measurement instrument when used several times by one appraiser, while measuring the identical characteristic on the same part. Variation obtained when the measurement system is applied repeatedly under the same conditions is usually caused by conditions inherent in the measurement system.
