Let us discuss the Measurement System Analysis.
Measurement
Attribute Screens – Attribute screens use two categories for determining data outcomes, acceptable or not acceptable, go or no go, pass or fail.
Gauge Blocks – Gauge blocks are used in manufacturing to set a length dimension for transfer or for tool calibration.
Measuring Tools
Various measurement tools are –
Calipers – They measure distance, depth, height, or length from either an inside or outside perspective. Most
calipers capture physical measurements which are transferred to a scale to determine the data. Calipers are of types like
- Spring calipers – The two sides are connected by a spring to measure difficult to reach areas. It’s accurate to a tenth of an inch and uses steel ruler to transfer measurement.
- Vernier calipers – It uses a vernier scale and are accurate to one thousandth of an inch.
- Digital calipers – It uses an electronic readout and are accurate to five thousandths of an inch.
Optical Comparators – It compares a part to a form that represents the desired dimensions by projecting a beam of light for a shadow of the object that is magnified by a lens for tolerance levels.
Micrometers – It is also called as “mics”, are handheld measuring devices with a C frame with the measurement occurring between a fixed anvil and a movable spindle.
Measurement System
Measurement Systems Analysis (MSA) refers to the analysis of precision and accuracy of measurement methods.
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.
Gauge R and R Studies – Assuming that a gauge is determined to be accurate (that is, the measurements generated by the gauge are the same as those of a recognized standard), the measurements produced must be repeatable and reproducible.
Range Method – The range method is a simple way to quantify the combined repeatability and reproducibility of a measurement system.
Average and Range Method – The average and range method computes the total measurement system variability, and allows the total measurement system variability to be separated into repeatability, reproducibility, and part variation.
Analysis of Variance Method – ANOVA is the most accurate method for quantifying repeatability and reproducibility and allows the variability of the interaction between the appraisers and the parts to be determined.
Measurement Correlation
It means the correlation or comparison of the measurement values from one measurement system with the corresponding values reported by one or more different measurement systems.
Bias
It is often due to human error. Whether intentional or not, bias can cause inaccurate or misleading results.
Linearity
It is the variation between a known standard throughout the operating range of the gauge.
Percent Agreement
Percent agreement between the measurement system and either reference values or the true value of a variable being measured
Precision-Tolerance Ratio
Precision/Tolerance (P/T) is the ratio between the estimated measurement error (precision) and the tolerance of the characteristic being measured
Metrology
It is the science of measurement. The word metrology derives from two Greek words: matron (meaning measure) and logos (meaning logic).
Measurement Error
Measurement error is the degree to which the measuring instrument differs from a true value.
Calibration
Calibration is the comparison of a measurement standard or instrument of known accuracy with another standard .
Calibration Schedule – Measuring equipment should be calibrated before initial use and periodically re-calibrated as often as necessary to maintain prescribed accuracies.
Calibration Standards – In the SI system, most of the fundamental units are defined in terms of natural phenomena that are unchangeable.
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