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.
Various gages and measuring instruments are used for measurement which are
- Gage (Gauge) Blocks – Carl Johansson developed steel blocks to establish a measurement standard to duplicate national standards and that could be used in any shop and had accuracy within a few millionths of an inch. Gage blocks are made from high carbon or chromium alloyed steel, tungsten carbide, chromium carbide or fused quartz. They are used to set a length dimension for a transfer measurement, and for calibration of a number of other tools.
- Calipers – Calipers are used to measure length. The length can be an inside dimension, outside dimension, height, or depth. Calipers are of four as spring calipers, dial calipers, vernier calipers and digital calipers.
- The Vernier Scale – Vernier scales are used on a variety of measuring instruments such as height gages, depth gages, inside or outside vernier calipers and gear tooth verniers.
- Optical Comparators – Comparators use a beam of light directed upon the part to be inspected, and the resulting shadow is magnified and projected upon a viewing screen. The image can then be measured by comparing it with a master chart or outline on the viewing screen or measurements taken. To pass inspection, the shadow outline of the object must fall within predetermined tolerance limits.
- Micrometers – Micrometers, or “mics,” are may be purchased with frame sizes from 0.5 inches to 48 inches. A 2″ micrometer readings are from 1-2″. Most “mics” have an accuracy of 0.001″ and using a vernier scale, an accuracy of 0.0001″ can be obtained. Supermicrometers when used in temperature and humidity controlled rooms, are able to make linear measurements to millionths of an inch.
- Dial Indicators – They are mechanical instruments for measuring distance variations. Most dial indicators amplify a contact point reading by use of an internal gear train mechanism.
- Surface Plates – They are a reference plane for dimensional measurements. They are customarily used with a toolmaker’s flat, angles, parallels, V blocks and cylindrical gage block stacks.
- Ring Gages – They are used to check external cylindrical dimensions and are often in “go, no-go” sets. A thread ring gage is used to check male threads.
- Plug Gages – They are generally “go, no-go” gages, and are used to check internal dimensions. The thread plug gage is designed exactly as the plug gage but instead of a smooth cylinder at each end, the ends are threaded.
- Pneumatic Gages – Pneumatic amplification gages types include one actuated by varying air pressure and the other by varying air velocity at constant pressure. Measurements can be read to millionths of an inch.
- Non-Destructive Testing (NDT) and Non-Destructive Evaluation (NDE) – NDT and NDE techniques evaluate material properties without impairing the future usefulness of the items being tested. The advantages of NDT techniques include the use of automation, 100% product testing and the guarantee of internal soundness. Some NDT results are open to interpretation and demand considerable skill on the part of the examiner.
- Visual Inspection – Visual examination of product color, texture, and appearance gives valuable information. The human eye is frequently aided by magnifying lenses or other instrumentation. This technique is sometimes called scanning inspection.
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.
Repeatability serves as the foundation that must be present in order to achieve reproducibility. Reproducibility must be present before achieving accuracy. Precision requires that the same measurement results are achieved for the condition of interest with the selected measurement method.
A measurement method must first be repeatable. A user of the method must be able to repeat the same results given multiple opportunities with the same conditions. The method must then be reproducible. Several different users must be able to use it and achieve the same measurement results. Finally, the measurement method must be accurate. The results the method produces must hold up to an external standard or a true value given the condition of interest.