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5. Measurement System Analysis - Overview and Objectives

Measurement System Analysis section overview and objectives By the end of this session, you will learn what is meant by precision and accuracy, what is meant by bias, linearity, and stability, and what is meant by gauge, repeatability, and reproducibility. What exactly are variable and attribute? MSA measurement system Analysis Precision and accuracy There are two critical characteristics to examine in a gauge system.

Accuracy is an unbiased true value and is normally reported as the difference between the average of a number of measurements and the true value. Checking a micrometre with a gauge block is an example of an accuracy check. Precision Engaged Terminology: repeatability is often substituted for precision. Repeatability is the ability to repeat the same measurement by the same operator at or near the same time. As you can see in the diagram in the first example on the left, all the dots are precise but not accurate. In the diagram in the center, the dots are accurate, that is, within the circle but not precise. In the diagram on the extreme right, the dots are accurate and precise.

The calibration of measuring instruments is necessary to maintain accuracy but does not necessarily increase precision. In order to improve the accuracy and precision of a measurement process, it must have a defined test method and be statistically stable. Measurement Methods Bias Linearity and stability Bias is the difference between the output of the measurement system and the true value.

It is often referred to as "accuracy." Bias has a direct impact on the process output and can change the inferences of data analysis linearity. Measurement system linearity is found by obtaining reference part measurement values throughout the operating range of the instrument and plotting the bias against the reference values. Linearity will cause problems at the extremes of the operating range. Stability Variation in measurement occurs when the same person measures the same unit over an extended period of time using the same measuring gauge or tool; if the stability is not consistent, it may necessitate additional costs for regular maintenance of the tools or instruments.

Repeatability and reproducibility Assuming that gauge accuracy and sensitivity are adequate, it is often desirable to determine the variance components of a gauge measuring system. Repeatability, reproducibility, and process There are three widely used methods to quantify measurement error. the range method, the average in the range method, and the ANOVA method. A brief description of each follows. Range Method reproducibility is the variability introduced into the measurement system by the biassed differences of different operators.

The range method does not quantify repeatability and reproducibility separately. The range method is a simple way to quantify the combined repeatability and reproducibility of a measurement system. To separate repeatability and reproducibility, the average and range method or the analysis of variance method must be used. 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. When a person measures the same unit repeatedly with the same measure, repeatability variation in measurement can cause gauge, repeatability, and reproducibility measurement system errors.

When two or more people measure the same unit with the same measuring gauge or tool, engage or tool reproducibility variation occurs. There are certain thumb rules for acceptable levels of measurement system variation for continuous data. If the percentage tolerance is less than eight, the percentage contribution is less than 2%, and the number of distinct categories is greater than ten, then the measurement system is acceptable.

Likewise, we would evaluate the risks of the measurement system. If the percentage tolerance is between 8% and 30%, the percentage contribution is between 2% and 77%, and the number of distinct categories is between four and ten, the measurement system is unacceptable. If percentage tolerance is greater than 30%, percentage contribution is greater than 7.5%, and the number of distinct categories is less than four, For discrete data, the measurement system is acceptable.

If accuracy is greater than 90%, repeatability is greater than 90%, and reproducibility is also greater than 90%, the measurement system is unacceptable. If accuracy is less than 90%, repeatability is less than 90%, and reproducibility is also less than 90%, measurement system in the enterprise. Enterprise performance can be measured and presented by using automatic counters.

Computer-generated reports internal and external audits, supplier assessments, management reports internal and external a variety of feedback reports Suppliers' number of product deviations and percentage of on-time deliveries percentage of early deliveries, shipment costs per unit, shipping costs per time interval, percentage of compliance to specifications, and current unit cost compared to historical unit cost.

Dollars rejected versus dollars purchased supplier timelines, technical assistance, marketing and sales timelines, and sales growth per time period, market share in comparison to the competition dollar amount of sales per month dollar amount of an average transaction time spent by an average customer on website effectiveness of sales events sales dollars per marketing dollar external customer satisfaction awaited comparison with competitors' perceived value as measured by customer ranking of product or service satisfaction evaluation of technical competency percentage of retained customers internal customer satisfaction Employee satisfaction with the company; job satisfaction rating feedback on major policies and procedures; an indication of training, effectiveness, and evaluation of advancement fairness knowledge of company goals and progress to reach them research and development number of development projects in progress percentage of projects meeting budget number of projects behind schedule development expenses versus sales income reliability of design change requests manufacturing key machine and process capabilities Machine downtime percentages and average cycle time for key product lines measurement of how housekeeping control adequacy of operator training and engineering evaluation of product performance; number of corrective action requests; percentage of closed corrective action requests; and an assessment of measurement control and the availability of internal technical assistance.

6. Measurement System Analysis - Variable and Attribute

The second is defined as the duration of the radiation corresponding to the transition between the two hyperfine levels of the Ground State of 9,192,000,631770 periods. Tate of The Caesium -13mass m Mastandardtunit Unit of Mass The kilogram me is equal to the mass of the international prototype, which is a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures at Sev, near Paris, France. A duplicate in the custody of the National Institute of Standards and Technology serves as the standard for the United States.

The only base unit that is still defined by an artefact electric current, or ampere, is this one. The ampere is a constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross section and placed 1 meter apart in vacuum, would produce between the conductors a force equal to two times ten newton’s per meter of length.

Temperature. That is Kelvin. The kelvin unit of thermodynamic temperature is a fraction over 270–316 of the thermodynamic temperature of the triple point of water. According to this definition, the temperature of the triple point of water is 270.316°F. 1. At standard atmospheric pressure, the freezing point of water is approximately 0°F (1°C) below the triple point of water. light or candle.

The candy is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 times 10 and has a radiant intensity in that direction of 1683 of a Watt per stiradian. amount of substance or mole. The mole is the amount of substance in a system that contains as many elementary entities as there are atoms in it. 012 carbon. Twelve. The elementary entities must be specified. and maybe atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Calibration. Throughout history, man has devised standards to support those common measurement tools used in trade between various parties. This standardization allows the world to establish measurement systems for use by all industries.

The science of calibration is the maintenance of the accuracy of measurement standards as they deteriorate with use and time. Calibration is the comparison of a measurement standard or instrument of known accuracy with another measurement standard or instrument to detect, correlate, report, or eliminate by adjustment any variation in the accuracy of the item being compared. The elimination of measurement error is the primary goal of calibration systems. Total product variability: the total product variability in a product includes the variability of the measurement process. The total variance is equal to the process variance plus the measurement variance. Measurement error.

True variance plus error variance equals measurement variance. Measurement Error There are many reasons why a measuring instrument may yield an erroneous variation, including the following categories: operator Variation This error occurs when the operator of the measuring instrument obtains measurements using the same equipment on the same standards and a pattern of variation occurs. Operator-to-operator variation This error occurs when two operators of a measuring instrument obtain measurements using the same equipment on the same standards. And a pattern of variation occurs between the operators regarding their bias between them. Equipment variation.

This error occurs when sources of variation within equipment surface through measurement studies. The reasons for this variation are numerous. As an example, the equipment may experience an occurrence called drift. Drift is the slow change of a measurement characteristic over time. Material variation: this error occurs when the testing of a sample destroys or changes the sample, prohibiting retesting. This same scenario would also extend to the standard being used. Procedural Variation: This error occurs when there are two or more methods to obtain a measurement, resulting in multiple results. Software Variation: With software-generated measurement programs, variation in the software formulas may result in errors even after identical inputs. Laboratory to laboratory This error is common when procedures for measurement vary from laboratory to laboratory.

The development of standardized testing procedures, such as the ASTM procedures, has helped to correct this type of error. Calibration Interval It is generally accepted that the interval of calibration of measuring equipment be based on stability, purpose, and degree of usage. We have often heard these words, but have failed to develop a system that truly adheres to these most basic of calibration principles. The ability of a measurement instrument to consistently maintain its metrological characteristics over time is referred to as its stability. This could be determined by developing records of calibration that would record the Astound condition as well as the frequency, inspection authority, and the instrument identification code. The purpose or function of the measurement instrument is important. Whether it were to be used to measure doorstops or nuclear reactor cores would weigh heavily on the calibration frequency decision. In general, critical applications will increase frequency, and minor applications will decrease frequency. The degree of usage refers to the environment as a whole. Thought must be given as to how often an instrument is utilised and to what environmental conditions an instrument is exposed. Contamination, heat, abuse, etc.

Are all valid considerations. Intervals should be shortened if previous calibration records and equipment usage indicate this need. The interval can be lengthened if the results of prior calibration show that accuracy will not be sacrificed. Intervals of calibration are not always stated in standard lengths of time, such as annually, biannually, quarterly, etc. A method gaining recent popularity is the verification methodology. This technique requires that very short verification frequencies be established for instruments placed into the system, i.e., shifts, days, weeks, etc. The philosophy behind this system is that a complete calibration will be performed when the measuring instrument cannot be verified to the known standard. This system, when utilised properly, reduces the cost associated with unnecessary scheduled cyclic calibrations. Two key points must be made about this system.

The measuring instrument must be compared to more than one standard to take into consideration its full range of use. This system is intended for those measuring instruments that are widespread throughout a facility and can be replaced immediately upon the discovery of an out-of-calibration condition. Calibration Standards Any system of measurement must be based on fundamental units that are virtually unchangeable today. A master international kilogramme is maintained in France. In the SI system, most of the fundamental units are defined in terms of the natural standard. In all industrialised countries, there exists an equivalent to the United States National Institute of Standards and Technology, whose functions include the construction and maintenance of primary reference standards. These standards consist of copies of the international kilogramme plus measuring systems, which are responsive to the definitions of the fundamental units and to the derived units of the SI table.

In addition, professional societies, for example the American Society for Testing and Materials, have evolved standardized test methods for measuring many hundreds of quality characteristics not listed in the SI tables. These standard test methods describe the test conditions, equipment, procedure, etc. to be followed. The various standardizing bureaus and laboratories then develop primary reference standards that embody the units of measure corresponding to these standard test methods. In practice, it is not feasible for the National Institute of Standards and Technology to calibrate and certify the accuracy of the enormous volume of test equipment in use. Instead, resort is made to a hierarchy of secondary standards and laboratories, together with a system of documented certifications of accuracy. When a measurement of characteristics is made, the dimension being measured is compared to a standard.

The standard may be a yardstick, a pair of callipers, or even a set of gauge blocks, but they all represent some criteria against which an object is ultimately compared to national and international standards. Linear standards are easy to define and describe if they are divided into functional levels. There are five levels at which linear standards are usually described. Working Level: This level includes gauges used at the work center. Calibrating Standards:

These are standards to which working-level standards are calibrated. Functional Standards: This level of standards is used only in the metrology laboratory of the company for measuring precision, working, and calibrating. Other Standards Reference Standard: These standards are certified directly to the NISH and are used in lieu of national standards. National and International Standards: this is the final authority of measurement to which all standards are traceable. Since the continuous use of national standards is neither feasible nor possible, other standards are developed for various levels of functional utilization.

National standards are taken as the central authority for measurement accuracy, and all levels of working standards are traceable to this grand standard. The downward direction of this traceability is shown as follows: standards established by the National Institute of Standards and Technology Quality assurance in laboratories control system or inspection department work centre The calibration of measuring instruments is necessary to maintain accuracy but does not necessarily increase precision. Precision generally stays constant over the working range of the instrument.

ISO 10012 Synopsis: an integral part of the quality system is the documentation of the control of inspection, measurement, and test equipment. It must be specific in terms of which items of equipment are subject to the provisions of ISO 10012, in terms of the allocation of responsibilities, and in terms of the actions to be taken; objective evidence must be available to validate that the required accuracy is achieved.

The following are basic summaries of what must be accomplished to meet the requirements for a measurement quality system by ISO and many other standards. All measuring equipment must be identified, controlled, and calibrated, and records of the calibration and traceability to national standards must be kept.

The system for evaluating measuring equipment to meet the required sensitivity, accuracy, and reliability must be defined in written procedures. The calibration system must be evaluated on a periodic basis by internal audits and management reviews. The actions involved with the entire calibration system must be planned. This planning must consider management system analysis. The uncertainty of measurement must be determined, which generally involves gauge, repeatability, reproducibility, and other statistical methods. The methods and actions used to confirm the measuring equipment and devices must be documented.

Records must be kept on the methods used to calibrate, measure, and test equipment, and the retention time for these records must be specified. Suitable procedures must be in place to ensure that nonconforming measuring equipment is not used.

A labelling system must be in place that shows the unique identification of each piece of measuring equipment or device and its status. The frequency of recalibration for each measuring device must be established, documented, and based upon the type of equipment and severity of wear. Where adjustments may be made that logically go undetected, a ceiling for the adjusting devices or case is required. Procedures must define the controls that will be followed when any outside source is used regarding the calibration or supply of measuring equipment.

Calibrations must be traceable to national standards. If no national standard is available, the method of establishing and maintaining the standard must be documented. Measuring equipment will be handled, transported, and stored according to established procedures in order to prevent misuse, damage, and changes in functional characteristics.

Where uncertainties accumulate, the method of calculating the uncertainty must be specified in procedures for each case. Gauges, measuring equipment, and test equipment will be used, calibrated, and stored in conditions that ensure the stability of the equipment. Ambient environmental conditions must be maintained. Documented procedures are required for the qualifications and training of personnel that make measurement or test determinations. Measurement System Analysis In this lesson, you learned about the following: What are precision and accuracy? What are bias, linearity, and stability? What are gauge, repeatability, and reproducibility? What are variables and attributes? MSA.

7. Process Capability - Overview and Objectives

Process Capability Overview and Objectives By the end of this session, you will learn how to perform capability analysis. What is the concept of stability? What are attribute and discrete capability, and what are the different monitoring techniques? Process capability, capability, and analysis The determination of process capability requires a predictable pattern of statistically stable behavior, most frequently a bell-shaped curve where the chance causes of variation are compared to the engineering specifications. A capable process is one whose spread on the bell-shaped curve is narrower than the tolerance range or specification limits. USL is the upper specification limit, and LSL is the lower specification limit.

As you can see in the figure, the lower and upper specification limits are described. The process is targeted at the center. It has minimum and maximum values as well. It is often necessary to compare the process variation with the engineering or specification tolerances to judge the suitability of the process. Process capability analysis addresses this issue.

A process capability study includes three steps: planning for data collection, collecting data, and plotting and analysing the results. The objective of process quality control is to establish a state of control over the manufacturing process and then maintain that state of control over time. Actions that change or adjust the process are frequently the result of some form of capability study. When the natural process limits are compared with the specification range, any of the following possible courses of action may result in doing nothing. If the process limits fall well within the specification limits, no action may be required.

Change the specifications. The specification limits may be unrealistic. In some cases, specifications may be set tighter than necessary. Discuss the situation with the final customer to see if the specifications can be relaxed or modified. The process should be centered. When the process spread is approximately the same as the specification spread, an adjustment to the centering of the process may bring the bulk of the product within specifications.

Reduce variability This is often the most difficult option to achieve. It may be possible to partition the variation, that is, from stream to stream, within a piece, batch to batch, etc. And begin with the most powerful defender. complicated process and experimental design may be used to identify the leading source of variation. Accept the losses. In some cases, management must be content with a high loss rate, at least temporarily. Some centering and reduction in variation may be possible, but the principal emphasis is on handling the scrap and rework efficiently.

Other capability applications provide a basis for setting up variables. control chart evaluating new equipment, reviewing tolerances based on the inherent variability of a process, assigning more capable equipment to tougher jobs, performing routine process performance audits, and determining the effects of adjustments during processing Identifying Characteristics The identification of characteristics to be measured in a process capability study should meet the following requirements:

The characteristics should be indicative of a key factor in the quality of the product or process. It should be possible to adjust the value of the characteristic. The operating conditions that affect the measured characteristics should be defined and controlled. Process capability would not normally be performed for all 14 dimensions of a part if it has 14 different dimensions. Choosing one or two key dimensions allows for a more manageable method of evaluating process capability. For example, in the case of a machined part, the overall length or the diameter of the part might be the critical dimension.

The characteristics selected may also be determined by the history of the part and the parameter that has been the most difficult to control or has created problems in the next higher level of assembly. Identifying Specifications and Tolerances The process specifications or tolerances are determined either by customer requirements, industry standards, or the organization's engineering department.

The process capability study is used to show that the process is within the specification limits and that the process variation predicts. The process is capable of producing parts within the tolerance requirements. When the process capability study indicates the process is not capable, the information is used to evaluate and improve the process in order to meet the tolerance requirements. There may be situations where the specifications or tolerances are set too tight in relation to the achievable process capability.

In these circumstances, the specification must be reevaluated. If the specification cannot be opened, then the action plan is to perform a 100% inspection of the process unless the inspection testing is destructive. Verifying stability and normality If only common causes of variation are present in a process, then the output of the process forms a distribution that is stable over time and predictable. If special causes of variation are present, the process output is not stable over time.

Note that the process may also be unstable if either the average or variation is out of control. Common causes of variation refer to the many sources of variation within a process that have a stable and repeatable distribution over time. This is called a state of statistical control, and the output of the process is predictable. Special causes refer to any factors causing variations that are not always acting on the process. If special causes of variation are present, the process distribution changes, and the process output is not stable over time.

When plotting a process on a control chart, lack of process stability can be shown by several types of patterns, including points outside the control limits, trends, points on one side of the center, line cycles, etc. Attribute and Discrete Capability The control chart represents the process capability once special causes have been identified and removed from the process. For attribute charts, capability is defined as the average proportion or rate of nonconforming products.

For Pcharts, the process capability is the process average nonconforming value and is preferably based on 25 or more control periods. If desired, the proportion should conform to specifications. One less than P may be used. For NP charts, the process capability is the process average nonconforming p and is preferably based on 25 or more control periods. For C charts, the process capability is the average number of nonconformities in a sample of fixed-sized N. For U charts, the process capability is the average number of nonconformities per reporting unit, or U. The average proportion of nonconformities may be reported on a defect per million opportunity scale by multiplying P times 1 million.

Capability Indices Capability Index Failure rates in Table 528: Ppm equals parts per million of nonconformance or failure when the process is centred on X, is normally distributed, has a two-tail specification, and has no significant shifts in average or dispersion with the drive. For increasingly dependable products, there is a need for failure rates in the C range of one to two. As the value of z increases to six, the value of CP increases from 0.33 to two, and the value of PPM decreases from 317,311 to capability indices, as shown in the table.

The capability index is defined as CP equal to the upper specification limit or USL lower specification limit divided by six standard deviations. As a rule of thumb, when it is capable, CP is greater than 1.33, when it is capable of tight control, and when it is incapable, CP is less than one. The capability ratio is defined as Cr, which is equal to six standard deviations divided by the upper specification limit and the lower specification limit. LSL Cr less than zero and up to ten is capable of tight control; cr greater than ten is incapable.

Note that the above formulas only apply if the process is centered, stays centred within the specifications, and CP equals CPK. Performance Indices The performance index is defined as PP, which is equal to the upper specification limit minus the lower specification limit divided by six standard deviations. The performance ratio is defined as PR, which is equal to six standard deviations divided by the upper specification limit (USL) minus the lower specification limit.

LSO Sigma I is a measure of total sigma and generally comes from a calculator or computer. PPK is the ratio given as the smallest answer between PPK being equal to the upper specification limit minus the mean divided by three standard deviations or PPK being equal to the mean minus the lower specification limit divided by three standard deviations.

Capability in the Short-Term and Long-Term When a process capability is determined using one operator on one shift with one piece of equipment and a homogeneous supply of materials, the process variation is relatively small. As factors for time, multiple operators, various lots of material, environmental changes, etc. are added, Each of these contributes to increasing process variation.

Control limits based on a short-term process evaluation are closer together than control limits based on a long-term process. When a small amount of data is available, there is generally less variation than is found with a larger amount of data. Control limits based on the smaller number of samples will be narrower than they should be, and control charts will produce false out-of-control patterns. The relationship between short-term and long-term process capability is that short-term Z is equal.

8. Process Capability - Monitoring Techniques

Monitoring techniques. Transfer Tools. Transfer tools such as spring callipers have no reading scale. Jaws on these instruments measure the length, width, or depth in question by making positive contact. The dimension measurement is then transferred to another measurement scale for direct reading. Attribute Gauges Attribute gauges are fixed gauges, which are typically used to make a go-or-no-go decision. Examples of attribute instruments are master gauges, plus gauges, contour gauges, thread gauges, limit-length gauges, assembly gauges, etc.

Attribute data indicates only whether a product is good or bad. Attribute gauges are quick and easy to use, but they provide minimal information for production control. Variable gauges and variable measuring instruments provide a physically measured dimension. Examples of variable instruments are rulers, berniers, calipers, micrometers, depth indicators, runout indicators, etc. Variable information provides a measure of the extent to which a product is good or bad relative to specifications. Variable data is often used for process capability determination and may be monitored via control charts. Reference or Measuring a reference surface means measuring the surface of a measuring tool that is fixed.

The measuring surface is movable. For an accurate measurement, both surfaces must be free of gritor damage, secure to the part, and properly aligned. Instrument Selection The terms measuring tool, instrument, and gauge are often used interchangeably in this text. An appropriate gauge should be used for the required measurement. Listed in the following slides are some gauge, accuracy, and application characteristics. Monitoring Techniques As you can see in the table, the adjustable snap gauges are usually accurate within 10% of the tolerances, and their application is that they measure diameters on a production basis where an exact measurement is needed. The air gauges' accuracy depends on gauge design. Measurements of less than 0.005 inches are possible. It is used to measure the diameter of a bore or hole.

However, other applications are possible. The automatic sorting gauges are used to sort parts by dimension and are accurate to within 0 inches. The combination square is accurate to within one degree and is used to make angular checks. The coordinate measuring machines have an accuracy that depends on the part. Its axis inaccuracies are within 35 millionths and its ti within zero five inches. It can be used to measure a variety of characteristics such as contour taper, radii, roundness, squareness, etc. When used properly, dial bore gauges have an accuracy of zero zero zero zero one inch and are used to measure bore diameters, tapers, and out of roundness. The dial indicator has an accuracy that depends on the part axis. Accuracies are within zero zero zero zero one inch and measure a variety of features such as flatness, diameter, concentricity, taper height, etc. The electronic comparator is accurate from 0 to 1 inch and is used when the allowable tolerance is less than 0 to 1 inch.

The fixed snap gauges have no set accuracy and are normally used to determine if diameters are within specifications. The flush pin gauges are used for high-volume, single-purpose applications and have an accuracy of about 0.022 inches. The gauge blocks have an accuracy that depends on the grade, and normally the accuracy is 0.008 inch or better. Gauge blocks are best adapted for precision machining and as comparison masters. The height Berniers are mechanical models that are measured to a thousandth of an inch. Some digital models can reach zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, zero, zero They are used to check dimensional tolerances on a surface plate. The internal and external thread gauges cannot provide exact readings. They will discriminate up to a specified limit. They are used for measuring inside and outside pitch thread diameters.

The internal micrometre has a mechanical accuracy of about 0.1 inch. Some digital models have a resolution of 0 0 0 0 5 inches. They are normally used to check diameter or thickness. Special models can check thread diameters. The micrometre outside has a mechanical accuracy of about 0.001 inches. Some digital models have a resolution of 0 0 0 0 5 inches. They are normally used to check diameter or thickness. Special models can check thread diameters. The optical comparator has an accuracy that can be within zero zero zero two inches. They measure difficult contours and part configurations. The optical flats depend on operator skill and are accurate to a few millionths of an inch.

They are used only for very precise toolroom work. best used for checking flatness. The plug gauges have accuracy that is very good for checking the largest or smallest hole diameter. They check the diameter of drilled or rimmed holes but will not check for out-of-roundness. The precision straight edge is used to check the flatness, weightiness, or squareness of a face to a reference plane. It has a visual zero of 10 inches and a feeler gauge zero of zero three inches. The radius and template gauges have accuracy that is no better than zero minus five inches, and they're used to check small radii and contours. The ring gauges will only discriminate against diameters larger or smaller than the print specification. The best application is to approximate the mating part in assembly. We'll not check for out-of-roundness. The split sphere and telescope have no better than.

The steel ruler or scale is only good for 0 0 5 inches. They are used to measure heights, depths, diameters, etc.

The flatness of the surface plates is expected to be no better than zero inches between any two points. They are used to measure the overall flatness of the object. An accurate micrometre is used to measure the tapered parallels. The precision is approximately 0.005 inches. In low volume applications, they are used to measure boresizes. The tool maker's flat has an accuracy that is no better than zero zero zero zero five inches, depending upon the instrument used to measure the height. They are used with a surface plate and gauge blocks to measure height. The Bernier callipers measure approximately zero, zero, one inch.

Some digital models have a resolution of 0 0 0 0 5 inches. They are used to check diameters and thicknesses. Bernier depth gauges measure approximately zero zero zero one inch. Some digital models are accurate to 0.01 inches. They are used to check depths. Attribute Screens: 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 tests must be of a nondestructive nature.

Screening programmes have the following characteristics: a clearly defined purpose high sensitivity to the attribute being measured, that is, a low false-negative rate. high specificity to the attribute being measured—that is, a low false positive rate. The benefits of the programme outweigh the costs. Measured attributes identify major problems. That is, serious and common results lead to useful actions. Common applications of screening tests occur in reliability assessments and in the medical screening of individuals. In reliability assessments, an attribute screen test may be conducted on separate production units that are susceptible to high initial failure rates. This period is also known as the "infant mortality period." The test simulates customer use of the unit or perhaps an accelerated condition of use.

The number of failures per unit of time is monitored, and the screen test continues until the failure rate has reached an acceptable level. The screen test separates acceptable items from failed items, and an analysis of the failed components is performed to find the cause of the failure. In medical screening, a specific symptom or condition is targeted, and members of a defined population are selected for evaluation. Examples of this type of screening include a specific type of cancer or a specific disease. In many cases, the members of the selected population may not be aware that they have the condition being screened. Medical screening tests have the ultimate objective of saving lives. Measuring instruments are typically expensive and should be treated with care to preserve their accuracy and longevity.

Some instruments require storage in a customised case or controlled environment. When not in use, even sturdy handtools are susceptible to wear and damage. Hardened steel tools require a thin film of oil to prevent rusting. Care must be taken in the application of oil since dust particles will cause buildup on the gauge's functional surfaces. Measuring tools must be calibrated on a scheduled basis as well as after any suspected damage. Gauge Blocks Carl Johansson of Sweden developed steel blocks to an accuracy thought impossible by many others at the time near the beginning of the twentieth century. His objective was to establish a measurement standard that would not only duplicate national standards but could also be used in any shop. He was able to construct gauge blocks with millimeter-level precision.

When first introduced, gauge blocks, or "Joeblocks," as they are unpopularly known in the shop, were a great novelty. Seldom used for measurements, they were kept locked up and were only brought out to impress visitors. Today, gauge blocks are used in almost every shop manufacturing a product requiring mechanical inspection. They are used to set a length dimension for a transfer measurement and for the calibration of a number of other tools.

We generally distinguish three basic gaugeblock shapes: rectangular, square, and round. The rectangular and square varieties are in much wider use. Generally, gauge blocks are made from high-carbon or chromium alloyed steel; tungsten carbide, chromium carbide, and fused quartz are also used. All gauge blocks are manufactured with tight tolerances on flatness, parallelism, and surface. Smoothness gauge blocks should always be handled on the nonpolished sides. Blocks should be cleaned prior to stacking with filtered kerosene, benzene, or carbon tetrachloride.

A soft, clean cloth or chamois should be used. A light residual oil film must remain on blocks for ringing purposes. Block sacks are assembled by a ringing process that attaches the blocks through a combination of molecular attraction and the adhesive effect of a very thin oil film. Air between the block boundaries is squeezed out. The sequential steps for the ringing of rectangular blocks are shown below.

Light pressure is used throughout the process. As you can see in the table, the US Federal Accuracy standard gauge block grades have new and old designations, as well as appropriate length accuracy. The old designation of AAA has a new designation of "zero five" and an accuracy of plus or minus zero zerozerozerozero one.The old designation of AA has a new designation of 1, and an accuracy of plus or minus zero, zero, zero, zero, two. The old plus symbol has been replaced by a two, with an accuracy of plus four and minus two. The old designation of A and B has a new designation of three and an accuracy of plus or minus 00008.

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