Transcript Chapter 20
Chapter 20 Measurement Systems Objectives • Define and describe measurement methods for both continuous and discrete data. • Use various analytical methods to analyze and interpret measurement systems. • Identify how measurement systems can be applied in functional areas. • Define and describe elements of metrology. Measurement Methods • Continuous or variable data result from characteristics that are measured rather than merely counted, such as length, diameter, and hardness. • Gage blocks are made of very hard material and provide basic references for setting and calibrating gages. • A set of gage blocks can be used to set precision measuring tools such as height gages. Measurement Categories • 1. Mechanical: Mechanical systems are used to amplify small movements. • 2. Pneumatic: Air pressure or velocity is used to detect dimensional variation. • 3. Electronic: Changes in characteristics such as resistance, capacitance, or inductance are converted to dimensional changes. • 4. Light technologies: Wave interference is used to provide standards. • 5. Electron systems: An electron beam microscope is used to make measurements. Measurement Tools • • • • • • • • • 1. Caliper: Whether equipped with a vernier scale, a dial, or digital readout, a caliper is essentially a very accurate steel ruler. 2. Micrometer: Uses precision screws to move a spindle relative to a fixed anvil to determine the distance between them. 3. Transfer Devices: Include spring calipers, spring dividers, and telescoping gages used to compare dimensions. 4. Height gage: Used with a flat surface plate to measure heights. 5. Precision protractor: Used to measure angles. 6. Coordinate measuring machine (CMM): Uses electronic probes to provide very accurate measurements. A CMM is usually interfaced with a computer to collect, analyze, and store measurement data. Many fields have special tools to meet their unique measurement requirements. Hardness of metals is usually measured by using a Brinnell or Rockwell tester. When selecting a gage for use in an inspection process, the rule of ten is often used: The smallest increment of measurement for the device should be less than or equal to 1/10 the tolerance. Measurement System Analysis • Accuracy: It is the closeness of the measured result and the true or accepted reference value. • Components of accuracy include: • 1. Bias: Difference between the mean of the test results and a true value. • 2. Linearity: Good linearity means constant bias throughout the range of measurements. • 3. Stability: of a measurement system represents the change in bias over time and usage. Measurement System Analysis • Precision: is the closeness of agreement between randomly selected individual measurements. • The components of precision include: • 1. Repeatability: precision of measurement results obtained with the same method, by the same operator, using the same equipment, in the same environment. • 2. Reproducibility: precision with different operators using different equipment. • Measurement system variation often represents the most cost effective way to reduce the total observed variation. Measurement Systems in the Enterprise • Human resources: Performance appraisals/ evaluations are widely used in most organizations usually on an annual basis. • Marketing and sales: Use customer surveys to gauge satisfaction, loyalty, needs, wants, and other behaviors. • Quality engineering: Conduct various quality studies, and are accountable for equipment calibration. • Supply chain management: Evaluating supplier performance is critical in many organizations to determine rework and repair cost allocations, reimbursements, and occasionally profit sharing. • Without adequate measurement systems, decision making may result in erroneous decisions. Metrology • Metrology: Whenever measuring equipment is used a calibration system should be in place to help ensure that the measurement system does its job. • Causes of measurement error: • 1. Machine (equipment): lack of accuracy, precision, and gage instability over time. • 2. Methods (procedures): wrong tool specified, improper procedure specified, or failure to use specified tool or procedure. • 3. Man or woman (appraiser): lack of training, lack of physical ability, or lack of motivation. • 4. Mother nature (environment): temperature, humidity, noise, or vibration. • 5. Materials (parts): wear and tear. • 6. Management: atmosphere of mistrust and fear. Summary • Continuous or variable data result from characteristics that are measured rather than merely counted, such as length, diameter, and hardness. • Mechanical, Pneumatic, Electronic, Light Technologies, and Electron Systems are 5 measurement categories. • When selecting a gage for use in an inspection process, the rule of ten is often used: The smallest increment of measurement for the device should be less than or equal to 1/10 the tolerance. • Accuracy: It is the closeness of the measured result and the true or accepted reference value. • Without adequate measurement systems, decision making may result in erroneous decisions. • Whenever measuring equipment is used a calibration system should be in place to help ensure that the measurement system does its job. Home Work • 1. Name 5 measurement categories. • 2. What are the advantages of using a coordinate measuring machine (CMM)? • 3. What is the rule of ten? • 4. What is accuracy? Name 3 components of accuracy. • 5. What is precision? Name 2 components of precision. • 6. Why are adequate measurement systems important? • 7. What can lead to appraiser measurement error?