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?