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?