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Metrology- The Science of Measurement

�What do we measure in normal life?– Time (sec, min, hour, �)– Mass or weight (lbf, kg, g, �)– Distance, height and length (mile, feet, m,..) – Speed (miles/hr, km/hr, �)– Temperature (F, C, �)

�What do we measure in this class?– Geometry (length, angle, �)– Surface finish (root mean square (RMS), �)

http://faculty.uml.edu/bkim/22.202/material.htm

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Questions to Ask When Measuring

�What is quality?– What do I need to measure?

– How do I quantify it?

�Where are my quality problems?

�How accurate– Does the measurement need to be?

– Is the measurement?

�How fast do I need to measure?

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Some Factors Affecting Piece Accuracy

�Machine and Control System Design– Structural, Drives/Control,�..

�Environment– Temperature, Vibration, Humidity, Pressure,...

�Tool– Wear, Stiffness, Build Up Edge effects,

Feeds,..

�Work Piece– Stiffness, Clamping, Thermal properties

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Some Basic Definitions

�Accuracy– A quantitative measure of the degree of conformance

to recognized national or international standards of measurement

�Repeatability– A measure of the ability of a machine to sequentially

position a tool with respect to a work piece under similar conditions.

�Resolution– Is the least increment of a measuring device; the least

significant bit on a digital machine.

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Accuracy, Repeatability and Resolution

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Accuracy and Repeatability

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http://www.online-stopwatch.com/full-screen-stopwatch/

Accuracy and Repeatability Test

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Establishing Machine Capabilities

�ANSI/ASME B89.1.12M-1985/1991– Methods for performance evaluation of

coordinate measurement machines

�ANSI/ASME B5.54-1991– Methods for performance evaluation of CNC

machining Centers

�These standards call for tests designed to evaluate parametric error sources of a machine.

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CMM(coordinate measuring machine )

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Errors in Measurement

�Nothing is perfect.– A 30 mm gage block.

• Measured error in the block=-0.0002 mm

• Accuracy of determination ±0.0004 mm

– Understanding tolerance to imperfection is critical.

�Some simple but embarrassing errors– Misreading an instrument

– Arithmetic errors

– Errors in units

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Errors in Measurement

�Controllable Errors– Environmental conditions

– Elastic deformation due to loading

– Alignment errors

– Parallax errors

�Non-Controllable Errors– Scale errors

– Hysteresis

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Parallax Error

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Scale Errors

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Hysteresis

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Hysteresis

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Error Compounding

�No measurement is exact.

�Therefore, it is necessary to state the accuracy of determination as well as the measured dimension.

�Example:– 1.445±0.002

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Units

�m

�cm

�mm

�um

�nm

�Å