Measurement Process Analysis: Principles and Techniques

Measurement Process Analysis

Measurement, regardless of the magnitude, involves decisions on:

Measurand

The measurand is of fundamental importance to the choice of instrument.

Measure or Check

Measure determines the numerical value of a quantity, while verification confirms if a magnitude is within preset limits.

Geometric Characteristics of the Scale

1. Provision of Space to Measure

• Exterior
• Interior
• Depth
• Distance

2. Geometric Shape

2.1 Form of Isolated Elements

• Straightness
• Roundness
• Form a line
• Flatness
• Cylindrical
• Form a surface

2.2 Guidance of Isolated Elements

• Parallelism
• Perpendicularity
• Angularity

2.3 Positioning of Associated Elements

• Position of an element
• Concentricity
• Symmetry
• Circular
• Total

Logistical Difficulties

Part size, specimen weight, mobility, measuring at different levels, measuring between unaligned references, ease of holding the workpiece, accidents between references, theoretical framework, narrow space for the instrument.

Reference Action

Marble, ruler, set square, V-wedge, fixed mandrels, expanding mandrels, mechanical or hydraulic adjustable coves or rules, or generating surfaces close to the theoretical axis.

Tolerances

The range of values in which a measurement must fall to meet specifications.

Key Terms

• Setting: A ratio of two parts.
• Nominal Size: The value indicated on a blueprint.
• Practical Action: A measurement of a built piece.
• Tolerance: The difference between the maximum and minimum allowable measurements.
• Tolerance Line: The line where tolerance zones are plotted.
• Maximum Size: The largest permissible measurement.
• Minimum Size: The smallest permissible measurement.
• Superior Difference: The difference between the measured and nominal maximum allowed.
• Lower Difference: The difference between the measured and nominal minimum supported.
• Game: Occurs when the shaft is smaller than the hole.
• Maximum Game: The difference between the hole diameter and the minor axis.
• Minimum Game: The difference between the smaller diameter of the hole and the largest in the shaft.
• Squeeze: When the shaft diameter is greater than the hole diameter.
• Maximum Torque: The maximum diameter difference between the shaft and the minor axis of the hole.
• Minimum Torque: The difference between the hole diameter and the minor axis.