Metrology and Measurement Concepts

Metrology and Measurement

• Metrology is the science of measurement, encompassing precision measurement of quantities.
• Calibration establishes the relationship between measured values and standard reference values. This is crucial for maintaining measurement precision, identifying/correcting instrument deviations, and ensuring uniform measurements.
• The calibration process involves selecting a standard, performing a comparison, identifying errors, making adjustments, and documenting results.

Accuracy vs. Precision

• Accuracy describes the correctness of a measurement; affected by systematic errors (calibrated out).
• Precision focuses on the consistency of measurements; affected by random errors (reduced by improving repeatability).
• The crucial difference is that accuracy targets correctness, while precision aims for repeatability.

Tolerance

• Tolerance refers to acceptable measurement variations.
• It defines limits ensuring functionality and is often expressed as a range.
• Tolerance types include unilateral, bilateral, geometric, limit tolerances.

Vernier Calipers

• Vernier calipers are precision steel instruments measuring 1 mm to 0.5 mm.
• Usage involves opening, holding, sliding, and reading.
• Important guidelines include clean surfaces free from burrs and gentle pressure for light contact.
• Common errors include incorrect insertion, soiled/damaged surfaces, improper pressure, and worn parts.

Micrometers

• Micrometers are even more precise, measuring 0.001 mm to 0.010 mm.
• Types include disk, screw thread, dial, and blade, for various situations.
• Errors include misreading scales, dirty spindles/anvils, damage, worn screws, miscalibration.
• Types include external, depth, internal, etc.

Gauges

• Gauges check if parts meet specified limits.
• Types include limit, plug, snap, ring, thread, hole, feeler, and straight edge gauges.
• Advantages include reduced skill needs, cost savings, speed, and portability.
• Disadvantages include limited info on component condition, manufacturing errors, and lack of direct measurements.

Gauge Blocks

• Gauge blocks are precision metal/ceramic blocks.
• Accurately ground and lapped for specific thicknesses; used as reference standards.
• Types include rectangular, square (larger surface), and square with central holes.
• Accuracy grades range from Grade 00 (0.00005 mm) to Grade 1 (0.00025 mm).
• "Wringing" is a phenomenon where smooth surfaces adhere due to molecular attraction.
• Proper cleaning, application of paraffin jelly, gentle handling and heat protection essential to maintain accuracy.

Measurement Errors

• Measurement error occurs when the measured result differs from the true size.
• Factors include calibration standards, workpiece conditions, instrument limitations, human errors, and environmental factors.
• Systematic errors (controllable) create a fixed deviation from the true value and can be corrected using calibration.
• Random errors cause unpredictable variations in repeated measurements (harder to remove).

Specific Instruments

• Sine Bar: Precise instrument for measuring/checking angles. Made of corrosion-resistant steel, available in standard sizes (100mm, 200mm, 300mm).
• Profile Projector: Optical instrument for inspecting/measuring workpiece dimensions using projected images. Provides magnified, two-dimensional images.

Measurement Error Sources

• Calibration Errors: Influenced by ambient conditions or thermal expansion.
• Workpiece Issues: Affected by conditions like cleanliness, ambient conditions, and surface finish.
• Instrument Errors: Caused by instrument limitations: hysteresis, friction, dents, or wear.
• Human Errors: Occur due to poor training, improper handling, lack of concentration, misreading.
• Environmental Factors: Impact measurement accuracy: temperature variation, heat radiation, vibrations, human activity.
• Error types include ambient errors (temperature), parallax errors (viewing angle), alignment errors (misalignment), reading errors (misrecording), zero errors, and deformation.

General Measurement Principles

• Measuring instruments like sine bars and profile projectors enable accurate measurements for various manufacturing and engineering applications.
• Appropriate handling, calibration, and control of environmental factors improve measurement reliability.