Precision Measurement Tools and Techniques in Engineering

Posted on Jan 22, 2025 in Electromechanical Engineering

N.P.L. Flatness Interferometer

The National Physical Laboratory (N.P.L.) Flatness Interferometer is an optical instrument used to measure the flatness of surfaces with high precision.

Principle

It uses the principle of light wave interference. When monochromatic light is directed on a flat reference surface and the surface to be tested, the light waves interfere, creating a pattern. Flat surfaces produce straight fringes, while deviations create curved fringes.

Construction and Working

  1. Consists of a monochromatic light source, beam splitter, reference flat, and the test surface.
  2. The light is directed through the beam splitter onto the reference and test surfaces.
  3. Interference patterns are observed on the surface, with the pattern shape indicating surface deviations.

Principle of Interference

Interference occurs when two or more light waves overlap, resulting in constructive (bright) or destructive (dark) fringes.

Flatness Measurement

  1. An optical flat is placed on the surface to be tested.
  2. Monochromatic light produces interference fringes, indicating the flatness level.

Parkinson Gear Tester

The Parkinson Gear Tester measures the accuracy of gear profiles and pitch errors.

Principle

This instrument compares the tested gear to a reference (master) gear, identifying deviations in gear profile and pitch.

Construction and Working

  1. It has a fixed reference gear and a test gear, mounted on the same axis.
  2. As the gears rotate, any deviations cause a probe or dial gauge to move.
  3. The gauge records pitch and profile errors by measuring the relative displacement between the gears.

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Taylor’s Principle

Also called the principle of functional gauge design, it states that:

  • For a “GO” gauge, all dimensions should check the maximum material condition (MMC) to ensure the part can fit within the limits.
  • For a “NO GO” gauge, it should check the least material condition (LMC).

Hole Basis and Shaft Basis Systems

Hole Basis System: The hole size is kept constant, while the shaft size varies to achieve different fits (clearance, interference).

Shaft Basis System: The shaft size is constant, and the hole size varies.

Encoders

Encoders convert mechanical motion into electrical signals, used for position, velocity, and direction feedback.

Incremental Encoder

  1. Generates a pulse train as it rotates.
  2. The number of pulses determines the displacement.
  3. A sketch would show a rotating disk with slots and an LED/photodetector that counts pulses.
  4. Example: Used in conveyor systems to track positions.

Absolute Encoder

  1. Provides a unique code for each position.
  2. The disk has multiple tracks with binary-coded patterns.
  3. Example: Robotics, where precise positioning is needed.

Taylor Hobson Talysurf

Taylor Hobson Talysurf measures surface roughness using a stylus-based method.

Working Principle

  1. A diamond-tipped stylus moves across the surface, detecting height variations.
  2. These variations are amplified and converted into an electrical signal to compute surface roughness.

Advantage: High accuracy for small, intricate features.

Disadvantage: Expensive and less portable than Tomlinson surface meters.

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Gauge Factor

Gauge Factor (GF): The ratio of the relative change in electrical resistance to the mechanical strain applied to the gauge.

Significance

Indicates the sensitivity of the strain gauge. Higher gauge factors mean greater resistance change for a given strain, allowing for more precise strain measurements.

Elastic Transducers for Pressure Measurement

Types: Bourdon, Tube Diaphragm, Bellows, Capsule

Example: Bourdon Tube: A curved tube that deforms under pressure. When pressure is applied, the tube uncoils, moving a pointer to display the pressure reading.

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Two-Wire Method for Screw Thread Measurement

The Two-Wire Method measures the effective diameter of a screw thread.

Working Principle

  1. Two small wires are placed in the threads, allowing for measurement across the thread’s pitch diameter.
  2. Micrometers are used to measure the distance over the wires, helping determine the effective thread diameter.

Magnetic Flow Meter

  1. Operates based on Faraday’s Law of Induction.
  2. Electrodes measure voltage induced by fluid flow in a magnetic field.
  3. Suitable for conductive fluids.

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Ultrasonic Flow Meter

  1. Uses sound waves; a transmitter sends sound across the pipe, and a receiver detects shifts in frequency.
  2. Measures flow rate by analyzing the time difference.