Unit 4: Standard of Measurements

Standard of Measurements

• Two standard systems for linear measurement: English (yard) and metric (metre) systems
• The metric system is widely accepted and used as the fundamental unit of linear measurement
• Scientists have always sought a suitable unit for length that remains constant regardless of environmental conditions
• The wavelength of monochromatic light is used as a natural and invariable unit for length

Line and End Measurements

• Line standard: measures distance between two engraved lines
  • Examples: yard and metre
  • Used for quick and easy measurements
  • Limited accuracy (±0.2 mm) and requires magnifying lens or microscope for high accuracy
• End standard: measures distance between two flat and parallel surfaces
  • Examples: slip gauges, end bars, and micrometer anvils
  • Used for precision measurements in workshops and laboratories
  • High accuracy (up to ±0.0005 mm) and requires skill and time for measurement

Characteristics of Line Standards

• Quick and easy measurements
• Limited accuracy (±0.2 mm)
• Engraved lines have thickness, making high accuracy measurements difficult
• Markings on the scale are not subjected to wear, but leading ends may wear, resulting in undersizing
• No built-in datum, making alignment difficult
• Subject to parallax effect, leading to reading errors

Characteristics of End Standards

• High accuracy (up to ±0.0005 mm)
• Measurements are time-consuming and require skill
• Measuring surfaces are subjected to wear
• Built-in datum, making alignment easy
• Not subject to parallax error

Comparison between Line and End Standards

• Line standards: quick and easy, but limited accuracy and no built-in datum
• End standards: high accuracy, but time-consuming and requires skill, with a built-in datum

Limits, Fits, and Tolerances

• Manufacturing processes inherently produce variations in size and shape
• Tolerances: the magnitude of permissible variation in a dimension or measured value
  • Upper limit: maximum allowable dimension
  • Lower limit: minimum allowable dimension
  • Tolerance: difference between upper and lower limits
• Fits: describe the relationship between mating parts, such as a hole and a shaft
  • Clearance fit: gap between mating parts, used for easy assembly and disassembly
  • Interference fit: overlap between mating parts, used for permanent or semi-permanent assembly
  • Transition fit: slight gap or overlap, used for accurate location and moderate tightness

Principle of Interchangeability

• Manufacture of identical parts in bulk, allowing for interchangeable assembly
• Essential for mass production and modern manufacturing techniques
• Enables the production of components with a desired fit, without further modification during assembly
• Increases productivity, reduces manufacturing cost, and facilitates ease of replacement

Tolerances

• Essential for manufacturing products with a desired level of accuracy and quality
• Tolerance principles: permit dimensional variations, achieve interchangeable assembly, and ensure functional requirements
• Tolerance is an absolute value, defined as the algebraic difference between the upper and lower acceptable dimensions
• Industry follows approved accuracy standards, such as ANSI and ASME, to manufacture different parts### Tolerances
• A letter indicates the fundamental deviation (position of tolerance zone relative to nominal size)
• A number indicates the tolerance grade (size of tolerance zone)

Basis Systems

• Hole Basis System: hole is kept as basic size, shaft size is varied to get desired fit
• Shaft Basis System: shaft is kept as basic size, hole size is varied to get desired fit

Gauges

• Essential for quality control in manufacturing processes
• Used to measure dimensions of manufactured parts and ensure conformity to specified limits

Types of Gauges

• Limit Gauges:
  • Check whether part's dimension falls within specified limits
  • Go Gauge: checks lower limit, part should fit into or pass through
  • No-Go Gauge: checks upper limit, part should not fit into or pass through

Plug Gauges

• Cylindrical gauges used to check dimensions of holes
• Go Plug Gauge: should fit into hole
• No-Go Plug Gauge: should not fit into hole

Ring Gauges

• Used to check dimensions of shafts
• Go Ring Gauge: shaft should fit into ring
• No-Go Ring Gauge: shaft should not fit into ring

Snap Gauges

• Used to check external dimensions of cylindrical parts, such as shafts
• Go Snap Gauge: part should pass through
• No-Go Snap Gauge: part should not pass through

Limits and Fits

• Limits: define permissible variations in dimensions to ensure parts fit and function correctly
• Fits: describe relationship between mating parts, ensuring correct assembly (clearance, interference, and transition fits)