Design for Manufacture

• Measurement and Marking Out

  • Measurement: Using tools like rules, tapes, and gauges to check dimensions.
  • Marking out: Transferring designs onto materials to indicate features like edges, holes, and bends for manufacturing.
• Tools for Measuring and Marking Out

  • Metal:
  • Scriber: High carbon steel tool for scratching metal surfaces.
  • Engineer’s blue: Ink applied to make scribed lines visible.
  • Dividers and beam compasses (trammels): For drawing circles or arcs.
  • Calipers: For checking dimensions.
  • Timber:
  • Marking knife or pencil (carpenter’s pencils have thicker leads).
  • Marker pen or scriber for polymer sheets.
  • Steel rule for measurement; measuring tape for distances over a meter.
  • Modern Devices:
  • Laser devices like cross-line levels: For projecting reference lines on large or complex fabrications.
• Guiding Tools for Precision

  • Surface Plates and Gauges: Used for accurate measurement and marking in engineering.
  • Marking Gauge: Marks a line at a constant distance from a timber edge.
  • Try Squares and Combination Squares: Used for marking right angles and mitres (45°). Combination squares also have a spirit level.
  • Protractors: For marking angles, ranging from simple to vernier protractors.
  • Centre Punch: Ensures accurate drilling by preventing drill wandering. Used for marking lines in heat processes like forging.
• Units of Measurement

  • The metric system (SI units) is standard globally, with millimetres (mm) commonly used in product design.
• High Precision Work

  Precision Measurement Tools

  • Micrometers, Vernier Calipers, and CMMs: Used for fine tolerances (see Chapter 2.9).
  • Profile Projector: For highly precise measurements.
  • Digital Test Gauge: A single-axis measuring device replacing mechanical dial indicators, offering easy-to-read displays and computer input.
• High Precision Work

  Machine Dials

  • Graduated Dials: Used in machining operations (milling, turning) to measure precise linear movements, achieving accuracy of 0.01 mm per graduation. Dials can be zeroed at the start of a cut to gauge distance moved.

  CNC Machining

  • CNC Processes: Controlled by G-codes from CAD/CAM software, eliminating the need for manual marking out.
  • Material Optimization: Ensuring optimal layout to avoid material waste, applicable to all component size decisions relative to stock material forms.
• The Importance of Accuracy

  Definition and SignificanceAccuracy ensures measurements conform to required values, crucial for fitting parts and functioning products, especially in miniaturized devices. Designers must determine acceptable accuracy levels, known as tolerances, which vary based on material, size, function, and interchangeability needs.

  Examples Requiring Accuracy

  • Cutting watch gears
  • Ensuring nut and bolt threads fit
  • Fitting glazing units in window frames
  • Positioning holes for self-assembly furniture
  • Selecting washers for tap repairs
  • Positioning holes for shelf brackets
• Tolerance Specification

  • Tolerance is expressed as X ± Y, where X is the required size, and Y is the allowable error. Components outside tolerance are rejected as part of QA/QC measures.

  Geometric Dimensioning and Tolerancing (GD&T)

  • GD&T, standardized by BS ISO 8015 and BS 8888, uses symbols on CAD models or drawings to specify ideal dimensions and allowable variations. General tolerances might be noted as linear ±0.2 or angular ±0° 30′. Specific tolerances are indicated using two figures or + and - symbols.
• Indicating Tolerances

  • Tolerances can be shown as 50± 0.2, 50.2/49.8, or 50 -0.2/+0.2 for linear dimensions.
  • Angular tolerances can be 45º 30′, 45º ± 0º 30′, or 45º -0º 30′/44º 30′.
  • A comma indicates a decimal marker on engineering drawings.

  Datum Surfaces and Edges

  • Using datum surfaces or edges for dimensioning improves accuracy by reducing cumulative errors compared to marking out dimensions individually.
• Tolerance ConsiderationsVaries based on the situation:

  1. Rectangular MDF piece for picture mounting: Tolerance is around ±0.5 mm, with slight deviations acceptable. Measurements are done with a ruler and try square, often using a circular saw with a fixed scale for accuracy.
  2. Mass-produced glass bottles: Precise tolerance is critical for screw top fit, shape, and volume. Automated systems and profile projectors with a ±0.015 mm tolerance check compliance.
  3. Mould for blow-moulding glass bottles: The mould requires high precision, typically ±0.002 mm, achieved through EDM.
• Jigs

  • Jigs control the motion of tools relative to a workpiece, ensuring precise hole placement. They eliminate the need for marking out and don't require high skill levels. Commonly, drilling jigs are clamped onto workpieces, guiding the drill with hardened bushes.
• Fixtures

  • Fixtures are robust frames that hold workpieces in place during machining, welding, or other operations. They ensure that each component or assembly is consistently positioned for every manufacturing process.
• Templates

  • Templates are rigid patterns used to transfer designs onto workpieces. By tracing the outline with a marking tool, manufacturers ensure consistent shape transfer for each part.
• Non-Contact Measuring Devices

  • These include lasers, material thickness sensors, and laser micrometers, which improve accuracy and eliminate human errors.