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Manufacturing Processes>Non-Traditional Machining > Electrical Discharge Machining

 

Electrical Discharge Machining(EDM)

 

Process description

The tool, usually graphite, and the workpiece are essentially electrodes, the tool being the negative of the cavity to be produced. The workpiece is vaporized by spark discharges created by a power supply. The gap between the workpiece and tool is kept constant and a dielectric fluid is used to cool the vaporized ‘chips’ and then flush them away from the workpiece surface.

Materials

  • Any electrically conductive material irrespective of material hardness, commonly, tool steels, carbides, Polycrystalline Diamond (PCD) and ceramics, but not cast iron.
  • Melting point and latent heat of melting are important properties, partially determining the material removal rate.
EDM

Process variations

  • Traveling wire EDM: wire moves slowly along the prescribed path on the workpiece and cuts the metal with sparks creating a slot of ‘kerf’. CNC control is common.
  • No-wear EDM: minimizing tool wear of steels by reversing the polarity and using copper tools.
  • Electrical Discharge Grinding (EDG): graphite or brass grinding wheel rotates relative to the rotating workpiece and removes material by spark erosion (no abrasives involved).
  • Ultrasonic EDM: increases production rate and gives less surface damage.

Economic considerations

  • Production rates very low. . Material removal rates up to 1.6mm³/min.
  • Cutting rate for traveling wire EDM approximately 0.635 mm/s.
  • Material removal/cutting rates a function of the current rate and material properties.
  • Lead time days to several weeks depending on complexity of electrode tool.
  • Tools can be of segmented construction for high complexity work.
  • Material utilization very poor. Scrap material cannot be recycled.
  • Disposal of sludge and chemicals used can be costly.
  • High degree of automation possible.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs high. High tool wear rates mean period changing.
  • Equipment costs generally high.
  • Direct labor costs low to moderate.

Typical applications

  • Tool and die blocks for forging, extrusion, casting, punching, blanking, etc.
  • Honeycomb structures and irregular shapes
  • Prototype parts
  • Burr free parts

Design aspects

  • High degree of shape complexity possible, limited only by ability to produce tool shape.
  • Traveling wire EDM limited to 2-dimensional profiles.
  • Suitable for small diameter, deep holes with length to diameter ratios up to 20:1. Can be up to 100:1 for special applications.
  • Undercuts possible with specialized tooling.
  • No mechanical forces used for cutting, therefore simple fixtures can be used.
  • Possible to machine thin and delicate sections due to minimal machining forces.
  • Minimum radius =0.025 mm.
  • Minimum hole/slot size =0.05mm.
  • Traveling wire EDM can cut sections up to 150 mm.

Quality issues

  • Burr free part production.
  • Produces slightly tapered holes, especially if blind, and some overcut.
  • Optimum tool to workpiece gap ranges from 0.012 to 0.51mm.
  • Surface layer is altered metallurgically and chemically due to high thermal energies.
  • A hard skin, or recast layer, produced may offer longer life, lower friction and lubricant retention for dies, but can be removed if undesirable.
  • Beneath the recast layer is a heat affected zone which may be softer than the parent material.
  • Finishing cuts made at lower removal rates.
  • Tool wear related to the melting points of the materials involved, and this affects accuracy. May require changing periodically.
  • Being a thermal process, residual stresses and fine cracks may form.
  • Removal rate can be increased with the expense of a poorer surface finish.
  • Surface detail good.
  • Surface roughness values ranging 0.4–25 µm Ra. Dependent on current density, material being machined and rate of removal.
  • Achievable tolerances ranging ±0.01–±0.125 mm. (Process capability charts have not been included. Capability is not primarily driven by characteristic dimension but by the material being processed.)

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