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Comparing EDM with conventional machining

Manufacturing engineers turn to EDM when milling, turning and grinding aren’t up to the task. It’s as if it’s a binary choice: if machining isn’t possible then chose EDM.

The reality is more complex. There’s a significant middle ground where EDM and conventional processes go head-to-head. Yes, the wire EDM cutting rate is lower than that milling or turning, but the process scores in other ways. Finished part geometry, workpiece hardness, surface finish and cost are all areas where conventional and EDM compete. Here’s a look at the advantages and limitations of EDM.

Advantages of EDM – part geometry

With EDM there are no cutting forces and neither tool nor workpiece rotate. In addition, wire EDM is done with wire typically 0.010” diameter. This all means:

  • Thin walls are possible.
  • Cut internal corners with very small radii.
  • Cut thin slots in extrusion dies with wire EDM.
  • Produce blind and high aspect ratio pockets with sinking EDM.
  • Produce non-round openings and cavities.
  • An alternative to broaching for 2D shapes like gears.

Advantages of EDM – hard materials

Material hardness has little effect on EDM wire cutting rate and doesn’t limit possible part geometries. Tungsten carbide, tool steel, Inconel and any metal harder than RC38 are all good candidates where conventional machining is slow and expensive.

A particular benefit is the ability to bring parts to their final size after heat-treatment. This takes out distortion brought about by hardening and stress-relieving.

Advantages of EDM – surface finish

Unlike conventional processes, an EDM surface has a very random texture. This is advantageous when directionality or “lay” could influence product performance. Additionally, EDM can create surface finishes as smooth as 5 RMS or around 4 micro-inches Ra. (This entails taking multiple skimming passes, for which the wire EDM cutting rate needs to be very low.)

Cost advantages of EDM

On the right job – complex 2D geometry, hard material – EDM offers cost savings over conventional processes. For example:

  • No special tooling (saves money and lead time.)
  • Skimming passes eliminate secondary finishing operations.
  • Laminate layers of material to cut multiple parts simultaneously.
  • Enables reuse of cut out slugs rather than producing chips.
  • Avoids moving a part through multiple conventional processes. The final shape is achieved in a single set up (which also improves accuracy.)

Limitations of EDM

EDM requires an electrically conductive workpiece. It will not work on wood, plastics or composites.

Material removal rates (MMR) are usually lower than for conventional processes. However, direct comparisons are difficult to make since material hardness is such a big factor. MMR is usually calculated in cubic inches per hour with 0.4 in3/hr a typical rate. Calculated this way, a narrow kerf makes the wire EDM cutting rate quite high. Also, workpiece thermal conductivity and melting point have significant impacts on EDM MMR.

Wire EDM can only cut 2D profiles, although a fourth axis to incline the wire enables conical forms and tapers.

An alternative to consider

EDM is more than just a fall-back for when conventional machining isn’t up to the task. It’s a process to consider whenever the workpiece is hard, electrically conductive, and has specific geometric challenges or surface finish requirements.