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EDM

Electro-Discharge Machining 

EDM – Electro-Discharge Machining – is a method of machining metals. It is sometimes called ‘spark erosion’ because the metal is removed by electrical discharge between an electrode and the work-piece. Hence EDM only works with materials that are electrically conductive. The removed material is taken away by a continuously flowing fluid. EDM is used for hard metals and is commonly used for cutting intricate contours or fine cavities which would be difficult to produce by other cutting tools. In NDT it is used to produce ‘EDM notches’, otherwise referred to as ‘spark eroded slots’. These notches are placed in testpieces so that the detection and sizing capability of NDT procedures can be assessed. In ultrasonics the EDM notch represents a smooth planar defect. The Figure shows a test-piece containing three EDM notches of various depths.

The advantages of using EDM notches as target defects are:
  • the position and dimensions of the EDM notch are known;
  • the insertion of the EDM notches is simpler, more cost efficient and less time consuming than manufacturing defects by fatigue or corrosion mechanisms;
  • the material surrounding the defect is left in the original state.
However, differences between EDM notches and the real defects may mean that the signals obtained from EDM notches differ significantly from those obtained from real cracks of the same size. For instance, the tip of an EDM notch may be wider than the tip of the real crack and the gape of a notch is usually much greater than the gape of real defects. Any such diff erences need to be considered when assessing the capability of an NDT procedure.

For more information on the design of test-pieces see:
ENIQ Recommended Practice 5: Guidelines For The Design Of Test-Pieces And Conduct Of Test-Piece Trials Issue 1 which can be obtained from:
http://capture.jrc.ec.europa.eu/capture/sites/capture/files/files/documents/eur18686en.pdf

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org
 
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