Electric Discharge Machined Surfaces
During Electric Discharge Machining (EDM), small volumes of metal are removed from the surface by electric sparks, which cause rapid thermal evaporation, followed by fast cooling through heat transfer into the comparatively large, cold work piece.
In high-speed steels and tool steels, this process generates a ‘recast layer’, consisting of a glassy, brittle, untempered martensite layer located over a partially softened region.
In cemented carbides, EDM often results in a damaged surface layer, with altered chemistry and structure, and inferior mechanical characteristics.
Although the coating quality on EDM’d tools is unpredictable, their performance can be optimized by observing the following guidelines:
- Keep the recast layer thickness to a minimum by specifying low burn currents and/or low energies/pulse. For specifics, consult the manufacturer of the EDM machine used.
- The finishing burn should produce as smooth a surface as practical, and remove the heavily damaged para-surface layer generated by the roughing burn
- Subject high speed steel EDM’d tools to an additional draw/stress relieve heat treat cycle
- Remove surface contaminations; achieve a clean and shiny metallic appearence. Use IVAC’s Standard Surface Preparation Instructions as a guideline to obtain optimum finish
Call us for more detailed instructions or explanations about any aspect of these instructions.
It should be noted, that adherence to correct EDM technology and tool preparation will improve the EDM’d tool performance, whether or not it is PVD coated.