Surface finish is often called out on part prints. Sometimes it’s because it influences appearance, more often because it affects how surfaces slide over one another. Bearing surfaces and those used for sealing are examples of where finish is important.
An EDM surface finish is different to that produced by conventional machining. Cutting processes produce a finish with directionality. This is determined by the movement of the cutting tool. At the microscopic level even ground surfaces have evidence of this “lay”, and it affects how a product performs. Sliding friction is typically lower in the direction of “lay” than across it. “Lay” also affects the long-tern durability and performance of seals acting on the surface.
With EDM there is no “lay” and EDM finish quality can be mirror-like.
Erosion rather than cutting
EDM works by creating arcs of electricity between the tool and workpiece. (Picture microscopic lightning bolts.) As each arc touches the workpiece intense heat and pressure vaporizes material and blasts it from the surface, leaving behind a small pit. Each pit is only microns deep, but thousands are created every second. The result is progressive erosion of the workpiece.
The material removed is carried away by a flowing dielectric (insulating) fluid and filtered out elsewhere in the EDM machine. Increasing electrical power and frequency results in more powerful arcs that remove more material. Higher flushing pressure is needed to remove the material and the resulting pits are larger and the surface rougher.
EDM is unaffected by material hardness. This means a range of exceptionally hard and tough materials are machineable. The only requirement is that they be electrically conductive. Tool steel, tungsten carbide and Inconel are all good examples.
EDM finish quality
As with conventional metal cutting, surface roughness is inversely proportional to cutting power and speed. In other words, the key to achieving better EDM finish quality is to cut slower.
When a very high quality finish is called for, usual practice is to follow a high-speed roughing cut with one or more finishing – skimming, to use EDM terminology – cuts. As a guide, a roughing pass results in a finish of around 60 – 120 RMS (about 60-120 micro-inch Ra.) Following up with three skimming passes will bring it down to 15 RMS (approximately 12 micro-inch Ra.)
With more passes it’s possible to approach 5 RMS (4 micro-inches Ra,) although this depends in part on material density. More dense materials, like tungsten carbide, can be EDM’d to a very smooth finish. In many applications taking several passes avoids the expense of secondary operations like grinding.
An EDM surface finish is different to one produced by conventional material removal processes. Rather than exhibiting directionality, the texture is random, which is beneficial in many applications. Furthermore, by taking multiple skimming passes, EDM finish quality can become almost mirror-like.