Surface Finish Achievable with EDM

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 (aka electrical discharge machining) 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 – how it all works

Electrical discharge machining 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 finish benefits and requirements

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 required, 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.

Unique attributes

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.

Why choose EDM for finishing?

There are several reasons why EDM works well for finishing. Here are a few:

  • More effective cutting capabilities
  • Capable of small radius cuts
  • No direct contact
  • Ability to cut many different types of conductive metals
  • Facility to cut delicate, intricate parts
  • Prototyping up to high volume machining
Manufacturing Locations Across National Time Zones
Manufacturing Locations Across National Time Zones
The Fathom Advanced Manufacturing Platform
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1050 Walnut Ridge Drive
Hartland, WI 53029
ISO 9001:2015


444 W. 21st St. Ste. 101
Tempe, AZ 85282
ISO 9001:2015
NIST800-171 Compliant


14000 N.W. 58th Court
Miami Lakes, FL 33014
ISO 9001:2015 Design Certified
ISO 13485:2016



620 3rd Street
Oakland, CA 94607
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1207 Adams Drive
McHenry, IL 60051

1401 Brummel Ave
Elk Grove, IL 60007
ISO 9001:2015 Design Certified


401 W. Shore Blvd.
Newark, NY 14513
ISO 9001:2015

1920 Slaterville Rd
Ithaca, NY 14850


1801 Rowe Lane
Pflugerville, TX 78660
ISO 9001:2015

1513 Sam Bass Rd
Round Rock, TX 78681
ISO 9001:2015
ISO 13485:2016


7770 Washington St.
Denver, CO 80229
ISO 9001:2015


13758 Johnson Street NE
Ham Lake, MN 55304