Di-Spark, The EDM Company: EDM Finish & Surface Integrity

Continuing our series of published extracts from the electrical discharge machining guide, ‘The EDM Handbook’, authored by David Light (managing director of renowned EDM company, Di-Spark Ltd) this section looks at the EDM finish and surface Integrity.

Di-Spark: The EDM Company

Di-Spark Ltd in terms of an EDM company supply both clean cut wire erosion EDM and spark erosion EDM – alongside advanced traditional machining such as large 5 axis machining. Being one of the first EDM companies in the country during the 1970’s, Di-Spark continue to innovate and inspire advanced precision machining to this day, with recent investments into clean cut wire erosion EDM.

Di-Spark are an EDM company supplying high-tech industries such as Formula 1

EDM Company: Blog posts and videos

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EDM Guidebook, by Di-Spark, The EDM Company

6. THE EDM FINISH AND SURFACE INTEGRITY

One of the most mis-understood aspects of an often mis-understood manufacturing process is the EDM finish. What is it? How is it different from conventionally machined finishes? How is it measured? What is recast? What is its effect on surface integrity?

During EDM each individual spark vaporises, then melts, a small amount of material and leaves a crater in the work-piece. This is consistent in both wire or spark EDM applications. As previously stated, high power and long spark duration (low frequencies) provide high metal removal rates and rough surfaces and conversely the combination of low power and short spark duration (high frequencies) results in finer finishes.

6.1 EDM SURFACE FINISH

There are several ways one can measure the finish of an EDM machined part. The simplest method is to use a set of surface finish blocks. These can be used to compare finishes both visually and by dragging a fingernail across the test piece and comparing the feel against the actual part. This is not a precise method and does not accurately measure the surface finish of the part.

A surface tester is a device that is used to measure the surface of a machined part. It is a very precise electronic instrument that utilises a diamond stylus that is traversed across the surface. The rise and fall of the stylus caused by surface roughness, waviness and other irregularities are amplified into electrical signals and subsequently converted into a surface finish measurement.

Another type of surface finish measurement device is the non-contact type using a laser beam. This device would be used on surfaces that could either be damaged by the diamond stylus or prove damaging to the stylus itself. A sharply focused beam of laser light, about 0.1 micron in diameter, is directed onto the work-piece. Surface finish is calculated by measuring the amount of movement made by the measuring arm while it maintains the correct focus and diameter on the laser beam.

6.2 EDM SURFACE FINISH MEASURING SCALES

There are many scales for defining surface finish, the most common are:-

Ra = Roughness Average. The average roughness Ra is the arithmetic average of all departures of the roughness profile from the centre line of the evaluation length. It is also known as the arithmetic average (AA) and the centreline average (CLA).

Rmax = Roughness Depth. The mean roughness depth Rmax is the largest of five roughness depths (peak to valley height) from five successive sample lengths of the roughness profile.

RMS = Root Square Mean. The perfect surface that would be formed if all the roughness peaks were cut off and used in filling the valleys below this surface.

After deciding upon an acceptable finishing scale there is often some confusion concerning the measurement of an EDM finish. Part of this confusion is caused by the lay of the surface finish. Lay is technically defined as the direction of the predominate surface pattern. In plain talk, conventional cutting tools leave directional tool marks depending on whether the material has been milled, turned, or ground.

The conventional machining processes of milling, turning and grinding all leave directional cutter marks. EDM does not physically produce chips, but removes materials electrically by random spark erosion. The surface finish produced by EDM is in the form of small, hemispherical craters with no distinct pattern or lay to influence measurement.

Measurement of a surface finish is typically made by tracing across the lay or across its worst direction. Since EDM finish has no lay, it can be measured from any direction or angle.

With everything concerning the EDM finish being different when compared to conventionally machined finishes, it also becomes important to examine the relationship of EDM to the surface integrity of the work-piece.

6.3 EDM SURFACE INTEGRITY

After the surface finish scale has been defined the next issue to contend with is something called the recast layer or white layer. In EDM, regardless of finish, the smoothest, shiniest finish does not mean a thing if, during roughing operations, the surface integrity of the material below the finished surface has been compromised.

The recast layer depending upon the dielectric (water or oil), can show a metallurgical difference from the parent material.

6.4 THE RECAST LAYER

One drawback of the EDM process is something called the recast layer or white layer. Being a thermal process the recast layer is an inherent by-product of EDM. Although new generator technology allows recast to be controlled to a very fine degree, the thermal nature of the EDM process makes it impossible to eliminate it completely.

In order to explain how the recast layer is formed it is important to understand the details of the metal removal process in EDM. Although the specifics can get very complicated the overall principles are relatively simple. Consider wire EDM where the wire electrode is brought into close proximity to a work-piece (without touching) and a voltage is applied between the two. The work-piece and the wire electrode are separated by a dielectric fluid and a spark will jump between them. The thermal energy of the spark will melt a small volume of both the wire and the work-piece and the dielectric fluid will flush the resultant debris away. This process, spark – melt – flush – spark –melt – flush, is repeated thousands of times per second. After a spark has removed some material and been switched off, not all of it will be flushed away and a minute quantity of the molten material will be drawn back onto the work-piece by surface tension and cooling effects. This molten material will solidify back onto the cooler surface. This layer tends to be highly carbonaceous and is called recast layer or the white layer. Just below the recast layer is the area called the heat affected zone (or HAZ). This area is only partially affected by the high temperatures. The thickness of the recast layer and the HAZ immediately below it depends upon the current and frequencies used during machining, and the heat-sinking ability of the material itself. Recast can affect the structural and/or surface integrity of the work-piece. There is a lack of modern research to substantiate the metal fatigue characteristics of a part produced by EDM compared to the same part produced by alternative machining methods.

In some cases, removal of the recast layer and/or stress relieving of the part is sometimes preferred. The recast layer was a major concern of the aerospace industry for many years, and only recently, with the improvements in modern power supplies, has acceptance of EDM machined parts become more widespread.

In order to increase the scope of applications of the process the EDM industry must address the issue of the recast layer and prove one way or another whether the process is any more detrimental to the performance of a component than other processes.

There are distinct differences between the recast layer left by wire EDM and the recast left by spark EDM. This is attributed to the different dielectric fluids used, which are usually oil for spark erosion machines and de-ionised water for wire erosion machines.

Dielectric oils change the parent material by producing an uncontrolled heat-treating process – heating the metal to a very high temperature and then quenching it in oil. The high heat breaks down the oil into hydrocarbons, tars, and resins. The molten metal draws the carbon atoms from the oil and they become trapped within the recast layer, creating a carburised surface. While this is a far cry from the carefully produced carburised surfaces obtained during professional heat-treating operations, it is nevertheless hard. Even when EDM machining hardened materials, the recast surface produced in oil will generally be several points higher in hardness than the parent material.

The recast layer produced in water dielectrics is different from oil because oxides produced by the vaporising water, along with electrolysis, can deplete carbon from the material’s surface. In addition, copper atoms released from the brass wire can be assimilated into the exposed surface of the work-piece, further contributing to an alloying process and softening the parent material. In ferrous materials, wire-cut surfaces can be several points lower in hardness than the parent material.

Carbides can be machined by EDM, but improper power settings will deplete the cobalt that holds the tungsten particles in place, resulting in flaking or cracking. Fortunately, most modern power supplies are capable of reducing this effect by controlling the thickness of the recast layer through constant monitoring and control of the spark conditions during high-frequency machining. Another reason for the improvements in the surface integrity of EDM carbides, especially in newer wire erosion machines, is the absence of capacitors in the machines power supply.

Do you  require a world-class EDM company? Contact Di-Spark today.