Continuing our series of published extracts from our EDM machining guide, ‘The EDM Handbook’, authored by David Light (managing director of precision machining company, Di-Spark Ltd) this section looks at Electrode Selection (Spark Erosion).

Di-Spark: The Spark Erosion Company

Di-Spark Ltd in terms of an EDM company supply precision EDM machining (spark & wire) alongside advanced traditional machining such as large 5 axis machining, multi-axis machining & multi-task mill-turn 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, making investments into spark erosion EDM machining technology, among other commitments to innovation.

Di-Spark are an EDM company supplying spark erosion to high-tech industries such as Formula 1, Oil & Gas, Medical, Aerospace, Satellite & Energy Markets

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. ELECTRODE SELECTION (SPARK EROSION)

Electrode material, electrode design and electrode manufacture are three of the many factors to consider in spark EDM. While users of wire erosion machines do have a selection process to consider, design and manufacture of the electrode are not involved as the wire is bought-out and is wound on spools and designed to be expendable. The spark EROSION operator faces quite the opposite challenge. He must select the proper electrode material, then design around flushing and finally ensure that the electrode can be economically manufactured. Then, when sparking the operator must strive to protect the electrode from excessive wear, while maintaining an efficient metal removal rate.

The three most popular types of electrode materials are brass, copper and graphite. Each material has its own unique properties and will fit certain applications better than others.

When EDM first became a reality almost fifty years ago, it was a very crude process used to remove broken drills, taps, bolts, etc., from work-pieces. Two Russian scientists improved upon this with the addition of a servo system, and the first EDM machine came about. Initially, brass was the electrode material. It was readily available and inexpensive but gave very high rates of wear. The next development led to the use of copper and copper alloys. They offered higher conductivity and much improved metal removal to wear ratios over brass. As improvements in equipment made EDM a more advanced machining technique the physical limitations of copper became evident. The element copper melts at approximately 1,083 degrees Centigrade, while temperatures in the spark gap may exceed 10,000 degrees Centigrade. With spark erosion, EDM spark temperatures many times higher than copper’s melting point, certain applications using copper electrodes suffer from unacceptable electrode wear.

With the advent of graphite as an electrode material, superior metal removal rates have been attained with far less electrode wear. Both of these advantages have been realised through the ability of graphite to resist thermal damage while discharge machining. Graphite is considered a metalloid as it does not melt like most metals, but undergoes a process called sublimation. That is, it goes directly from a solid to a gas, bypassing the liquid state entirely, this occurs at approximately 3,500 degrees Centigrade. This superior resistance to thermal damage is the main reason graphite resists wear better than copper.

In addition to providing superior wear resistance, ease of manufacture is another benefit of graphite. It is far easier to machine than copper, with no resulting burrs to remove. Graphite electrodes can be hand worked easily if blend radii need to be manually dressed.

If difficulty in machining and burrs are the negative aspects of copper, then the clouds of dust (and the wrath of fellow workers) generated by machining is the negative aspect of graphite. An efficient vacuum system to control this problem needs to be a major consideration. While this dust is not actually toxic, it can cause respiratory problems and allergic reactions, especially the graphite’s infiltrated with copper. In general, machining graphite is a housekeeping nuisance, for both personnel and equipment.

The graphite used for the spark erosion EDM electrode materials is synthetic and very abrasive. The graphite material does not cut in the conventional sense. It does not shear and flow across the tool edge like metals do. It fractures, or crushes, under the cutting tool pressures, and floats away as a fine powder or dust. Under a microscope, these particles appear crystalline in structure and are very sharp. When these particles mix with the film of oil on machine ways, it forms an abrasive slurry that will cause premature wear and decrease the machine tool’s life and accuracy.

Another criterion for spark erosion electrode selection is the raw material costs. A premium grade of graphite is, on average, three times more expensive than copper. Logic would indicate purchase of the less expensive material. However, this must be balanced against the complexity or degree of difficulty in manufacturing and/or redressing the electrode and the subsequent metal removal rate that it will offer. Despite initial raw material cost savings, if the electrode shape is complex, then manufacturing costs can quickly offset any savings realised by purchasing cheaper materials. Therefore electrode selection is seldom simple. All aspects of the job from an EDM perspective must be examined before electrode material selection can be made.

There are many metal alloys and infiltrated materials available that can be used as electrodes for the spark erosion EDM process.

7.1 METAL ELECTRODES

1. Brass. The first metal electrodes were made of brass. Due to its high rate of wear, brass is no longer a viable electrode material for spark erosion use (with the exception of small-diameter tubing for EDM hole drilling).

2. Tellurium Copper. The most commonly used metallic electrode material. The element tellurium has been added (0.5 to 1%) for ease of machining. It has acceptable metal removable rates and reasonable wear. Sometimes called “Telco”.

3. Copper Tungsten. Used when the safety of copper is desired, but it provides much better wear resistance. Also used in the EDM machining of carbides and refractory metals (cobalt, molybdenum, etc.). When mixed and sintered with the element tungsten, copper shows a marked increase in resistance to wear. This is due to the high melting point of tungsten. A tungsten/copper ratio of 70/30 is most commonly stocked by suppliers.

4. Silver Tungsten. Used where the high conductivity of silver is desirable As the name suggests, this material is also very expensive, about five times the cost of copper tungsten. It is obviously only for very specific applications.

5. Tungsten. While not a common selection, tungsten is used when wear and/or strength is a consideration, not speed. Tungsten will EDM very slowly when compared with other metals. It is very expensive compared with other alloys, and is limited in the sizes available. Due to the difficulty in machining tungsten, electrodes are usually supplied pre-formed. Its most common application would be small hole sparking operations.

7.1.2 EVALUATION OF METAL ELECTRODES

Advantages: Low cost, high strength, suitable for most spark erosion machines, good for entry-level or inexperienced operators, spark eroded mirror finishes can be produced, wire erodes easily, few housekeeping problems.

Disadvantages: difficult to grind, burrs produced, slower EDM metal removal rates, higher wear.

7.2 GRAPHITES

1. Straight Graphite. These are commercially available in many grades. The density and the grain size will be a key factor in its performance and cost. If the application does not require a good finish or fine detail, then satisfactory results can be obtained with cheaper graphite. If these are the primary considerations then a material with small grain size and high density should be the choice. As a general rule in choosing graphite, for improved wear resistance and surface finish – then the finer the grade of graphite required.

2. Infiltrated Graphite. A mixture of superfine copper particles in a graphite matrix can used in specific applications where good machinability of the electrode is desired (a complex shape), but with the forgiveness of copper in marginal cutting conditions (deep slots, poor flushing). The greater the percentage of copper, the lower the chance of arcing, due to its higher conductivity. However a proportionately greater degree of wear will be incurred, and the metal removal rate will also decrease slightly. Due to its expense, in comparison with straight grades, careful consideration must be given in its selection.

7.3 EVALUATION OF GRAPHITE ELECTRODES

Advantages: High strength, good machinability, high metal removal rates, and excellent wear resistance.

Disadvantages: High cost, lower degree of safety in difficult machining conditions, dust from machining, wire erodes more slowly than metals.

Of course, every application is different and must be individually evaluated. Details to be considered are: How much material must be removed, and what is the surface finish required? What is the electrode raw material costs? How Many electrodes will be needed, and how difficult will it be to construct them? What about redressing?

7.4 THE ELECTRODE SELECTION PROCESS

The electrode material selection process should start with the consideration of five important factors:

1. Metal Removal Rate. This is the amount of material removed in a given period of time; most commonly measured in cubic millimetres per minute.

2. Resistance to Wear. This is one of the most important aspects of electrode material selection. There are four types of wear to consider: volumetric, corner, end and side wear, with corner wear usually being the greatest concern.

3. Surface Finish. The surface finish required can sometimes dictate the selection of the electrode material. For a fine finish using graphite, the electrode material should be dense. Mirror finishes are usually obtained with multiple metal electrodes or graphite electrodes and motion (orbiting or rotation).

4. Manufacturing Costs. This depends upon the complexity of the electrode. The more complex the electrode is, the higher the manufacturing costs will be

5. Raw Material Costs. This is usually only a small part of the total EDM time/cost evaluation. Unless it is an unusually large or high volume application, all four of the above points need to considered first before this becomes a major factor.

The next step is to consider some basic facts concerning the nature or properties of the work-piece and the electrode materials.

The approximate melting temperature of the work-piece material should be known. With all of the different alloys in use today, this can prove a difficult task, although material suppliers are a good source of information. Knowledge of the approximate melting temperature alone will usually be enough to start with, but it is also useful to know the material’s specific gravity or, in layman’s terms, its density. Knowing the specific gravity will give a good idea of how many molecules will have to be removed, and if they are big or small in molecular terms.

7.5 MELTING TEMPERATURES

Knowledge of the melting point of the exact material is not necessary; often knowledge of the range of a similar group is sufficient. For example, most ferrous metals have similar melting points, as do most austenitic materials. While there are exceptions to every rule (especially in EDM/Spark Erosion), for the most part any differences in melting temperatures between similar alloys would be too small to affect the selection process.

Often, only the melting point of the work-piece and the working temperature of the electrode material need to be known. If the alloy has a low melting point – for instance, aluminium – the first choice of electrode material would be copper. Its melting temperature is 1,082 degrees Centigrade while that of aluminium is 660 degrees Centigrade. Graphite could be used but it has a sublimation temperature that is approximately 3,500 degrees centigrade. While graphite does prove satisfactory in eroding aluminium it is almost overkill to have such a wide difference in working temperatures.

The chart below is a guideline for some common work-piece materials and some of their characteristics.

(Conductivity values based on Silver = 100.00)

Material Specific Degrees Degrees Conductivity

Gravity Centigrade Fahrenheit

Aluminium 2.70 660 1220 63.00

Cobalt 8.71 1480 2696 16.93

Copper 8.89 1082 1980 97.61

Manganese 7.30 1260 2300 15.75

Molybdenum 10.20 2625 4757 17.60

Nickel 8.80 1455 2651 12.89

Carbon Steel – 1371 2500 12.00

Titanium 4.50 1820 3308 12.73

Tungsten 18.85 3370 6098 14.00

Based upon the melting temperatures alone, the best results will usually be obtained by matching the electrode’s melting temperature to that of the work-piece. Low-temperature alloys such as aluminium, brass, and copper should be EDM machined with low-temperature metal electrodes. Higher-temperature alloys such as carbon and austenitic steels will warrant the use of graphite, which has a much higher melting temperature.

A general rule of thumb is:

Low temperature alloys = metal electrodes

High temperature alloys = graphite electrodes

7.6 Spark Erosion: ELECTRODE WEAR

When predicting electrode wear, several combined parameters will determine the surface finish of the work-piece and consequently the amount of electrode wear. Primarily, wear is a function of the ability of the electrode material to resist thermal damage. The electrode’s density, polarity, and the frequencies used will also be a major part of the wear equation.

Graphites have a much higher resistance to heat and wear at lower frequencies, but will wear significantly more during high frequency and/or negative polarity applications. The use of high frequencies with graphite is usually reserved for finishing operations or when the electrode is expendable. This might be generating clearance holes quickly or for roughing simple shapes when the electrode can be easily manufactured or redressed.

7.7 SURFACE FINISH

Without delving too deeply again into EDM theory or the phenomena involved during EDM operations, suffice is to say that all finishing operations are done using reduced power settings in conjunction with high frequencies. These parameters will always produce more wear to the electrode, regardless of the electrode material. The degree of wear will depend upon the type of grade of material and the finish desired.

Wear can be incurred with every single spark that leaves the electrode. Low frequencies provide fewer sparks in a given unit of time – hence low or no wear. High frequencies will allow many individual pulses within a given unit of time. The result is small sparks producing small craters and better surface finish with more electrode wear.

Graphite, while being far more resistant to damage from heat, is much less dense than copper. It is made from very fine (depending upon the grade) carbon powder and bonding agents and is sintered in a furnace. Even with today’s advanced methods of refinement the structure of graphite will always contain air and so they will always wear more than metal electrodes during finishing operations.

Copper has a much finer grain structure, since it and its alloys are metals. Compared to the atomic structure of copper (copper is an element), graphite can never approach the same density. Although copper is more susceptible to thermal damage than graphite, there is very little heat generated in high-frequency operations (finishing); therefore, copper’s density will yield less wear in high-frequency applications than will graphite electrodes.

When finishing cavities on manual spark erosion machines, electrode wear for both copper and graphite will be high, but fine finishes are obtained more readily when using copper electrodes. This also is due to the metal’s higher density. The coarser the grain, the coarser the surface finish.

In CNC EDM applications, the wear for both copper and graphite electrodes will decrease significantly. Finishes from graphite electrodes are comparable to metal electrodes due to the movement of the electrode during machining. Movement of the electrode will average out the size and depth of the electrodes grain size and thus the depth of the craters.

One final consideration concerning finishing operations is D.C. arcing. The increased chance of D.C. arcing during finishing is due primarily to the exceedingly small spark gaps encountered in such operations. Since the grain structure of graphite is much larger in comparison to copper, the particles dislodged from graphite electrodes have less room to be flushed from the gap. Since electricity will take the path of least resistance, the next spark will be a continuous discharge of current, and almost instantaneously create a false cut in the work-piece.