EDM electrode materials need to have properties that easily allow charge and yet resist the erosion that the EDM process encourages and stimulates in the metals it machines. Alloys have properties which provide different advantages based on the needs of the application.
- Brass is an alloy of copper and zinc. Brass materials are used to form EDM wire and small tubular electrodes. Brass does not resist wear as well as copper or tungsten, but is much easier to machine and can be die-cast or extruded for specialized applications. EDM wire does not need to provide wear or arc erosion resistance since new wire is fed continuously during the EDM wiring cutting process.
- Copper and copper alloys have better EDM wear resistance than brass, but are more difficult to machine than either brass or graphite. It is also more expensive than graphite. Copper is, however, a common base material because it is highly conductive and strong. It is useful in the EDM machining of tungsten carbide, or in applications requiring a fine finish.
- Copper tungsten materials are composites of tungsten and copper. They are produced using powder metallurgy processes. Copper tungsten is very expensive compared to other electrode materials, but is useful for making deep slots under poor flushing conditions and in the EDM machining of tungsten carbide. Copper tungsten materials are also used in resistance welding electrodes and some circuit breaker applications.
- Graphite provides a cleaning action at low speeds. Carbon graphite was one of the first brush material grades developed and is found in many older motors and generators. It has an amorphous structure.
- Molybdenum is used for making EDM wire. It is the wire of choice for small slot work and for applications requiring exceptionally small corner radii. Molybdenum exhibits high tensile strength and good conductivity, making it ideal where small diameter wire is needed for demanding applications.
- Silver tungsten material is tungsten carbide particles dispersed in a matrix of silver. Silver offers high electrical conductivity and tungsten provides excellent erosion resistance and good anti-welding characteristics in high-power applications. This composite is thus the perfect choice for EDM electrode applications where maximizing conductivity is crucial.
- Tellurium copper is useful in EDM machining applications requiring a fine finish. Tellurium copper has a machinability that is similar to brass and better than pure copper.
EDM has grown up. EDM has taken its place as a proven, precision technology, chosen for what it can do, rather than what conventional machining can’t do. EDM machine technology has spawned a world of new applications wherein increased importance is placed on the graphite electrode material utilized.
While there are many methods used to determine the right material for a job, we believe there are five factors that mean the difference between success and failure, profit and loss.
Metal Removal Rate (MRR)
Metal removal rate is usually expressed as cubic millimeters per hour (mm3/hr) or cubic inches per hour (in3/hr), but in fact could just as realistically be expressed as €/hr. Achieving an efficient MRR is not simply a matter of the right machine settings. It also involves direct energy dissipated in the EDM process. Graphite is generally much more efficient than metallic electrodes, however metal removal rates vary widely between graphite types. With the proper electrode material/ work metal/application combination MRR can be maximized.
Wear Resistance (WR)
There are four types of wear: volumetric, corner, end, and side. Of the four, we believe that corner wear is the most important since the contours of the final cut are determined by the electrode’s ability to resist the erosion of its corners and edges. It follows that if an electrode can successfully resist erosion at its most vulnerable points, then overall wear will be minimized, and maximum electrode life achieved. Electrode erosion cannot be prevented, but it can be minimized by choosing the proper electrode material/work metal combination and machining at the optimum settings.
The ability of an electrode to produce and maintain detail is directly related to its resistance to wear and its machinability. Minimizing corner wear requires choosing an electrode material that combines high strength with high temperature resistance.
Surface Finish (SF)
Fine surface finish is obtained by a combination of the proper electrode material, good flushing conditions, and the proper power supply settings. High frequency, low power and orbiting produce the best finish, as these conditions produce smaller, less defined craters in the work metal. The final surface finish will be a mirror image of the electrode’s surface, so Angstrofine and Ultrafine particle, high strength graphites are the best choices for finishing electrodes.
Any machinist who has ever machined graphite is aware that graphite cuts very easily. Simply being easy to machine doesn’t necessarily make a material the best choice for an electrode. It must also be strong to resist damage from handling and from the EDM process itself. Strength and small particle size are important so that minimum radii and close tolerances may be achieved. Material hardness is also a factor in graphite machinability, as the harder electrode materials will be more prone to chipping during the machining process.
Electrode material cost generally represents only a small part of the total EDM job cost. What is too often overlooked, however, is that electrode material cost considered outside the total job cost is completely meaningless.
Fabrication time, cutting time, labor, electrode wear - all these factors depend on the electrode material more than on any other factor. Thus it is critical that you know the properties and performance characteristics of the available electrode materials as they affect the work metals you are machining. Only with this data is it possible to make a cost/performance analysis to determine the true cost of an EDM job.
Graphite is the material of choice for the majority of EDM electrodes produced today in the Western World. Selecting the best grade of graphite for a particular application can be difficult if the differences between graphite grades are not understood. Depending on the graphite grade selected and the application, the graphite can be the limiting or the key factor in achieving the desired results from the equipment.
There is a wide range of grades from a number of manufacturers to choose from. Each manufacturer uses different processing techniques, source materials and process controls, which mean the end product, will be quite different. Each manufacturer's grades are designed for optimum performance for specific types of applications. To aid in material selection, each manufacturer publishes technical specifications on their material, but there are no standard test methods.
Since any grade of graphite looks similar to another grade, appearance is not part of the selection criteria. Each grade should be selected by its physical characteristics and properties. To make this process easier, various grades of graphite are grouped into six classifications that are segregated by average particle size. Only four of the six classifications are suitable for use as EDM electrodes. How the different grades rank within the classifications is an indicator of their performance potential.
Advances in Graphite
The graphite industry is constantly striving to produce higher quality grades. Graphite materials have continued to evolve along with the other aspects of the EDM industry, but at a less dramatic pace. Advancements have been made in the microstructure of the graphite materials, as this is the key to performance.
Coarse graphites with particle sizes over 100 microns have never been suitable as an electrode material. During the last decade, medium grades with a particles size between 21-100 microns have all but disappeared from the market as an EDM material. In the last few years many of the low-end grades within the fine classification (11-20 micron particle size materials) also have disappeared. The superfine classification (6-10 particle size) materials have remained stable. Some of the manufacturers of these materials allow their graphite grades to be sold as house brands, which can be confusing to the end user. Confusion occurs when consumable distributors change the names of their house brands, but still use the same material or change the material, but keep the same name for the house brand. The same grade may be offered under many house brand names.
The ultrafine classification (1-5 micron particle size) is where most of the real development efforts are targeted. Many of the plastic consumer products require molds with fine detail and finishes that can easily be achieved with ultrafine materials. Materials in this classification are very difficult and expensive to make and it is even more difficult to produce consistent material batch after batch and year after year.
There are very few grades in the angstrofine classification (< 1 micron particle size). The grades are available in small blocks to control the uniformity of the graphite. These grades are the most expensive to produce and have limited use. Generally, they are used for fine detailed engraving electrodes and small featured electrodes that produce very high surface finishes without the use of powder additives when polishing of cavities is not possible.
The lower end graphite grades are slowly disappearing from the market as EDM applications change. Large cavity mold work without fine detail and forging dies can easily be done by high-speed milling thus reducing the need for grades in the fine classification. At the same time intricate detailed cavity work requires graphite with small particle sizes, uniform microstructure and high strengths to produce complex, small-featured cavities.
Grades within Classifications
The physical properties of each grade of graphite determine the ranking within classifications. The properties that influence performance are particle size, flexural strength and shore hardness. These properties along with a photomicrograph of the microstructure are the best tools for predicting graphite performance.
The best graphite in any classification has tightly packed particles with little variation in size. This kind of uniform material resists wear caused by the thermal nature of the EDM process. Particle size is generally stated as an average size. When particle size spans a small range, the microstructure of the material becomes more uniform with reduced porosity. The porosity in the graphite is boundary between particles. The particles are bound together by chemical or mechanical means and the failure of this system is what releases particles into the gap when EDMing. If the material's particles are small, uniform in size and tightly packed, erosion of the electrode is minimal. Particle size has a bearing on the minimum surface finish that the material will produce. Since the electrode reproduces its structure in the cavity, fine surface finishes cannot be obtained with graphite grades that have large particle and non-uniform microstructure.
The microstructure of the graphite grade is often the limiting factor determining EDM performance. A non-uniform micro-structure with a wide range of particle and pore sizes can have soft spots that are large areas of porosity and/or hard spots which are conglomerates caused by inconsistent blending. Hard spots also can be caused by impregnating the open porosity of the material with pitch and then reprocessing the material giving the particles and pitch impregnated areas different hardness values. Since the unaided eye cannot see the microstructure, there is no way to detect these problems prior to the machining process. Identification of the cause of machining problems involves destructive testing and the examination of photomicrographs.