EDM-ing thin ribs is a difficult application. High-speed machining of the electrodes has made it possible to fabricate these electrodes from grades that are not capable of maintaining the detail in the cavity. The ideal graphite grade for thin ribs should have a flexural strength above 700 kgf/cm². Materials that have these properties are normally found in the ultrafine classifications. Without adequate flexural strength, thin-ribbed electrodes can be deflected by flushing pressure or can break during orbiting. Graphite made of tightly packed small particles, will be able to resist erosion at the corners and edges of the electrode better than a material with large particles and pores. A thin ribbed electrode from material in the ultrafine classification can successfully complete the cavity, when the same electrode shape in a superfine or fine classification may crack or break during the cut.
Surface finish in the cavity is a mirror image of the surface of the electrode; therefore, grades with large particle and pore structure will not produce as fine a finish as small particle/pore size grades in the ultrafine classification. The operator can request a particular surface finish from the control menu and the operating parameters are adjusted to achieve that finish. If the graphite grade is not physically capable of producing the requested surface finish due to excessive porosity or particle size, the machine will continue to run without ever achieving the desired finish. When the proper grade of graphite is used for the electrode the desired surface finish can be obtained.
Decreased metal removal rates also can be a problem when the optimum grade of graphite is not used. Because the machine's sensors monitor the gap and adjust the machining parameters as necessary to maintain a stable cut, excessive or large particles in the gap will cause the ram to retract and advance slowly as cutting resumes. When there are excessive particles in the gap, it is due to high electrode wear.
Graphite selection is the key to achieving optimum performance from the equipment on the shop floor. The equipment cannot run any faster than the quality of the graphite material allows it to run. Ultrafine graphites with uniform microstructures tend to be high-performance materials that allow aggressive settings to be used. Using these high-performance materials can trim hours off each operation and eliminate the waste caused by a graphite that has excessive wear, slow machine speed or failure in the tank. Understanding the capabilities of the graphite grade chosen for an application is critical to the EDM performance.
There has been a long standing debate about whether graphite or copper is a better EDM electrode material. One of main reasons people have historically used copper within Electrical Discharge Machining (EDM) is because it is cleaner in their working environment, but is that where the advantages of the material stop? With over 70% of the global market choosing to use graphite electrode materials over copper today (total as high as 95% in the United States and 72% in Europe), the question is why choose graphite over copper in your EDM applications? Here, we take a closer look at the differences between the two materials and outline 5 reasons why graphite is likely to be the most appropriate for your EDM application needs.
In terms of material cost, it is commonly assumed that copper is lower priced than graphite. This is often the result of comparing the price of copper material with more expensive grades of graphite available. Due to the wide range of graphite materials available, it is proven that some EDM grades are more economical than copper. Furthermore, most comparisons do not consider the cost of machining the electrode. Significant cost savings can be made choosing graphite over copper, generated through reduced machining times and speed of cut, less production time to create electrodes, faster EDM times and better throughput from EDM machines. For example, due to the soft 'ductile' characteristics of copper, the material is often 'gummy' and conventional machining practices, such as feeds and speeds must be altered to successfully machine this material. This results in longer machining times and increased costs. In comparison, graphite can be conventionally machined much more easily and quickly and even with more expensive graphite materials, the machining costs often offset any savings that are realized when choosing copper.
Graphite is produced with a wide range of material characteristics to allow matching the electrode material properties to the EDM application. Less critical applications with electrode features containing a large radius, an open tolerance or minimal EDM requirements would use an electrode with large particles, lower strength and economical price. A highly detailed EDM electrode however with critical features, extreme tolerance and stringent EDM requirements would entail a more premium graphite electrode to fit the needs of this application. On the other hand, the types of copper available on the market are few and minimize the ability to match the material characteristics to the EDM application, thus limiting optimum performance.
Copper does not have the ability to handle current density as effectively as graphite, which performs exceptionally well at a high current density even with complex geometry, allowing for various intricate machined details to be designed on the same electrode. The result is that the number of electrodes required to perform a job is significantly reduced.
With the large number of graphite grades offer today, customers can carefully select the correct material for a specific job. Graphite can be chosen depending on required surface finish, electrode life, speed of cut or metallurgy of the job. When working with copper however, there is only one form and precisely selecting the optimum material specification is not possible.
EDM operators know that excessive wear results in the use of extra electrodes or frequent redressing. Graphite can achieve electrode wear of less than 1% in relation to the depth of cut, while working to more aggressive machine parameters. This means, unlike copper, the high amperage and longer on-times preserve the graphite electrode.
In the aerospace sector when working with very thin, fine detail electrodes, copper is vulnerable to any rough handling and physical damage. Pressure applied to a thin section of copper will cause movement that can go unnoticed, causing further performance issues at a later stage. Graphite on the other hand is either in the right condition, or is clearly broken, vastly reducing the risk of the electrode being used in production.
Some firms wire erode their own electrodes and are misled into believing that you cannot wire erode graphite. Contrary to this, the wire erosion of graphite has been tested and has done so without any breakages and at comparable speed times to that of copper.
Copper electrodes do provide very fine surface finishes. With the sophistication of today's EDM machine technology, the surface finish gap between graphite and copper has narrowed significantly. For example, fine grain graphite electrodes can deliver similar surface finishes to that of copper, while offering much faster speeds and vastly reduced electrode wear.
As EDM industry is growing year-on-year, and steadily the outlook towards it has changed during this period. Market looks at EDM as proven and precise machine, which can do wonders as compared to conventional machining.
While there are many methods used to come to conclusion as to what material is to be used, we at Novick have identified 5 key factors that for it. Lets take a look at the same.
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.
Typically there are 4 types of wears – volumetric, corner, end, and side. Corner wear impact more as contours of the final cut are determined by the electrode’s ability to resist the erosion of its corners and edges. Minimizing corner wear requires choosing an electrode material that combines high strength with high temperature resistance.
Producing the best finishing can be achieved by using electrode with combination of materials. Final surface finish is usually a mirror image of electrodes, proper power supply settings, high frequency, low power, and orbiting are key aspects of final finish. High strength graphite are the best choices for finishing electrodes.
Harder electrode materials will be more prone to chipping during the machining process. Most machining personnel know Graphite is easy to cut. So strength of the graphite has to be defined as per the machining process while selecting a electrode.
Cost of electrode material is often considered as point of decision. However, this cannot be considered in isolation. Fabrication time, cutting time, labor, electrode wear— all these factors depend on the electrode material more than on any other factor. Hence, it is pertinent to understand properties and performance characteristics of the electrode in detail, and not to select the electrode on upfront costs.