An Added Layer of EDM Knowledge
Understanding electrode material selection and the relationship among the electrode, workpiece material and machine parameters can improve EDM efficiency.
Sinker EDM technology has improved significantly over the years, changing many of the processes and decisions that an EDM operator will face when operating the machine. Even though operators may have access to the latest in machine tool technology, it is still important for them to receive adequate training to understand machine parameters and the impact each one has on the finished product.
Most EDM training is separated into basic and advanced levels of operation, and covers topics such as EDM theory, preventive maintenance, operational practices, programming and advanced technologies. However, these are primarily focused on the sinker EDM itself and not on the consumable materials associated with this machine. For example, the dielectric oil used in the EDM process, the filters used to keep the dielectric fluid clean and even the electrode material used to complete the burn.
Did you know that the electrode material used in the EDM process has a direct correlation to the performance and efficiency of the EDM burn? This is similar to the food we eat to maintain good health and remain at the best physical condition possible. If we eat healthy and exercise, then we can expect to have energy and fewer illnesses than if we were to consume mostly junk food and live an inactive life.
In this same regard, the electrode material used in the EDM burn must be matched to the application to guarantee success at the highest level of productivity. Sure, even the incorrect electrode material can complete the job, but at what cost? To ensure the most effective operation, everyone from the shop owner to the machine operator should receive training on the impact of the electrode material on the EDM process. This is not meant to replace the training by the sinker OEM, but rather to supplement that training and enhance the knowledge of the operator with an added layer of knowledge.
Let’s review five main factors of electrode material selection that can have an impact on EDM performance and that should serve as a basis for the training you should offer your EDM operators.
Machinability. Simply being easy to machine doesn’t necessarily make a material the best choice for an electrode. For example, any machinist who has ever machined graphite is aware that it cuts very easily. However, the electrode material must also be strong enough to resist damage from handling and the EDM process itself. Strength and small particle size are also important so that minimal radii and close tolerances can be achieved. Material hardness is also a factor in graphite machining, as the harder electrode materials will be more prone to chipping during the machining process.
Metal removal rate (MRR). MRR is usually expressed as cubic millimeters per hour or cubic inches per hour, but could just as realistically be expressed as dollars per hour. 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 electrodes are generally much more efficient than metallic electrodes, however metal removal rates vary widely among graphite types. With the proper electrode material/work metal/application combination, MRR can be maximized.
Wear resistance. The four types of wear are volumetric, corner, end and side. Of the four, corner wear appears to be 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 any 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. Fine surface finish is obtained with a combination of the proper electrode material, good flushing conditions and the appropriate 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.
This means, for example, that graphite grades with a particle size of less than 1 micron and very high tensile strength, isotropy and uniform structure, or those with a particle size from 1- to 5-micron isotropy, uniform structure and high strength would be optimal choices for finishing electrodes.
Material cost. Generally, electrode material cost represents only a small part of the total EDM job cost, which is why it is often overlooked. Fabrication time, cutting time, labor and electrode wear are all factors that depend on the electrode material more than on any other factor. So, it is critical that you know the properties and performance characteristics of the available electrode materials so you can fully understand how they can affect the work metals being machined. This data is essential when doing a cost/performance analysis to determine the true cost of an EDM job.
Regardless of the machine you are using, each of these performance factors will be impacted by the electrode material. Whether the job calls for a specific surface finish, electrode wear or MRR, a consistent effective electrode material is critical to achieving the desired result.
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