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.