EDM Technical Manual
CHAPTER 7: PRACTICAL APPLICATIONS OF THE FIVE KEY FACTORS IN ELECTRODE MATERIAL SELECTION
MINIMIZING THE GUESSWORK
Information on the properties and characteristics of the various materials is available. Any reputable manufacturer of electrode materials will be happy to make specification sheets available to you. Those who are specifically involved in the EDM market should also be able to provide you with EDM performance data under a variety of conditions. Ask the person who sells you your electrode materials for performance data under a variety of settings and against a variety of work metals. If this information is not available, it doesn't mean that the material won't make a good electrode. It does mean that the user will have to run some tests or gamble on the results.
Machinability is the only factor, other than material cost, that can be accurately deduced without running tests. As explained previously, no material may be machined to a dependable tolerance greater than its largest pores and voids, so reliable information as to the particle size, pore size, and strength is a good indicator of machinability. And, of course, knowing that the material is isotropic gives assurance that those properties will remain constant in all directions.
Metal removal rate, wear resistance, and surface finish can be reliably ascertained only through precise, repeatable, thorough testing. Obviously, few shops can afford to dedicate a machine to nothing but electrode materials testing, and yet that is the only way to determine a material's performance characteristics. Through the Objective Comparison Testing program, POCO has assumed this task for the EDMer.
Machining Dependable Tolerances
Except for thin slots and ribs, good machinists can machine detail into material from any graphite classification. The problem is that the material may not be able to reproduce or maintain the detail in the cavity that was machined into the electrode.
The following examples (figures 7-1 and 7-2) show what happens as electrodes from the Ultrafine, Superfine and Fine classifications try to reproduce a sharp angle. Normally the larger particle material (Fine) will give a better metal removal rate, but due to the sharp angle, the smaller particle material gave the best metal removal rate (figure 7-3). Because the energy was concentrated at the sharp angle of the electrode, excessive wear in the larger particle material caused the gap to become clogged. Side flushing was unable to keep the gap free of particles and unstable machining conditions kept the metal removal rate low. The tip of the electrode became rounded and the bottom of the cut is actually broader than the cut made with the small particle graphites. This is why it is important to use material from the proper classification to handle the detail for the application.
Figure 7-1. Left to right—First cut is with an Ultrafine material and the cavity shows straight angles that come to a point. The second cut is with a Superfine material and the bottom of the cavity is starting to round and the angle is a little wider and deeper. The third cut is with a Fine material and the bottom of the cavity is very rounded and the overcut is larger.
Figure 7-2. Electrodes after the cut - Only the Ultrafine electrode on the left has retained the original angle. The middle and right electrodes show wear at the points.
Figure 7-3. Comparison of metal removal rates for three electrode materials.
Figure 7-4. Wear comparison for two grades of graphite electrodes selected to perform the same job. Note the high wear of the teeth of electrode B on the right. (Electrodes courtesy of OAR Tool & Die; Providence, R.I.)
Resistance to Wear
The electrodes above (figure 7-4) are both from material in the Superfine classification. Both electrodes produced splines through the inside diameter of 30" of stainless steel.
The difference is that these two grades were produced by different manufacturers. Although these two grades are classified as Superfine materials, the performance is not the same. Variations in manufacturing techniques result in significant differences in structure and physical properties within a classification (figure 7-5). Therefore, their resistance to wear is obviously different. The material with better wear resistance would also reduce the number of electrodes needed.
A cavity cut with the electrode on the right (B) will have a rougher surface finish than the electrode on the left (A). The cut probably was slower due to unstable machining conditions from the amount of eroded electrode material in the gap. Excessive material in the gap can also cause the walls of the cut to be tapered and the overcut to be larger.
Figure 7-5. Photomicrographs (100x) of the two grades of graphite from the Superfine classification that were used to produce the electrodes in figure 7-4. Material A that had less electrode wear has a more consistent structure.