EDM Technical Manual
CHAPTER 9: ELECTRODE DESIGN CONSIDERATIONS
Figure 9-1. CAD stations can be connected directly to EDM to increase production. (Courtesy of Delta Tech Mold, Inc.; Arlington Heights, Illinois)
TYPES OF ELECTRODES
The choice of configuration of the electrode(s) for a job will bear heavily on the time and number of electrodes used, metal removal rate, final surface finish, and running stability. A number of electrodes may be run from a single lead, bearing in mind that there will still be only one spark at a time. Also, multiple electrodes means that a problem with any one of them means they all stop cutting - simply determining the culprit may be very frustrating.
Some jobs may be most efficiently run with exact duplicates of the basic electrode. In this method, known as staged electrodes, a lesser grade of material is used for hogging the initial shape, and others of higher quality follow to reduce the heat affected zone or produce a fine surface finish.
There are times when pieced electrodes are more cost effective than a solid electrode. The requirements of the job will determine the method used to make the electrode. Aluminum backing plates can be used as a base and attached with screws to fabricated electrodes to eliminate the need for a large piece of graphite for a complete electrode. Also a coarser grade of graphite can be used as a base for a premium grade. An adhesive can be used to join the two pieces of graphite. Large electrodes can be made from several pieces of graphite that have been bonded together. Sometimes it is practical to repair electrodes by cutting in another piece of graphite to replace a damaged area.
It is important to use the same grade when repairing an electrode or making a large electrode from several pieces. If the grades are not the same, an uneven wear pattern could develop as the cut progresses. When the bonded areas are part of the leading edge of the electrode, orbiting the electrode may help eliminate a hairline seam in the cavity where the joint of the two pieces occurred.
Figure 9-2. This complex pieced electrode produced the mold for the plastic case shown in the upper right. (Courtesy of Jerald Umbaugh, Inc.)
Partial Area Electrodes
Partial area electrodes are being used increasingly in CNC/tool changer operations. This allows the electrode designer to choose the optimum material and electrode shape for every portion of a job. This flexibility opens up a whole new world of productive, profitable possibilities to the innovative EDMer.
Figure 9-3. An EDM machine equipped with a tool changer can produce complex molds or multiple cavity molds without operator intervention.
Figure 9-4. Computer Aided Design (CAD) systems offer the ability to preview an electrode in three dimensions, thus allowing the EDMer to spot potential flushing problems. (Courtesy of CENTECH, Inc.; Glenview, Illinois)
DESIGNING FOR FLUSHING
The time to start thinking about flushing is the first time you look at the drawings of the finished article. Whether speed or surface finish is a primary consideration, flushing conditions will be of great importance in determining the degree of difficulty the job presents. Premium Angstrofine and Ultrafine graphites are considered to be more "forgiving" materials than lesser grades, and much of their "forgiving" nature can probably be traced to their finer particle size, which makes flushing easier. For this reason, it will be cost effective to use them in many jobs that, given better flushing conditions, would ordinarily be accomplished well by lesser grades.
In CNC EDM operation, flushing may be the primary consideration in electrode design. In such repetitive, automated situations, maximum productivity with minimum operator intervention is the key. Carefully designed partial area electrodes made of premium materials can provide trouble-free, unattended performance. An automated operation that is constantly requiring attention due to unstable operation or arcing is of little benefit.