Electrode fabrication — tips and tricks
This is the first of a four-part series written by certified tool & die journeyman Mark Benoit. Mark is employed at Waltech Engineering Ltd. (Wallaceburg, Ont.) and has been an instructor in the St. C...
This is the first of a four-part series written by certified tool & die journeyman Mark Benoit. Mark is employed at Waltech Engineering Ltd. (Wallaceburg, Ont.) and has been an instructor in the St. Clair College apprenticeship program for the metal cutting trades for nine years.
WHAT IS THE PURPOSE OF THE ELECTRODES? WHY SO FUSSY?
The electrode is the cutting tool through which the electrical discharge of the EDM process takes place. Its form or shape is a mirror image of the desired form in the work piece. The accuracy of the electrode directly affects the accuracy of the finished work piece.
All dimensions on the electrode must be adjusted by the same amount of over-cut. Over-cut is the amount the electrode cuts larger than itself.
An electrode that has varying amounts of over-burn on its dimensions cannot be easily compensated for in the EDM process. Therefore when fabricating an electrode every attempt should be made to keep all adjusted dimensions within a range of chosen accuracy.
The number of electrodes needed to finish a desired cavity in a work piece depends on the following factors: the finish required, the shape of the cavity, the amount of material left in the work piece, the electrode material used, and the parameters used on the EDM process.
WHAT TYPES OF MATERIAL ARE COMMONLY USED FOR ELECTRODES?
The first and most important rule for the selection of an electrode material is that it must be electrically conductive. There are a great number of materials used for the fabrication of electrodes.
The European market has primarily used copper-based electrodes, while the North American market has widely embraced graphite. Each material has specific pros and cons depending on the application.
Graphite itself has varying degrees of density and grain size. The selection of the grade of graphite used depends on the number of cavities required in the work piece, the amount of material to be removed, the finish of the cavity and the cost.
HOW ARE ELECTRODES FABRICATED?
Electrodes can be fabricated in a wide variety of methods. Because of the specific nature of the material, there are “rules of thumb” that should be considered. Graphite is both brittle and abrasive. These two properties lend themselves to consider the use of primarily good grades of carbide during the machining process. There are, as well, specific coatings that have proven themselves capable of providing longer cutting tool life during the fabrication of an electrode.
High-speed machining centres equipped with vacuum systems are used for electrode production. Graphite can be milled, turned, ground, wire-cut and sawed.
When machining, graphite standard practices should be employed. These include keeping tools sharp, using relief angles, positive rake angles, and rigid setups. Graphite should never be machined without the proper dust control method in operation.
EXAMPLE: THREADED ELECTRODE
As an example we will look at producing a threaded electrode for a 1/2-13 UNC. The following process should be followed:
Decide on the amount of overburn you require on the electrode. For our example I will use 0.012 in. overburn per side.
Determine the tap drill size for the thread required. Tap drill size for 1/2-13 UNC is 27/64 in. or 0.4219 in. diameter.
Because the electrode should be dropped to the bottom of the hole and expanded on the EDM in order to spark the threads into the material, we therefore must fit inside our tap drilled hole.
Example: 0.4219 in. diameter – 0.024 in. total overburn = 0.3979 in. diameter
So therefore we begin by turning the electrode to an outside diameter of 0.3979 in.
Then continue to cut the thread to the proper depth from that diameter as if you were cutting threads on steel. Example: 1/2-13 UNC double depth of thread is 0.100 dp.
Note: Use a center support if a long thread is required. Your threading tool bit should be very sharp as well when working with graphite.
WHAT OTHER TYPES OF THREAD ELECTRODES CAN BE PRODUCED?
A wide variety of thread types and styles are possible. Pipe threads for water lines that are in hardened material, finished threads on an injection mold that become part of the cavity, as well as special thread forms can be produced easily and effectively through the EDM process. Each of these areas can be expanded depending on the complexity of the thread. In some cases several identical electrodes must be produced in order to account for the wear that occurs on the electrode depending on the EDM process used. CPL