Abstract

EDM carries a major role in today's mold shop. The manufactures of electrodes is an integral part of the Machining process. This paper will address some of they techniques used to build electrodes. Proper electrode design and construction are crutial to make the job how more easily Use of computer-aided design/computer-aided machining (CAD/CAM), model-making, pantographing and conventional machining methods give the mold maker a varied array of tools to use. Only by using all of them can the mold maker be assured of the best electrodes possible

Introduction

Proper planning is a necessary first step with any job. In the case of EDM machining planning the electrodes alone can be a job in Itself! The type of EDM’ing to be done, the Material to be EDM’ed, and the number of parts needed directly affect the electrodes. These factors determine the electrode material, the amount of over-cut (how much to under-size the electrodes to account for spark gap), and the number of electrodes needed. Through proper adesign and construction the manufacture of electrodes can be made easy and efficient.

O.A.R. Too] and Die. Inc. In Providence, RI has been Involved with EDM machining since 1973. This shop has developed many methods for electrode fabrication and it Is this experience that this paper will share. It is noted that there are many different ways to do a single EDM job and it is hoped that the techniques expressed here will benefit the industry as a whole.

Materials

There are some basic guidelines which can be used as a starting point In the electrode selection process. The type of EDM cut to be made will usually be a major factor in deciding which material to use for the electrode. Machining processes to be performed, electrode mounting configurations, and the type of EDM Machine to be used (manual or CNC) will also be determining factors. The amount of materials available allow the electrode to be a very task-specific or a general all-purpose tool. EDM Machining ranges from the heavy duty, large over-cut, "hogging" cuts to the fine detail, excellent finish. very light duty cuts. Any of the commercially available materials will do any EDM job. Depending on the application, some will be easier to work with than others. Commonly used electrode materials Include graphite, copper, copper impregnated graphite, copper tungsten, silver, and others. The two most common materials are graphite and copper.

Graphite is available in different grades from coarse and porous to fine and dense. Roughing cuts would generally call for a coarse grade while finish cuts would call for a fine grade. Graphite is desirable for many reasons. The first reason Is its wear resistance. During the EDM process a certain amount of electrode wear occurs as the material being EDM'ed Is burned sway. The amount of heat being generated at the burn site is what causes this wear and graphite resists this heat better than any other material. A second reason is machinability. Although graphite is very abrasive it is relatively easy to machine. Carbide tools are recommended for the machining process. Milling, drilling, and grinding all provide excellent finishes in graphite. One must be careful not to chip the outside edges (due to the brittle nature of the material) but otherwise machining Is straight-forward.

Copper Is the next choice of electrode material. This material EDM's differently than graphite but is more forgiving In the deeper burns. It is more difficult to machine than graphite but Is still soft compared to copper tungsten. One machining advantage is how easily complex shapes can be wire EDM'ed onto copper electrodes. Another advantage is the ability to coin electrodes from copper. These methods will be addressed later.

Copper impregnated graphite is a cross between the two materials. This tends to be a harder material than graphite alone but is still easily machinable. It has the forgiving Characteristics of copper with the wear resistance of graphite. Copper-graphite was developed before

Today’s CNC machines when control of the burn was more difficult. Unfortunately EDM machines are usually set for either graphite or copper. The material tends to confuse the EDM machine as the machine does not know which material It is burning with. Today's CNC EDM machines with their automatic settings allow much more control over the burn. The use of copper or graphite alone has become much more efficient. Because of this the use of copper-graphite is becoming less frequent.

Copper-tungsten is a very hard material. This makes it ideal for very intricate detail EDM'ing, long thin blades, or cavities with very sharp corners. It tends not to deflect as the other materials would and is easily wire-EDM'ed.

As stated earlier there are many electrode materials available the main groups have been briefly touched on here. The application and machining capability of the end user will determine which material is best for a particular job.

Design and Machining Concepts

Good electrodes are the result of a team effort. The engineer, designer, machinist, and EDM operator all contribute valuable input. This interaction is necessary due to the amount of specialization In today's shop. After the type and number of electrodes has been determined for the job, work can start on their design. A standardized holding system should be the basis for this design. This way all electrodes use the same base-coordinate (for example, the center of the electrode's mounting shank) as they move through the manufacturing process. This also helps streamline the work flow as the electrodes move from machine to machine.

There are a number of machines used to build electrodes. Computerized-numerical-control (CNC) -machines head the list with mills, machining centers, lathes, wire EDM's, and ram type EDM machines. Manual machines include mills, lathes, grinders, ram type EDM's, and pantograph machines. It should be noted that CNC machines or manual methods should not be used exclusively. Each method compliments the other and in this way a cost and time effective way of producing electrodes emerges.

Computer aided design (CAD) is an invaluable asset. CAD drawing packages are available to run in anything from the least expensive personal computers (PC's) up to work-stations costing thousands of dollars. Through 3-D computer modeling and precise line drawings the

machinist knows exactly what an electrode will look like before he even begins to build it. The designer can position the electrode on the holder and then use this information to establish locations for the holder center over the burn site. Possible errors are decreased because of the fewer dimensions needed to define positioning moves. Another advantage of CAD is its ability to generate CNC programs directly from the drawing. These programs can be used on CNC mills, wire-EDM's, etc. to out the electrodes with great precision and repeatability.

CNC mills and machining centers will generally handle most of the work in the CNC department. These machines have the ability to cut complex shapes in 2-D and 3-D that would be difficult an a manual machine. A single program can be used to out multiple electrodes which are all exactly the same. Many of these machines come equipped with multiple tool changers enabling a program to perform many different operations completely unattended.

CNC mills and machining centers will generally be 2-1/2 to 3 axis machines. A 2-1/2 axis machine will produce true arcs in 2 axes with the 3rd axis being linear. A 3-axis machine will cut true arcs in all 3 axes at the same time. The 3-axis machines generally have rig id rams and generate their motions with coordinated straight line table feeds. The 4- and 5-axis machines introduce rotation of the spindle head and allow the tool to remain perpendicular to the work-piece at all times. These machines would be used to cut electrode shapes such as propellers and turbine blades. A 5-axis machine will probably cut any electrode one can imagine.

While a 5-axis machine is desirable often it is just not practical. There are ways to get around this type of machining. Accessory equipment such as CNC (or manual) rotary tables and indexing heads allow one to closely approximate many of the functions performed by the 5-axis machines. Add fixturing such as a compound sine plate and one can orient an electrode into almost any position for cutting. Most CNC's have commands which allow the program to access external equipment and coordinate it during the cutting process.

There are many times when a contour is just too complex to CNC onto an electrode. This is where the "art and science" of mold making work hand in hand. The artist/model-maker has been a Part of mold making since the beginning. Before CNC machines the model-maker would make a 2:1 or larger model by hand. He could then use this model to pantograph a 1:1 electrode. While this is still a very viable option today (depending on the job), CNC. can take this one step better. Instead of the model being pantographed by hand, CNC has the ability to digitize the model. Digitizing Is the process by which a model Is probed either by a mechanical contact or by laser. This probing determines all the coordinates of the model and stores them In a computer file. These coordinates can then be retrieved to cut an exact duplicate of the existing model. Through CNC these coordinates can be scaled up or down, mirrored, or even produce a male copy from a female model (or vice-versa). This single file can be used to cut multiple electrodes with CNC accuracy and unattended operation. This Is a major health benefit when cutting dust producing graphite electrodes. The digitized file can also be used to draw the part using CAD. With CNC digitizing a part may come into the shop with no drawings or dimensions but still have electrodes and drawings made from it.

CNC Wire EDM machines are just as important as the CNC mills. The wire EDM machine Is essentially a vertical saw which uses an electrified wire (.010"-.020" dia.) to make its cut. A very high degree of accuracy is available because of the small diameter of the wire and CNC control. The wire makes Its cut between an upper and lower guide. These guides can move independently of each other enabling tapered cuts or even an entirely different shape on the top of the piece than on the bottom. This amount of control makes the wire EDM a strong tool for use in electrode manufacture. The wire EDM cuts very hard materials easily and therefore allows otherwise difficult to machine materials, such as copper-tungsten, to be machined quickly and efficiently. Complex shapes can be programmed through CAD or manually and fed directly to the machine. As with the mills many electrodes can be produced from a single program.

Often the cost of CNC machines puts them out of reach of many shops. The difficulty of learning a CNC programming language and the learning curve involved for machine operation may not be cost justifiable to a shop owner. Fortunately there are many numerical control (NC) retrofit units on the market today that are geared specifically to solve this problem. These units are fit to a manual milling machine and allow basic NC functions to be performed. Programming is straight-forward and Is written with the machinist In mind, not the mathematician. These units enhance the abilities of the manual mill and can be a very valuable and successful part of the electrode making process. They are the middle ground between CNC and manual machining methods.

Another method to build electrodes is fabrication. Parts of the electrode are machined In separate operations. Milling, drilling, turning, and grinding are used to build sections of the total electrode. These sections are then screwed or glued together to form the complete electrode.

Cast or sintered electrodes involve essentially the same process. A model is used to produce a mold of the desired electrode shape. Electrodes are then cast from the mold using any casting material. This method is generally used to form electrodes from the harder, more difficult to machine materials such as copper-tungsten. Benefits include high accuracy and repeatability as all electrodes are coming from the same source (the master mold). Some disadvantages are that the model for the master mold has to be perfect as any flaws or marks will be transferred to the electrodes. Also there may be a lead time for the electrodes as sintering is a specialized process.

Coining electrodes may involve all of the above processes. A model is used to pantograph or CNC digitize a graphite electrode which is then used to EDM a master die. The easily machined graphite which may lose its Intricate detail quickly is used to EDM one cavity. Material such as annealed copper is then pressed into the die to create the electrodes for the job. The copper electrodes which hold their detail better are quickly manufactured from a pressing operation. A substantial savings can be realized by eliminating the machining time involved for multiple copper electrodes.

A standard conventional machining method for electrodes is form grinding. The desired shape is dressed onto a grinding wheel which is then used to make the electrodes. Grinding is one of the best ways to machine graphite because of the excellent finish and decreased edge chipping. CNC's can also be introduced here to dress complex contours onto the wheel.

Health Considerations

EDM'ing As stated earlier graphite is probably the most commonly used electrode material. The ease with which this material can be machined and its EDM'ing properties make it generally the best all-around material for most jobs. The major drawback of graphite is the fine dust it produces during machining. Precautions must be taken to assure that a health hazard is minimized to the machinist during the manufacture of the electrodes. This dust is also very abrasive. It it is able to settle on the ways of the machine and mix with the machine's oil it will act like a lapping compound to eventually destroy the accuracy of the machine. For these reasons it is a good practice to establish an electrode making room. A CNC mill, grinder, and pantograph will generally be the minimum machines required to equip this room. An excellent vacuum system is necessary to evacuate the majority of the graphite dust both from the machining site and the surrounding environment. It possible negative pressure should be established in the room to vent the room's air (and dust) Into the vacuum system and not out into the rest of the shop. This way the graphite dust's destructive potential is kept in check both locally and shop-wide.

Summary

Successful electrode manufacture begins with good engineering and design. Input from all key people will assure that the best materials and procedures are used to build the electrodes. Planning all electrodes to be built using a standard holding system through-out the process will aid In the overall work-flow. The use of CAD from the beginning of the process will aid in visualization of the end product before the first material is cut. CAD will also help in determining how many of each electrode Is needed and where. Finally, CAD can be used to generate CNC programs to cut complex contours In the electrode material.

Both manual and CNC machines are integral parts of the machining process. Neither type of machine is better than the other and both should be exploited to their fullest potential to guarantee the best electrodes possible. CNC has the benefits of multiple tool changing, complex contouring, digitizing, and unattended operation. Manual machines allow short runs with no programming, pantographing and form grinding. While both have their strong points, using the two together makes for greater efficiency.

With the many materials available and the different techniques expressed here, electrodes should be able to be made for almost any application. Proper fore-thought and imagination are necessary to assure success. At the same time, one's imagination is the only limitation as to what can be achieved on today's EDM machines.