Abstract Expanding EDM's role in a contract shop with the advent of viable EDM technology a new way of doing business was given to the contract shop. Previously difficult to machine materials could now be easily machined by burning the stock away with an electrode. But the process was time consuming and messy. Since that time great advances have been made. But the contract shop is still being sluggish in its implementation of EDM as a fore-runner of machining on the shop floor. This paper will discuss how O.A.R. Tool and Die, Inc. Turned that around and made EDM a primary force in the shop. Introduction EDM has been around for many years. The benefits of this type of machining are well known. Cavities which would be difficult to machine can be easily burned into almost any material including most of today's "hi-tech" alloys. Electrodes are usually made from graphite, copper, or even silver. All these materials have very desirable free machining properties. Even the EDM machines themselves are becoming easier to run with user friendly programming and automatic multiple tool changers enabling them to run unattended for long periods of time. Why then do many shops still use this method of machining only as a last resort? This paper will attempt to give some guide lines as to how to expand EDM in a contract shop environment putting it in line as one of the first options and not as one of the last. The Need Remember the "good old days" before EDM? More often than not you had to laminate the mold together. You had to insert just about everything. You had to broach square corners. You had to grind and fit everything to match. Not to mention the design nightmare (say if you had cams). The toolmaker had to have working knowledge of machining do's and dont's, mold can's and cannot's, etc. It was only then that you could take the tool maker and make him a designer if you had a designer who was not familiar with the shop end you generally had trouble. Then there was the time factor. Many of these procedures were not difficult but were sometimes very tedious and repetitive in multiple cavity molds. There had to be another way. The First Breakthroughs With EDM life became much simpler the first machines were equipped with tubes and capacitors making for very bulky power supplies. Solid state technology was a major advance making the units more compact in size in addition to making the working currents smoother, more steady, and easier to control. Then came the orbital head. This enabled the operator to "orbit" or displace the electrode limited distances in a circular pattern thereby making it easier to size the cavity. EDM was now establishing a foothold in the shop. The old methods of building a mold were slowly being left behind. Instead of laminating small pieces together, now one could burn a cavity into a single block of steel making the mold stronger than before. Square corners were no longer a problem. Just mill or grind up a square male electrode and burn them in. The benefits were beginning to be realized. It was at this point that O.A.R. tool and die decided to put EDM at the forefront of its machining arsenal. With this decision some priorities had to be changed. Greater attention to detail had to be paid to the electrodes being made. Locations became critical. All electrodes for a particular burn had to be in exactly the same location on multiple holders. These concerns introduced new problems. Originally only the EDM operator would make the electrodes. While his machine was running, he could be off preparing the electrodes for the next job. If EDM was to be the fore-runner of the shop, this meant that more than one EDM machine would probably be involved. Obviously, to cost justify this investment, these machines would have to be kept running almost continously. This in turn meant more electrodes would have to be set up. The one EDM operator could no longer handle this task alone. As more men are put on the task of electrode set-up, a certain degree of error is introduced. Everybody has their own way of doing things but in the end it always comes back down to the one operator running the machine. He has to inspect each and every electrode to be sure that "Joe’s" electrodes are exactly the same as "Don's". To minimize the effect of individual *style" in electrode set-up, the system 3r machining method was adopted. System 3r involves small round and square hydraulic chucks which clamp onto a round shank. Electrode blanks are mounted on the shanks. The chucks are picked up once on a mill or grinder. With the shank locked in place in the chuck, location is guaranteed to be within (.0002") for all electrodes. These chucks are also mounted to all EDM machines assuring that close tolerances are carried all the way through the machining process. Continuous Change The introduction of NC/CNC machines opened up even more avenues to help make the project successful. The more intricate and complex electrodes were originally done by building a model and 3D pantographing them onto the electrode blanks using a Deckel model GK21 pantograph. Two Bridgeport series 1 2-1/2 axis CNC mills eliminated some model building by enabling a programmer to go directly to a finish electrode making the process much less time consuming. A Sharnoa 3-axis CNC machining center (model SDC 20) with 3D digitizer was bought. Now even models that were too difficult to program could be used to make electrodes. It should be noted that the pantograph is still a viable option in the electrode set-up process. For very few or very small electrodes, the pantograph is usually the way to go. But for very many or very large electrodes, the CNC mills can run unattended and don't care if they see the same piece for days at a time. This is very desirable when cutting a material such as graphite which is can be dusty and hazardous to breath. Also, once a CNC program has been de-bugged, it usually won't make a mistake. With the CNC mills happily doing their jobs, this now freed up the mill operator to go set up more electrodes. To quickly recap, there is one EDM operator running two machines and inspecting electrodes. There is also one CNC mill operator running two machines. And there may be other men setting up more electrodes - all of which are interchangeable within very close tolerances through the system. It was only a matter of time before CNC came to EDM. EDM was a slow process to begin with. It was slowed down further by the fact that you could make only one burn, then had to crank the table to a new location before starting the next burn. With CNC multiple locations could be programmed freeing up the operator even more. But the EDM process wears electrodes at a good rate (which explains why there are so many of them) and this factor defeated the purpose of CNC. For a multiple cavity burn a single electrode would be all but gone by the time it reached the last cavity. Enter the automatic tool changer. Now many electrodes could be loaded into the machine. These electrodes could be all the same or all different depending on the burns. The CNC program could be written to locate the correct electrode over the correct location. This electrode could even be rotated into position if need be via a rotating head (the "c' axis) in the machine. The CNC program includes everything the machine needs. Cutting codes for rough and/or finish burns, tool changes, location and rotation codes, even mirror imaging could be involved. O.A.R. has found that it is easier and faster to use conventional machining practices (either with manual or CNC machines) to put intricate detail on the electrodes. These electrodes are then used with a simple locate-burn-locate-burn program in the EDM machine to do the job. The CNC EDM machine is capable of doing the same job with a straight electrode and programming the 3D contour to be burned out, but this practice should probably be avoided due to the time (and thereby cost) involved. The first CNC EDM machine O.A.R. decided to purchase was a Sodick model a3c-r with a 18 station tool changer. With the introduction of CNC and, tool changers to EDM, O.A.R. was finally able to safely and confidently start EDM machining jobs 24 hours a day. Making It Work The achievement of a goal had been reached. The original idea had been tested true and now the focus changed from getting the machine on line to keeping it there. The system 3r proved invaluable in helping to keep tolerances tight. Through extensive use of the CNC mills and machining center very high accuracy and repeatability from one electrode to the next was relatively easy to achieve and maintain. With the number of electrodes being produced, the CNC milling department started to become overtaxed. It was decided that many of the simpler electrodes could be done on manual machines, but the speed and accuracy of CNC was still desirable. Because the relative simplicity of these electrodes did not warrant the purchase of another CNC mill, the problem was remedied with an NC retro-fit to one of the manual Bridgeport series I mills. This NC retrofit, the Proto-Trac by Southwestern Industries, Inc., was so easy to use that no special CNC training or knowledge was required. This made it perfect for its application. At a fraction of the cost of a full blown CNC mill, the integrity of the electrodes was maintained. In addition, any toolmaker could run the Proto-Trac. The concept was so readily accepted and worked so well that there are presently three Proto-Trac’s in use at O.A.R. With the ever-increasing number of men cutting poco graphite electrodes it was found that the atmosphere of the shop could very easily become contaminated. This mandated an upgrade of the existing vacuum system already in the shop. Also, in addition to one portable vacuum already in use, two more torrit model 75 vacuums were purchased to help out. Different materials have been tried as electrode material and all have been used successfully depending on their application. Copper is used almost as much as graphite. For example, there may be a graphite roughing electrode and a copper finisher. Cast sterling silver has been used with great success and does a very nice job but due to its cost is only used when absolutely necessary. Graphite remains the primary material. It was soon necessary to buy a second CNC EDM. As before, a tool changer was mandatory. It was decided to buy a Mitsubishi model C4G35 with a 16-station tool changer. This was a larger machine with an increased programmable area. The type of work that could be done in the machine now included entire mold plates, not just individual cavity blocks. As the shop was becoming more and more computer oriented, a PC based CAD/CAM system was purchased. The auto-hold system by cad-master uses a 386 PC with AutoCAD as its base design software. This took most of the design work that was being done outside and brought it in-house. This was a major step that helped streamline the entire operation of the shop. Incoming jobs could now be taken, in-house, from their original purchase order/product drawing format through design and into the final programming stages. Many programs came directly from drawings created within the PC. Better efficiency was realized due to less errors being made during the transfer of information from drawing to program. This not only helped the EDM's but the CNC mills as well. Direct up- and downloading of programs was accomplished by tying all CNC machines into the PC via RS-232 communications. The latest machine to be purchased is a Mitsubishi model dwc90c wire EDM machine. Using the same strategy, programs can be developed within the cad system and fed directly to the machine. The wire EDM added a new aspect to the way the shop machined certain materials. Carbides and hardened tool steels are very easily machined. As with the first EDM die sinkers, once the concept was proved out, a second wire machine identical to the first was purchased. Depending on the job, these machines have the capacity to run 24 hours a day. Summary At present O.A.R. tool and die has one Bridgeport CNC mill, one Sharnoa CNC machining center, two Mitsubishi wire EDM machines, three Proto-Trac NC retro-fits, one Japax/Bridgeport wire EDM hole burning machine, and two EDM die sinking machines (one Sodick and one Mitsubishi). There are two in-house PC based cad systems with two more PC’s on the shop floor (three IBM compatibles and one Hewlett-Packard). In addition there are many manual machines including two Charmilles bench top EDM machines, Bridgeport mills, reed surface grinders, lathes, deckel pantographs and one Moore jig grinder. With the amount of computer-controlled machinery that exists in this 22-man contract shop, it should be obvious that O.A.R. is committed to staying on the leading edge of the CNC wave. With EDM die sinking being the backbone of the shop, O.A.R. has developed a system that works and will continue to work in the future. |
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