Abstract Correct and efficient project management Is critical In today's fast paced computerized mold shop. Gone are the days of putting a piece on a CNC (computerized numerical control) machine, writing a "quickie" program, and machining It. Time tables, delivery dates, and on-going multiple projects make machine time scheduling a most important part of the overall manufacturing process. It Is increasingly evident that this scheduling must start at the design phase of the project, not at the machining stage. From the mold design, electrode design and CAM (computer aided machining) inputs can be directly generated for the CNC machines. Auxiliary programs can be produced as needed but possibly 80-90% of the Job can be planned for before it has even started. This paper will address this process planning from the design stage through to the finished mold with emphasis on the CNC EDM aspects of the job. Through proper design of the electrodes and the "TARGET ZERO" concept, each step of the process will be analyzed. It will be shown how work "flow" can be streamlined and still be able to accommodate unexpected scheduling emergencies which may arise. Introduction CAD/CAM/CAE has revolutionized the way the plastic -mold Industry works today. A product from concept and design to finished mold is almost entirely done through computers. The methods used to machine many types of cavities has changed with the evolution of CNC EDM machines (both ram and wire types). Multiple tool changers and automated programming of cutting parameters has enabled mold builders to achieve 24 hour-a-day unattended machining. But there Is much more to the process than just "burning a cavity". The unattended aspect of CNC EDM calls attention to a new set of problems. Extra steps must be taken to avoid contamination. The type of material being burned and the complexity of the electrode will determine the material for the electrode (generally graphite, copper, or sterling silver). Proper flush is a major factor. Set-up's and the order in which burns are made also enter in. Through proper planning the design department can be Instrumental in attaining the most efficient machining times as the work piece progresses from one work station to the next. This concept follows not only for the CNC EDM department, but for all other departments as well. This paper shall attempt to establish guidelines toward achieving this goal. The Importance of Design Without design there would never be an end product. No matter what the application there has to be some sort of design to produce the part. Design of the mold tool is an obvious and important first step. The mold builder is usually supplied with product drawings from the molder. Often it is to the molder's advantage to allow the mold builder to participate in the product design. In this way the mold builder can help design a part around his machines making the entire process more cost effective. The mold builder can immediately establish guidelines for or the entire project to follow. Potential problems of all mold components, electrodes, set-up's, etc. can be determined In advance through design. The Target Zero Concept Today's new 3-dimensional CAD systems enable a mold designer to view a product and its mold from any angle and/or direction. He can use this ability to see where special electrodes may be needed during the machining process. He can also establish an electrode's "target zero" which Is Its location In all axes (X,Y, and Z) to make a correct burn. A simplified set-up method for holding and locating electrodes is another important factor. Electrodes must be able to be machined in different machines (lathe, mill, grinder, etc.) without the lost time of indicating them each time. This facility’s need for an accurate and repeatable holding system. There are many standardized electrode holding systems on the market today. O.A.R. uses a modular system which consists of a circular shank with a locating dowel on which the electrode is mounted. It is well known that CNC EDM's are able to burn In 3-dimensions. For the sake of this discussion we will assume that the electrode Is a finished contour of the cavity shape and that the burn consists of a positioning move in X and Y (the horizontal plane) and a burn in the Z direction (the vertical axis). The target zero of the electrode Is the center of the shank. Correct positioning of the electrode Is done by simply programming the holder's target zero In the proper place according to the blueprint. This simplifies operations at the CNC machine. Each electrode is labeled and all Information about that electrode is entered Into a master chart. This chart Includes the following information: electrode label, target zero coordinates (X, Y, and Z), blueprint reference number, and a brief description of the burn which Includes electrode material and cutting parameters. Perfect inspection of every electrode is what makes this system work. All mistakes in contour or position must be caught b before the electrodes make it to the CNC machine. This part of the process is so important that O.A.R. has set up an inspection station specifically designed to check electrodes. All electrodes must pass through this station. A target zero is marked on the master chart only after the electrode has passed inspection. Project Management The use of a master chart Is not a new idea. Work flow charts are used in all businesses to help in planning. The use of these charts gives a visual representation to the progress of a job. Using this tool the mold builder call estimate machining times for both electrode fabrication and burning. He can be certain that electrodes will be ready when they are needed. This eliminates costly machine downtime. At the same time emergency break-in points in the schedule can be planned. It is very easy to see which series of burns should be done together and where the breaks between these series will be. There is only so much room on the worktable of a CNC EDM machine. As long as the machine can reach all of the burn locations, it dries no matter where on the worktable the piece is set up. By setting up the work piece off to one side of the worktable, room Is left to do emergency work If need be. Long burns can be run overnight for many nights without breaking down the work piece. Short burns can be run during the day by vatting up on the opposite side of the table. Both long and short runs can be made In the same machine with a minimum amount of setups. Naturally the size of the work pieces determine whether or not this type of preplanning procedure can be used. The flow chart method is very successful because It works not only for project managers but also for the men on the shop floor. Anyone can look at the chart and see ahead to the next phase of the project. This helps them plan their own schedules (depending on which part of the project they have been assigned) and to keep the work moving. Questions can usually be answered ahead of time because the chart helps show how the entire project fits together. In summary, the master chart becomes a catalog of necessary electrodes and also a flow chart for the entire project with "escape holes" built in. Onto the Shop Floor In order to achieve the most efficient burns at the CNC. Communication must be established between the electrode designer and the EDM operator. The wealth of information that the operator has to offer can be Invaluable. He can help to design electrodes that will most efficiently do the job. The operator can draw on his experience to supply over-burn dimensions for roughing and finishing burns, flush hole locations through the electrodes, and how the electrode should be mounted due to direction of the burn. lie can also specify preferred electrode material due to type of burn and the numbers of electrode needed. How the work piece is set up on the worktable is very important during electrode design. Hold down straps and other fixturing may not enable an electrode to travel directly to its target zero. Different height levels in the mold may also contribute to this condition. Where these fixtures will be is very critical to the designer In determining tool paths to target zeros. Of all the factors involved in electrode design nothing is more important than flushing. Excellent and efficient flush is the single most important factor in achieving a good burn. Proper finish in the cavities and good machining times are directly related to good flushing. Less contamination to the work piece and to the electrode results in better wear ratios (electrode to piece) and better all around machining. Serious thought should be put into flushing the burn to prevent a build up of sludge and grit between the cavity and the electrode. To clear the immediate area just to have the sediment collect in a future burn area defeats the purpose of flushing. Flush should be directed through the electrode as close to the center deepest part of the burn as possible. This way the flush acts like an upside-down umbrella to take burnt material up out of the cavity toward the top of the maid plate. Secondary flush lines could take the material horizontally off the mold plate where it would eventually be taken out by the machine's filtering system. Flush lines are added through the electrodes as needed to achieve this effect. This procedure takes a large amount of material out of the way very quickly. The multiple tool changer has become a tool the CNC EDM operator cannot do without. An entire multiple cavity job can be loaded Into the tool changer a at one time (tool changers are usually able to hold from 8 to 100 electrodes). The speed with which a tool changer can process a given number of electrodes makes the reason for prior planning clear. Also, the unattended machining ability that tool changers allow shows the reason for proper electrode and flush design. Typically the work crew is in the shop for only 8-9 hours a day. If the CNC's are running 24 hours a day they are always hungry for electrodes. To keep them "well fed" means the mold builder can get 24 hours of work for only 8 hours of man time. This situation is good for both molder and mold builder. The unattended machining time helps to keep delivery times low. Costs can also be lower due to more machine time with less paid labor. Flexible man hours help in controlling work flow. There will be times when the machine does not have a long enough burn to keep it running all night. The EDM operator can plan his work day around the machines using the master chart to approximate machine down times. HP would know when he would have to be in the shop to start the next series of burns. In this-way the machine can be realistically kept running for 24 hours a day. Along the same lines, there will be times when the operator Is in the shop while the machine is involved in a lengthy burn. Using everything that has been discussed so far, an operator has a number of options open to him. He can work on electrodes (either fabrication or inspection), he can generate programs for future burn series, or he can even met up and/or run another machine. In actuality, planning the job from the design stage has already alerted the operator as to when these periods would be. If long enough these periods could warrant secondary master charts so the time could be used most efficiently. As mentioned earlier, these breaks would be the logical choices for any emergency burn jobs. This-would mean minor scheduling changes In the primary job but this approach is still able to accommodate almost any situation. By programming cutting parameters into the CNC almost any type of finish can be obtained. The EDM operator Is able to produce a part that needs a minimum amount of finish work. This cuts man-hours even further. Given the right combination of programming, electrode accuracy, flush lush, and time, the CNC EDM can be a major force In the shop. Summary This presentation has touched briefly on some of the factors Involved in CNC EDM operation. The methods described here work for O.A.R. and can be applied in any mold shop. How this technology Is used Is limited only by one's Imagination. The importance of planning ahead cannot be overemphasized. The sometimes random nature of how jobs are contracted makes this a necessity. Even with emergency jobs trying to break tip the schedule, a plan gives direction and makes it easier to get back on track after the emergency has passed. Good electrode design and inspection helps to streamline the operation. Eliminating mistakes In the electrodes during fabrication lets the EDM operator concentrate on the quality of the burn, not whether or not things are in the right place. Proper flush hole location Is probably the most Important part of electrode design. Finally, the aspect of one man running multiple jobs, or machines is a benefit of CNC EDM. As the CNC's are running the mold builder can be preparing the next operations to keep the work Moving due to the many factors involved in mold making it safe to say that there will always be something for the mold builder to do. Today, CNC EDM's are commonplace. Unfortunately many shops do not use their enhanced features to their fullest potential. By using a no-nonsense approach as O.A.R. does these advanced tools can help to take today's mold shop into the 21st century and beyond. |
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