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Multiaxis EDM makes large dies.

Electrical-discharge machining (EDM) came to metal-working shops in the early 1950s and has become a key process for producing small and medium-size precision molds and dies. Numerous capabilities and accessories have been developed to increase the accuracy, repeatability, and productivity of standard-size EDM machines, making them invaluable for creating intricate tools. Until recently, however, progress in developing sophisticated EDMs for large-part machining has not been as great.

To a large degree, this fits with the old adage, "necessity is the mother of invention." In the '50s, large tools generally meant forging or die casting. The tolerances for both were open, and the larger dies were not extremely intricate. It was simply a matter of cutting an electrode and sinking it straight into the die block.

Although improvements in metal formability have changed the nature of both forging and die casting, the development of reaction injection molding (RIM) has had the largest effect. With this process, it's possible to create large injection-molded plastic parts. Most noticeable in the automotive industry for making bumper fascias, the RIM process also plays a big role in the housewares industry.

Five-ton electrodes

The dies involved are extremely large, yet they demand very tight tolerances to maintain intricate details and mating characteristics. The need for multiple duplicate molds increases the demand for precision.

Dies weigh as much as 20 tons, and the electrodes used to make them weigh as much as 5 tons. Dimensions exceed 7 ft wide X 4 ft deep in a single part, and tolerances are in the 0.001" range, with details measured in hundredths of an inch.

Most of the demands can be met with larger, more-rigid machines, larger work tanks, sophisticated generators with precise controls, and precision-machined electrodes. However, creating the large electrodes is expensive and time consuming. Also, the repeatability needed to create precisely matched duplicate molds requires a high level of process control.

The mold-making process with a ram-type EDM has been fairly straightforward; all the design and production details were put into the electrode. The EDM operator was interested only in optimizing the cutting process for least wear, finest finish, and best speed. Although this produced good results for individual mold sets, the inherent variables in the electrode-manufacturing process and burning with different electrode sets can mean unacceptable variations between duplicate molds. Simply put, when you add the tolerances of the machining process to the EDM process, you may not be able to meet the specifications for the final tool.

As has been taught in many quality seminars, there are two ways to fix this problems. The first is to rework the out-of-tolerance parts till they match. The second is to not create the problem in the first place. Orbital EDMing can meet this second objective.

Orbiting all sides

Orbital machining on an EDM, such as the Charmilles-Nassovia, allows the machine to orbitor or rotate the electrode using X and Y movements to create complex cavities with a single electrode. Also, it can burn multiple, identically shaped cavities grouped around a centerpoint. The primary result is more-even wear on the electrode.

For example, with the straight sinking method would concentrate the wear on the bottom of the electrode, thereby rounding the edges. But when the electrode moves on the X and Y axes as well, the sides of the tool also burn the cavity. Balancing these movements results in even, predictable wear and little change in electrode shape. Introduced a few years ago for smaller machines, orbiting is now available on machines with electrode capacities to 5.5 tons.

A typical application is the manufacture of die-casting dies for transmission housings at BMW. The firm uses a Charmilled-Nassovia 535 with a work tank measuring 98" X 49" and table capacity of 42,000 lb. Typical electrodes can weigh up to 2000 lb.

Predictable enhancement

The basis of orbital EDM machining, as with other multiaxis methods, is the computer numerical control. A CNC program describes the desired part, and the EDM follows the program to cut the part. The software can direct an electrode changer to switch variour electrodes in and out of service to cut specific details or shapes. In this way, the EDM cuts the die or mold directly, rather than by duplicating a part model, and the result is increased accuracy.

Another key to a accuracy is the predictability of the electrical-discharge machining process. Based on a given current and on-time period, the control can predict electrode wear very closely. Also, it can determine electrode condition, based on machine performance while cutting the workpiece.

An adaptive control system adjusts machine and program settings to compensate for electrode wear and thus maintain precision. When wear tolerances are reached, the toolchanger provides a new electrode, and automatic setup cycles then feed new electrode data to the control to maintain untended operation.

Orbital techniques can also enhance control of the EDM process itself. Straight sinking electrodes allow only minor adjustments during the cut. Because most of these affect overburn and surface finish, no allowances can be made for fine-detail areas as opposed to large flat areas. With orbital machining, however, the program can be adjusted during EDMing to optimize the process.

The control sets a high current and large gap for major rough cutting, then sets low current and a smaller gap for the same electrode to provide a fine finish. Operators can edit the program while the machine is running--to adapt to actual cutting conditions or make changes in the part.

Crankshaft cavities

Another European application of orbital techniques took place at Smith-Clayton Forge in the UK. Machines used include a Charmilles-Nassovia 535. One of the latest dies to be produced makes a crankshaft for a truck engine. In addition to the orbital process, the firm used a four-channel, multiple-lead electrode with a maximum power of 2000 A. This allowed cutting large tools from solid, hardened, tool-steel blocks with no pre-milling.

In most cases, however, orbital techniques eliminate the need for multiple electrodes for one cavity. That's one way to improve the economics of the EDM process. Programming the EDM is equivalent to programming most multiaxis machines and usually can be done either on-line at the machine or by CAD/CAM and DNC link.

Finally, the overall reliability of the EDM diesinking process boosts productivity. Except for loading tools and an occasional check, the machines can run untended for very long periods, and even tool loading can be automated.

Just as the move to multiple-axis machining and CNC programming made major strides in standard machining practices and tolerances, the addition of orbital techniques to large-tool EDM diesinking promises to increase the versatility and productivity of these machines.
COPYRIGHT 1991 Nelson Publishing
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:electrical-discharge machining
Author:Gardner, Phil
Publication:Tooling & Production
Date:May 1, 1991
Words:1110
Previous Article:Muscle YAGs: precision plus power.
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