Wire EDM In The Extreme

Cutting metal with an energized strand of wire is still one of the most dynamic frontiers in metalworking, and this shop goes to extremes to keep up with the latest advances. It has even developed some of its own.

Article From: 2/1/1997 Modern Machine Shop, ,

Although it was founded in 1978 as a wire EDM shop in Minneapolis, Minnesota, Specialty Tool & Engineering has roots that go back to some of the earliest days of wire EDM in the United States. A number of the people who were involved in its startup had been part of Andrews Engineering, one of the first companies to introduce wire EDM equipment in this country.

Specialty Tool & Engineering was originally called EDM Specialty Tool. It was established as a job shop, but its main mission was to support the sales of EDM equipment by demonstrating how wire EDM worked and what it could do.

In those days, wire EDM was not well known, not well understood, and not widely used. For many applications, wire EDM was a last resort and few shops could justify having a wire EDM of their own.

Changing Times

But times have changed. Today, wire EDM is considered a mainstream metalworking process and it is not unusual to find a wire machine in the average job shop or typical toolroom. Although the basic principle of wire EDM has not changed (as the wire feeds from reel to reel, it uses sparks of electrical energy to progressively erode an electrically conductive work-piece along a path determined by the relative motion of the machine's axis), the process has advanced dramatically in the size, complexity, speed and accuracy of the metal-cutting it can perform even as the equipment has become more affordable, more reliable, and easier to operate.

Over the years, Specialty Tool & Engineering has changed, too. In the early 1980s, the shop's role as an EDM showroom gave way to that of a fully functional wire EDM job shop. As customers looked for additional manufacturing and engineering capability, the shop moved into complete mold, die, prototype and complex tooling work. The shop changed its name in 1995 to reflect its broader scope and today, the shop does milling, turning, grinding, ram EDM, and small hole EDM as well as wire EDM. The majority of the machine tools have computer numerical control (CNC) or other automated features and electronic readouts.

Progressive And Compound Dies

Nevertheless, wire EDM does not play a diminished role at Specialty. It is still very important and is a cornerstone of the shop's operation, but most of the wire work is generated in-house as a versatile technique for making the molds, dies and special tooling its customers source from Specialty Tool. In fact, wire EDM represents the shop's finest specialty. That's largely because the shop continues to take wire EDM into new frontiers of metalworking, going to extremes in the process. Literally.

But what may be considered extreme today will be the norm tomorrow. Although wire EDM is moving in many directions, Specialty Tool & Engineering's applications provide an interesting glimpse of where this technology is headed.

Extremely Fine Wire

A human hair is about 0.0035 inch in diameter. Specialty Tool routinely cuts with wire 0.001 inch in diameter. Greg Johnson, general manager of the shop, would say that the shop regularly cuts with wire that small because the jobs that call for wire that small are never routine. Often, the jobs come in from other shops not able (or willing) to cut with 0.001-inch wire. And Mr. Johnson admits that not every job they attempt is successful on the first try.

Wire this size must be made of tungsten to cut successfully. Tungsten has the tensile strength to withstand the tension placed on it in the EDM, and that tension has to be exactly controlled. Too much tension and the wire breaks; not enough and it bows and cuts inaccurately. That is true of any EDM wire, of course, but the difference between too much and not enough tension in this case is very slight. Whereas a commonly used electrode wire measuring 0.010 inch may require a tension somewhere between 1100 and 1900 grams on certain wire EDMs, Mr. Johnson reports that 0.001-inch wire on the same machines takes less than 100 grams in a range only 25 grams wide from highest to lowest.

Likewise, power settings are limited to a very small range. Because 0.001-inch wire has such a small surface area in any given length (about a third of that for 0.002-inch wire), it cannot carry very much electrical energy. According to Mr. Johnson, the shop has learned from experience where that band of settings lies on the machines it uses for this kind of super-small wire.

And those machines have to be meticulously maintained. Specialty has several older EDMs from Agie U.S.A. (Davidson, North Carolina) that it relies on for fine wire work. These machines lend themselves to fine wire because they have a three-point wire guide system that allows quick, accurate positioning of the wire. Also important is the wire tensioning system on these models. It is a mechanical arrangement that lets operators develop a "feel" for the proper wire tension.

The wire guides for wire of this diameter as well as matching rollers and power contacts are maintained separately and reserved only for work with this wire. It takes about two hours to set up a machine for this size wire and about the same to tear it down, so scheduling is an issue. Usually, the shop allows jobs calling for extremely fine wire to accumulate until enough is on hand to fill one or two days' time on the EDM.

"We do a lot of work for the computer hard drive industry and the trend is to greater and greater compactness of drive units -- so components and assemblies are shrinking in size," Mr. Johnson reports. "That drives up the demand for tooling that can handle smaller and smaller pieces." One example he cites is a carbide guide used to insert wires on hard drive components. Manufacturing this guide involved cutting a 0.0018-inch slot in a 0.5-inch-tall carbide workpiece -- that's 500 times the diameter of the EDM wire used to make it. The shop had to hold tolerances within a "tenth" -- 0.0001 inch.

Medical instruments are another application where Mr. Johnson sees a growing role for extremely fine wire. Forming the teeth on cutting devices used in endoscopic surgery is an example. "This is not the place for untended operation," warns Mr. Johnson, referring to wire EDM in this size range. "It calls for the full attention of our most experienced operators."

Extremely Small Holes

Riddle: When is the electrode on a wire EDM the same as the electrode on a ram EDM? Answer: when Specialty Tool is using its custom-built micro hole machine. The same 0.001-inch tungsten wire mentioned above serves as the electrode on this uniquely designed piece of equipment. It allows the shop to make holes 0.002 inch in diameter in workpieces up to 0.02 inches thick. Larger electrodes that measure up to 0.02 inch in diameter can also be used for micro hole making.

Specialty Tool started out with the X-Y axis table of a very early model Japax wire machine. Precision glass scales were retrofit to this table, giving it the level of positioning accuracy they were looking for. A Z-axis feed mechanism feeds sections of the wire into the workpiece. The wire acts as a miniature ram electrode that "sinks" a round cavity into and through the workpiece. A special power supply was also engineered for this machine. This power supply allows extremely fine adjustment of the current. On the other hand, its top amperage is relatively low -- electrodes over 0.02 inch can't draw enough current to erode material effectively.

For holes this size and larger, the shop's small-hole EDM units take over. The shop has two machines, one a CNC and the other a manual model, from Sodick EDM (Buffalo Grove, Illinois). These and other commercially available "hole poppers" are designed for "drilling" small holes at high speed. At Specialty, the electrodes used on these units are hollow brass tubes ranging in diameter from 0.008 to 0.187 inch.

With this shop's micro hole machine, however, the emphasis is not on speed. It's on precision. The shop achieves positioning and dimensional accuracy to ±0.0001 inch with this machine.

One especially interesting application characterizes the kind of work for which the shop utilizes this capability. All three types of EDM -- small hole, wire, and ram -- play a part. The micro hole machine is used to make a 0.002-inch start hole in a tool steel extrusion die, using 0.001-inch wire electrodes. The die opening is then wirecut with 0.001-inch wire to make a three-lobed shape that would fit within a 0.02-inch circle.

This die goes to another shop that uses it to extrude foot-long electrodes of pure silver. These electrodes come back to Specialty Tool, where they are used in ram EDMs to burn openings in a stainless steel spinneret extrusion die for the carpet-fiber indus-try. There may be as many as 300 openings in one spinneret die. Both the shape of these openings and their position on the die are highly engineered in response to the unique material flow and cooling behavior of the material extruded in these dies. All edges in the die openings must be sharp and totally burr-free.

Mr. Johnson sees precision small-hole EDM having a significant impact on wire EDM. "The size and precision of the start holes you can make determines what you can do with wire cutting, especially with very fine wire. It gives us an option when "hole poppers" don't have the accuracy, precision, fine finish or consistency for work with tolerances in the `tenths.'"

An Extremely Reliable Wire Threader

Wire EDM is a peculiar process. Sometimes you want to cut a good part and leave behind a hole; other times you want to cut a good hole and leave behind a part or "slug." And when it's hundreds that you have to make, whether they are holes or parts, wire EDM has become an important "high production" metalcutting technique. Here's another area where Specialty Tool has been pushing the limit.

"What's really helped us here is the reliability of today's automatic wire threaders in extremely difficult jobs," says Mark Kriedermacher, a manufacturing engineer at Specialty Tool. Although the shop has automatic wire threaders on four of its ten wire machines, the automatic threader on the shop's one wire machine from Fanuc/Methods EDM Division (Sudbury, Massachusetts) best represents the state of this technology.

The essence of using wire EDM for "production" jobs is the ability to complete one cut, sever the wire, move to a new position over the next start hole, thread the wire through the start hole and resume cutting. What makes this an especially efficient operation is doing the threading automatically. Automatic threading allows the machine to run without the constant attention of an operator as it moves from cut to cut.

"In the past, we wouldn't consider trying automatic threading unless the start holes were at least 0.060 or 0.090 inch in diameter," Mr. Kriedermacher recalls. "With all the work we do in tight tolerances and smaller wires, that was a real limitation. With the Fanuc, we'll consider work with start holes 0.030 inch and smaller and wire down to 0.004 inch in diameter."

The job that convinced them to reconsider auto threading was a plate of 174 stainless with 150 shaped holes for extruding carpet fiber. Start holes 0.015 inch in diameter had been drilled with one of the Sodick "hole poppers," and 0.004-inch wire was to be used to cut finished holes 0.022 inch in diameter. When running these holes overnight on the Fanuc wire machine was proposed, Mr. Johnson remembers thinking it wouldn't work. "The operator had a lot of confidence that it would, so we went ahead. The next morning, every hole had been completed. It hadn't missed a beat."

The Fanuc wire threader severs the wire by annealing and stretching it. This operation straightens the wire and leaves it slightly tapered with a rounded tip. High pressure waterjets guide the wire to the start hole without emptying the dielectric tank (the machine is designed for submerged cutting). If the wire misses the hole on the first try, the machine will make a user-determined number of additional attempts before it moves on to the next hole. A list of missed locations is posted on the CNC display for the operator.

According to Mr. Kreidermacher, they've come to count on threading to be 80 to 90 percent successful with 0.030-inch start holes, even in difficult situations such as tapers or where the wire guides cannot get close to the workpiece surface. A 100 percent rate, however, is common. "This opens up a lot of new possibilities," he says, "in terms of what is technically feasible as well as what is economically attractive."

The threading cycle is short because the dielectric tank remains filled. Frequent attention of the operator isn't required, so labor costs go down. And the machine can run 24 hours day. "We figure that around-the-clock operation gives us 50 percent higher utilization," Mr. Johnson says. Other EDM builders have been improving auto threaders. "No matter where our next wire EDM comes from, the advances in this technology make it indispensable, certainly at this shop."

Extremely Tight Tolerances

Very small workpieces or work-piece features with very tight tolerances are one end of the spectrum. At the other end, large workpieces with tight tolerances have their own set of challenges, and wire EDM is a problem-solver here, too.

The complexity of these jobs is often deceptive. A good example is the hydraulic valve sleeve made of high carbon steel, about 18 inches long and 9 inches in diameter, shown in the illustration above. Four rectangular openings have to be cut through to the ID, which is about 4.5 inches, at both ends of the sleeve. In essence, the wire is making a 9-inch-tall interrupted cut, which is difficult to begin with. The added challenge is cutting the openings so all four are perpendicular to 0.000125 inch to the center bore. Location of the sets of holes is toleranced at 0.00025 inch.

Specialty Tool's solution is to fixture the part in special tooling for precise indexing and allow the entire setup to stabilize at the temperature of the dielectric fluid, which is chilled to counter thermal growth during EDMing. This is the main advantage of submerged cutting in this case. A two- or three-degree change in the temperature of the dielectric fluid is enough to create thermal growth and lose the tolerances. One important consideration that Mr. Johnson acknowledges is agreeing on carefully defined inspection routines with the customer.

By the way, this sleeve is a repeat job for the company. It makes 50 or so a year for a firm that makes sophisticated testing equipment.

Back To Non-Traditional

For a long time, users and suppliers of EDM worked mightily to escape the classification as a "non-traditional" metalworking process. Non-traditional implied that it was exotic, experimental, or on the fringe. For the most part, this educational campaign has been successful. EDM is no longer a mystery, even to non-users.

Non-traditional can also mean out-of-the-ordinary and exceptional. For many shops, that was one of the lures of working with ram or wire EDM when these technologies were very new. It was exciting and challenging. And that seems to be the way everyone at Specialty Tool & Engineering still likes it to be. So at this shop, taking wire EDM to its limits, which they define for themselves if they can, is hardly extreme at all. It's a tradition.

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