Mighty Fine: General Industry Coverage

Courtesy of Sodick

Cemented carbide sample cut with an oil-dielectric wire EDM, accurate to +0µm/-1µm with a surface finish of 0.29µm R_z.

Fine finishing with a wire EDM eliminates need for secondary operations.

For years, conventional machining wisdom has held that achieving a superior surface finish after EDMing—2µin. Ra or better—requires time-consuming grinding, honing or polishing. Worse, it means removing the workpiece from the EDM to perform these processes. This adds cost and possibly causes dimensional inaccuracy.

In the past, conventional wisdom has said abrasive finishing is required because of inherent EDMing problems, such as the recast layer, where globules of molten workpiece and bits of wire reweld themselves to the workpiece surface, leaving it to resemble a sausage pizza left in the oven too long. Also, surface cracking has long been an issue when EDMing punch tools. Unless you’re making sledgehammers, surface cracking is a cause for concern in any tool and die shop.

Then there’s cobalt depletion of carbide workpieces, caused when the water dielectric typically used in wire EDMing attacks the cobalt binder, weakening it and leading to premature failure. And the heat-affected zone (HAZ) causes softening of the workpiece material in the area immediately surrounding and below the affected surface, which might have a negative impact on part life.

Before you fire up that grinder, however, consider that, for the most part, these problems belong to the past. Modern EDM technology not only imparts fine finishes, it does so fast and predictably, according to its proponents.

One EDM builder offering fine-finish capabilities is Makino Inc., Mason, Ohio. Brian Pfluger, EDM team leader at Makino’s die and mold technology center in Auburn Hills, Mich., said, “We have several EDMs that can achieve fine surface finishes, but even our standard machines are getting down to 6µin. to 10µin. R_a in five-pass machining of tool steel and tungsten carbide.”

But for true fine-finish applications, Makino developed U-Series wire EDMs, capable of imparting a 1µin. R_a finish and holding +0.0001 “/-0.0000 ” tolerances. “There’s a lot more to it than just circuitry,” Pfluger said. “You need a very rigid machine with no mechanical hiccups and special emphasis on the axis-drive system. Machines for fine finishing are designed from the ground up.”

Imparting a fine finish with a wire EDM, however, is trickier than achieving a smooth surface when sinker EDMing. Pfluger explained that a sinker EDM has a relatively large electrode, which allows the spark to disperse over a greater area, making high-quality finishes easier to achieve. “But in a wire machine, you need to drastically reduce power on the skim passes,” he said. “And as the power goes down, it makes it increasingly difficult to control the spark gap. Because the spark jumps a shorter distance, the machine’s mechanical system, wire tensioning and guide quality can create problems.”

Technology that can counter those problems sounds like it would be expensive, but according to Pfluger it’s not as much as you might think: a standard EDM might cost in the neighborhood of $150,000. You can anticipate an additional $40,000 for fine-finish capability.

EDM applications are also changing. “Of course EDMs play a big role in tool, die and mold making, but we’re seeing wire EDMing used more frequently in general machining, especially for aerospace, medical and defense parts,” Pfluger said. “About 70 percent of the wire EDMs we sell today go into job shops.”

And how do wire EDMs avoid metallurgical problems? There are several strategies, according to Pfluger. Achieving fine finishes on a wire EDM typically require additional passes at lower power, which increases cycle time, but also produces less and, in some cases, negligible microcracking, recast and a HAZ. “It’s not all about the finish, because if you look at the metallurgical characteristics of a part coming off a standard-finish machine vs. one coming off a fine-finish machine, the fine-finished part is going to be better, perhaps as much as 25 to 30 percent better.”

But what does better mean? Certainly cracking and recast are problems to be avoided, but the bane of any EDMed surface is the HAZ. Pfluger said: “Twenty years ago, the HAZ may have been 0.005 ” to 0.010 ” deep. In a stamping die application, that would be a problem because tools couldn’t hold a sharp edge long enough. In aerospace parts, you wouldn’t want to soften that material, because any small cracks might propagate through the workpiece. But today, the ‘pudding skin’ of soft material in the HAZ is no longer a problem because new generator and adaptive power control technology have drastically improved workpiece metallurgical characteristics.”

This means that, compared to 20 years ago, the HAZ layer produced by fine finishing today is virtually nonexistent, opening the doors to EDMing critical aerospace, medical and, especially, stamping components.

Moon Tool & Die Co. Inc., Meadville, Pa., uses a Makino U-Series EDM to produce stamping dies, mold cores and cavity work in hardened steel for the automotive, electronics and energy industries. Jack Moyer, the company’s general manager, said: “We have an EDM with Pico guides (precision round guides). A typical part for us ranges from 0.1 ” to 2 ” thick, and we hold tolerances inside of 0.0002 ” with four to five passes using the factory settings. We’re very impressed with the straightness and finishes we can achieve.”

Vibration Control

Another part of the fine-finish puzzle is controlling vibration. “Construction is critical,” said Dave Thomas, president of EDM builder Sodick Inc., Schaumburg, Ill. “You need to start with a very rigid, solid machine, and you have to eliminate any vibration in the machine, or any inaccuracies that can affect the finish.”

Sodick accomplishes this with a Meehanite cast iron base and a design that ensures no axis overhang. Powering the axes is equally important. “Linear motor technology allows us to drive the axes without any mechanical influence and eliminate any vibration. This means you can achieve finer finishes than with a machine using ballscrews,” Thomas said.

Courtesy of Mitsubishi

Digital circuitry enables wire EDMing 2 “-thick D-2 to a 4μin. R_a surface finish with an accuracy to ±0.00004 “.

He added that linear motors are sometimes misunderstood in the machine tool world. Because linear motors have a small amount of thrust relative to their size, builders of chip-making machines using linear motor technology typically compensate by using larger motors, which consume more energy and generate more heat—never a good thing in a precision machine tool. But in the case of the noncontact EDMing process, much less thrust is required than in conventional machining. “In an EDM, you need precise, smooth movement, and that’s what small, low-power linear motors give us,” Thomas said.

And Sodick, like some other EDM builders, uses oil dielectric for its fine-finish machines. “Oil dielectric machines were developed by Sodick 30 years ago,” Thomas said. “Because oil doesn’t conduct electricity to the same level as water, the spark intensity is much lower in oil, and if the spark intensity is much lower, the cavity each spark burns into the workpiece is much smaller. Therefore, we can achieve a better surface finish. And when machining carbide, the use of oil eliminates cobalt depletion, which can occur with a water dielectric. It also eliminates corrosion. All this means a better surface finish and better surface integrity, with an almost immeasurable recast layer.”

Courtesy of Makino

A carbide test piece shows a surface finish of 0.8μin. R_a cut with 0.004 “-dia. wire and 14 passes.

Oil-dielectric machines can run 0.001 “-dia. and thinner wires due to their “much more docile spark,” according to Thomas. The disadvantage is that cutting speeds are up to 50 percent slower than in water—especially when roughing. However, when using oil, fewer passes are required to achieve the same surface finish. “It’s not how fast you’re cutting, but how fast you can make the part to the correct tolerances and desired surface finish,” Thomas said.

One user of Sodick’s oil-dielectric machines is Marlow Roberts, vice president of manufacturing at Advanced Research Corp., White Bear Lake, Minn. ARC performs contract manufacturing for mold, tool and die customers, and manufactures gages and touch probes.

“Our customers like the finish we offer,” Roberts said. “We can get to a 1µm R_a finish or better in different workpiece materials, and hold tolerances of 0.0001 ” or better taking seven to nine passes. And since we don’t have to polish, it reduces bench time.”

Spark Control and Monitoring

Fanuc Corp., well-known for its CNC and servo equipment, takes a different approach to fine-finish technology. According to Steve Bond, national sales manager for Methods Machine Tools Inc., Sudbury, Mass., the exclusive U.S. importer of Fanuc RoboCut wire EDMs, Fanuc’s proprietary method of monitoring and controlling the spark is critical to generating fine surface finishes. “Fanuc’s Ai Pulse Control monitors each spark discharge and adjusts the cutting parameters automatically based on the effectiveness of the spark,” Bond said. This approach differs from the method used by most EDM builders, in which spark-gap voltage is monitored and cutting parameters are changed manually or automatically to maintain a consistent spark gap.