October 2011 / Volume 63 / Issue 10|
By Kip Hanson
Courtesy of Sodick 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. Ra 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. Ra 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
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
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 Ra 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.
Courtesy of Sodick
Bond explained that conventional spark-gap monitoring method often produces micro lines and wall-thickness variations in the workpiece, requiring several additional passes to correct. “When using Fanuc’s approach, the discharge is monitored thousands of times a second and minor cutting condition changes are made continuously, which produces a surface requiring much less work by the power supply to overcome the damage produced by a rough cut,” he said.
Bond added that the improved Ai Pulse Control II incorporates a device in the lower head that keeps the lower power feed contact, or brush, from discharging against the wire. This produces lower impedance in the cutting zone and a finer surface finish, according to Bond.
The Fanuc EDM also virtually eliminates recast. Methods reports that independent laboratory results have shown no visible recast at 1,000× magnification in nickel-base alloys, titanium and stainless steel. “This has opened up new opportunities for EDMing where it was once forbidden as a finishing process,” Bond said.Water Dielectric
Mitsubishi offers yet another fine-finish approach. MC Machinery Systems Inc., Wood Dale, Ill., offers Mitsubishi EDMs with a power supply that reduces workpiece corrosion while imparting fine surface finishes using water dielectric.
Greg Langenhorst, technical marketing manager, said Mitsubishi’s anti-electrolysis (AE) system creates a reverse-voltage pulse during the off time of spark generation. “This eliminates the stray DC voltage in the work tank that causes corrosion, oxidation and cobalt depletion in carbide. AE is on all the time, from the rough cut to the final skim, providing zero-voltage protection throughout the cutting process.”
The Mitsubishi power supply offers two levels of fine-finish circuitry. The first level is the PF circuit, standard on Mitsubishi’s FA series, which can achieve surface finishes down to 9µm Ra in hard tool steel and carbide. The next level is the Digital-FS circuit, available only on the company’s NA series machines, which produces a finish as fine as 2.4µm Ra in tool steel and carbide.
“Any time you want a finer finish, the process adds passes,” Langenhorst said. “Each pass offsets to remove less and less material at a progressively lower power setting. Each lower setting creates a smaller spark, thus creating a finer finish and inducing less destructive energy into the workpiece surface, which reduces the recast layer.
“The trick to producing fine finishes without sacrificing a great deal of speed is high frequency,” he continued. “The more small sparks you can pack onto the wire, the faster you can skim cut and still maintain the desired finish.”
Courtesy of Makino
High-speed stampers that use mostly carbide dies are good candidates for fine-finish wire EDMs. According to Langenhorst, most carbide punches and dies processed using AE require no secondary finishing, and cobalt binder is not depleted along the cutting edge, allowing the die to make more parts before requiring resharpening.
Mold inserts are another good application for fine-finish wire EDMs due to reduced polishing time and a more uniform surface for the polisher to work with, thus maintaining a true-to-cut profile. “Aircraft engine builders love AE and the fine-finishing process because it creates an almost zero recast surface on the root forms that hold the blades in the engine hubs,” Langenhorst said.Medical Marvels
One of Mitsubishi’s customers is a medical manufacturer in the eastern U.S. The 650-employee shop produces bone plates and screws, intramedullary nail systems, and spine repair and bone replacement products.
The shop uses Mitsubishi FA machines with FM circuitry to produce high-accuracy tools. The wire EDM can impart a 3µm to 4µm Ra finish, which eliminates the need for most lapping and honing. For example, a 180mm-tall part required flatness and squareness within 5µm with an N4 finish (3.5µm Ra). After one roughing pass and six skims, the part only required grinding to remove the cutoff tab.
The EDMed punch is highly repeatable, and even if it doesn’t last as long as a ground punch, it can be replaced cheaper and faster, according to the shop.
Machine maintenance—including filters and power feeders (indexable carbide inserts that conduct electricity to the wire)—is critical to EDM performance, according to the shop. It uses extended power feeders, which cost 10 percent more than traditional power feeders, but last two to three times longer. The shop also uses clean, temperature-controlled areas for its EDMs, which helps it produce more accurate parts and protect its machines.
Achieving fine surfaces with wire EDMs requires a mix of the right equipment, maintenance, environment and personnel—the most critical element. One good analogy is a stock car race. Drivers might have the same car, tires and engine, but someone is going to win. In the end, it’s all about the guy behind the wheel and the pit crew. CTEAbout the Author: Kip Hanson is a freelance writer and manufacturing consultant. Contact him at (520) 548-7328 or firstname.lastname@example.org.
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