Burning Higher Volumes

Better technology boosts use of EDMs for production applications.

The phrase “high speed” has never been associated with EDMs. Relatively low cutting speeds have kept many shops from considering EDMs for production work. 

However, as EDMs’ usable cutting speeds continue to increase and the machines keep evolving, part manufacturers are finding more production applications for electrical discharge machines. “As the machines got more capable, part manufacturers started applying EDMing more to production and designing parts that would lend themselves to wire EDMing,” said Jeff Gubbins, co-owner of Xact Wire EDM Corp., Waukesha, Wis.

Gubbins added that when he and John Dora established the EDM job shop in 1984, tool, die and mold makers understood the technology and therefore EDMed their low-volume work. In contrast, higher-volume manufacturers didn’t use the technology nearly as much, “mainly because the cutting speeds were a lot slower back then,” he said. “However, once an engineer is aware of what you can do with wire, he designs features into the parts and specifies tolerances so wire EDM is the way to make the part, and it’s easily made by wire EDM.”

Over time, production applications filled a larger percentage of Xact Wire’s workload, most of which consists of jobs with tolerances of ±0.0005 " or tighter, Gubbins noted. That includes medical instruments that are not economically feasible—or even possible—to machine with more conventional methods, according to Gubbins.

Material, Volume Issues

In addition to parts for the medical industry, Dave Thomas, president of EDM builder Sodick Inc., Schaumburg, Ill., indicated that higher volumes of aerospace components are being EDMed. According to Thomas, that’s because EDMs can cut hardened and difficult-to-machine materials faster than relatively soft and easy-to-cut ones and therefore are used when the workpiece material doesn’t lend itself to conventional machining. “Aerospace and medical are the real growth industries as far as production EDMing is concerned,” he said.

Although the majority of EDM production runs are wire EDMed, Steve Bond, national sales manager for EDM products at distributor Methods Machine Tools Inc., Sudbury, Mass., noted that manufacturers are applying sinker EDMing more for production parts made of nickel-base superalloys, such as turbine engine components for the aerospace industry. “Die sinkers are burning out details that are complicated or hard to get to in traditional machining,” he said. “The details tend to be small, so machining time is not real long. That production is shifting to EDMs.”

Courtesy of Xact Wire EDM

Xact Wire EDM produces an array of surgical instruments. The large block shows a good example of chaining and stacking.

What qualifies as a production run varies. “A high volume for an EDM would be lot sizes of 10 to 500,” Bond said.

Others feel higher volumes are appropriate—as long as they are not too high. Thomas noted that EDMing is suitable for the part volumes seen in the medical and aerospace industries, for example, but not for those found in many automotive applications. “It still could be a 24/7 operation, but I would say thousands instead of tens of thousands,” he said.

On the other end of the production spectrum are single parts with a repeating feature. For example, MGS Manufacturing Group Inc., Germantown, Wis., EDMs multiple cavities in molds for plastic-injection molding. “When you’re making 96 of the same cavity and trying to hold a couple tenths, it becomes high-precision production,” said Scott Spitza, MGS’ EDM manager. The majority of the molds are made of stainless steel and S-7 and H-1 tool steels.

Because the finished plastic parts produced with the molds MGS makes usually have tolerances of ±0.0003 ", Spitza pointed out that the moldmaker receives a tolerance specification of ±0.00015 " and the machine’s tolerance typically needs to be half that. “It’s a trickle-down effect,” he said. “Everything keeps compounding as you build it.”

To enhance machine accuracy while reducing maintenance, Thomas said linear motors are standard on all Sodick EDMs. “We eliminated ballscrews, which are a mechanical drive and therefore can induce wear,” he said. “Especially if you’re in production and working constantly in the same area of the machine, a machine with linear motors can go longer without maintenance.” 

Thomas added that ballscrews can cause inaccuracy after a year or two, especially when performing production applications and consistently working in the same area of the machine. In contrast, linear motors provide a noncontact, frictionless drive. Regardless of the number of hours a linear motor-driven EDM is run, Sodick reports that it provides a 10-year positioning guarantee.

Minimizing Recast

One barrier to using EDMs in production is that burning, or vaporizing, material to remove it creates the possibility of the material remelting onto the workpiece, known as recast. Manufacturers would have to completely remove recast from many parts, such as knee and hip implant components, so there’s no flaking of the material once inside a body, but EDM builders are developing technology that prevents recast from forming.

A new power supply on the Fanuc RoboCut iE wire EDM is a good example, according to Methods’ Bond. “Recast is virtually undetectable at 1,000× magnification in high-nickel alloys,” he said. “As a result, we’re breaking into areas we were never able to get into before.”

Courtesy of Methods Machine Tools

Methods Machine Tools designed and built this production EDMing cell, which uses a Fanuc robot on a rail to load a Fanuc wire EDM, an exeron sinker EDM and a Fanuc TrodeMaster graphite mill. The rack in the front left corner is for handling electrodes made on the mill for use in the sinker EDM. The rack in the back holds workpieces.

That includes turbine engine components, which previously were only broached or electrochemically machined at a higher cost per part than EDMing to avoid failure-inducing microcracking from the recast, according to Bond. He noted that refining the metal-removal spark by changing the technology in the power supply also imparts finer surfaces and increases cutting speed. Fanuc reports that it developed a method to isolate the lower arm power feed contact during skimming on the iE series machines. This significantly reduces stray capacitance during final skim cuts, according to the company. Additionally, the discharge pulse frequency is more than five times faster than Fanuc’s previous design.

Courtesy of MGS Manufacturing Group

Production EDMing can include producing a single part with multiple repeating features, such as this mold with 96 cavities.

In addition to boosting throughput, the higher speed reduces a drawback of wire EDMing: the creation of slugs, which can get trapped between the workpiece and the machine’s lower head. Rather than removing a slug, such as one for a 0.090 " hole, a fast cutting speed allows a wire to take multiple passes around a feature, incrementally increasing in size until the final dimension is achieved and thereby obliterating the material, according to Gubbins of Xact Wire EDM. “Because the machines are faster, it makes more sense to do that.”

When a slug is too large to economically eliminate with that method, an EDM can be programmed to leave a thin web about 0.020 " thick (or larger for large slugs) so the slug doesn’t drop, Gubbins added. Later, an operator can remove the slugs by auto threading the wire and continuing the cutoff while carefully monitoring the cut and then skim cutting the cavities. 

After Dark

High-volume production and lights-out machining typically go hand in hand. Although Xact Wire EDM operates three shifts, jobs are often set up for unattended EDMing. That primarily involves chaining and stacking parts to cut as many parts per setup as possible. “The operator just has to check in on it once in awhile,” Gubbins said. “This takes careful planning and experience to know what your tolerances will allow.”

Stacking sheets of material is straightforward, but when the workpiece shape is more complex, such as tubes for endoscopic surgical instruments, Xact Wire EDM designs and produces fixtures to hold them. The shop also uses Erowa indexable chucks to preset outside the machine, to rotate a part 90° or to move a part from machine to machine. “We might put our own fixtures on those chucks,” Gubbins said. “Most wire jobs we do all in one setup in one machine.”

He noted that the company has evaluated integrating robots into its facility but determined that its methods for presetting and fixturing provide most of the benefits of robotic automation. “We just don’t have the robot there to take the chuck out and put the next one in,” Gubbins said.

Robots, however, can significantly enhance productivity, depending on the application. Methods Machine Tools’ Bond noted that one toolmaker uses a robot to feed PCD tools to up to 10 wire EDMs. “They did have a machining center in the cell, but pulled it out because a wire machine was producing the part complete with no secondary operations,” he said.

A robot could also be incorporated into a work cell to transfer graphite electrodes from the machining center where they’re produced to a sinker EDM, enabling a continuous flow of electrodes as needed. For example, an electrode might wear and need replacement after EDMing only one or two cavities. “It used to be the case that you’d have a machining center down the hall or across the shop and you’d machine an electrode, bring it to the die sinker and put it in a traditional electrode changer with 10 or 20 positions,” Bond said. “That was about as long a run as you could do.”

Courtesy of Methods Machine Tools

A machining cell concept with three Fanuc wire EDMs, a Fanuc 710i robot, an in/out load station (the circular table) and a multiple-level rack for holding workpieces.

The protective technologies in EDMs minimize wire breakage, but lights-out EDMing requires an effective auto threader just in case. According to Bond, Fanuc’s second generation of auto threaders enables threading through a submerged workpiece up to 800mm thick. 

He also noted the desire to rethread at the point where the wire breaks rather than at the beginning of the toolpath. The latter means the wire must retrace a potentially complex shape, which consumes time and can cause the wire to drag on a feature and break again. Also, some workpiece materials, such as Inconel and stainless steel, tend to move after EDMing relieves stress in the material, and that movement can close the gap a wire would have to navigate if rethreading wasn’t done at the point of breakage. “You can’t run production if your threader can’t do the job,” Bond said.

Thomas concurred that an automatic wire feeder is essential for unattended production and noted that Sodick offers a wire feed system that uses a pipe that can pass through the workpiece to the bottom guide, eliminating any chance of the wire not entering the bottom guide. “Also, the machines are intelligent and if there’s a problem, such as bad material, you can set the number of times it will attempt to feed the wire,” he said. “If it fails to recover, it goes on to the next part.”

Another technology that enhances production applications on EDMs is a drop tank, which Thomas noted is standard on Sodick machines. Rather than having three fixed sides and a manual door, a drop tank allows three sides of the tank to drop, providing easy access to the workpiece and work area for operators and robots, according to Thomas.

Except when micromachining, Spitza noted that MGS primarily performs EDMing unattended. “We’re probably one of the most highly automated shops in the U.S.,” he boasted. Nonetheless, that doesn’t mean the workers there are unimportant or being eliminated. “It doesn’t reduce the labor content,” Spitza added. “We redirect the labor and are able to get more done.” CTE

About the Author: Alan Richter is editor of CTE, having joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.

Courtesy of Makino

Makino offers its Edge2 sinker EDM with a wire EDM option.

'Multitask' electrical discharge machining 

Production EDMing can involve applying a wire in one machine and a sinker electrode in a second EDM, or dressing or otherwise preparing an electrode with a wire before transferring the electrode to a sinker machine. To eliminate this production roadblock, Makino Inc., Mason, Ohio, offers a sinker EDM with a wire option.

Although water is the preferred dielectric fluid for a wire EDM because it provides a faster cutting speed than an oil dielectric, the machine uses oil for both the wire and sinker operations in the same tank, noted John Bradford, micromachining R&D team leader. He is based at the machine tool builder’s technical headquarters in Auburn Hills, Mich. “Machining speed is not necessarily a prime objective,” he said. 

Instead, the hybrid machine enhances EDMing flexibility while reducing floor space and eliminating manual electrode handling and resetting of tools, according to Bradford. “The individual dressing or machining operations are not necessarily faster than doing them on a dedicated machine,” he said, “but the collective throughput cost is considerably lower because you don’t have a person handling and transferring the tools from one machine to another.”

To maximize return on investment, Bradford recommends operating the machine at least 12 to 16 hours a day, with about 85 percent of that time being unattended.

Both the wire and sinker electrodes use the same spindle, which moves from one operation to the other as needed. Bradford emphasized that combination machines increase electrode and part accuracy because of the common fixturing. “You have a machine that’s taking the place of a wire EDM, taking the place of a lathe or even a cylindrical or surface grinder,” he said. “You’re essentially consolidating machining operations or electrode preparatory operations.”

In addition to the wire, other options include an indexing rotary table to provide a fifth and sixth axis and fine-hole machining for assisting in guiding smaller, more fragile electrodes.

Less than a dozen of the machines are in operation in the U.S., primarily for production of medical, aerospace and power generation parts at a rate of about 100 to 1,000 a day. “Another segment is the molding industry,” Bradford said. “One customer is preparing special electrodes with the wire option and then burning cavities into a mold.”

—A. Richter

Contributors

Makino Inc.
(513) 573-7200
www.makino.com

Methods Machine Tools Inc.
(978) 443-5388
www.methodsmachine.com

MGS Manufacturing Group Inc.
(262) 255-5790
www.mgstech.com

Sodick Inc.
(847) 310-9000
www.sodick.com

Xact Wire EDM Corp.
(262) 549-9005
www.xactedm.com