Innovation Boosts EDM's Appeal
Improvement and progress
By Robert B. Aronson
Electrical Discharge Machining (EDM), now a main-line manufacturing process, continues to attract new users. This popularity is due to improvement in the basic design of EDM equipment and advanced concepts that open new markets.
Methods Machine Tools (Sudbury, MA) has found that tailoring their EDM machines more closely to the needs of customers is key to success. "Some of our clients in the medical field do not need all the features offered on our standard machines," explains Steve Bond of the company's EDM group.
Removing certain features that do not influence cut quality, such as those related to process monitoring, greatly increases the appeal of the machines in some markets.
An advance that has increased versatility was the addition of the Fanuc 310i CNC, which has the ability to simultaneously control seven axes. The main payoff of the control is the ability to do more work in a single setup, which is particularly important for small complex parts where additional fixturing can introduce inaccuracies. These parts are often precise, highly delicate medical parts, such as special fasteners for implants.
"Through our work with PCD, we have also made advances in the cutting of nonconductive materials. The binder in these materials can be conductive, which allows the introduction of enough electric current to effectively EDM the PCD.
Makino (Mason, OH) has two new offerings for the EDM market: the DUO 43 and 64. The main difference in their design is the wire-guide system, either Pico or Split Precision V. The Pico is generally for high-precision applications, such as automatically threading holes smaller than 0.025" (0.64 mm) in a matrix with a center-to-center distance of 0.060" (1.5 mm). The V-guide system is suitable for about 85% of most other applications.
A lot of research is investigating the influence of EDM on the surface integrity of aerospace alloys regarding recast layer and HAZ. All EDM parts have some recast, even though it is not visible to the eye.
"The issue to be resolved is: 'Do today's EDMs leave a small enough amount of recast and HAZ so as to be safely used to complete more parts without requiring secondary operations," says Jeff Kiszonas, EDM Product Manager.
To determine the influence of EDM processes on recast layers on commonly used aerospace alloys, Makino engineers ran tests on aluminum, titanium, stainless, and Inconel, using both wire and sinker machines. "Indications were that recast layers averaged less than 0.0002" (0.005 mm), there was no measurable HAZ, and micro cracking or changes in hardness were negligable," says Kiszonas.
According to Kiszonas, one of the drivers of EDM research is fuel injector nozzles. Fuel economy for both gasoline and diesel engines can be greatly improved if the fuel is injected and finely atomized during the combustion cycle. The key to this achievement is fuel injectors with holes in the 60–90 µm range. EDM may be the answer.
There is an even increasing demand from industry—chiefly aerospace electronics, and medical—for very small, precisely made parts. In many cases, micro EDM is the answer because of the ability of this process to produce fine features with good surface finish.
A product that uses some EDM concepts is the EDS (electrical discharge saw). The "sawing" is done by a metal band of mild steel that is carried on two pulleys. As sodium-silicate dielectric is sprayed on the band, sparks between the saw band and workpiece erode the metal.
The EDS unit is limited to straight-through cuts and, for this type of work, is said to be less costly than conventional EDM cutting. Band width is 0.78 mm and the HEZ is said to be 0.5 mm.
The EDS 300 is available from Ross Group Equipment Technologies (Troy, MI).
SmalTec International (Lisle, IL) has developed a variety of products for this market. For example, their micro-EDM machine can combine with micromilling, drilling, turning, grinding, polishing, and even inspection in a single machine. Industry standard G-code programs that control the operation and tool paths for each of these processes can be automatically generated by a wide variety of CAD/CAM packages.
Many of the advances at SmalTec have come from improvements of established technologies. The spark erosion that does the cutting in EDM uses a basic RC (resistor/ capacitor) circuit. The control of the capacitance size directly impacts the spark size and frequency.
This technology can create a spark size, and corresponding spark gap, ranging from 0.5 to 10 µm and beyond. This wide range allows a large volume of material to be removed (relatively speaking) as well as a super fine surface finish. Because the pulse duration can be as short as 5 nanosec, a 50-nm Rmax surface finish is possible.
To achieve the highest accuracy, the company's GM703 micromachine has a step-over motion of 10 nm that ensures a good shape and surface finish. A surface roughness of 10-nm Rmax is achieved utilizing PCD. Tooling with motion control is based on conventional industry-standard G-code.
The EDM's code is designed to capture several conditions such as discharge, near short, and short. The software automatically responds to these conditions to protect the tool as well as the part. An operator that has operated or programmed conventional CNC machines can usually master the new system in hours.
"One area of research is the use of highly purified deionized water as an electrolyte," explains SmalTec General Manager Jerry Mraz. "This has the potential for reduced tool wear and better surface finish. Maintaining the purity of this fluid can be a challenge, because conventional filtering systems cannot remove the particles from the water."
Tool wear is another issue. With conventional dielectric oil, tool wear is 10–12% relative to the volume of material removed. Making a single hole can take 10 min or more. With the deionized water, production time is reduced by 25 x and tool wear drops to less than 0.1%.
Near-dry EDM is an area of interest. In this process, the electrolyte is a conductive medium "floating" in a gas. This version of EDM may reduced tool wear and improve productivity because, there is better control over the electrode-workpiece gap.
In the future, SmalTec may do some major research on EDM's inability to cut nonconductive material. According to Mraz, they will attack the problem from the material's side. "We want to investigate the potential for finding a way to establish higher conductivity within nonconductive materials. All materials have some form of conductivity because they are all made from atoms. The goal is to find some way to work with the material's atomic structure to make them more conductive. This may be done using nano techniques."
Another avenue to more versatile applications of EDM is to combine that process with waterjet or laser cutting. This may have the benefits of improved production speed and quality. Plus there is the benefit of cost savings and improved accuracy that a single-setup work cycle offers.
Wire EDMs have limited cutting speed due to the size of the wire electrode, while waterjet machines have limited cutting accuracy due to the waterjet itself. These disadvantages may be overcome with the hybrid waterjet/EDM machine from Sodick (Schaumburg, IL). Linear-motor axis drives with an axis speed of 1440 ipm (3657 cm/min), and 1.2-g acceleration are said to further increase cutting speed and accuracy.
The Hybrid Wire EDM has an axis travel of 22 x 14 x 10" (559 x 356 x 254 mm), maximum workpiece size is 30 x 22 x 10" (762 x 559 x 254 mm), and maximum workpiece weight is 1540 lb (699 kg). The waterjet cuts up to an 8° angle while the wire EDM cuts up to a 30° angle.
Agie Charmilles (Lincolnshire, IL) has made several moves to reduce the power needed for its line of EDM equipment. "Our new GL Digital generator can save customers as much as $6000 annually in power costs," says Business Development Manager Gisbert Ledvon.
The Resin water treatment system on a wire EDM ensures a specific conductivity level, controlling the water conductivity is critical to wire EDM performance. AgiesCharmilles' ResinMax water systems offers efficiency, increased resin life over standard systems, elimination of resin "channeling," easy installation, no direct handling of resin, no disposal of used resin, customer friendly refill program. This means a time saving because there are fewer resin changes and resin disposal cost problems are minimized.
In a move that goes counter to present trends, AgieCharmilles has stayed away from linear motors in some of its latest designs. "Though a redesign of our axis drives, our new machines move as fast as those driven by linear motors but consume less energy," says Ledvon.
"With linear motors, it's usually necessary to keep current flowing, because the motor has to be cooled by chillers that reduce error-causing heat transfer into the machine. This version consumes about 18 kVA when idle. Our version uses between 6 and 8 kVA," he says.
For a further savings, electrical components of the Agie EDMs are not left to idle when not in use. Integrated machine functions in the CNC shut high-energy-consumption components-down.
To improve cutting speed, Agie-Charmilles developed a series of wires with their key suppliers that have a specific coating with microscopic "pockets" that carry water. This allows the user to apply more power to the wire, and the additional water in these pockets cools the wire more efficiently and allows increases in cutting speed, typically from 30 to 42 in2/hr (194–271 mm2/hr) industrial cutting speeds.
Software modifications are the path for EDM performance improvement. For example, the newest wire machine from Mitsubishi EDM (Wood Dale, IL), uses the new M 700 series control, which takes a 3-D CAD Parasolid file and converts it into commands for a 2-D cutting path. In addition, the control reviews the parasolid file to find those areas where metal thickness varies. It then modifies the feed speed and spark power to match the part thickness being cut.
"Because of the increased complexity of many parts and the lack of competent EDM operators, there is a strong effort to incorporate what would be the skills of a competent operator into the controls," explains Greg Langenhorst, manager for MC Machines.
"With sinker machines, the effort has been to minimize the flushing needed to maintain the gap between the electrode and the spark. In addition, we have minimized the need for side flushing by increasing fluid circulation in the work tank."
"Keep it simple" is one of the important ideas in the design of the company's new line of mid-size machines, such as the BA24 wire machine. "This unit has reduced some of the bells and whistles, while retaining its cutting capabilities," says Langenhorst. This machine also fits into the company's program to emphasize machines for doing piece-part work over tool-and-die work.
For the potential buyer, Langenhorst cautions, "Don't get carried away by every claim of 'innovation.' You may not be aware of what adverse side effects will show up after a system has been used for a while. Many ideas that work well in the lab or on the first prototype will not survive long-term."
This article was first published in the November 2008 edition of Manufacturing Engineering magazine.