Continual Improvement

The stagnation or decline of an industrial system occurs when managers and engineers begin to believe the phrase, “If it ain’t broke, don’t fix it.”

Individuals who think that any material-removal system is perfect should be discouraged from working in the metalworking industry. Every machining operation is a process that needs to be continually improved—and that includes electrical-discharge machining.

EDM has made tremendous advances since its “tap-busting” days 50 years ago. Some of today’s EDMs, for example, come equipped with 6-axis CNCs and the newest power-supply systems. They produce parts with mirror finishes and “split-tenth” accuracy.

There have been many other improvements to the EDM process in recent years, of course. And many others are in the works. Discussed below are five of the newest developments: an electrode material that’s highly resistant to spark erosion; replacing dielectric fluid with a gas; electroconductive powders that can be added to dielectric fluids to improve surface finish; EDMing of nonconductive materials; and a method for producing complex, 3-D parts with simple electrodes.

Today, EDM is used to produce high-accuracy parts with mirror finishes.

Erosion-Resistant Electrode Material
EDM researchers have long sought to develop an electrode material that can withstand the high temperatures developed in the spark gap without suffering excessive wear. Researchers at Texas A&M University have developed such a material.

It is zirconium boron/copper (ZrB₂/Cu), a metal-matrix ceramic. The material has wear rates that are several times lower than those of plain copper and are comparable to copper tungsten’s rates. Additionally, ZrB₂/Cu costs less to manufacture than graphite and produces higher workpiece-erosion rates (Figure 1).

Researchers have come to understand that the best electrode has both a high melting point and high thermal conductivity. Copper coated with a material that has a high melting point, such as tungsten, ZrB₂, or titanium boron (TiB₂), works well if the coating remains in place. If the coating wears away, though, the copper surface evaporates. That means the coating cannot be so thin that the copper at the copper/coating interface melts, or boils, causing the coating to break.

For this reason, a metal-matrix ceramic makes an ideal electrode material. As sparks evaporate surface copper, microscopic cavities form in it and fill with the molten ceramic of the matrix.

Figure 1: ZrB₂/Cu produces higher workpiece-erosion rates.

Manufacturing the new electrode material is a four-step process. First, the raw ZrB₂ powders are coated with a polymer optimized for use in a selective laser sintering (SLS) machine. Second, the SLS tacks the powders together by sintering their respective polymer coatings and creating the desired electrode shape. Third, a high-temperature furnace vaporizes the polymer coatings and sinters the ZrB₂powder. Lastly, the 35% to 70% dense network of ZrB₂ is infiltrated with an appropriate copper alloy. What is usually considered to be a liability in the SLS production of metals and ceramics—specifically, the lack of 100% density—is not only acceptable here but necessary for producing ZrB₂/Cu by infiltration.

Besides EDM, ZrB₂/Cu can be used for many other tasks, including spot welding, processing micro-electronics, and plasma-spray coating.

Gas Replaces Dielectric Fluid
Most people in the industry know that for the EDM process to work a dielectric fluid must fill the gap between the electrode and workpiece. The fluid acts as an insulator and provides the high-pressure flushing needed to remove molten material.

Researchers at the Tokyo University of Agriculture and Technology recently discovered that a gas can also perform these functions. They determined the following:

• a clean gas is as good an insulator as dirty EDM oil;

   

• a high-speed gas flow can transport molten metal away from the workpiece surface and immediately remove ions produced by the discharge; and

   

• a clean flow of pressurized gas can be introduced into the discharge gap through a hole in the electrode.

The researchers confirmed these findings by successfully producing a small, 3-D cavity on an EDM equipped with a CNC (Figure 2). The compressed gas was applied through a copper-tube electrode.

A potential benefit of using gas for EDMing is that it would free shops from the environmental problems associated with oil-based fluids.

Figure 2: A gas replaced the dielectric fluid in the EDMing of this cavity.

Conductive Powders Improve Finish
When EDMing a wide area, the large electrical capacitance between the electrode and the workpiece adds undesired energy to each discharge pulse. This makes it difficult to produce a fine surface finish.

Researchers have discovered that the addition of a silicon powder to the dielectric fluid results in a very smooth finish. This suggests that a high electrical impedance around the discharge gap helps to suppress the contribution of stray capacitance to the energy of a discharge pulse.

The discovery has led to the use of EDM oils containing suspended electroconductive powders. Although not yet fully understood, it has been proven that the technique improves surface finish as effectively as using a silicon electrode. However, utilizing the powders is much more practical because it allows the use of complex-shaped electrodes—shapes that are too detailed to be made from silicon (Figure 3).

Figure 3: An electroconductive powder suspended in a dielectric fluid allows the use of complex-shaped electrodes that can produce 3-D surfaces, like the one shown above.

Some recently introduced commercial machines let users take advantage of electroconductive powders. A benefit of the technology is that it significantly reduces the amount of hand polishing required for molds and other highly polished, machined cavities.

A drawback is that the powders require the gap distance to be wider than if plain EDM oil were used. As a result, some degradation of machining precision may occur.

Machining Nonconductive Materials
A long-accepted belief about EDM is that the workpiece must be electrically conductive. That is now being questioned.

Figure 4: Nonconductive ceramic produced on a conventional EDM.

It was discovered in 1993 that an electrically nonconductive material such as silicon nitride (Si₃N₄) can be machined by a conventional die-sinker EDM. Until recently, most insulating materials had been considered unworkable by EDM.

Researchers in Japan are testing a technique that allows the EDMing of nonconductive materials such as ceramic. The researchers achieve this by coating or plating the ceramic with a highly conductive metal. So, for example, even though natural Si₃N₄ cannot be EDMed, metal-plated Si₃N₄ can. When the EDM spark strikes the conductive metal, thermal energy from the spark not only removes metal but is released into the ceramic surface. This energy erodes the ceramic workpiece. Zirconium oxide (ZrO₂) and some other types of ceramics have also been EDMed (Figure 4).

A big advantage of the technique is that nonconductive materials can be EDMed on an unaltered conventional machine. The researchers developing the technique are currently working to improve machining speed and accuracy.

Keeping It Simple
Another area that EDM researchers are looking at is the production of complex, 3-D shapes with a simple electrode. Accomplishing this would decrease the time and expense needed to prepare electrodes for sinker-EDM applications.

Electrical-discharge milling with a round, rotating electrode is one way to achieve this goal. But researchers at the University of Tokyo Institute of Industrial Science have developed a more advanced method.

They combined two processes into a single system by fitting a wire-electrical-discharge-grinding (WEDG) machine onto a micro-EDM. The WEDG makes the micro-electrodes and the micro-EDM produces the parts.

Layer-by-layer machining is used. A cylindrical, rotating electrode moves back and forth over the workpiece surface, removing a layer of material with each pass. This permits the use of simple electrodes with round or polygonal sections to produce complex, 3-D microcavities—even ones with sharp corners (Figure 5).

In order to perform this type of electrical-discharge milling, the researchers developed a way to determine how much workpiece material was being removed in relation to the uniform wear rate of the electrode. The EDM automatically compensates for these wear rates.

Figure 5: A simple electrode can be used for electrical-discharge milling microcavities with sharp corners.

Most materials used for dies and molds can be machined by this method, and it has been used to produce standard-size mold cavities. It’s expected that the process will make it practical to mass-produce 3-D microparts from metals and plastics.

This development—along with those mentioned earlier—help reinforce that EDM is one of the most innovative and highly advanced machining processes in use today.

About the Author
R. Dean Brink is the technical director of EDM Technology Transfer, an Orem, UT, organization that tracks information pertaining to EDM.

New Developments in EDS

Electrical-discharge sawing (EDS), or arc sawing, is a modified EDM process that involves a low-voltage (~20v), high-current (~500a) arc operating in a highly conductive electrolyte solution.

Machining applications for EDS include the cutting of fragile aluminum honeycomb structures, stainless steel, zirconium, and titanium. Compared to conventional bandsawing, the process can reduce material waste, improve component distortion, and produce a burr-free machined surface.

Diagram of electrical-discharge saw built at the Univerity of Birmingham.

A great deal of the current interest in EDS stems from work carried out in the United Kingdom during the late 1980s and early ’90s. In the past five years, industrially funded research in the U.K. has resulted in the development of a simple single-axis EDS machine. It has been used to demonstrate the EDS process, develop a current-monitoring servo-control system, and conduct initial surface-integrity work on Nimonic 80A and Inconel 718.

A good deal of today’s EDS research is being conducted at the University of Birmingham, England. Researchers there are testing a 2-axis, servo-controlled EDS machine designed for the rapid sectioning of profiles in Inconel 718.

The machine is based on a converted Jones & Shipman 310 surface grinder. The insulated shaft spindle is belt-driven by a motor in the base of the machine. The spindle speed is fully controllable, from 0 to 5300 rpm, by an IMO Jaguar-drive, 3-phase speed controller. The acceleration and deceleration time is also fully programmable. The power supply allows digital control via an RS-232 port. The DC power control ranges from 0v to 20v and 0a to 500a. The design of the servo controller allows accurate setting of the discharge current to within ±0.5a. The machine has a traverse of 200mm in the y-axis and 450mm in the x-axis.

The new machine has improved the EDS of Inconel 718 three ways:

1. When cutting the material submerged, there is a significant reduction in the white-layer thickness and cracking depth. The thickness of the white layer was reduced by increasing the sodium-silicate concentration.

    

2. The higher electrolyte concentration also raised the material-removal rate, due to the system’s more stable and continuous arcing.

    

3. The stability of the arc and the overall machining performance was enhanced by operating under submerged conditions. — R. Brink