Spark success with EDM: Medical Manufacturing

When the going gets tough, the sparks fly at many machine shops. Those sparks are the signature of electrical discharge machining, a process called on by those tackling some of the biggest challenges presented by metal cutting.

Common techniques of this kind include wire EDM and sinker EDM. The choice between the two depends on the machining task at hand. Both have their strengths and downsides, and both have been enhanced by recent developments unveiled by companies that make EDM machines.

Suitable for machining conductive materials, EDM works by producing current discharges between two electrodes — a tool and a workpiece — with different polarities that are separated by a dielectric fluid. When the electrodes are close to each other, sparks are generated, causing high-temperature spikes that melt and remove material from the workpiece.

The tool electrode in the wire EDM process is normally a thin wire. Today, the diameter of an EDM wire is typically 0.01″, said Eric Ostini, business development manager at Lincolnshire, Illinois-based GF Machining Solutions LLC, which sells EDM machines. As the EDM process melts material, the wire continues to unspool and move through the workpiece to cut the desired through-cavity shape. The process can include an initial roughing pass and a number of “skim” passes to improve the accuracy and surface finish of the cavity, he explained.

Wire EDM can include an initial roughing pass and a number of skim passes to improve accuracy and surface finish. Image courtesy of GF Machining Solutions

Unlike wire EDM, sinker EDM — also known as die-sinker EDM and ram EDM — normally is used to create blind holes or cavities. The shape of the tool electrode in sinker EDM is the inverse of the cavity shape that the machinist wishes to create. The electrode is plunged into the workpiece material, which is immersed in a dielectric fluid.

“You have to manufacture the electrode out of materials like graphite, copper and copper tungsten, and you need to machine the shape that you want to burn, typically on a CNC mill,” said Brian Coward, EDM product manager at Makino Inc., a machine manufacturer in Mason, Ohio. “So the cost for sinker is going to be higher than it would be for wire.”

In fact, he added, wire EDM is actually very cost-efficient compared with sinker.

“There is a cost for the spool of wire,” Coward said, “but it’s much cheaper than manufacturing an electrode.”

Adding to the electrode costs of sinker EDM is the fact that more than one of them probably will be needed.

“Usually,” Ostini said, “you’re using one electrode as a rougher, then a second electrode as a semifinisher and maybe even a finishing electrode to get to the final size and surface finish that you’d like to achieve.”

Pluses and Minuses

So why use EDM? When an application calls for very high accuracy and precision, there may be no other choice.

When EDM is employed, “we’re talking about holding ±0.0002″ in the operations,” said George Brown, Northeast and Southeast regional manager at Pittsburgh-based Vollmer of America Corp., which sells EDM machines for making cutting tools. “Basically, it’s on the same level as grinding, but with grinding you can’t always get the precision contours that you can with EDM.”

Wire EDM is employed to create parts with fine details and intricate geometries. Image courtesy of Makino

He also noted that EDM is a good fit for automated processes, including lights-out operations.

“There doesn’t need to be someone there babysitting the machine,” Brown said. With wire EDM, “as long as you have wire on the spool, the machine is going to run. But with a milling process, you’ve got to be concerned about how long your cutting tools are going to last.”

On the other hand, EDM is slow compared with other machining options.

“EDM is the last process you want to use to make something because it’s the most time-consuming process,” Brown said. “So if you can make it another way, you probably want to do it on a mill or lathe.”

Even though EDM machines might generate 300,000 sparks per second, Ostini noted, typical wire EDM speeds today are in the range of 37 to 42 sq. in. per hour, whereas milling speeds could be up to 200 sq. in. per hour.

Coward pointed out, however, that certain shapes and features simply cannot be milled. Or maybe you can mill them, he said, but if they’re really intricate, they actually could take longer to mill than to make with a wire EDM machine.

“Wire EDM is very easy to program,” he said, “and you can get things like very tight corner radii that you’re not going to be able to do in a mill unless you go down to very small cutters and then go in and pick out those corners. But that’s going to be very inefficient in terms of cost because those cutters are very expensive and they’re going to wear fast.”

In addition to its speed disadvantage in most cases, Brown mentioned another EDM downside that can be a cause for concern, especially in the aerospace industry. Although wire EDM makes it easier to put precise contours in titanium aerospace parts and wastes much less of that expensive material than conventional machining, he noted that the process also creates a heat-affected zone in parts, so aerospace users must make sure that the resulting properties in that zone are within allowable limits.