EDM Hole Drilling
Understanding noncontact drilling
May 4, 2011
Holemaking using erosion.
Electrical discharge machining (EDM) drilling is a little more complex than typical chip-removal processes.
In EDM work, energy is directed into an electrode and the workpiece is submerged in a dielectric fluid. When the two are separated by a small space—the spark gap—a pulsed electrical discharge from the electrode creates a spark that removes material from the workpiece through a process of melting and evaporation.
In typical applications, brass or copper electrodes with the correct finish-hole diameter are used to create holes in the workpiece.
The electrode is charged by the EDM power supply and is fed forward to create the machining feed rate. The power usage and electrode feed rate, as well as the motion of the machine's axes, are all controlled by the machine's CNC.
A rotating spindle holds the electrode, and a guide ensures the correct location is held during drilling. As the electrode creates a hole in the material, a high-pressure dielectric flush surrounds it. This high-pressure flush forces the eroded material to leave the hole quickly, enabling deeper holes to be made.
As holes become deeper, maintaining the high-pressure flush becomes even more important because the removed material has farther to travel up the hole.
If flushing conditions are poor, or if the pressure is not high enough, material can build up and will begin acting like an extension of the electrode. If this occurs, sparks will arc across the dielectric fluid and strike the workpiece in an unwanted area. This short-circuit, or DC, arc creates pitting on the workpiece and is the first indicator of poor flushing conditions.
The only limitation to workpiece material choice is that it must be conductive.
In mechanical drilling applications, the tool makes physical contact with the material and is more of a friction proc-ess. The drill can "wander" or "walk" as it makes contact with the workpiece, especially if it is drilling into a concave or convex part. Prehardened parts can also cause wandering to occur. Hardened metal, material with a hardness of 70 and higher on the Rockwell hardness C scale, can also be drilled with EDM.
Alternatively, EDM is a thermal process in which the material is eroded away rather than mechanically cut. The electrode does not come in contact with the workpiece.
"We maintain a gap between the electrode and the workpiece where the spark is created. It's noncontact machining," explained Dave Thomas, president of EDM manufacturer Sodick. "No physical stress is put on the material and no burrs are created."
According to Thomas, one of the most common drilling applications for EDMs in the marketplace is the creation of starter holes for wire EDM work.
These EDMs, often referred to as "hole poppers," are a special type used almost entirely to make starter holes. However, there are also some production applications in both the aerospace and medical industries.
"EDM was born as a die/mold process, but certainly over the last 15 years, it has become a system for production applications as well," said Thomas.
These machines have been designed to create holes quickly, and hole accuracy is typically within ±0.0005 in.
Precision Machines
The other type of holemaking EDM is for small, precise holes with diameters as small as 0.003 in.
"The machine changes when it becomes more of a high-precision machine. In these operations we use oil as a dielectric fluid rather than water," said Thomas. "And we can control the accuracy of the hole to 5 microns in terms of both diameter and position. We can even control accuracy down to 2 microns in some applications."
High-precision machines predominantly make use of linear-motor technology.
"By applying this technology to the drilling axis, we can better control the spark gap, flushing, and spark location," said Thomas. "There is a more delicate spark needed at these small diameters, and a ball screw-driven machine can't respond as well. Linear motors allow you to respond to changing conditions very quickly."
The control technology is also very important when performing precision applications.
The motion of the EDM's axes will be familiar to anyone who has run a milling machine. Therefore, moving the electrode through the work envelope is pretty basic. However, the control also manages the power supply, spark creation, and automatic electrode feed.
"One of the most important parts of the control is the power supply system that supplies the energy to the electrode," said Thomas. "Proper energy control is important to create a very fine spark, and the smaller the spark, the smaller the hole you can produce. It is necessary to make the correct movements, but far more importantly, it's about controlling the entire process."
Power levels are adjusted automatically by the control, which helps improve accuracy and surface finishes.
Hole Creation
EDMs are adept at creating very small holes in difficult-to-machine materials. For example, when using a 0.003-in. electrode on an EDM machine, you do not have to worry about tool breakage, which can occur easily at these diameters in conventional drilling.
Electrodes can be used in these applications that are tubular, not solid.
"While it is true that there is a hole through the center of the electrode, it is actually spiral-shaped," explained Thomas. "If it was a true straw-shaped tube, we would create a 'hollow,' a needle-shaped remnant standing up on the workpiece. The spiral shape, combined with the flushing through the tube, ensures that we don't actually leave any material in the hole."
Flushing is important, and so is the fluid delivery.
"Not only do you have to use the flush, but the amount of pressure required to put oil through a small tube like that is tremendous," explained GF AgieCharmilles Product Manager Ken Baeszler.
The two types of EDM drills, hole poppers and precision machines, create holes for different applications and in different diameters. Hole poppers typically create hole diameters between 0.006 and 0.200 in.
Precision EDM machines can create holes with diameters of 0.003 in. and even smaller.
"Generally speaking, the larger the hole diameter you are creating, the faster the feed rate can be, and the deeper you can drill," said Baeszler.
As the electrode enters the workpiece it will begin to wear away. Depending on the material being cut, electrode wear ranges from 50 to 250 percent. For example, at 100 percent wear rate, for every inch eroded, the electrode will wear by 1 in. too.
"This is such a big variable, and has always been an issue in EDM drilling," said Baeszler.
However, electrode wear is consistent and predictable.
"Electrode wear is so consistent during hole creation on our hole-popper EDMs that we actually have automatic electrode wear compensation on these machines," said Baeszler. "Being able to control wear more effectively is a key area of development for us. By reducing electrode wear, consumable costs are reduced, as is the time wasted changing electrodes."
Because electrode wear can vary so significantly, Baeszler's advice is to make sure electrode cost is figured into the job-quoting process.
"Calculate your wear," Baeszler advised. "If you have 50 holes to drill, determine how many holes you can get per electrode and budget accordingly. When you use electrodes that are smaller than 0.006 in. in diameter, it can be shocking to some users how expensive these electrodes can be. So make sure to talk to your electrode supplier when you are quoting jobs."
The larger-diameter electrodes typically are made of brass and are less expensive. As diameter decreases, the electrodes generally are made of tungsten or carbide and become more expensive.
Also, smaller-diameter electrodes typically wear faster. This means that the smaller the hole, the more expensive it is to create.
Wear varies depending on material type as well.
The control used for EDM can increase the power intensity, which enables faster drilling. However, it also creates more electrode wear. Depending on the application, especially in a production environment when cycle time is extremely important, you can sacrifice a little electrode wear to improve productivity.
In a die/mold environment, if you are making only two or three parts, time may not be the biggest factor. This will allow you to preserve the electrode—and consumable costs—by slowing down.
For more information, visit www.gfac.com and www.sodick.com.
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