Small hole edm is a specialized component of electrical discharge machining. A small hollow electrode spins about a spindle much like a drill and drill bit (edm drill). The electrode is electrically charged by a servo-controlled generator producing the spark. Water based dielectric flushes through and around the electrode providing a controlled environment for the spark to jump to the work piece.
The electric spark erodes the surface of the work piece creating very small pockets. Eventually millions and millions of these microscopic pockets create the small hole. The size of the hole is controlled by the diameter of the electrode. The location and depth of the holes are driven by CNC ISO codes, reducing and eliminating operator error. Machines shown have automatic electrode changers for unattended machining.
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Small hole EDM (electrical discharge machining) drilling, also known as fast hole EDM drilling, hole popper, and start hole EDM drilling, was once relegated to a “last resort” method of drilling holes. Now, small hole EDM drilling is used for production work. Drilling speeds have been achieved of up to two inches per minute. Holes can be drilled in any electrical conductive material, whether hard or soft, including carbide. See below Figure for various small hole EDM machines.
Applications
Small hole edm is in many ways similar to a drilling operation but it offers the advantages of edm. Very low machining and work piece hardness are not barriers to performance. A short list of common applications follows.
1. Addition of wire edm start holes after heat treating
2. Removal of broken drill bits
3. Placement of holes too small/difficult for conventional drilling
4. Coolant holes in hardened machine tool bits
5. Taps
6. End Mills
7. Drill
Please see the small hole edm capabilities chart for size and thickness.
For high-production small hole drilling, machines are also available with tool
changers, as illustrated in Figure.
Small hole EDM drilling is used for putting holes in turbine blades, fuel injectors, cutting tool coolant holes, hardened punch ejector holes, plastic mold vent holes, wire EDM starter holes, and other operations. The term “small hole EDM drilling” is used because conventional ram EDM can also be used for drilling. However, ram EDM hole drilling is much slower than machines specifically designed for EDM drilling. See Figures.
Small hole EDM drilling, as illustrated in Figure 14:5, uses the same principles as ram EDM. A spark jumps across a gap and erodes the workpiece material. A servo drive maintains a gap between the electrode and the workpiece. If the electrode touches the workpiece, a short occurs. In such situations, the servo drive retracts the electrode. At that point, the servo motor retraces its path and resumes the EDM process.
A. Dielectric and Flushing Pressure: The dielectric fluid flushes the minute spherical chips that were eroded from the workpiece and the electrode. The dielectric fluid also provides an insulating medium between the electrode and the workpiece so that sufficient energy can be built. When the dielectric cannot resist the applied energy, a spark jumps from the electrode to the workpiece and causes the spark to erode the workpiece and the electrode. The servo mechanism provides the proper gap for spark erosion to continue.
Deionized water is preferred dielectric, but some manufacturers recommend an additive to aid in cutting. To accomplish small hole EDM drilling, high-pressure flushing is used (up to ten times the pressure for conventional ram EDM). Flushing pressure is one of the most important variables for high speed EDM drilling. The dielectric should be clean. Some manufacturers use the dielectric only once; others reuse it. When the dielectric is reused, it should be filtered carefully to remove eroded particles.
B. The Electrode: A round hollow electrode is constantly rotated as the dielectric fluid is pumped through the electrode. The rotating electrode helps in producing concentricity,
causing even wear, and helps in the flushing process. See Figures. Since the eroded particles are conductive, removing them from the hole is important to prevent shorting between the electrode and the workpiece, and to prevent EDMing the sides of the hole.
The high flushing pressure helps keep the
workpiece and electrode cool. See Figure 14:8.
This helps to keep the heat-affected zone, or
depth of recast level, at a manageable level. The pressure also aids in producing a reasonably good finish. Regular ram EDM's, with weaker flushing pressures are unable to duplicate the results of small hole EDM machining. Hollow electrodes allow dielectric fluid to flow through the electrode center. However, larger electrodes with a single hole can create problems. As the electrode erodes the workpiece, the center of the electrode does not remove material, thereby leaving a spike.
The spike can cause the machine to short. A short causes the machine to retract, which lengthens the cutting time. To overcome this problem, electrodes with multiple channels were developed to eliminate center slugs
C. Electrode Guides: The electrode guide keeps the electrode on location and prevents drifting while the rotating electrode is cutting. The electrode guide prevents electrode wobbling and aids in minimizing the EDM overcut, generally .001 to .002" (.025 to .05
mm) per side. The guides are above the workpiece; this allows the high pressure
dielectric to escape from the hole.
D. Servo Motors: The servo motors are controlled by a microprocessor which measures the gap voltage. By monitoring the gap voltage, the servo motor maintains the proper gap for spark erosion. If the gap voltage is too high, as in a short or accumulation of debris, the microprocessor signals the servo motor to retract the electrode. When the gap voltage is reduced, the servo motor advances the electrode and resumes cutting.
Due to the high-pressure removal of the EDM chips, the servo motor needs no constant retract cycle as in conventional ram EDM. The constant forward motion allows for rapid EDMing of holes.
Metal Disintegrating Machines Compared to Small Hole EDM Drilling
Metal disintegrating machines use the same principles as EDM, but these machines are used primarily for removing various types of broken taps, drills, and fasteners. Small hole EDM drilling is a much more precise method for drilling.
A metal disintegrating machine uses a hollow electrode to erode broken tools or fasteners. A coolant flows through the electrode and flushes the metal particles. Since the surface finish is unimportant, these metal disintegrating machines can remove within 1 minute a broken 1/4" (6 mm) tap that is 1" (25
mm) in the workpiece, and within 2 minutes a 1/2" (13 mm) tap that is 1" (25 mm) in the workpiece. These machines also come in portable models and can cut upside down.
Other Methods to Produce Holes
Besides small hole drilling, ram EDM, lasers, and photochemical machines can produce holes, even into hardened materials. Conventional drilling machines using carbide drills can also drill many hardened materials.
Disadvantages in Small Hole EDM Drilling
A. Electrode Wear: Considerable electrode wear results from EDM drilling. The electrode wear can equal or exceed the depth of the hole. For example, a 2 inch (51 mm) depth can
wear the electrode 2 inches (51 mm) or more.
B. Reduced Speed for Large Holes: Although large holes can be EDMed, the drilling time is often not competitive with conventional drilling or with wire EDM. For some difficult drilling applications, like carbide, a starter hole can be drilled with small hole EDM and then machined with wire EDM. Small hole EDMing is also used for holes that cannot be deburred due to obstructions.
C. Blind Holes Are Difficult to Control: Due to the high electrode wear, the depth of blind holes is difficult to control. Whenever possible, conventional drilling should be used for blind holes.However, if a blind hole is needed, the electrode needs to be dressed or a new
electrode used. Otherwise, electrode wear causes a bullet-shaped hole at the bottom.
Advantages in Small Hole EDM Drilling:
A. Drilling on Curved and Angled Surfaces
When holes must be drilled on curved or angled surfaces, great difficulties arise with conventional drilling. Drills tend to walk off such surfaces. To prevent drills from walking, fixturing and guide bushings are used on these irregular surfaces to guide conventional drills. But in EDM drilling, the electrode never contacts the material being cut. This non-contact machining process eliminates the tool pressure when drilling on curved or angled surfaces; however, water pressure coming from the electrode can cause slight deviation on curved surfaces. In starting, use lower water pressure to prevent water pressure movement of the electrode.
B. Drilling Hardened Materials: Some materials are too hard to drill using conventional methods, i.e., hardened tool steel, difficult alloys, and carbide. But material hardness does not affect the EDM process. However, some materials, like carbide, cut slower, not because of hardness, but because of conductivity properties of carbide.
C. Materials That Produce Chips that Cling to Cutters: Materials such as soft aluminum and copper can produce chips that cling to cutters. EDM drilling easily machines such materials.
D. Drilling Deep Holes: Drilling deep small holes with conventional drilling is often extremely difficult, and many times impossible. Small EDM hole drilling is often the only practical method for producing such holes.
E. No Hole Deburring: Deburring of holes from conventional drilling can take longer than drilling the holes. As in conventional EDMing, small hole EDM drilling creates no burrs when
drilling. This burr-free drilling is especially important when difficult holes, such as turbine blades, require deburring.
G. Creating Straight Holes: Due to the non-contact process of EDM, the deep hole EDM drilling produces straight holes. In contrast, conventional deep hole drills tend to drift.
Accuracy of Small Hole EDM Drilling Because eroded particles from the holes are flushed, variations can occur in the hole diameter. These are the reported results of small hole EDM drilling with a 0.040" (1 mm) drill in D2 tool steel.
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