Machining operations like milling, turning, drilling and sawing are familiar to most people in manufacturing. Even those who are not machining experts typically know the basic functions of each of these activities and when they are used. Less familiar are some nontraditional machining operations, such as electrical discharge machining.
EDMing is sometimes called spark erosion machining, and toolmakers routinely use the word “burning” to describe what happens in the machine. The process removes material with an electric arc. The system has two electrodes, one of which is the workpiece, and the other is the tool. The work area is submersed in a dielectric fluid that electrically insulates the components from each other. A high-voltage charge is applied to the tool, causing the dielectric properties of the fluid to break down until an electric arc is created between the tool and workpiece. Material from the tool and workpiece is melted and carried away by the fluid, thereby generating the cutting action.
EDMs come in one of two varieties: ram-style or wire cut.
EDMing processes have numerous advantages, and accuracy is the most significant.
Ram-style machines sometimes are called die sink, sinker or conventional. Ram-style EDMs use a solid tool cut from copper, graphite or other conductive materials. The tool is machined so that it matches the desired shape of the workpiece in reverse. Shaped electrodes then are fed slowly into the workpiece, resulting in a cavity with the desired shape. Ram-style EDMing is used extensively in the tool and die industry to build sheet metal forming tools and in the injection molding industry to make injection dies.
EDMing drilling machines — or hole poppers as some people call them — are a subset of the ram-style machines. These machines use hollow cylindrical electrodes to drill holes. The electrodes are hollow so fluid can be pumped into the work zone to remove swarf and supply fresh dielectric fluid. Hole poppers are an integral part of gas turbine blade and vane manufacturing. Cooling air holes that protect the surfaces of these turbine parts are small, usually very deep and made often in curved surfaces. Drilling these holes by conventional means would be impossible.
Wire-type EDMs use the same technology and physics as ram machines. Although the cutting action of the arc and the flushing of the work zone are much like ram-style machines, the tool is different. Wire machines use — as you probably guessed — a wire electrode. Wire machines are CNC-controlled and programmed like machining centers. A small-diameter wire is driven along a programmed path, and the arc between the wire and the workpiece generates a small kerf along the path. Wire is consumed during the process as the wire continuously is unspooled and fed through the cut zone while traversing the programmed path. Wire cutting is used when there is no need to leave a cavity in the workpiece. Making complex punch and die sets is a common use of wire EDMs.
EDMing processes have numerous advantages, and accuracy is the most significant. Unlike conventional machining, which relies on contact between a tool and workpiece to form a chip, EDMing is noncontact. Because the cutting action is completed without contact between the component and tool, no pressure is exerted on the tool, workpiece or machine. No contact means no deflection, so only a few variables can introduce errors into the cutting path. The result is an extremely accurate process. Working to 0.0025 mm (0.0001") tolerances with EDMs is routine.
Zero cutting force means that EDM tools can be small and that workholding does not need to be complex or rigid. This makes EDMing ideal for tiny parts, such as those found in medical devices and miniature mechanical assemblies.
Ram-type machines can create geometries that would not be possible by any other means. Cavities with small corner radii, deep undercuts and other complex workpiece geometries may be impossible to make with conventional machining techniques. Ram machines are capable of multi-axis interpolation, which gives the ability to make threads and other complicated shapes.
Wire machines, like ram-style ones, can produce geometries that cannot be created by conventional machining processes. However, wire machines are not capable of working in cavities as the wire must extend through the part, making wire EDMing ideal for tools like extrusion and punch dies.
As with other CNC machines, wire EDMs can be outfitted with rotary devices, quick-change workholding and robotic loading and unloading, which makes wire machines appropriate for many applications. A quick search on the internet for wire-cut parts will show creative uses of wire EDMing.
EDMing has a lot of benefits but also drawbacks.
EDMing is slow compared with other machining processes, making it inherently more expensive.
Using a ram machine usually means that someone needs to machine the electrode before EDMing begins, thereby adding costs and lead time.
The machines are expensive. A machining center with a 508 mm × 762 mm (20"×30") work envelope can be purchased for less than $100,000. A wire machine with the same envelope costs over $200,000.
Recast forms when the molten metal on the cut surface solidifies. This undesirable condition is thought to harm the mechanical properties of the metal. Concerns about recast often disqualify EDMing from consideration in some industries, such as aerospace.
EDMing does not work on nonconductive material. When a workpiece cannot conduct electricity, there cannot be an arc between the part and electrode.
But once you realize that EDMing is needed, you will have recognized that it is the only process that will get the job done, so its unfavorable aspects are never really a concern.
It may sound negative to say EDMing is the machining process of last resort, but this actually is a testament to the effectiveness of the operation. When nothing else will work, burn it.
Related Glossary Terms
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- cutting force
Engagement of a tool’s cutting edge with a workpiece generates a cutting force. Such a cutting force combines tangential, feed and radial forces, which can be measured by a dynamometer. Of the three cutting force components, tangential force is the greatest. Tangential force generates torque and accounts for more than 95 percent of the machining power. See dynamometer.
- electrical-discharge machining ( EDM)
electrical-discharge machining ( EDM)
Process that vaporizes conductive materials by controlled application of pulsed electrical current that flows between a workpiece and electrode (tool) in a dielectric fluid. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Useful in diemaking.
Conversion of an ingot or billet into lengths of uniform cross section by forcing metal to flow plastically through a die orifice.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.
Width of cut left after a blade or tool makes a pass.
- machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
- mechanical properties
Properties of a material that reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness and fatigue limit.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- nontraditional machining
Variety of chemical, electrical, mechanical and thermal processes for machining workpieces. Originally applied to new or emerging processes, it designates any process developed since 1945.
Machining operation in which a powered machine, usually equipped with a blade having milled or ground teeth, is used to part material (cutoff) or give it a new shape (contour bandsawing, band machining). Four basic types of sawing operations are: hacksawing (power or manual operation in which the blade moves back and forth through the work, cutting on one of the strokes); cold or circular sawing (a rotating, circular, toothed blade parts the material much as a workshop table saw or radial-arm saw cuts wood); bandsawing (a flexible, toothed blade rides on wheels under tension and is guided through the work); and abrasive sawing (abrasive points attached to a fiber or metal backing part stock, could be considered a grinding operation).
Metal fines and grinding wheel particles generated during grinding.
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
- work envelope
Cube, sphere, cylinder or other physical space within which the cutting tool is capable of reaching.