MACHINE TOOL BUYING GUIDES

Wire electrical discharge machining (EDM) is widely used to create dies, punches, mold components, special tooling, extrusion dies, airfoils, gears, medical instruments, carbide cutters, tool holders, jewelry and thousands of workpieces too numerous to list. Wire EDM can be used to cut electrically conductive materials to make parts that require a level of accuracy, intricacy and fine surface finish that traditional machining methods cannot achieve. A wire EDM unit can be programmed to cut complex shapes (small or large) to a dimensional tolerance of ±0.0001 inch and can be trusted to do so repeatedly and reliably. Unlike other types of machining, wire EDM exerts no cutting force on the workpiece and introduces no residual stress. There is little or no change in the mechanical properties of the material. Today’s wire EDM technology is capable of leaving virtually no thermal effects on the surface.

Wire EDM is best at cutting extremely hard materials such as tool steels, carbide, PCD (polycrystalline diamond), special alloys and parts requiring complex shapes, angles, tapers and sharp internal corners. Wire EDM can provide a surface finish as fine as 4 microinch Ra that may require no additional finishing or polishing. The wire EDM process leaves no burrs on the workpiece, a result which greatly reduces the need for subsequent operations. Wire EDM can be considered a time-saving, one-step process.

Wire EDM rarely calls for expensive workholding fixtures. One operator can run multiple machines.

How Wire EDM Works

A typical system consists of a CNC unit, a power supply with anti-electrolysis circuitry, a mechanism for automatic wire threading, a tank to contain the dielectric fluid, a chiller to keep the fluid at a steady temperature and a filtration system to keep the dielectric clean.

Wire EDM uses an electrically charged strand of wire fed from a reel and moved through upper and lower guides. The wire is energized as it passes across a power contact at the top and bottom. The distance between the guides can be adjusted to accommodate the thickness of the workpiece. A tightly controlled sequence of electrical discharges between the wire and the workpiece creates hundreds of thousands of sparks per second to remove the workpiece material. The heat of each electrical spark, estimated at around 15,000 to 21,000°F, melts away a microscopic bit of the workpiece material. Although the volume of metal removed during this short period of intense heat, lasting 1 millionth of a second or less, it is quite small, the frequency and multitude of sparks is an effective method to cut a narrow slot through very hard materials. The action of the spark may also remove material from the wire at the same time.

Submerging the wire and part in deionized water allows cutting debris to be flushed away. Splash flushing can be used when the part cannot be fully submerged, but a top and bottom nozzle must be constantly directed at the wire to help wash away debris. When machining submerged, cutting occurs in a more easily controlled environment and the flushing nozzles need to be adjusted only once.

A CNC unit moves the machine in an X-Y plane and can position the upper guide independently in the U-V axis, giving the machine the ability to move all four axes (X, Y, U, V) simultaneously to cut tapers or shapes that transition from top to bottom. A programmable Z axis enables workpieces with different thickness to be machined automatically.

Types and Sizes of EDM Wire

The wire used in the EDM process may consist of brass, zinc-coated brass, brass-coated copper, tungsten, molybdenum or brass with a steel core, to name a few. Each type has its own purpose, benefit and cost. EDM wire ranges in diameter from 0.0008 to 0.013 inch. A wire with a diameter of 0.0008 inch will produce a 0.00044-inch radius in a corner, a feature nearly impossible to manufacture any other way. Such precision may be required for openings in dies to produce electrical components, for example. However, wire this fine is rarely used, because of the unusual challenges it creates. Typically, the smaller the wire, the lower the power settings and the slower the cut. Most shops do not want to pay for this lengthy machine time unless the application absolutely requires it.

Plain brass wire 0.010 inch in diameter is used in more than 80 percent of EDM work. Three types of brass wire exist: hard, half-hard and soft. Soft brass is typically used for cutting tapers because a low tensile strength is needed. This type can bend while cutting at an angle without breaking.

Hard brass is best for both roughing and skimming when higher tensile strength is desired. Hard brass withstands aggressive flushing and enables a high voltage to be applied to the wire without breaking. A faster cutting rate is the result.

Brass wire is available with a zinc coating. Because zinc melts at a lower temperature than brass, the zinc absorbs heat as it boils away. Less heat enters the wire, so it retains its strength. Whereas brass wire is very smooth on the outside, zinc-coated brass has a rougher outside finish. This rougher finish improves flushing, resulting in increased-speed. Typically, coated wire delivers a 10 to 15-percent improvement in speed.

Wire with a copper core and a defused zinc outer layer is often referred to as high-speed wire, or stratified wire. The high electrical conductivity of copper enables doubling the cut speed. However, the price of the wire can be double that of other types. Stratified wire is recommended for roughing cuts. Plain copper wire is now rarely used because it is too soft and is too expensive.

No matter which type of wire is selected, it can only be used once. All wire is degraded by the wire EDM process and can be sold for scrap afterward.

Wire Tensile Strength

Specifying the tensile strength of EDM wire is also important. Even though the wire does not touch the part during cutting, it is stretched by the machine’s wire drive feed mechanism, which consists of wire tensioner, roller guides, and upper and lower feed contacts (where the electric current is applied). Tension is preloaded onto the wire and can then be increased or decreased to accomplish different cutting techniques. Tensile strength determines the ability of the wire to withstand the tension imposed during cutting. The lower the tensile strength, the easier it is to break.

Wire with less tensile strength will cut angles without breaking. By displacing or offsetting the wire guides in the UV axis, large tapers and angle cuts of up to 45 degrees can be generated. Wire with more tensile strength can be stretched more tightly when using the UV axis to closely control the perpendicularity of vertical walls for precise cutting.

For roughing, lower wire tension enables the machine to cut faster without breaking the wire. Skim cuts require higher tension, so slower speeds and less power are applied to achieve maximum surface finish and precision.

EDM wire is sold by the pound and typically comes in an 8-pound spool. Under normal cutting conditions, one spool will last eight to 10 hours of uninterrupted cutting. Most EDM manufacturers offer a large-spool option which will hold a spool of wire weighing as many as 35 or 50 pounds, making it possible to cut more than 40 hours without interruption. Provided the EDM unit has an auto rethreading system, the machine can operate for a week or more with little attention. However, a robot or automatic workpiece loading device is needed to support such long run times.

Automatic Rethreading

All wire EDM units now come with an automatic wire rethreading system. This feature works exceptionally well with wire as small as 0.002 inch in diameter, but the larger the wire diameter, the better. A high-pressure stream of water guides the wire through the workpiece.

Automatic threading enhances unattended operation by making it possible to produce multiple openings or features within a part in a single setup. Cutting and rethreading of the wire are controlled by codes in the program. If the wire breaks accidentally, the machine senses the break, rethreads the wire and resumes cutting where the wire broke. If wire smaller than 0.004 inch is required, a fine wire kit is available as an option.

Cutting Speed, Accuracy and Surface Finish

The two things every wire EDM user wants are speed and accuracy. Unfortunately, these objectives are usually incompatible. You don’t get speed with precision and you can’t achieve high accuracy without also achieving a fine surface finish. Accuracy and surface finish go together. Speed and accuracy do not.

EDM units from the early 1980s might achieve cutting speeds of 3 to 4 square inches per hour. With changes in machine design and power supplies, speeds of 17 square inches per hour became attainable in the 1990s. Today, with improved power supplies, working in conjunction with sophisticated adaptive controls, it is not uncommon to achieve 24, 37 and in some cases 45 square inches per hour.

The type of material and the height of the part being cut are critical as well. It is generally easier and faster to cut hardened tool steel than cold-rolled steel, for example. The harder the material the better. Typically, tool steels, carbide and special alloys have fewer impurities and lower porosity, making them easier to cut. Cold-rolled steel may contain impurities, so wire cutting is slower, and the surface finish is poorer. Although aluminum is easy to cut at higher speeds, the material is so soft that it is very difficult to get a good surface finish. Even a 30-microinch surface finish is difficult to achieve in aluminum. In contrast, it is possible to cut a three-inch thick carbide workpiece, with accuracies of ±0.0001 inch, and still produce a of 5-microinch Ra surface finish.

A typical wire EDM process consists of several passes, traveling at varying speeds. The first pass is generally a roughing pass designed to cut as quickly as possible, while accuracy and surface finish are less of a concern. Each subsequent skim cut travels at progressively faster speeds, takes less and less material while steadily improving dimensional accuracy and quality of the surface finish.

During the finish cuts, the tension on the wire is increased, the current is reduced, and the voltage gap narrowed, allowing the user to refine the spark and the distance the spark jumps from the wire to the part. The offset applied to the last finish pass might be as small as 3 microns. To achieve a 4- or 5-microinch Ra finish, as many as six or seven skim cuts might be necessary. Whereas the diameter of a cutting tool determines the offset in milling, the EDM controller applies a cutter comp based on the diameter of the wire. For example, if a 0.010-inch diameter brass wire is used, the cutter comp will approach 0.005 inch plus a spark gap as the wire gets closer and closer to the part surface, and possibly finish at 0.0051 inch.

To achieve these close tolerances and super-fine surface finishes, every parameter must be properly set. The right type of EDM wire must be selected. The wire must have the right diameter and tensile strength. The power setting and tension of the wire must also be right. The condition of the deionized water and flushing arrangements must be optimized, as well.

EDM Machine Accuracy

When attempting to hold ±0.0001-inch positional accuracy with wire EDM, the shop environment becomes a factor. For example, both steel and carbide have a thermal expansion coefficient of ~6.8 ppm per degree Fahrenheit. This means that, for every 2°F change in shop temperature, a 12-inch part could grow as much as 0.00016 inch, putting the operation over the 0.0001-inch tolerance it is trying to hold. To be successful under these conditions, a shop must be able to hold its ambient temperature within 1°F in either direction during an eight-hour period. Controlling the temperature of the dielectric solution to ±1°F also helps control the temperature of the machine and the workpiece.

The two most common machine designs use either ballscrews or linear motion systems. In terms of machine accuracy, each design has pluses and minuses, which must be explored when choosing a wire EDM unit.

High-precision glass scales are used to negate the effects of pitch error or backlash on the linear feedback. On the best machines, high-resolution servodrives with fine increments are used to position the wire, thus improving surface finish and accuracy. Adaptive controls can compensate for thermal growth. High-speed circuitry in servomotors enables them to react instantaneously for finer control of the spark. High-peak power supplies can now put more electrical energy into the wire, greatly enhancing productivity.

Dielectric Fluid and Flushing

The dielectric fluid, or coolant, used in the wire EDM process, is deionized water. It serves several purposes. First, it acts as a semiconductor between the energized wire and the workpiece to maintain stable and controlled conditions for ionization in the spark gap. Second, it can be chilled to keep the wire, workpiece, worktable and fixtures at a steady temperature. This limits thermal growth of the workpiece and machine in order to hold tight tolerances. Third, it acts as a flushing agent to wash away the ashy debris created as cutting occurs.

When a machine is commissioned, distilled or deionized water with low conductivity is used first. Subsequently, tap water can be used if first passed through a deionizing resin bottle to filter out any contaminants and neutralize particles with an electrical charge. The water then circulates through a 3- or 5-micron paper filter to remove any remaining particles. Most machines are equipped with 5-micron filters.

Because EDM creates microscopically small particles during the cutting process, removing these “chips” becomes a key factor in maximizing cutting speed as well as attaining part accuracy and surface finish. To be an effective flushing agent, the dielectric fluid must flow freely into the zone where the cutting action occurs. Because each spark melts away a microscopic bit of the workpiece, the fluid helps solidify the molten particle and keep it from adhering to the wire or the workpiece surface.

Adjustable flushing nozzles, which are located close to the top and bottom of the workpiece, direct a stream of fluid into the cutting zone from opposite directions. When the flushing pressure and nozzles are set properly, the two streams meet in the middle, creating a “rooster tail” effect that is visible on the slug removed after the roughing pass. If debris is coming mostly out of the bottom of the part when cutting a solid, flushing may be unbalanced. This creates poor cutting conditions where the wire has been most weakened as it passes through or across the workpiece surface. Frequent wire breakages can result.

Setting the flushing pressure too high can induce vibration and deflection of the wire, especially in tall workpieces. This condition will affect the surface finish and accuracy of the part.

Filtration – Maintaining Cleanliness of Dielectric Fluid

To get the best performance from a wire machine, the user must maintain the cleanliness of the dielectric fluid. If it gets too dirty, some materials will start to rust in the tank, and surface finish may deteriorate. For example, cutting aluminum raises the conductivity of the dielectric fluid quickly. When this happens, the deionizing bottle must be changed promptly. Good maintenance practice includes periodically replacing the paper filter cartridges and sending out the deionizing bottle for regeneration.

Cutting certain materials will clog filters rapidly. For example, when cutting additively manufactured parts, an internal pocket of loose, unsintered metal powder may be penetrated. The sudden release of this powder into the fluid can clog the filter unexpectedly.

Today, the cost of a filter ranges from $85 to $120.

Electrical Discharge Machine Maintenance

For the most part, wire EDM units require little maintenance. Some important maintenance items include cleaning the orifices in the wire guides and the surfaces of the power contacts. Power contacts should be indexed every 50 to 100 hours of cutting time. The wire rethreading mechanism should be checked and adjusted periodically. Follow the OEM’s recommendations faithfully. Expect to spend a modest amount of time on maintenance for each machine on a weekly basis, more often if machines are running more than eight hours a day.