FundaMetls

EDMs are only as good as their consumables

Operators need to know how to select wire and graphite

By Chris Frost
Manager
Mitsubishi Consumable
Products Group
Wood Dale, IL

Wire and diesinking electrical-discharge machines (EDM), long respected for their precision-machining capabilities, operate at optimum levels only when quality consumable products are used.

Wire EDM machines, for example, work most efficiently with the best selection of wire. Diesinking EDM machines, too, must have quality graphite electrodes for superior burning capabilities.

The following are guidelines for selecting two different consumable products--wire and graphite.

Down to the wire

The type and quality of wire used in wire EDM machines have a substantial influence on cutting speeds, as well as part quality.

While wire selection will be determined by the variables that affect the final outcome of the workpieces--such as material type, cutting rates, contour precision, and tended and untended operations--machinists with a knowledge of different wire characteristics are most likely to make a good wire selection for each application.

Wires all have different conductivity ratings, tensile strengths, wire melting temperatures, vapor pressures, material compositions, and elongation. The following list describes the various characteristics that must be considered when selecting a wire for a particular application.

Conductivity: Wire electrical-discharge machine generators require wire to handle large machining currents that easily exceed 700 peak amps, or 45 average amps. This energy transfer must be done efficiently to deliver the high-frequency, low-amperage pulses required for surface finishes of 0.2 Ra or less. This is often expressed as electrical conductivity of the wire. Different wires have different conductivity ratings. The type of wire selected will depend on the conductivity requirements of your specific application.

Tensile strength: Tensile strength is also an important factor in selecting wire. Tensile strength is a measure of a material's ability to resist breaking when placed under a longitudinal load. Different wires exhibit different tensile strengths, and different tensile strengths are selected for different applications. Copper, for example, has one of the lowest and molybdenum one of the highest (35,000 psi vs 280,000 psi) tensile strengths.

In addition, wires are sometimes referred to as "soft" and "hard." A soft wire does not have the ability to stay straight when if comes off the spool. While this is not critical during cutting (because of the tension applied to the wire), it is important when trying to automatically thread it during its relaxed state. A soft wire, however, is beneficial when making taper cuts in excess of 7 deg on machines with nontilting upper and lower guides. A hard wire is ideally suited for automatic threading machines. Because of its higher tensile strength, it is also valued for its ability to resist deflection under cutting forces.

Elongation: This is the percentage of change in length due to tension forces and thermal forces during EDM machining. It is a desirable characteristic in some applications. A soft wire may experience greater than 20% elongation, while a hard wire might stretch less than 2%. Wire with higher elongation will more correctly follow the intended toolpath and will introduce less vibrations in the cut as it travels over the entry/exit points of the guides.

Wire-melting temperature: The melting point of wires is an important consideration. It is desirable to have some wire wear to promote an open gap at the front of the EDM slot (to prevent/reduce short circuits). In addition, this wear opens up a gap at the rear of the slot to improve flushing action.

Vapor pressure: During the EDM cut, a lot of heat can be generated, some of which is lost or soaked up by the wire. This loss of potential heat reduces cutting performance. If too much heat is lost to the core of the wire, then the wire suffers heat stress and breaks. The wire must vaporize quickly, releasing the energy into the workpiece. All materials possess a melting point and create vapor pressure when heated. One of the desired qualities of an EDM wire is a low melting point and high vapor pressure. When the wire vaporizes instead of melts, gases are created instead of particles of molten metal in the cutting zone. This, in turn, enhances the flushing process, since there are fewer particles to flush away. A material with a low melting temperature will be easier to vaporize.

Material composition: Wires are made of different materials suitable for different applications. The following is a list of various wire types:

Copper wire--The wire of choice for use in the first EDM machines, copper wire conducts electricity well and is readily available. But copper wire's flushability and low tensile strength of 34,000 to 60,000 psi led to the development of "engineered" wires.
Brass wire--The first "engineered" wire for EDM, brass wire produced a dramatic breakthrough in cutting speed from 1.1눼²/hr. to more than 2.3눼²/hr. in 2눼 workpiece thickness. Today's highly evolved wires can cut at 28눼² to 30눼²/hr. Brass wire is available in several different tensile strengths to suit various machine and application characteristics. Plain brass wire tensile strengths range from 70,000 to 130,000 psi. This is a cost-effective wire that can produce superior workpieces when used in EDM machines manufactured specifically to handle brass wire, such as Mitsubishi EDM machines.
Molybdenum wire--Dubbed "moly" wire, molybdenum has some characteristics which made it popular at one time. It's now considered an expensive solution to cutting intricate workpieces with small internal radii. With high tensile strength of 280,000 psi, moly wire is especially beneficial when using wires of 0.002눼 to 0.004눼 dia. Moly wire isn't widely used today because of its high cost and high melting temperature (4757 F), which results in poor flushability. Moly wire is being replaced in many applications by the more cost-effective and productive hard aluminum-brass wires.
Steel-clad wire--An engineered wire, steel-clad wire contains a steel core for strength up to 135,000 psi, a copper coating for conductivity and an outer brass coating for flushability.
Zinc-coated copper wire--Zinc-coated copper wire has a high conductivity copper core. And with the addition of 0.4% magnesium, it has improved performance. It is rough with micro-cracks on the surface layer which improves flushing performance. The machinist can cut high workpieces and cut graphite and aluminum alloys with superior accuracy. This wire has a high resistance to breakage and is suitable for production runs.
Zinc-coated brass wire--Zinc-coated brass wire has excellent mechanical straightness, high tensile strength of 130,000 psi, and is very accurate. These properties make it an excellent general purpose wire where greater speed and excellent finish are required. Its high tensile strength makes it very suitable for cuts involving 0.003눼, 0.004눼, and 0.006눼 dia wire.
Silver-coated brass wire--This wire was developed in 1990 to offer the traditional brass wire user a mid-priced wire that costs slightly more than brass, but less than coated wires. Silver-coated wire increases machining speed on high-performance machines at an economical price. The key to the wire's performance is the efficient transfer of electrical energy from the carbide power contacts to the wire itself.
Diffusion-annealed wires--With a thin surface coating of 50% copper/50% zinc, diffusion-annealed wires have superior flushability and greater speed.

Guide to graphite

Graphite is the material most commonly used for diesinking EDM machine electrodes. Selecting the right electrode material will have a dramatic effect on the quality of the finished part. When selecting an electrode, machinists should review the electrode's metal removal rate, wear, surface finish, machinability, and cost. These characteristics must be taken into account when purchasing graphite for specific EDM applications. Following are guidelines provided by Poco Graphite Inc from the company's EDM Selection Guide:

Metal removal rate (MRR)--Measured in cubic inches per hour, the MRR is the speed with which the electrode wears down. Achieving an efficient removal rate is not simply a matter of the right machine settings. The proper material choice for the work performed raises and lowers the MRR.

Wear resistance (WR)--There are four types of wear on the electrode: volumetric, corner, end, and side. The corner wear is important since the final cut contours are determined by the electrode's ability to resist erosion of its corners and edges.

Surface finish (SF)--Surface finish is obtained by a combination of the proper electrode material, good flushing conditions, and the appropriate power-supply settings. High frequency and low power produce the best finish.

Machinability--Graphite cuts very easily, but simply being easy to machine doesn't necessarily make a material the best choice for an electrode. It must be strong to resist damage from handling and from the EDM process. Strength and small particle size are important so that minimum radii and close tolerances may be achieved.

Material cost--Fabrication time, cutting time, labor, electrode wear -- all these factors depend on the electrode material. Thus it's important to know the properties and characteristics of the electrode material as they affect the metals being machined.

To select an electrode for a large cavity with no detail, for example, a machinist would typically select a material that would provide a high metal removal rate, good wear, and be available in a large size at a reasonable cost. At the other extreme, a very small cavity with razor-sharp detail would typically require an electrode material where good surface finish, good wear resistance, and machinability.

EDM graphites are classified by their particle sizes, ranging from finest (which is less than a micron) to coarse (which comes in particle sizes greater than 100 microns.) Machinists should use graphite with a fine particle size for detailed workpieces.

Information for this article was supplied by Mitsubishi CPG, Wood Dale, IL, GISCO Equipment Inc, Hauppauge, NY, and Poco Graphite Inc., Decator, TX.

This article was originally published in the November 1997 issue of Metlfax.