Continuing our series of electrical discharge machining articles (extracts from our EDM machining guide, ‘The EDM Handbook’), this section looks at Electrode Selection (Wire Erosion).
Di-Spark: The Wire Erosion Company
Di-Spark Ltd in terms of an EDM company supply precision EDM machining (spark & wire electrical discharge machining services) alongside advanced milling & turning technologies such as large 5 axis machining, 5-axis milling & CNC mill-turn machining. Di-Spark were one of the first EDM companies in the UK during the 1970’s, making recent investments into clean cut wire erosion EDM machining technology, among other commitments to machining innovation.
Di-Spark are an EDM company supplying clean cut wire erosion machining services to high technology industries such as Formula 1, Oil & Gas, Medical, Aerospace, Satellite, Defence & Energy Markets
Di-Spark are a wire erosion EDM company supplying high technology industries such as Formula 1, Aerospace/Aviation, Oil, Gas and Energy markets, Medical and Surgical sectors, defence/SC21 and Satellite & Space Agency
8. ELECTRODE SELECTION (WIRE EROSION EDM)
In the early years of wire erosion EDM, choosing wire was very simple as copper was the only choice. Within a very short time the limitations of copper wire were discovered and various alloys were experimented with. Brass proved to be adequate, and today it remains the most widely used material. More recently, the increasingly difficult-to-machine alloys and the demands for faster cutting speeds brought about new wire materials that can support the high-performance power supplies available today.
8.1 WIRE EROSION: EDM WIRE PROPERTIES
Tensile Strength: This is the load-bearing capability of a material based upon its ability to resist stretching and breaking. High-tensile wire is used to reduce breakage when cutting tall parts or when using small diameter wire. It is also good for skim cuts, aiding in part straightness and geometric accuracy.
During wire erosion EDM operations, the wire has a natural tendency to lag behind the wire guides. The amount of lag can vary depending on cutting speed and work-piece thickness. This is called the bicycle effect, after the way the wheels of a bicycle behave whilst executing a turn. Two things will aggravate this condition, fast cutting speeds and the distance between the wire guides. Speed is perhaps the most obvious cause but tall work-pieces also increase this tendency. To help reduce the bicycle effect, a high-tensile wire should be used. This allows the wire to be run under higher tension, reducing the amount of lag.
Some controls have the ability to read ahead many lines within a program and slow down automatically, allowing the wire to catch up to the guides producing an accurate corner.
Another condition EDM users must contend with is the barrel effect. Often referred to as bow or belly. This is caused by the wire vibrating within the cut like a bowstring. This vibration is usually greatest in the middle of the part. The taller the part the greater the effect. Again, a high tensile strength wire can be pulled tighter and will help defeat this condition.
Fracture resistance: This might better be described as the ability of the wire to resist the effects of the incredibly hostile environment of the spark gap. Many factors contribute to reduce the effective tensile strength of a wire. When EDM machining with wire, the cross section of the wire is gradually being reduced. Small craters are burnt into the wire and these flaws act as stress-risers that can allow cracks to propagate and cause wire breakage. Too much current, poor flushing, secondary discharge, or too slow a wire speed will all further stress to the wire. Eventually, all these factors will exceed the wire’s toughness and it will fail. Therefore selecting a wire with a high fracture resistance is the best insurance against breakage.
Conductivity: This is the measure of a material’s ability to carry electrical current. The higher the conductivity, the more power can be delivered to the work-piece. In most cases, this means increased cutting speed. Copper wire offers the highest conductivity, but it has limitations due to their relatively low tensile strength.
Vaporisation Point: In wire erosion EDM operations, a low melting/vaporisation temperature of the wire will prove the best for flush-ability. Flushing is just as important to wire erosion EDM as it is for spark EDM, and a low vaporisation point is good for several reasons.
1) The initial spark impulse vaporises material from both the work-piece and the wire. The quicker the wire vaporises the faster heat will be transferred to the work-piece removing more material. The erosion of the wire is often not a problem because it is continually renewed by fresh wire from the spool.
2) When the surface coating of the wire can vaporise rapidly, it protects the body, or core, of the wire by not transferring heat into it.
3) When wire melts instead of vaporising, it will create chips instead of gases. These molten chips quickly re-solidify in the water and, since they have mass, they take up valuable space within the spark gap. When the metal is vaporised, gases are produced and escape from the spark gap much easier and with less influence on flushing. These gases condense in the cool dielectric but will always have a much smaller mass than a melted chip.
4) During the on-time part of the cycle, gas bubbles are constantly being created. These bubbles form and grow quite rapidly only to collapse and implode during the off-time part of the cycle. This rapid agitation of large volume gas bubbles growing and collapsing is an aid to flushing.
5) Due to lower melting/vaporisation temperatures, the craters produced on the surface of a low temperature wire such as brass will be larger than those found on a high temperature wire such as molybdenum. This greatly improves flushing because the large craters will leave more room for the wire to act as a conveyor belt, carrying away amounts of contamination and damaged dielectric from the gap.
Hardness: The hardness of a wire in EDM terms refers to its ductility, its ability to undergo elongation. A wire’s hardness or softness determines how much elongation can occur. In this category, EDM wires are called soft, half hard or hard.
1) Soft wire can undergo elongation that allows it to stretch and deflect without breaking. Elongation of soft wire averages around 15% to 17% and some even exceeds 30%. Soft wire should be used when taper-cutting beyond a 7 degree angle because it elongates as it passes across the guides.
2) Hard wires cannot stretch more than 2% or 3%. They are better suited for use with automatic wire feed systems.
8.2 WIRE EROSION: EDM WIRE TYPES
Wire selection was easier in the early days of EDM – there simply were not a lot of choices. Today there are different wires that have specific properties keyed for use against different work-piece materials. Some EDM generators perform better with a particular wire brand or alloy.
Copper Wire: When wire erosion EDM was in its infancy, copper wire was the natural choice. Pure copper wire for EDM has a very low tensile strength and is subject to elongation and excessive breakage. Probably the greatest shortcoming of copper wire is that it has poor flush-ability due to its high vaporisation temperature.
Although copper wire has excellent electrical conductivity it also has a high thermal conductivity. This inhibits cutting speed because it sinks heat away from the cut and into the body of the wire. This in turn slows the melting process and reduces efficiency because larger amounts of heat are transferred to the wire instead of the work-piece.
Brass Wire: This is by far the most widely used wire type. Brass is made by alloying copper with zinc. Compared to copper, brass wire provides higher tensile strengths and a lower vaporisation temperature. This allows smaller chips to be formed and also allows larger craters to be formed on the surface of the wire to carry them away.
High-tensile brass wires are made by alloying brass with small amounts of aluminium or titanium. Flush-ability will suffer as strength increases.
Coated Wire: Coated wires have a thin coating of zinc that is applied by electroplating using special equipment to ensure a homogenous coating with uniform thickness. Diffusion annealed wires have a thicker coating of zinc applied to the wire’s core and it undergoes a heat treating process that changes it to brass and fuses this outer coating to the core.
Coated wires combine the tensile strength of one material, and the conductivity or flush-ability of another.
Zinc has become the widely used wire coating and alloy material. It provides faster cutting speeds and reduced wire breakage because it has a much lower melting/vaporisation temperature than brass. In fact, the zinc coating will boil at 906 degrees Centigrade, long before brass can melt at 930 degrees Centigrade. The lower melting temperature of zinc allows larger craters to be formed on the wire, improving flushing. It also helps to ensure that the brass core can never burn through. Wire breakage is reduced and cutting speeds go up.
Molybdenum Wire: This is a very high strength wire, far in excess of brass wire. While it offers a very high tensile strength and high resistance to breakage, molybdenum wire has a very high melting temperature of 2,625 degrees Centigrade. Its high boiling point of 5,560 degrees Centigrade causes it to cut very slowly because of its poor flush-ability. The EDM craters formed on molybdenum wire are quite small and cannot contribute much to flushing.
Molybdenum wire is generally used when near-sharp internal corner radii called for that require fine diameter wire typically of 100 microns or less.
8.3 WIRE QUALITIES
The wire must meet the following standards to ensure success:
1) Material Quality. The drawing and heat treatment processes must produce wire surfaces that are smooth and uniform. The wire surface must be free from any flaws that can propagate cracks under tension and cause wire breakage. The cleanliness of the finished product is of the utmost importance. A dirty wire will contaminate rollers, wire feed systems and wire guides, delivering poor performance and increased maintenance.
2) Accuracy. The wire must have a precision uniform diameter. Besides the obvious effects on work-piece accuracy problems can occur through the wire snagging in the guides.
3) Tensile Strength. The tensile strength must match the application or excessive wire breakage can occur. Wire breakage is typically not directly related to tensile strength and this is seldom a problem because all quality wires have a generous safety margin in their ratings.
4) Hardness. The hardness or ductility of the wire must be uniform and consistent. While tensile strength is important, if the wire is too hard or brittle, it cannot withstand shock and will break. Different hardness is used for different applications. For example soft wire is advised when cutting steep tapers and machines using automatic wire feed will often prefer a hard wire.
5) Winding. The winding of the wire must be uniform. Producers go to great lengths to ensure the spools are wound without snags, overlapping, pinching or variations in tension.
6) Coating. Any coating on a wire must be uniform in depth and density. If the wire is improperly plated, it can have thin spots resulting in erratic cutting performance, unnecessary breakage, and potentially inconsistent dimensional and surface finishes. If the plating or coating of the wire has not been processed correctly, the zinc surface can flake or scale off as it passes through the rollers and guides, causing additional wire breakage and wire threading problems.
7) Packaging. While not exactly a quality of the wire itself, packaging is important in assuring that all the above characteristics are used to their best advantage. All spools should be sealed in plastic bags to avoid oxidation and contamination, and they should be unwrapped only when ready to use.
8.4 WIRE EROSION: OTHER POINTS TO NOTE
Terminology for EDM erosion wires may vary from manufacturer to manufacturer. One manufacturer can consider a certain grade of wire to be hard, while another maker might consider that same grade to be half-hard.
Selecting the correct wire for the job comes from a basic knowledge of the EDM process, experience and the recommendations of the wire and the machine tool suppliers. Experience alone used to suffice, but in the past few years, wire technology has developed greatly mainly due to the newer wire erosion EDM machines with their improved power supplies, flushing capabilities, tapering and tensioning devices and threading mechanisms.
Wire machines require regular preventative maintenance, without which performance will drop off rapidly no matter what kind of wire is being used. Cleanliness is mandatory. Check the power feed contacts, guides, bushings, rollers, and power cables daily. Do not forget to monitor the cleanliness and condition of the water, and check the condition of the filters and resin regularly. With wire machines running unattended longer these days, we can easily take for granted just how many hours they actually are working. Charting these hours and scheduling regular preventative maintenance will go a long way in assuring optimum, or at least predictable, cutting performance, not to mention prolong the life of other consumables and the machine tool itself. Only then can a fair evaluation of any wire’s performance be made.
