Di-Spark – The EDM Machining Company – Dielectric & Flushing in EDM
Continuing our series of electrical discharge machining articles (extracts from our EDM manufacturing guide, ‘The EDM Handbook’), this snippet examines the technical challenges towards running a success EDM machining company.
Di-Spark: The EDM Machining Company
Di-Spark Ltd are an ‘EDM Machining company’ supplying precision Electrical Discharge Machining Services, such as spark & wire erosion. We supply high technology companies and typically manufacture world-class, mission-critical components and sub-assemblies destined for aerospace and high stress environments such as oil & gas.
We serve companies requiring an ‘EDM machining company – committed to ongoing investment towards current & innovative machining technologies such as clean-cut wire erosion & spark erosion machining.
10. DIELECTRIC AND FLUSHING:
Di-Spark, The EDM Machining Company
There is an old EDM adage – “The keys for running a successful EDM machining company are flushing, flushing and flushing”, and it applies to both wire and spark EDM. Other than knowing the fundamentals of on-time/off-time and basic electrode material selection, the most important element of successful electrical discharge machining is flushing. There are several different methods, but the concept is the same for any application.
Flushing is the process of introducing clean dielectric fluid into and through the spark gap. This serves several purposes:
1) introduces fresh dielectric to the cut
2) flushes away the debris form the spark gap
3) cools the electrode and work-piece
There must be sufficient off-time to allow the dielectric time to recover. The combination of spark duration, dielectric strength and flushing efficiency are all factors determining the amount of time the power is off. Since during off time no material is being removed it should be kept to a minimum. The better the flushing conditions, the less off-time is required, and the efficiency of the entire operation is increased.
10.1 DIELECTRIC – OIL
Di-Spark, The eDM Machining Company
Oils have been used as dielectric for as long as the process has existed, but only in the past decade has any appreciable research been made as to their composition and compatibility with people and the environment. Recent health and safety concerns have accelerated this overdue interest.
There are many different types of fluids available to an EDM machining company from EDM suppliers. Typically hydrocarbon based oils have been used. It is strongly recommend that only dielectrics that have been produced specifically for EDM are used. Many dielectrics that have been sold by non-specialist suppliers have actually been by-products from other processes that just happened to work for EDM. The dielectric fluid must be compatible not only with the EDM operation but also with EDM operators.
The following list details key points concerning dielectric oils:
Flash Point. Simplistically this is the temperature at which the vapours of the fluid will ignite. Generally, the higher the temperature, the better.
Dielectric strength. This is the fluid’s ability to maintain high resistivity before spark discharge and, in turn the ability to recover rapidly with a minimal amount of off-time. Oil with a high dielectric strength will offer a finer degree of control throughout the range of frequencies used especially those when machining with high duty cycles or poor flushing conditions. This will provide better cutting efficiency coupled with a reduced potential for arcing.
Viscosity. The lower the viscosity of the dielectric oil, the better accuracy and finishes can be obtained. In mirror finishing or close tolerance operations, spark gaps can be only a few microns. With such tight, physical restrictions it is much easier to flush small spark gaps with lighter, thinner oil.
When EDM applications require high metal removal rates, or moderate surface finishes heavier oils can be used. Viscosity can be heavier in these applications because of the naturally longer spark gaps, and it will also help prevent excessive fluid loss through vaporisation.
Specific Gravity. The lighter the oil or the lower its specific gravity, the faster the debris that is suspended within it will settle out. This reduces gap contamination and the possibilities of secondary discharge and/or D.C.arcing.
Colour. All dielectric oils will eventually darken with use, but it seems only logical to start with a liquid that is as clear as possible to allow viewing of the submerged part.
Odour. Besides the obvious reasons for choosing a fluid with no discernible odour, oils that smell are usually a strong indication of the presence of sulphur, which is undesirable in EDM oil.
10.2 DIELECTRIC – WATER
Di-Spark, the EDM Machining Company
Whilst running an EDM machining company, one of the most overlooked facets of successful wire-cutting is the water that is used as the dielectric, despite the fact that it is integral to the EDM process. The main considerations for water quality are filtration, de-ionisation and temperature
1) Filtration.
Filters should have the finest mesh economically feasible for the job. This will allow faster cutting because of clean cutting conditions, which means less secondary discharge. Fine-mesh filters will also ensure longer, more efficient life from the de-ionising resin.
2) De-ionisation.
The resin system is an integral portion of any wire EDM machine. The level of de-ionisation is important because as the ionisation level increases so does the number of ions. This means it becomes easier for sparks to pass across the gap between the work-piece and the electrode. This means the wire will effectively be closer to the work-piece, so short circuit arcing may become more common leading to wire breakage.
As the ionisation falls, so the number of ions decreases, sparks find it harder to cross the gap and the potential voltage rises to maintain the same current flow, if this potential rises too much the energy per pulse can cause wire breaks.
3) Temperature Control.
It is a matter of fact that a wire EDM machine will cut parts faster and produce more consistently accurate parts with a temperature-controlled dielectric system than without one.
10.3 WHY OIL? WHY WATER?
Di-Spark, the EDM Machining Company
In either case, spark or wire, metal removal will be much faster using water as a dielectric because it is a much better flushing medium. It is less viscous than oil. Instead of insulating and retaining heat in the spark gap, water dielectrics sink heat from the spark gap much better than oils. This is because water is naturally thinner than oil and possesses what is referred to as better gap penetration. This is the speed and ability at which a liquid can naturally move within a confined space. In EDM’s case, of course, there are extremely small spark gaps. Mirror finishing settings are so low and spark gaps are so small that even capillary action gap penetration is welcomed and water does this matter faster and better than oil.
If machining speeds are always faster using water as a dielectric, then why do spark EDM machines use oil? Primarily to reduce electrode wear.
In years gone by the standard polarity was a negative electrode. This provides excellent machining speeds but at the sacrifice of high electrode wear. Since the design and manufacture of a formed electrode is often more costly than the actual EDM operation, it is very desirable to find ways to protect the electrode from excessive wear. This can be achieved by using positive polarity electrodes although machining speed is slow. Today, in favour of protecting the electrode, positive polarity is considered standard.
Experimentation with water dielectric in spark EDM began more in trying to eliminate oil contamination of certain types of parts rather than as a quest for machining speed. It became apparent that cutting speeds in spark EDM operations can be increased up to five times using a water dielectric. However, water dielectric used in such applications boils away quickly, so it must be replenished more often than EDM oils. Further, to keep up with the high metal removal rates, a cumbersome filtration and dielectric system must be maintained consuming large amounts of both filters and de-ionising resin. Unfortunately, the faster machining speeds realised using water in spark EDM do not offset high electrode wear, rougher finishes and high operating and maintenance costs. These negative aspects have made this type of machine cost prohibitive and production machines using water instead of oil have been limited.
10.4 HIGH-PRESSURE AND SUBMERGED FLUSHING
Di-Spark, the EDM Machining Company
While in most instances flushing is more difficult to address in spark EDM, it is still imperative for efficient wire EDM.
De-ionised water is used in wire EDM and is an integral part of EDM theory. The dielectric fluid provides insulation against premature discharging, cools the machined area, and flushes away the debris. Without flushing, EDM simply will not work. Special nozzles are used to direct a stream or jet of water around the wire and through the work-piece.
Partial credit for the fast cutting speeds of today can be given to improved generators and control systems. However, these would not function without high-pressure flushing which is necessary to flush the large amounts of debris from the kerf created by the wire.
High-pressure or co-axial flushing has been the solution for high-speed cutting, but it does not solve all of the flushing-related problems. Steep tapers, interrupted cuts, changing sections and round or tubular parts are just a few examples of applications that pose flushing problems. In these instances, flushing is inconsistent and perhaps non-existent, so cutting speeds decrease and wire breaks are prevalent. Under these conditions, it is recommended that the part be submerged during machining. Submerged cutting greatly improves the poor flushing conditions encountered in the above examples, allowing faster and more reliable machining.
Submerged cutting also enables improved cutting accuracy over non-submerged machines because temperature changes have less influence since the entire work-piece, control arms, worktable, and tooling can all be held at a constant temperature by the dielectric.
For pure speed, nothing is better than high-pressure flushing. Usually, when cutting submerged, flushing pressures will seldom approach the pressures found in speed cutting, but the entire cut remains wet, providing reduced likelihood of wire breakage.
To submerge or not to submerge, that is the question. Almost all wire EDM builders provide machines with submerged cutting capabilities. Some people insist that submerged cutting is the single greatest answer to all wire EDM problems, and recommend a submerged machine in every case. Others dismiss submerged cutting as being only a marginal solution. In actuality, it all depends on the application.
