Functions of the Dielectric
Insulation
One important function of the dielectric is to insulate the workpiece from the electrode. The disruptive discharge must take place across a spark gap which is as narrow as possible. In this way efficiency and accuracy are improved.
Ionization
As quickly as possible optimum conditions for the production of an electrical field must be created and then a spark path must be provided. After the impulse the spark path must be deionized quickly so that the next discharge can be made. The dielectric ought to constrict the spark path as much as possible, so that high energy density is achieved, which increases discharge efficiency at the same time.
Cooling
The spark has a temperature of 8000 – 12000° C when it punctures the workpiece and so the dielectric must cool both the electrode and the workpiece. Overheating of the electrode must be avoided, so that excessively high electrode wear cannot occur. It must be possible for the metal gases which develop during spark erosion to condense in the liquid.
Removal of waste particles
Metal particles that have been eroded away must be removed from the area of erosion by the dielectric to avoid disruptions in the process
The Importance of Dielectric Fluid
Being a disposable commodity, dielectric fluid is easily overlooked, its importance to EDM underestimated and cheap alternatives for it sought. This harms and underestimates the central role that dielectric fluid plays in the EDM process. Dielectric fluid is a liquid insulator, resisting electrical discharge. A liquid dielectric becomes conductive for a short period when the applied voltage exceeds a certain threshold, rendering inherently unpredictable forces predictable. Consequently, a fluid without these dielectric properties is inherently dangerous, and does not provide the controlled discharges that make EDM characteristically accurate.
USE APPROPRIATE FLUID
Use the appropriate fluid - in sinker EDM machines use dielectric fluid. Sounds simple enough, but many shops use a cheap mineral oil in an effort to save money. Fluids not designed for use in an EDM system may be dangerous to the operator, the workplace, and could damage your EDM equipment. Mineral oil does not have the fluid properties to be effectively used in EDM. The savings of a couple hundred Euros isn’t worth the risk. Remember, use fluid specifically designed for EDM. For low amp fine finishing work, choose a fluid with a low viscosity. The low viscosity allows the fluid to get into the cut of material and help flush out particles. Generally, because these fluids have a low viscosity they also have lower flash point - not for rough cutting with high amperage. For roughing work, choose a fluid with a high flash point. If you are like most shops and the machine is used for every application, choose a good overall performer. Dielectric quality fluids provide good overall performance in most EDM situations from fine finishing to roughing.
PETROLEUM OR SYNTHETIC
Generally, a synthetic fluid is better: longer life, better performance, and excellent operator acceptance. However, synthetic fluids cost more and depending on what applications you are using your EDM for, sometimes it makes sense to choose petroleum over synthetic. But, be careful when selecting a synthetic. Some low-cost synthetics are not fit to be called synthetics. A low grade “synthetic” or “synthetic blend” is not any better than a good petroleum fluid...and it may be worst! Keep in mind that highly refined petroleum EDM fluids perform almost as good as many synthetic EDM fluids on the market. However, a petroleum fluid life span is usually less than a synthetic. In addition, semisynthetic fluids provide some of the benefits of synthetic and is economical for most shops. Generally, if your budget allows, choose a quality synthetic fluid. It will last up to 6 times longer than petroleum fluids and will pay for itself in the long run.
FILTERING
A simple way of extending the performance of EDM fluid is to replace filters often and to replace them with quality filters. Some filter sizes are available in “cheap” alternatives from an automotive, truck, or industrial supplier but the quality is low. Remember, your EDM machine is only as strong as its weakest link - using a cheap filter affects the performance and shortens the life of the fluid.
COMBINING FLUIDS
When dielectric fluids are combined, a mixture of a wider distillation range is generally the result. Distillation implies that lower boiling components of a mixture are among the earliest to vaporize with heat. The resulting material is often higher in viscosity than the initial mixture. Possible effects of mixing dielectric fluids of different distillation ranges are slower removal rates, rougher surface finish, and higher fluid evaporation rates. Generally mixing fluids is not recommended.
REPLACING FLUID
If you are replacing your EDM fluid, there are a few things you can do to maximize the performance of the new fluid. Always try and remove as much as possible of the old oil. Clean out your system including any sludge or waste at the bottom of the work tank, base, and reservoir. Drain hoses and pump housing. Remove old filters and clean out the filter canister and replace with a new filter. If you are using an external filtration system or centralized system, take appropriate actions to clean out any hoses, tanks, and reservoirs it may have. These suggestions are especially important if you are replacing a petroleum fluid with a synthetic fluid. Some minor contamination will not affect performance, but heavy contamination will.
RECYCLE FLUIDS
It is important that you dispose of fluids properly. There are many organizations that will recycle used dielectric fluid.
AVOID CONTAMINATION
Care should be taken to prevent water from contaminating the dielectric fluid. Water in dielectric fluid lowers its dielectric strength and, in some cases, can cause excessive arcing making it necessary to replace the fluid. In addition, if drums are being stored outside place them on their sides to prevent any water from leaking into drum. Hydraulic fluid should also be prevented from contaminating the dielectric fluid. In addition to possible filter system damage, it could also cause harm to your machine. Dielectric that has significant hydraulic fluid contamination should be replaced immediately.
HEALTH AND SAFETY
If you have been around EDM machines for a while, you know some EDM oil can smell and irritate the skin. Today, quality EDM fluid is available which can minimize odor and the effects on the skin. High quality petroleum products have virtually no odor and usually do not irritate the skin. Synthetics and most semi-synthetics also have virtually no odor and do not irritate skin. Of course, there are some individuals that are so sensitive to chemicals that they may react to even the purest of fluids. Many individuals skin is not irritated by the dielectric fluid, but by contaminates and particles introduced during the EDM process. Wiping hands with oily rags or towels which may contain small EDM made parts could cause many very small cuts which can lead to skin irritation. It is strongly recommended, as with all chemicals, that you limit exposure. Like house-hold products, it is suggested that you wear gloves and have adequate ventilation. Do not put hands in the dielectric fluid when it is not necessary. Use of a barrier lotion will aid in the protection of skin.
Di-electrical fluids
Though the definition of aromatics has been expanded to any molecule with a particular flat and circular structure, aromatic content refers to the presence of hydrocarbons such as benzene. Aromatics can cause skin irritation in operators, and can affect the longevity of rubber EDM machine components. Since aromatics are commonly found in petroleum and its distillates, synthetic dielectric fluid has lower aromatic content than comparable petroleum-based fluids.
Colour normally does not imply anything about performance, as sometimes manufacturers add dyes to their dielectric fluid. The exception is yellow fluid, which is a sign of oxidation. Oxidation changes the chemical properties of dielectric fluid and negatively affect performance.
Sparks are responsible for the erosive effects of EDM, and a spark is the temporary creation of a conductive channel in an otherwise insulating medium. This depends on the strength of an electric field. Electric field strength is proportional to the inverse square of distance—known as the spark gap in EDM—and this gap needs to be very small in order to ensure consistent sparking. The most effective way to reduce spark gap size is to drill in a medium with a high dielectric strength, which is the measurement of a material’s ability to curb an electric field.
All dielectric fluids have a high dielectric strength by definition. Dielectric strength can be too high, however. A spark gap that is too small can increase electrode wear and slow down machining.
Flash and fire points are related, but there are subtle differences. The flash point is the temperature at which dielectric fluid will ignite in the presence of a spark or flame. At the flash point, there will not be continuous flame.
The fire point is the temperature at which a fire will be sustained. Ignition will still not be spontaneous, and needs an ignition source.
Fluids with higher flash points are safer. Fluids with high flash points also have greater viscosity. Because of the role that dielectric fluid plays in flushing, a high viscosity can be a problem, particularly when the spark gap is small. Applications requiring a small spark gap and lower viscosity fluid also demand less current, which contributes greatly to heat.
High quality dielectric fluids are generally odorless. The presence of odor may be a sign of aromatics or other undesirable impurities.
This is the chemical reaction between dielectric fluid and oxygen. Heating and contamination exacerbate oxidation, though high quality dielectric fluid resists it.
The pour point is the lowest temperature at which a dielectric fluid will pour. The cloud point is the temperature at which certain compound in the dielectric fluid precipitate out, lending a cloudy appearance. Neither of these points change dielectric fluid characteristics at normal operating temperatures.
Products refined from petroleum will have some amount of sulfur. Dielectric fluid with too much sulfur are more hazardous when burned than highly refined dielectric fluids. Sulfur content should be lower than 5 p.p.m.
Synthetic dielectric fluid is generally more viscous than petroleum-based fluid, with a high flash point and better resistance to oxidation.
Viscosity is a liquid’s resistance to flow. Due to the importance of flushing in EDM, low viscosity is generally preferred over high. High viscosity may have to be accepted in some circumstances since dielectric fluid with a high flash point also has a high viscosity.