Despite the growing variety of EDM hole-drilling technologies available on the market, there exist two primary drilling platforms, oil dielectric and water dielectric, which offer very distinct performance characteristics. Key points of differentiations include speed, quality, hole size, machine features and maintenance.
WHY DRILLING WITH OIL?
Oil is used to provide the highest accuracy and best surface-quality holes. It is typically reserved for super precision machining or special applications not possible with the tolerances and surface finish of water machines.
WHY DRILLING WITH WATER?
Water, typically deionized to control conductivity, is an effective and inexpensive dielectric fluid that is the most commonly used in EDM drilling machines. In order to maintain consistent water quality for the most controlled burn, these machines use filtration and deionization systems. Water is an excellent choice when the workflow calls for a dedicated process machine, one that drills only holes, and requires the fastest possible machining speeds. A perfect balance between electrode, machining discharge power and dielectric characteristics exists at typical hole sizes (0.5~3.0 mm) and yields the best speed.
OIL DIELECTRIC EDMs
Dielectric Properties
As opposed to the conductive properties of water, oil is an insulator and therefore provides both a smaller spark gap and much more precise control over the spark during machining. These factors lead to slower processing times and provide stable control over low-power finishing processes for improved overall accuracy and surface finish. With lower power sparks, the resulting HAZ and recast characteristics are dramatically improved (minimized), and smaller diameters holes can be reliably produced.
Processing Methods
Most oil EDM drilling machines are based on standard sinker EDM platforms and provide automatic tool change (ATC) capabilities. Unlike water dielectric, all oil-based machining operations are performed completely submerged. Oil-based machine platforms typically use copper and tungsten electrodes, depending on hole requirements.
Machining Characteristics
Oil dielectric drilling operations are typically limited to L:D ratios of 150:1, but can produce accuracy tolerances of ±0.005 mm or finer. Entry and exit holes made via oil-based systems are typically more consistent than those of water-based systems, producing highly accurate hole location, straightness, roundness and surface quality. Finishing operations on the machine are possible to improve part accuracy, quality and surface finish.The produced accuracy and surface quality from the roughing hole-drilling operation in oil is substantially enhanced when compared to water-based systems. Given the higher viscosity of oil combined with the smaller spark gap, machining speed can be up to five times slower than water. As a result, oil-based EDM drilling is typically reserved for hole diameters under 0,8 mm.
WATER DIELECTRIC EDMs
Dielectric Properties
The speed of water-based EDM drilling is attributable in large part to the conductive nature of the water allowing for both stable high-power EDM discharge and a large spark-gap area that increases flushing ability during machining. Additionally, the light viscosity of water provides for efficient cooling and debris evacuation in flushing. Most water-based machines use di-resin to control the water conductivity.
Processing Methods
Water-based systems can perform machining operations submerged and non-submerged by using through-electrode flushing and an external flush hose. The stable high power discharge and flush properties of water dielectric offer the fastest possible machining speeds with the deepest L:D ratios using brass electrodes, either simple hollow tubes or multiple internal channel design.
Machining Characteristics
In water EDM drilling, L:D ratios upwards of 300:1 are possible with accuracy usually held to ±0.050 mm or greater. The size, shape and quality of entry and exit holes produced via water-based processes tend to see greater variance than oil dielectric processes. Water dielectric machines are typically designed for one process and do not provide any ability for finish operations; the final surface finish is what is possible with the high-power rough machine setting only.