Spark erosion and dielectrics – much too involved to be a commodity decision!

Citation

(2004), "Spark erosion and dielectrics – much too involved to be a commodity decision!", Aircraft Engineering and Aerospace Technology, Vol. 76 No. 5. https://doi.org/10.1108/aeat.2004.12776eab.001

Spark erosion and dielectrics – much too involved to be a commodity decision!

Spark erosion and dielectrics – much too involved to be a commodity decision!

Keywords: Erosion, Dielectric devices

Removal rates, cutting times, surface integrity and finish, accuracy and electrode wear are all affected by EDM users' choice of dielectrics.

EDM consumables and wear parts specialist – Intech – part of Agie Charmilles is, through its recently launched “Consumability” initiative, challenging EDM users to revisit their understanding and use of dielectrics within the spark erosion process – and to question whether their existing dielectric purchasing and application decisions deliver optimum performance.

Tony Steels, Managing Director at Agie Charmilles says:

“From our experience it seems that EDM users' knowledge and understanding of how dielectric selection can affect their overall spark erosion performance is patchy to say the least. Often treated as a commodity decision – dielectrics do not appear to be as high on the agenda as say deciding on the correct choice of electrode material for a specific application – and yet, dielectrics can and do affect all aspects of performance. The relationship Intech has with our EDM brands – Agie and Charmilles – means it is better placed than most other EDM consumable suppliers to help customers make strategic decisions and informed choices about the use of dielectrics”.

Theoretically any insulating liquid can be used as a dielectric. However, there are a number of essential performance characteristics required from dielectrics which mean generally that only deionised water (for polishing applications) and hydrocarbons are used today.

Every EDM user knows that dielectrics perform a number of vital functions in the spark erosion process. In the first instances dielectric helps insulate the workpiece from the electrode until the optimum spark gap size (i.e. the narrowest possible) is created and the discharge can occur.

The dielectric also must be capable of facilitating rapid deionisation within the spark path to enable the next discharge to occur.

Because of the high temperatures created during the sparking process – (8,000-12,000°C) the dielectric must act as a coolant for both workpiece and electrode.

The dielectric also acts a flushing agent – helping remove waste metal particles that have been created during the cutting process – thereby reducing the potential for disruptions and short circuits.

Yet, knowing the function of a dielectric is only half the issue – of equal importance is both an understanding and being able to assess the relative merits of different dielectrics and their impact on the spark erosion process. The following criteria (below) can be used as an aid to decision-making.

Health issues

The effects of industrial hydrocarbons on health are increasingly important. From a skin irritation perspective products consisting of completely saturated hydrocarbons with as few aromatic compounds should be specified (an aromatic content of less than 1 per cent in volume is desirable). Hydrocarbons derived from paraffin C12-C14 cause skin irritation and should be avoided.

Similarly dielectric toxicity is also under scrutiny especially the degree to which certain aromatic compounds develop during the erosion process (aging stability). Dielectrics that create polycyclic aromatics (e.g. benzpyrene) are today considered carcinogenic.

The amount of smoke produced by a dielectric during erosion is dependent on the varying metal removal rates required. Thin-bodied dielectrics give off less smoke than viscous ones – and the higher the flow of dielectric over the erosion area... the less it smokes.

Unused dielectric should be odourless. Even after it has been used for some time a faint ozonic smell, created by electrical discharges, may be detected but this should not be excessive.

Flash points

The flash point is the lowest temperature at which a dielectric gives off sufficient vapours to produce an inflammable mixture of air and gases. The higher the flash point... the safer the dielectric. Dielectrics that display flash points below 21°C (benzene) are clearly unsafe to use.

Density

The influence of dielectric density is greater during finishing and trim cuts rather than in roughing operations. “Heavy” products remove more metal and dielectrics used today have a density between 0.750-0.820.

Changes to the specific gravity of a dielectric before and after use can indicate that contamination has occurred (e.g. that hydraulic fluid has entered the dielectric).

Evaporation rate

The evaporation rate of a dielectric used for polishing applications should have an evaporation rate or number of 500-1,000. For economic reasons, substances that evaporate more quickly are not suitable as dielectrics.

Particle suspension

Waste particles eroded from the workpiece, the electrode and carbon particles created from electrical discharges must be moved away from the working area. Satisfactory particle suspension is required for this to happen. If particle suspension is too high – impurities will not separate from the dielectric during filtration. Too many impurities lead to arcing.

However, there need to be a few micro-particles in the dielectric to enable ionisation to occur. With new dielectric these particles can be added artificially to improve erosion from the outset.

Compatibility

Dielectric fluids must remain neutral towards EDM machine components, e.g. seals, tubes, etc., and must not cause these materials to shrink or deteriorate.

Aging stability

The longer a dielectric can be used the more economical it becomes. In ordinary erosion processes it may be possible to use a dielectric with paper filtration for up to 2 years. It is clearly worthwhile checking as dielectrics with an acid number of more than 1 mg/KOH/g ought to be renewed as soon as possible.

Details available from: Agie Charmilles Ltd. Tel: +44 (0) 2476 538666; Fax: +44 (0)2476 530023; Web site: www.agie-charmilles.com