High speed EDM makes complex parts

High speed EDM machine tools provide high accuracy and high quality surface finishes in producing complex parts with thin walls, multi cavities, ribs and ultra-fine details.

Some engineers consider spark erosion - or EDM - a relatively slow machining process.

The spark erosion process will deliver exceptional accuracy and superior surface finishes when machining complex, high-precision parts such as fine and deep blades and components that have thin walls, multi cavities, ribs and ultra-fine details.

Issues concerning EDM cutting speeds and the viability of EDM in production environments, as opposed to mould and tool manufacture, still persist in certain quarters.

Such thinking is in the past thanks imostly to EDM specialist Charmilles' introduction of a range of High-Speed Spark Erosion Machines (HSEDM) onto the market.

* The spark erosion process and the element of speed - the main factor that affects speed and performance in spark erosion concerns the 'spark gap' and the energy of the spark on the active surface between the electrode and the workpiece.

In different machining operations and applications the relationship between the status of the gap and the surface between the electrode and workpiece can vary depending on the followng.

* The depth and dimension of a cavity - it can become more of a challenge to evacuate eroded material and particles from the gap depending on the depth of a cavity.

If eroded material is not removed effectively from such cavities - workpiece geometry, accuracy and finish are likely to be affected.

* The under-size of an electrode and the zone that surrounds it: if the zone around the electrode is large - contamination within it caused by eroded particles - can be removed relatively easily.

But if the undersize is small (often the case for intricate and fine details) - removal of debris is more difficult.

* The type of material being machined - different materials have different properties - and these properties include the pollutants that are produced during the EDM process.

This is especially true when materials such as titanium and carbides are being machined.

* The geometry of the electrode - during the downward plunge of the electrode the surface area in contact with the electrode increases.

As a result there is more material removed - but as the spark energy remains constant - the plunge speed diminishes.

** HSEDM principle - the new machines developed by Charmilles have faster axis acceleration and speeds.

Secondly they are equipped with PILOT EXPERT software capability - intelligent software that adapts to the prevailing machining conditions.

** Speed and acceleration - faster speeds and higher acceleration result in a much more efficient cleaning of the spark gap (via a suction effect) and ensure that conditions in the gap between the workpiece and the electrode are optimised at all times.

Increased speeds and acceleration obviate the need for flushing operations - which in themselves can be a barrier to speed and machining productivity and can also cause cavity deformation.

** Intelligent and adaptive software - PILOT EXPERT - most electrodes used in die-sinking operations have variable geometries (tapers or other free form shapes).

These different shapes and features all create variable conditions in the spark gap.

To control these variables Charmilles' PILOT EXPERT system modifies the retraction distance of the electrode and adapts the power of the spark accordingly.

During machining operations the frequency of the pulse and the retraction distance are variable - i e, they change according to the depth of cut (shallow, medium or deep cavity).

The PILOT EXPERT system adapts to these changing conditions and machining requirements by optimising both the spark duration and the timing between each spark - ensuring that during roughing operations more energy is directed to the surface contact area.

As well as optimising the EDM machining process this adaptation also ensures less electrode wear.

HSEDM capability is standard on Charmilles ROBOFORM 350 and 550 EDM machines.

Said Peter Pearce, Charmilles product specialist: "HSEDM provides precision manufacturers with the optimum spark erosion solution".

"Faster speeds and intelligent software working in combination means improved performance, increased productivity and greater process reliability".

* Examples are as follows.

1 - Deep blade 100mm - in this example using a graphite electrode (1mm x 15mm) with an undersize of 0.10mm to machine 100mm deep blade from steel to a surface finish of CH40 - the machining time recorded without HSEDM was 5h 13 min.

The HSEDM reduced the machining time to 4h 30 min.

2 - Fine blade 18mm - in this example using a copper electrode (20mm x 0.28mm) with an undersize of 0.10mm to machine 18mm fine blades from steel to a surface finish of CH27 - the machining time recorded without HSEDM was 3h 42 min.

HSEDM reduced this time to 2h 16 min.

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