No cartridges allows EDMs to work long cycles

EDM die sinkers that can filter electrolyte without having to change any cartridges means that they are able to operate continuously over long periods when machining nickel alloy turbine parts.

EDM die sinkers that can filter electrolyte without having to change any cartridges means that they are able to operate continuously over long periods when machining nickel alloy turbine components.

Capability of ONA die sinking EDM machines to filter their electrolyte without the need for replaceable cartridges is critical to their successful application at EDM Precision Technologies Ltd, Silverstone.

The company operates two such Spanish-built Techno H400 machines supplied by sole UK agent, Warwick Machine Tools.

They are used exclusively for machining turbine nozzle segments that form part of land-based gas turbines for power generation.

Each machine is set up to machine a pair of segments in a fully automated cycle utilising up to three graphite electrodes.

A cycle time of around 18 hours allows manufacture of one complete turbine set per week.

Roy Marks, managing director of EDM Precision Technologies, said, "The components are machined from nickel alloy castings and require around 10kg of material to be removed from each component.

If we used a cartridge-based dielectric filtration system we would probably spend as much time changing the cartridges as we did machining.

However the ONA machines have a self-flushing filter bed that enables us to machine continuously, and also saves the cost of replacement filters." The ONA machines use a fully automated ecological filter not requiring any maintenance for more than 10,000 hours of machining with a filtering capability of 1micron particle size.

This is designed for large machine applications and centralised installations where high flows of dielectric are involved.

Filtration, backwashing and sludge discharge is performed through several paper filter rods in an automatic operation, including the sludge discharge into disposable bags.

EDM Precision Technologies, as the name implies, specialises in providing electric discharge machining services to a broad customer base.

As well as the die sinking machines and a graphite machining facility for electrode production, the company operates eight wire EDM machines as well as a CNC high precision EDM drill.

Founded as a toolmaker, the company has specialised in EDM applications since 1991.

Mr Marks commented, "The types of machines that we have installed allow us to operate on a minimally manned basis.

We only have six employees including myself but the factory operates round the clock, seven days a week." The company tends to specialise in high value-added work for the motorsports, aerospace, medical and defence sectors, as well as subcontract work for toolmakers.

However, no job is too small or too large, and batch quantities range from one- to thousands-off, processing the full range of materials including titanium.

While most jobs require either wire EDM or sink EDM or drilling, occasionally all three processes are involved.

Workpiece capacity ranges up to 1000mm by 750mm by 400mm; tank dimensions on the H400s are actually 1200mm by 800mm by 500mm.

"The ONA H400s were bought to process the component that is currently running on the machines," Mr Marks explained.

"We had gained the supply contract and needed a machine with the necessary capacity and a vendor who was willing to assist us in developing the application.

In this respect, Gerard Berger at Warwick Machine Tools is particularly good, both in terms of technical competence and willingness to help.

Essentially we formed a partnership with the supplier which helped us to realise the manufacturing objectives in line with our customer's requirements." The component being machined is a large nickel alloy investment casting with two blades per segment.

21 of these form an annular nozzle guide vane assembly within a gas turbine.

EDM Precision's task is to machine the non-aerofoil sections of the casting to remove excess material and machine pocket forms on the inner and outer circumference.

To that end the machines are equipped with purpose-built fixtures to locate the castings.

Processing from then on is fully automatic.

The machine detects the true position of the castings by reference to datum points and then proceeds to erode the required features with automatic adjustment to tool offsets as necessary.

The programmable C-axis on the quill of the machine allows the electrodes to approach the workpiece from three directions.

An innovation is the use of staggered electrode holders to minimise axis movement, allowing use of a smaller tank than would otherwise be necessary.

Pre-set programmable flushing arrangements include nozzles sited at strategic points in the tank to ensure good surface finish.

"We had quite a bit of assistance from Warwick Machine Tools to tailor the machines to our application - in fact it was Gerard Berger who suggested that we use the staggered electrode holders," Mr Marks revealed.

"However, we developed our own part programs and fixtures.

The CNC system on the machines is easy to use and well integrated with the EDM power supply so that roughing and finishing parameters can be defined as a continuous program under automatic control.

As a result, we can set the machines going and then leave them alone until the time comes to unload the finished component." EDM Precision Technologies operates the ONA Techno H400s with three sets of electrodes per machine of which one is in use, one is available as a spare and the third is in the process of being reprofiled on the adjacent machining centre.

Electrode life between reprofiling is 6 to 8 components.

"We have been very pleased with the results from the machines, and so has our customer," Mr Marks concluded.

The machines have been highly reliable and back-up from Warwick Machine Tools has been first class.

If it became necessary to expand production of these components we would certainly buy the same type of machine again.".

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