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Tech Focus: EDM Wire EDM machine for turbine disc fir-tree roots

| Editor: Steffen Donath

The AV35 aims to be one of ONA’s most ambitious projects in recent years: It represents their technological development of WEDM technology applied to the manufacture of highly demanding parts.

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The most commonly used machining techniques to manufacture turbine disc fir-tree roots have traditionally been physical machining processes: Broaching, grinding and milling.
The most commonly used machining techniques to manufacture turbine disc fir-tree roots have traditionally been physical machining processes: Broaching, grinding and milling.
(Source: Ona)

The company explains that one of its great virtues is that it is a standard machine. It has a basic structure to which modules/accessories can be added depending on the type of component to be manufactured. This allows for fully automated machining. Just replace or adapt some of these elements to manufacture different types of parts.

One of ONA’s greatest challenges has been to develop EDM technology adapted to the process in order to convert electrical discharge machining into a competitive and high-quality machining process, even when compared to some of the most common processes in the industry, such as milling or broaching. An example of the latter is the cutting of fir-tree roots for aviation turbine discs, mainly manufactured by broaching. A successful project to highlight is the work carried out in collaboration with a company in the aeronautics industry. This was a project in which ONA manufactured fir-tree roots for aviation turbines on the ONA AV35 wire-cutting EDM machine (WEDM).

According to the company, there are currently few alternatives on the market to the AV35. What distinguishes the ONA AV35 from other EDM machines for the manufacture of fir-tree roots for turbine discs, is that the ONA AV35 is a standard machine to which accessories/modules can be added, which makes it able to adapt to the manufacture of other components. In other words, this equipment is not manufactured exclusively for one purpose.

Some of AV35’s accessory items include:

  • Touch probe; to ensure the correct positioning of the part.
  • Script programming; tool for the creation of smart 'in-process' measurement programs.
  • 2-axis rotary table with pneumatic chuck. Automatic and repetitive clamping of the part; it enables the erosion of the part in a single bundle.
  • Full part traceability and process analysis thanks to the ONA Smart Connect platform; option to monitor the machining of each section of the part, perform statistics, prevent errors, foresee maintenance tasks, trace the parts, etc.
  • Automation of the whole process; last phase: A robot has been added to keep the machine working day and night with the same accuracy.

“The most complicated part was coordinating with different departments as well as with the client. The machine was designed according to the specific needs of the clients. We had to use state-of-the-art software tools to comply with the traceability requirements,” one of ONA’s technicians describes the process

The AV35 offers important advantages to companies, such as a lower initial investment in machinery, thanks to this adaptability, and more autonomy. The entire machining process is automated and controlled so that the part meets the requirements set out: The continuous monitoring of the provided data ensures progress is being made according to the initial programming. In the event of an error (a change in water temperature, etc.), a warning signal is emitted.

Technology at the level of broaching and milling

Discs are critical rotating parts in a gas turbine: They reach up to 60,000 revolutions per minute and must be able to withstand extreme temperatures and pressure fluctuations, while being lightweight and safe. The quality of the material used (usually nickel-based superalloys) and the machining process (high geometrical and surface quality requirements) are therefore essential. The most commonly used machining techniques to manufacture turbine disc fir-tree roots have traditionally been physical machining processes: Broaching, grinding and milling. Broaching being, perhaps, the best known and most used process.

With regard to electrical discharge machining, the development of new generators has been the main drive that has made EDM a viable and competitive manufacturing process for the manufacture of components in the aeronautics industry. The main advantages of developing new EDM generators are: The capacity to provide a higher power output, thus increasing productivity; and the adaptation of the technology and machining strategy to reduce the heat affected zone as well as the white layer, which are indispensable, especially in the aeronautics industry. ONA’s WEDM process also offers turbine manufacturers greater flexibility in terms of deadlines and references: It allows them to adapt faster than conventional machining processes would allow them to.

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