GF Machining Solutions, the EDM, milling and laser ablation machine tool specialist and automation and tooling systems supplier, has introduced a new high-performance wire EDM machine specifically designed for aerospace applications which includes the machining of fir tree patterns on jet engine turbine parts.
The machine – the CUT 200 Dedicated – provides aerospace part manufacturers with a proven and reliable alternative to conventional machining methods, such as broaching, when machining high-precision fir tree geometries.
In recent machining trials conducted by GF Machining Solutions, the wire EDM process was compared and evaluated, from a cost perspective, against broaching. The results indicated that a 40% reduction in costs (with no loss in productivity) could be achieved by adopting the wire EDM route offered by the new CUT 200 ‘Dedicated’.
Machining Fir Tree Patterns
Cutting the fir tree pattern on both the disc and the blade root presents problems for manufacturers.
A modern jet engine may have as many as 40 or more turbine discs, each disc holding as few as two dozen or as many as 200 blades. Cutting the numerous fir-tree-patterned slots on the discs poses several difficulties because the geometries are complex; the dimensional tolerances are extremely tight; the work-piece material is very hard, but sensitive to damage; and the surface finish must meet exacting standards
With potentially many slots to produce for each disc, the manufacturing method selected must be effective and efficient, and the productivity of this particular operation is a significant factor in the total jet engine cost.
Broaching vs Wire EDM
Broaching, traditionally used for machining fir tree patterns is a machining process that removes material via a ‘shearing action’. However, the process hardens part surfaces meaning that broach tools wear out quickly and need to be frequently replaced, adding to the cost of the process.
Other downsides with broaching tools include their long lead times (for new tools) and their high cost.
Wire EDM, by comparison, is a more cost-effective and versatile machining option and, unlike broaching tools which are application-specific, can be used to cut virtually any shape in virtually any material, making it suitable for a wide and diverse range of applications.
Wire EDM is a process that also lends itself to automation, and advances in generator technology mean that it no longer produces micro-cracking or a re-cast layer.
A ‘Dedicated‘ machine
The new CUT 200 ‘Dedicated’ is a standard wire EDM machine with a significant number of design and build modifications. (The fact that the machine model was based on a standard design and proven technology helped accelerate its time to market and reduce overall development costs).
The new model is based on the AgieCharmilles Cut 200 wire EDM machine that has been re-engineered to accommodate an innovative rotary tilting table and enable automated production to be achieved.
Other features include in-process probing, customised power-generator settings and process monitoring/tracking software.
The innovative table
The CUT 200 ‘Dedicated’ is equipped with a non-trunnion-style rotary tilting table designed to machine large, heavy aerospace work-pieces (up to 250Kg) without flexing.
The table provides full C-axis rotation and +/-20 degree B-axis tilt – which when combined allow parts to be moved in a horizontally-orientated crescent-shaped axis.
Furthermore the table is built down into the machine (as opposed to sitting on top of the machine’s standard table) and as such reduces the need for an overlong Z-axis travel. This configuration, as well as being rigid, also improves flushing capability as the machine’s lower flush nozzles remain close to the work zone – enabling higher cutting speeds and part accuracies to be achieved.
The positioning of the table also negates issues with part weight as the tilt motion occurs directly underneath the work-piece clamping surface thereby providing improved load support and accurate table motion.
The machine’s base with its polymer granite construction provides more than ample strength and rigidity for the new table design.
Another advantage of the new table design and position is that enables production on the CUT 200 ‘Dedicated’ to become automated – a particular issue for aerospace component manufacturers.
In the past machining large aerospace parts on wire EDM machines with standard configured A/B-axis tables always proved difficult to automate because such tables occupy a significant proportion of the work envelope available and restrict a robot’s access for load/unload operations.
The table design of the CUT 200 ‘Dedicated’ machine make things easier, and the rerouting of power cables behind the work envelope has also helped free-up more space too.
Furthermore, the machine’s upper and lower guide heads can be moved to allow robot access, and then moved back into position for EDM machining.
Possibly the most important issue concerning the CUT 200 ‘Dedicated’ machine is that it confirms the acceptability and appropriateness of the wire EDM process in jet engine component manufacture.
The digital generator functionality on the Cut 200 Dedicated produces no detectable “white layer,” and is therefore suitable for machining aerospace alloys such as titanium and Inconel.
The CleanCut generator on the CUT 200 Dedicated manages the heat created in the EDM process by manipulating the parameters of the electrical discharge that produces each spark.
The generator therefore produces a pulse of electrical energy that reaches the ideal temperature very rapidly, lasts long enough for material removal to occur, then “shuts off” before material (melted away from the work-piece) has time to re-solidify on its surface. Instead, the material is quenched in the dielectric fluid and is washed away as hard particles with only a minimal chance to fuse onto the work-piece surface.
The cutting strategy applied to the fir tree patterns involves an initial roughing pass that severs a solid slug from the disc followed by three skim cuts.
In each skim cut, the wire passes along but not in contact with the newly exposed surface of the work-piece. For each successive skim cut, spark parameters are set so the electrical discharges selectively remove unwanted peaks left by the previous pass.
The CUT 200 ‘Dedicated’ also features an e.Tracking proprietary software capability to record all of the process parameters and link them to each operation. This software feature, essentially onboard data acquisition, monitors each process in detail and ensures traceability, which is essential in aerospace work. The e-Tracking system also provides a real-time dashboard to monitor an entire machine shop, as well as machine and consumables status for preventative maintenance purposes.
The design and technology developments represented by the Cut 200 ‘Dedicated’ wire EDM machine do not restrict its application to jet engine turbine discs. GF Machining Solutions believes these developments will expand the use of wire EDM wherever work-pieces of similar size, weight and complexity are encountered in tough work-piece materials. This includes many components made from titanium and high-nickel alloys used in the energy and power generation sectors.