A new high-performance wire-cut EDM machine designed specifically for aerospace applications, including the machining of fir-tree features on jet engine turbine parts, has been introduced by GF Machining Solutions — the EDM, milling and laser ablation machine tool specialist.
The machine — the CUT 200 Dedicated — gives aerospace part manufacturers 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 (
www.georgfischer.com), the wire-cut EDM process was evaluated against broaching from a cost perspective. The results indicated that a 40% reduction in costs — with no loss in productivity — could be achieved by adopting the wire-cut EDM route offered by the new CUT 200 Dedicated.
Cutting the fir-tree pattern on both the disc and the blade root presents problems for manufacturers. A modern jet engine can have as many as 40 turbine discs, each 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 workpiece 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, as producing these features is a significant portion of the total jet engine cost.
Processes compared
Broaching, which has traditionally been used for machining fir-tree patterns, 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 negatives with broaching are the long lead times for new tools and their high cost.
By comparison, wire-cut EDM 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 diverse range of applications. Moreover, wire-cut EDM lends itself to automation, while advances in generator technology mean that the process no longer produces micro-cracks or a re-cast layer.
The new CUT 200 Dedicated is a standard wire-cut EDM machine with a significant number of design and build modifications; the fact that the machine is based on a standard design and proven technology helped to reduce its time to market — and its overall development costs.
The machine is based on the Agie Charmilles Cut 200, but it has been re-engineered to accommodate a rotary/tilting table — and allow automated production to be achieved. Other features include in-process probing, customised power-generator settings and process-monitoring/tracking software.
The non-trunnion-style rotary/tilting table has been designed to accommodate large heavy aerospace workpieces (up to 250kg). The table provides full C-axis rotation and ±20deg B-axis tilt. Furthermore, these axes are built into the machine, as opposed to sitting on top of a standard table, which reduces the need for an over-long Z-axis travel.
As well as being rigid, this configuration also improves flushing'; the machine’s lower flush nozzles remain close to the work zone, thereby allowing higher cutting speeds and part accuracies to be achieved. The positioning of the table also negates issues associated with part weight, as the tilt motion occurs directly underneath the workpiece clamping surface, thereby optimising load support and the accuracy of table motion. Furthermore, overall performance is enhanced by the machine’s polymer granite construction.
Automation-ready
Another advantage offered by the design of the table and its position is that it allows production on the CUT 200 Dedicated to be automated — a notable benefit for manufacturers of aerospace components. In the past, machining large aerospace parts on wire-cut EDM machines with conventional A/B-axis tables proved difficult to automate, because such tables occupy a significant proportion of the work envelope and restrict a robot’s access for load/unload operations.
Furthermore, the routing of power cables behind the work envelope has freed up more space on the CUT 200 Dedicated — plus the machine’s upper and lower guide heads can be moved to allow robot access, and then moved back into position for EDM machining.
However, possibly the most important issue concerning the CUT 200 Dedicated’s application for jet engine component manufacture is that its digital generator ensures there is no detectable ‘white layer’, which means the machine is suitable for cutting aerospace alloys such as titanium and Inconel.
The CleanCut generator manages the heat created in the EDM process by manipulating the parameters of the electrical discharge that produces each spark. The generator produces a pulse of electrical energy that allows the ideal temperature to be reached very rapidly.
Moreover, the pulse lasts just long enough for material removal to occur; it then ‘shuts off’ before the material that has melted away from the workpiece has time to re-solidify on its surface (the material is quenched in the dielectric fluid and washed away as hard particles, with only a minimal chance of it fusing 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. With each skim cut, the wire passes along — but not in contact with — the newly exposed surface of the workpiece. For each successive skim cut, spark parameters are set so that the electrical discharges selectively remove unwanted peaks left by the previous pass.
The CUT 200 Dedicated also features the manufacturer’s E-Tracking software, which records all of the process parameters and links them to each operation. This software, which is essentially onboard data acquisition, monitors each process in detail and ensures traceability, which is essential in aerospace work.
E-Tracking also provides a real-time ‘dashboard’ that allows the monitoring of an entire machine shop, as well as machine and consumables status for preventative maintenance purposes.