Method and apparatus for providing rotor discs
A method for providing a disc of a bladed rotor for use in a gas turbine engine including the steps of determining the location of blade fixing slots to be formed on the disc to receive the roots of respective blades, determining a virtual profile of each slot on the disc, and forming, prior to the slots being formed in the disc, a narrow, axial pilot slot centrally of the virtual profile of each slot.
1. A method of making blade fixing slots in a disc of a bladed rotor for use in a gas turbine engine, the method comprising:
determining a plurality of circumferential locations on the disc for receiving the blade fixing slots, the blade fixing slots being adapted to receive roots of respective airfoil blades of the bladed rotor;
determining a virtual full slot profile of each of said blade fixing slots, the virtual full slot profile being adapted to correspond to a profile of the roots of the airfoil blades;
using wire electro-discharge machining (EDM) to form a single-cut pre-slot in the periphery of the disc at each of said plurality of circumferential locations, the single-cut pre-slots being planar slits which extend radially inwardly from an outer periphery of the disc, each planar slit having a uniform width corresponding to a diameter of a single EDM cutting wire from the wire EDM and thus a shape which does not correspond to a shape of the full slot profile of the blade fixing slot to be made, the single-cut pre-slots reducing residual stress in the disc prior to forming said blade fixing slots; and
forming the blade fixing slots having said full slot profile in the periphery of the disc at each of said circumferential locations, using the single-cut pre-slots as pilot guides by circumferentially aligning a center of the full slot profiles with the single-cut pre-slots.
2. The method as defined in claim 1 , wherein the step of forming the single-cut pre-slots includes extending the single-cut pre-slot to within a predetermined distance from a bottom of the virtual full slot profile.
3. The method as defined in claim 2 , wherein the virtual full slot profile is firtree shaped and defines a number of lobes, further comprising selecting said predetermined distance to correspond to a center of a radius of curvature of a lowermost one of said lobes of the firtree shaped full slot profile.
4. The method as defined in claim 2 , wherein the predetermined distance is 0.10″.
5. The method as defined in claim 1 , further comprising forming the disc by forging a metal, the metal being a forged or powder metallurgy nickel alloy.
6. The method of claim 5 , further comprising performing a thermal treatment on the disc.
7. The method as defined in claim 1 , further comprising using wire electro-discharge machining to form the blade fixing slots having said full slot profile in the disc.
8. The method as defined in claim 1 , wherein the width of the pre-slot is about 0.010 in.