Method and apparatus for electric-discharge machining of a turbine blade

1. A method for electric-discharge machining a turbine having a plurality of blades formed along a circumference thereof, the method comprising:

a) preparing a disc material divided into a first portion and a second portion on a basis of a symmetrical line;

b) fixing the disc material, with the first portion facing upward;

c) forming an initial profile of at least one of the plurality of blades at the first portion of the disc material using at least two roughing tool electrodes to concurrently form a roughing space at the first portion, the at least two roughing tool electrodes moving generally downward and circumferentially along a desired roughing lead angle with respect to a surface of the first portion;

d) leading at least two finishing tool electrodes into the roughing space to concurrently machine the at least one of the plurality of blades, the at least two finishing tool electrodes moving generally downward and circumferentially along a desired finishing lead angle with respect to a surface of the first portion and each of the at least two finishing tool electrodes pivotally reciprocate about a reciprocation axis generally parallel with the surface of the first portion;

e) rotating the disc material and fixing the disc material, with the second portion facing upward;

f) forming an initial profile of the at least one of the plurality of blades at the second portion of the disc material using at least two roughing tool electrodes to concurrently form a roughing space at the second portion, the at least two roughing tool electrodes moving generally downward and circumferentially along the desired roughing lead angle with respect to a surface of the second portion; and

g) leading at least two finishing tool electrodes into the roughing space formed at the second portion to concurrently machine the at least one of the plurality of blades, the at least two finishing tool electrodes moving generally downward and circumferentially along the desired finishing lead angle with respect to a surface of the second portion and each of the at least two finishing tool electrodes pivotally reciprocate about a reciprocation axis generally parallel with the surface of the second portion, and

wherein the at least two finishing tool electrodes cut through the symmetrical line by about 0.1 mm to about 0.3 mm.

2. The method according to claim 1 , wherein the at least two roughing tool electrodes and the at least two finishing tool electrodes are formed corresponding to a shape of the blade, but are formed in a different shape to each other.

3. The method of claim 1 , wherein the turbine includes a shroud integrally formed with the disc material along the circumference thereof.

4. The method according to claim 1 wherein the desired roughing lead angle is different than the desired finishing lead angle.

5. An apparatus for electric-discharge machining a turbine having a plurality of blades formed along a circumference thereof, comprising:

an electrode holder including a plurality of mounting grooves, in which at least two tool electrodes are arranged and mounted in a circumferential direction of the turbine corresponding to the blade; and

a mounting frame opposed to the electrode holder for supporting a disc material.

6. The apparatus according to claim 5 , wherein:

the at least two tool electrodes comprise a plurality of tool electrodes that are arranged corresponding to the plurality of blades; and

the plurality of tool electrodes are spaced apart from each other at a gap corresponding to one pitch of at least one of the plurality of blades.

7. The apparatus according to claim 5 , further comprising:

an electric-discharge voltage generating unit for applying an electric-discharge voltage to the at least two tool electrodes;

a tool electrode transferring unit for moving the electrode holder in a vertical direction and a circumferential direction; and

a numerical control unit for controlling the electric-discharge voltage generating unit and the tool electrode transferring unit.

8. The apparatus according to claim 5 , wherein the electrode holder is concurrently controlled in a vertical and circumferential direction.

9. A method for electric-discharge machining a turbine having a plurality of blades formed along a circumference thereof, the method comprising:

a) providing a disc material having a first side and a second side;

b) positioning the disc material, with the first side facing a machining device;

c) forming an initial profile of at least one of the plurality of blades on the first side of the disc material using at least two roughing tool electrodes to concurrently form a roughing space on the first side, the at least two roughing tool electrodes moving generally downward and circumferentially along a desired roughing lead angle with respect to a surface of the first side;

d) leading at least two finishing tool electrodes into the roughing space to concurrently machine the at least one of the plurality of blades, the at least two finishing tool electrodes moving generally downward and circumferentially along a desired finishing lead angle with respect to the surface of the first side and each of the at least two finishing tool electrodes pivotally reciprocate about a reciprocation axis generally parallel with the surface of the first side;

e) rotating and positioning the disc material, with the second side facing the machining device;

f) forming an initial profile of the at least one of the plurality of blades on the second side of the disc material using at least two roughing tool electrodes to concurrently form a roughing space on the second side, the at least two roughing tool electrodes moving generally downward and circumferentially along the desired roughing lead angle with respect to a surface of the second side; and

g) leading at least two finishing tool electrodes into the roughing space formed on the second side to concurrently machine the at least one of the plurality of blades, the at least two finishing tool electrodes moving generally downward and circumferentially along the desired finishing lead angle with respect to the surface of the second side and each of the at least two finishing tool electrodes pivotally reciprocate about a reciprocation axis generally parallel with the surface of the second side.

10. The method according to claim 9 , wherein in the leading operations the disc material proximate a symmetrical line bisecting the disc material into a first half including the first side and a second half including the second side is cut by the at least two finishing tool electrodes to be about 0.1 mm to about 0.3 mm lower than the symmetrical line.

11. The method according to claim 9 , wherein the at least two roughing tool electrodes and the at least two finishing tool electrodes are formed corresponding to a shape of the blade, but are formed in a different shape to each other.

12. The method according to claim 9 wherein the desired roughing lead angle is different than the desired finishing lead angle.