GB2385815A

GB2385815A - Method of manufacturing turbine blades

Info

Publication number: GB2385815A
Application number: GB0204818A
Authority: GB
Inventors: James Stewart Rogers
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 2002-03-01
Filing date: 2002-03-01
Publication date: 2003-09-03
Anticipated expiration: 2022-03-01
Also published as: GB0204818D0; GB2385815B

Abstract

A method of manufacturing turbine blades comprises rotating a rotary cutting tool (3 fig 1) to give a cutting speed of at least 500 m/min; moving the rotary cutting tool and a workpiece (1, 2 fig 1) such that the tool shapes the profile of a blade by performing a generally orbital motion while moving incrementally along the workpiece length, the tool rotation axis remaining at a fixed angle to the direction of incremental movement; and changing the speed of the relative motion at least once during each orbit. The speed of the relative motion may be changed at the start and end of each face of the blade, being less across a concave face than across a convex face and greatest at leading and trailing edges of the blade. The tool may make a series of parallel cuts, each 0.01 to 0.04 mm deep. The tool may be moved out of contact with the workpiece to be moved incrementally along the workpiece. The tool may have a constant rotational speed and a cutting speed of at least 700 m/min, preferably 733 m/min.

Description

METHOD OF MANUFACTURING TURBINE BLADES Field of the Invention This invention relates to a method of manufacturing turbine blades.

Background to the Invention Turbine blades, for example gas turbine blades, are typically manufactured by machining a solid metal block, using a 5-axis rotary cutting or milling machine under computer control. The cutting tool, typically carbide tipped, is moved in a continuous spiral path around the metal block. It is generally understood that, for carbide-tipped tools cutting stainless steel, the cutting speed, that is the speed of the cutting edge of the tool in contact with the workpiece, should not exceed about 150m/minute to avoid the risk of tool failure and a reduction in cutting efficiency.

Current manufacturing methods produce turbine blades which typically require hand finishing to remove machining marks and the like, and the manufacturing cost is high, both because the rate of machining is slow and because hand finishing is costly.

Since a typical engine will need a large number of blades, the contribution of the blades to the overall cost of the engine is significant. Any reduction in the manufacturing cost of the blades will therefore have a significant effect on the overall engine cost.

A further factor is that newer designs of turbine require blades with a significant degree of twist in the aerofoil shape to maximise efficiency. Such blades cannot readily be hand-finished using such tools as belt grinders.

While there has been discussion of the possibility that, if one increases cutting speed substantially, a point is reached at which cutting again becomes effective, in practice this has not been achieved. One reason for this may be that vibration of the workpiece and the tool tend to have a deleterious effect. It has now been found that it is possible to increase the cutting speed substantially, while moving the cutter in only three axes, thereby simplifying and accelerating the manufacture of turbine blades.

Summary of the Invention According to the invention, there is provided a method of manufacturing turbine blades, comprising:

(a) rotating a rotary cutting tool at a speed such that the cutting speed is at least 500m/min ; (b) causing relative motion between the rotary cutting tool and a metal workpiece such that the tool shapes the profile of the blade from the workpiece by performing a generally circular motion around the profile, while moving incrementally along the length of the workpiece, the axis of rotation of the tool remaining at a fixed angle to the direction of incremental movement; and (c) changing the speed of the relative motion at least once during each generally circular motion around the profile.

Preferably, the method comprises changing the speed of the relative motion at the start and end of each face of the aerofoil profile. More preferably, the speed across the concave face of the blade is less than the speed across the convex face, while that at the leading and trailing edges is greatest. Suitable feeding speeds are 4m/minute across the concave face, 6m/min across the convex face and 6.6m/min across the leading and trailing edges.

It may be desirable to change the speed of the relative motion repeatedly according to the position of the tool on the workpiece during each generally circular motion around the profile. Thus, the feed rate may be repeatedly changed as the tool moves across each face of the aerofoil profile of the blade.

While the tool may be moved in a continuous spiral motion around the workpiece, while remaining in contact with the workpiece, in a preferred method the tool is moved relative to the workpiece so as to perform a series of parallel or layer cuts, each preferably of 0.01 to 0. 4mm depth, the tool preferably performing the incremental move along the length of the workpiece while moved out of contact with the workpiece, before being brought again into contact with the workpiece for the generally circular cutting motion. It has been found that the preferred method may be advantageously used for the production of blades with a relatively thin profile, where the vibration set up by continuous contact of the tool with the blade may result in a significant reduction of the surface finish of the blade.

The rotational speed of the tool is conveniently kept constant, while the speed of the tool's axis relative to the workpiece may be varied so that, for example, the speed across the concave face of the blade may be less than the speed across the convex face, while that at the leading and trailing edges may be greatest.

The cutting speed of the tool is preferably at least 700 m/min, for example 733 m/min.

It will be understood that the method of the invention concerns not only the shaping of the blade surfaces, but also the root or platform of the blade in the same operation.

It will usually be convenient for the workpiece to be held stationary and the tool moved around the workpiece, although it would also be possible for the workpiece to be moved, either in addition to. or instead of, the movement of the rotating tool.

It has been found that, by using the method of the invention, the high cutting speed produces blades which have an acceptable surface finish which does not require any further machining or finishing. In addition, only a 3-axis machine is required for the operation, saving considerably on costs. As a consequence of these advantages, the cost of manufacturing gas turbine blades may be reduced substantially, for example by a factor of ten.

Brief Description of the Drawings In the drawings, which illustrate diagrammatically exemplary embodiments of the invention: Figure 1 is a perspective view of a turbine blade and a typical cutting tool ; Figure 2 illustrates the cutting pattern in accordance with one method of the invention; and Figure 3 illustrates the cutting pattern in accordance with an alternative method of the invention.

Detailed Description of the Illustrated Embodiments Referring first to Figure 1, a typical turbine blade comprises a curved blade portion 1 extending from a slotted root 2 by which it is mounted in the turbine rotor or stator, depending on the type of blade. The blade portion 1 is an aerofoil whose profile

typically changes along the axis of the blade. The design of the blade shape is a complex process which will not be discussed here in detail, since the manufacturing process applies to all blades regardless of variations in shape. In accordance with the present invention, a rotary cutter 3 is rotated at high speed while being moved relative to the workpiece from which the blade is to be formed. The illustrated cutting tool 3 is an"elephant's foot"cutter surfaced with tungsten carbide particles. Such cutters are well-known in the shaping of metals. A typical rotary speed for the cutting tool in accordance with the invention is 6500 rpm. This may be compared with the conventional maximum speed of 1500 rpm for the same tool.

In a preferred method, illustrated in Figure 2, the cutting tool 3 performs a series of generally circular movements in planes parallel to each other, the tool being brought into contact with an initially generally rectangular block of metal in an inwardly curving motion to commence the cutting of the profile at a position corresponding with the midpoint of the convex face 4 of the blade, moving to cut the leading edge of the blade, followed by the concave face, the trailing edge and the remainder of the convex face, the tool 3 being moved outwardly away from the block to an end position A, from where it moves across to a start position B, to be incremented downwardly by a small distance, typically 0. 01mm to 0. 4mm, conveniently around 0.2mm, to repeat the circular motion (it will be understood that, as the profile may change gradually along the length of the blade, the circular motion of the tool may not be identical from one incremental cut to the next, but may change very slightly). The movement of the tool over the metal surface will suitably be varied according to the part of the profile being cut, for example being of the order of 4m/minute over the concave face of the blade, about 6m/minute over the convex face, and about 6.6m/minute over the leading and trailing edges. The feed rates vary to maximise cutting efficiency and to avoid the creation of resonance in the blade/tool.

An alternative method in accordance with the invention is illustrated in Figure 3.

In this method, the cutting tool remains in contact with the metal throughout the shaping operation, performing a continuous spiral movement whose pitch is comparable to that incremental movement of the tool in the method illustrated by Figure 2, namely about 0.01 to 0. 4mm. Thus, the tool requires a continuous feed movement parallel to the blade

axis in addition to the movements perpendicular thereto. It has been found that this method is acceptable for larger, less flexible, blades.

Claims

1. A method of manufacturing turbine blades, comprising: (a) rotating a rotary cutting tool at a speed such that the cutting speed is at least 500m/min ; (b) causing relative motion between the rotary cutting tool and a metal workpiece such that the tool shapes the profile of the blade from the workpiece by performing a generally circular motion around the profile, while moving incrementally along the length of the workpiece, the axis of rotation of the tool remaining at a fixed angle to the direction of incremental movement; and (c) changing the speed of the relative motion at least once during each generally circular motion around the profile.

2. A method according to Claim 1, comprising changing the speed of the relative motion at the start and end of each face of the aerofoil profile.

3. A method according to Claim 2, wherein the speed across the concave face of the blade is less than the speed across the convex face, while that at the leading and trailing edges is greatest.

4. A method according to Claim1, comprising changing the speed of the relative motion repeatedly according to the position of the tool on the workpiece during each generally circular motion around the profile.

5. A method according to any preceding claim, wherein the tool is moved relative to the workpiece so as to perform a series of parallel or layer cuts.

6. A method according to Claim 5, wherein each cut is of 0.01 to 0.4mm depth.

7. A method according to Claim 5 or 6, wherein the tool performs the incremental move along the length of the workpiece while moved out of contact with the workpiece, before being brought again into contact with the workpiece for the generally circular cutting motion.

8. A method according to any preceding claim, wherein the rotational speed of the tool is constant.

9. A method according to any preceding claim, wherein the cutting speed of the tool is at least 700 m/min.

10. A method according to Claim 9, wherein the cutting speed of the tool is 733 m/min.

11. A method of manufacturing a turbine blade, substantially as described with reference to the drawings.