Manufacturing method

Abstract

A method of manufacturing a component is provided. The method includes performing a machining operation by moving a rotating, abrasive grinding tool along a feed direction to remove material from the component. At least the part of the component from which the material is removed is formed of composite material. The abrasive grinding tool follows a trochoidal path along the feed direction.

Claims

1. A method of manufacturing a component, the method including: performing a machining operation by moving a rotating, abrasive grinding tool along a feed direction to remove material from the component; wherein at least the part of the component from which the material is removed is formed of composite material, and wherein the abrasive grinding tool follows a trochoidal path along the feed direction, wherein the composite material is a ceramic matrix composite material, wherein the trochoidal path has a step over t, which is the distance in the feed direction between two equivalent points on adjacent loops of the trochoidal path, such that: $t<\sqrt{{\left(\frac{d}{2}\right)}^2-{\left(\left(\frac{d}{2}\right)-\mathrm{ae}\right)}^2}-\sqrt{{\left(\left(\frac{d}{2}\right)-g\right)}^2-{\left(\left(\frac{d}{2}\right)-\mathrm{ae}\right)}^2}$ where d is the diameter of the tool, ae is the depth of cut made by the tool, and g is the maximum distance by which abrasive grit particles protrude from the surface of the tool, wherein the component is a seal segment of a gas turbine engine, and the machining operation forms a groove in the seal segment.

2. The method according to claim 1, wherein the trochoidal path lies in a plane and the abrasive grinding tool rotates about a rotation axis which is perpendicular to the plane.

3. The method according to claim 1, wherein the rotation of the abrasive grinding tool is directed such that the surface of the tool in contact with the component rotates towards the a most recently cut surface of the component.

4. The method according to claim 1, wherein the trochoidal path contains loops having a loop radius r, which in terms of the tool diameter d of the abrasive grinding tool has a value which, to within ±10%, is determined by the expression: $r=\left(\left(\frac{d}{0.7}\right)-d\right)0.5.$

5. The method according to claim 1, wherein the machining operation includes forming a slot in the component.

6. The method according to claim 5, wherein:
d<0.7w where d is the diameter of the tool and w is the width of the slot measured perpendicularly to the feed direction.

7. The method according to claim 1, wherein the machining operation includes removing an external face of the component.

Description

DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIG. 1 is a sectional side view of a gas turbine engine;

(3) FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

(4) FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

(5) FIG. 4 shows schematically a slot being machined by trochoidal cutting using a rotating, abrasive grinding tool;

(6) FIG. 5 shows schematically a flank being machined by trochoidal cutting using a rotating, abrasive grinding tool; and

(7) FIG. 6 illustrates parameters of the machining operations.

DETAILED DESCRIPTION

(8) FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

(9) In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

(10) An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

(11) Note that the terms “low pressure turbine” and “low pressure compressor” as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the “low pressure turbine” and “low pressure compressor” referred to herein may alternatively be known as the “intermediate pressure turbine” and “intermediate pressure compressor”. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

(12) The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the present disclosure. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

(13) The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

(14) It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

(15) Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

(16) Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

(17) Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20 meaning that the flow through the bypass duct 22 has its own nozzle 18 that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

(18) The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

(19) The turbine section of the engine has components such as seal segments, a seal rings, nozzle guide vanes, and/or turbine blades formed from CMC material. FIG. 4 shows schematically a slot in one of these components, e.g. a groove in a seal segment, being machined by trochoidal cutting using a rotating, abrasive grinding tool, such as a grinding wheel or pin. The trochoidal path of the tool results in the slot being cut with a width w which is greater than the diameter d of the tool. This width w is determined by d and the radius r of the loops of the tool on the trochoidal path. FIG. 5 shows by comparison trochoidal cutting along a flank of one of these components to remove surface material using the abrasive grinding tool. In the machining operation of FIG. 4 the direction of tool feed is the same as the direction of maximum depth of cut, while in that of FIG. 5 it is perpendicular to the direction of maximum depth of cut. Nonetheless in both operations similar considerations apply.

(20) In particular, in both operations the tool moves along a feed direction (indicated by a bold arrowed line) having a trochoidal path, two loops of which are indicated in each of FIGS. 4 and 5 by dash-double dotted lines. This path is defined by a step over t which is the distance along the feed direction between two equivalent points on adjacent loops (i.e. points A and B in FIG. 5), and the radius r of the loops. The tool rotates around an axis which is perpendicular to the plane of the trochoidal path. The rotation direction of the tool is typically such that the surface of the tool in contact with the component rotates towards the most recently cut surface of the component. In this way chips can be preferentially evacuated in a rearward direction relative to the movement of the tool into the component, which assists chip removal. However, a further consideration for determining the rotation direction of the tool is to avoid inducing strong delamination-inducing forces in the composite material as a result of the tool rotation. In general the rearward evacuation of chips is compatible with this further consideration.

(21) The trochoidal path produces a cycle of engagement and disengagement of the tool and the component surface being cut which helps to prevent overheating of the tool and the component. The cycle also facilitates chip evacuation and cutting fluid access.

(22) In the slot cutting operation of FIG. 4, the width w of the slot determines a preferred upper limit for the tool diameter d in order that the tool can follow an effective trochoidal path. In particular, preferably:
d<0.7w

(23) Additionally or alternatively, it is preferred that:
d>0.5w

(24) Such a preferred upper and lower limits for the tool diameter d do not apply in the case of the flank cutting operation of FIG. 5. In respect of both these operations, however, there exists a preferred relationship between the tool diameter d and the path loop radius r. In particular, r may have a value which, to within ±10%, is determined by the expression:

(25) $r=\left(\left(\frac{d}{0.7}\right)-d\right)0.5$

(26) Moreover a preferred upper limit can be set on the path step over t dependent on the following parameters: the tool diameter d, the depth of cut ae, and the maximum distance g by which abrasive grit particles protrude from the surface of the tool, these parameters being illustrated in FIG. 6. Thus:

(27) $t<\sqrt{{\left(\frac{d}{2}\right)}^2-{\left(\left(\frac{d}{2}\right)-\mathrm{ae}\right)}^2}-\sqrt{{\left(\left(\frac{d}{2}\right)-g\right)}^2-{\left(\left(\frac{d}{2}\right)-\mathrm{ae}\right)}^2}$

(28) Advantages of performing the machining operations using a trochoidal path for the tool is that cutting forces can be reduced, chip evacuation improved, and the risks of overheating, delamination, chipping, fibre damage etc. can be reduced. Overall machining rates and tool life can also be increased, reducing cycle times and costs.

(29) To reduce the likelihood of machining operations causing surface damage, a known approach is to encapsulate the component in a protective encapsulating material, such as wax or glass fibre reinforced plastic, prior to machining. The encapsulant can then be removed after the operation is completed. However, an advantage of the trochoidal grinding process is that it can eliminate the need for encapsulation.

(30) Although described above in relation to machining of a CMC component, the method can also be applied to other composite material components, for example formed from polymer matrix composite materials (e.g. carbon fibre reinforced composite fan blades), or metal matric composite materials.

(31) It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.