GB2435440A

GB2435440A - A grinding wheel

Info

Publication number: GB2435440A
Application number: GB0603490A
Authority: GB
Inventors: Gordon Mark Lanes
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-02-22
Filing date: 2006-02-22
Publication date: 2007-08-29
Also published as: GB0603490D0; WO2007096295A1

Abstract

A grinding wheel 1 for grinding a required profile in the side of a channel 17 that extends through a body, the wheel 1 taking the form of a cup, the cup having a base 3 and a wall 7 that delimit the hollow 5 of the cup, the wall 7 being circular in form, the circular form of the wall being centred on the axis of rotation A of the wheel, the exterior 13 of the wall being abrasive and having a profile corresponding to the required profile in the side of the channel, in use of the grinding wheel a section of the wall 7 being placed in the channel 17 so that the abrasive exterior of the section bears against the side of the channel 17 in which the profile is to be ground, grinding of the profile being achieved by rotation of the wheel 1. The body containing the channel 17 to be ground may be a central disc 19 of a turbine for a gas turbine engine and the channel to be ground may be a slot 17 in the periphery of the disc 19 for accommodating a root part of a turbine blade.

Description

-i -2435440 A grinding wheel The present invention relates to a

grinding wheel.

The present invention finds particular application in the manufacture of turbines for gas turbine engines.

The turbine of a gas turbine engine typically comprises a central disc, and, extending radially outwardly from this disc, a series of turbine blades. In order to secure the blades to the disc, a series of slots or channels must be cut in the periphery of the disc to accommodate root parts of the blades. The slots cut must have a profile corresponding to the profile of the root part of the blades.

It is known to form the required slots by so called broaching. This involves the progressive cutting away of the disc by using in turn a large number of cutting blades. The blades are sized such that each successive blade cuts away more of the disc so as to finally arrive at the required slot profile.

Problems encountered with broaching are as follows. It is expensive to make the large number of cutting blades. It takes a long time to assemble the large number of cutting blades for cutting. It takes a relatively long time to cut a finished slot. Broaching causes some tensile stress in the central disc of the turbine, reducing turbine life.

According to the present invention there is provided a grinding wheel for grinding a required profile in the side of a channel that extends through a body, the wheel taking the form of a cup, the cup having a base and a wall that delimit the hollow of the cup, the wall being circular in form, the circular form of the wall being centred on the axis of rotation of the wheel, the exterior of the wall being abrasive and having a profile corresponding to the required profile in the side of the channel, in use of the grinding wheel a section of the wall being placed in the channel so that the abrasive exterior of the section bears against the side of the channel in which the profile is to be ground, grinding of the profile being achieved by rotation of the wheel.

In a grinding wheel according to the preceding paragraph, it is preferable that the base of the cup is substantially planar.

In a grinding wheel according to either of the preceding two paragraphs, it is preferable that the wall includes an abrasive material applied to the exterior thereof.

The present invention also extends to a grinding wheel according to any one of the preceding three paragraphs in combination with the body containing the channel to be ground, the grinding wheel having a diameter which exceeds a predetermined length which depends on the length and width of the channel to be ground.

In a combination according to the preceding paragraph, the body containing the channel to be ground may be a central disc of a turbine for a gas turbine engine, and the channel to be ground a slot in the periphery of the disc for accommodating a root part of a turbine blade of the turbine.

The present invention also extends to a grinding machine including mounted therein a combination according to the preceding paragraph.

In a grinding machine according to the preceding paragraph, it is preferable that the mounting of the grinding wheel is such that the wheel is capable of translational movement along three mutually perpendicular axes, one of which is the axis of rotation of the wheel, and the mounting of the central disc of the turbine is such that the disc is capable of being positioned about first and second axes by rotation about these axes, the first axis passing through the centre of the disc and being perpendicular to the plane of the disc, the second axis passing through the centre of the disc and lying in the plane of the disc.

In a grinding machine according to the preceding paragraph, it is preferable that the mounting of the central disc of the turbine is such that the disc is capable of being positioned about a third axis by rotation about this axis, the third axis passing through the centre of the disc, lying in the plane of the disc, and being perpendicular to the second axis.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a perspective view of a grinding wheel in accordance with the present invention being used to cut turbine blade root slots in the central disc of a turbine for a gas turbine engine; Fig 2 is a view taken on the arrow II in Fig 1 and illustrates the grinding being done by the grinding wheel in Fig 1; Figs 3a, 3b and 3c are schematic views taken on the arrow III in Fig 2, and illustrate respectively the grinding wheel in a start position, mid position, and end position when grinding the profile of one side of a turbine blade root slot; and Figs 4a, 4b and 4c correspond to Figs 3a, 3b and 3c, and illustrate respectively the grinding wheel in a start position, mid position, and end position when grinding the profile of the other side of the turbine blade root slot.

Referring to Figs 1 and 2, the grinding wheel 1 takes the form of a cup, the cup having a substantially planar base 3 and a wall 7 that delimit the hollow 5 of the cup. The wall 7 is circular in form, its circular form being centred on the axis A of rotation of the grinding wheel. The cup comprising base 3 and wall 7 is formed of steel. Other suitable materials for forming the cup are aluminium, and carbon fibre composites. Secured to the centre of the exterior 9 of base 3 is a hub 11 that enables grinding wheel 1 to be rotated about axis A. The exterior 13 of wall 7 is formed so as to have a profile corresponding to the required profile of the sides 15 of the turbine blade root slots 17 of the central disc 19 of a turbine for a gas turbine engine. Exterior 13 is coated with an abrasive material such as cubic boron nitride (CBN) or polycrystalline diamond (POD) Formation of a turbine blade root slot 17 will now be described. The slot 17 is first roughly formed using a conventional grinding wheel or wire Electrical Discharge Machining (EDM) . The sides of the slot are then finished using the grinding wheel 1 of Fig 1.

Referring also to Figs 3a to 3c, grinding wheel 1 is manipulated so as to place in the slot 17 to be ground a section 21 of wall 7. As shown in Fig 3a, the section 21 is positioned so that the abrasive radially outer side 13 of the section bears against one end 23 of one side l5a of the slot.

Rotation of the grinding wheel 1 grinds end 23 with the profile of the exterior 13 of wall 7.

The grinding wheel is manoeuvred so as to move its point of contact with side 15a along side 15a from the one end 23 of side 15a to the other end 25 of side 15a. Fig 3b shows the grinding wheel when approximately midway between ends 23 and 25. Fig 3c shows the wheel when at end 25. The rotation of the wheel whilst being moved grinds the entirety of side 15a with the profile of the exterior 13 of wall 7.

Side 15a is repeatedly ground in the manner described dependent on the size of slot required.

The diameter of grinding wheel 1 must be such as to allow proper positioning of section 21 of wall 7 within slot 17 throughout the grinding of side 15a. As can be seen in Fig 3a, when grinding end 23 of side iSa the radially inner side 27 of wall 7 passes close to end 29 of the other side 15b of slot 17. If the diameter of grinding wheel 1 were any smaller then correct positioning of wall 7 for grinding end 23 would not be possible due to obstruction of wall 7 by end 29 of side 15b. Similar considerations apply when grinding end 25 of side 15a as regards obstruction of wall 7 by the other end 31 of side 15b, see Fig 3c. Thus, the diameter of the grinding wheel must exceed a predetermined length which depends on the length and width of the slot to be ground.

Grinding of the other side 15b of slot 17 is carried out in similar manner to the grinding of side 15a. Grinding wheel 1 is repositioned from its position shown in Fig 1, wherein the bottom of wheel 1 grinds downward on side 15a, to a position wherein the top of the wheel grinds upward on side 15b. In other words, referring to Fig 1, bottom region 33 of wheel 1 is removed from the slot being ground, wheel 1 is then lowered, and then top region 35 of wheel 1 is placed in the same slot being ground.

Fig 4a shows the position of grinding wheel 1 when grinding end 31 of side 15b, Fig 4b shows the position of grinding wheel 1 when grinding a mid part of side 15b, and Fig 4c shows the position of grinding wheel 1 when grinding end 29 of side 15b.

An alternative way to grind the other side 15b of slot 17 would be to leave grinding wheel 1 in the position shown in Fig 1, and to flip disc 19 by 180 degrees so that the front of disc 19 in Fig 1 becomes the back.

The slot 17 being ground is finally finished by using a conventional grinding wheel to grind the base 37 of the slot, see Fig 2.

Positioning of grinding wheel 1, positioning of turbine central disc 19, rotation of wheel 1, and movement of wheel 1 along a slot 17 to grind a side of the slot, may all conveniently be carried out by use of a standard five axis grinding machine. The five axes concerned are: (i) translational movement of wheel 1 horizontally in Fig 1, see axis X in Fig 1; (ii) translational movement of wheel 1 vertically in Fig 1, see axis Y in Fig 1; (iii) translational movement of wheel 1 into and out of the paper in Fig 1, see axis Z in Fig 1; (iv) positioning of disc 19 about vertical axis Yl in Fig 1 (i.e. disc 19 may be rotated about axis Yl to enable it to be moved to various positions about axis Yl); and (v) positioning of disc 19 about axis Zi in Fig 1, axis Zi being into and out of the paper (i.e. disc 19 may be rotated about axis Zi to enable it to be moved to various positions about axis Zi).

Side 15a of slot 17 would be ground as follows. The plane of wheel 1 is disposed vertically in Fig 1. Disc 19 is positioned about vertical axis Yl so that its plane is perpendicular to the plane of wheel 1. Disc 19 is then positioned about axis Zi so that the particular slot 17 to be ground is located to the extreme right of disc 19 in Fig 1.

Wheel 1 is moved along axes X, Y and Z so that bottom region 33 of wheel 1 enters the particular slot. Wheel 1 is then moved along axes Y and Z in order to position the wheel as shown in Fig 3a. Finally, wheel 1 is moved along axes Y and Z to grind side 15a of the slot. The movement of wheel 1 and disc 19 is suitably controlled by computer.

It is mentioned above that the diameter of grinding wheel 1 must be sufficiently large that there is no obstruction of the wheel, when grinding, by either side of the slot being ground. If positioning of disc 19 were possible about horizontal axis Xl in Fig 1 (i.e. if it were possible to rotate disc 19 about axis Xl to enable it to be moved to various positions about axis Xl), then, during grinding of a slot, as the wheel moves along the slot along axes Y and Z, positioning of disc 19 about axis Xl could be used to avoid obstruction of the grinding, thereby enabling a smaller diameter wheel to be used. By way of example, in Fig 3a, in order to avoid end 29 of slot side 15b obstructing radially inner side 27 of wall 7, disc 19 would be moved about axis Xl (which is into and out of the paper in Fig 3a) in the direction indicated by arrow 39. A corresponding arrow 41 is shown in Fig 3c.

It can be seen from Figs 3a to 3c and 4a to 4c that slots 17 do not extend through disc 19 in a direction that is precisely perpendicular to the plane of disc 19, but that slots 17 are skewed with respect to the perpendicular to the plane of the disc. It is for this reason that grinding wheel 1 when grinding a slot 17 must move along axes Y and Z in Fig 1. If slots 17 did extend precisely perpendicularly to the plane of disc 19 then a slot could be ground by movement of wheel 1 only along the axis Z.

Claims

Claims: 1. A grinding wheel for grinding a required profile in the side

of a channel that extends through a body, the wheel taking the form of a cup, the cup having a base and a wall that delimit the hollow of the cup, the wall being circular in form, the circular form of the wall being centred on the axis of rotation of the wheel, the exterior of the wall being abrasive and having a profile corresponding to the required profile in the side of the channel, in use of the grinding wheel a section of the wall being placed in the channel so that the abrasive exterior of the section bears against the side of the channel in which the profile is to be ground, grinding of the profile being achieved by rotation of the wheel.

2. A grinding wheel according to claim 1 wherein the base of the cup is substantially planar.

3. A grinding wheel according to claim 1 or claim 2 wherein the wall includes an abrasive material applied to the exterior thereof.

4. A grinding wheel according to claim 1 or claim 2 or claim 3 in combination with the body containing the channel to be ground, the grinding wheel having a diameter which exceeds a predetermined length which depends on the length and width of the channel to be ground.

5. A combination according to claim 4 wherein the body containing the channel to be ground is a central disc of a turbine for a gas turbine engine, and the channel to be -10 -ground is a slot in the periphery of the disc for accommodating a root part of a turbine blade of the turbine.

6. A grinding machine including mounted therein a combination according to claim 5.

7. A grinding machine according to claim 6 wherein the mounting of the grinding wheel is such that the wheel is capable of translational movement along three mutually perpendicular axes, one of which is the axis of rotation of the wheel, and the mounting of the central disc of the turbine is such that the disc is capable of being positioned about first and second axes by rotation about these axes, the first axis passing through the centre of the disc and being perpendicular to the plane of the disc, the second axis passing through the centre of the disc and lying in the plane of the disc.

8. A grinding machine according to claim 7 wherein the mounting of the central disc of the turbine is such that the disc is capable of being positioned about a third axis by rotation about this axis, the third axis passing through the centre of the disc, lying in the plane of the disc, and being perpendicular to the second axis.