GB2458738A

GB2458738A - Holding device for non planar work piece

Info

Publication number: GB2458738A
Application number: GB0806155A
Authority: GB
Inventors: Gordon Hampstead
Original assignee: HELLER MACHINE TOOLS Ltd
Current assignee: HELLER MACHINE TOOLS Ltd
Priority date: 2008-04-04
Filing date: 2008-04-04
Publication date: 2009-10-07
Also published as: GB2458738A8; GB0806155D0

Abstract

The holding device 20 is for holding the work piece 10 having a first non planar face 16 and a second non planar face 18. The device includes a plurality of first holding surfaces 28 which are moveable in use to a holding position in contact with the first non planar face 16. At least one second holding surface 36 is moveable in use to a holding position in contact with the second non planar face 18. There may be more than one second holding surface and each holding surface may apply a contact force to the work piece. The holding device may include hydraulic cylinder actuators for moving the holding surfaces. The work piece may be part of a gas turbine engine such as an aerofoil.

Description

A Holding Device The present invention relates to a holding device, particularly but not exclusively a holding device for holding a work piece.

It is known to provide holding devices for holding work pieces, for machining of the work pieces or other purposes. Typically, such holding devices comprise a pair of holding surfaces which locate against the work piece, applying a clamping force to the work piece to hold the work piece securely in position while the work piece is machined. Such conventional devices work relatively well for work pieces having planar faces against which the clamping surfaces can engage. However, for work pieces which comprise non planar surfaces, such as aerofoils, such devices are unsatisfactory, since they apply relatively high point clamping loads which can damage the work piece. To overcome this problem, it is known to encapsulate part of the work piece in wax. The wax encapsulation serves to distribute the clamping forces, preventing damage to the work piece. However, the encapsulation with wax can introduce inaccuracy, the wax can contaminate the machine tool and the holding device, and the wax encapsulation provides less rigid clamping than metal to metal contact. The wax must also be removed from the work piece after machining.

According to a first aspect of the present invention, there is provided a holding device for holding a work piece, the work piece including a first non planar face and a second non planar face, the holding device including a plurality of first holding surfaces which are movable in use to a holding position in contact with the first non planar face and at least one second holding surface which is movable in use to a holding position in contact with the second non planar face.

Possibly the holding device includes one or more actuators for moving the first holding surfaces to and from the holding position.

Possibly the holding device includes a plurality of second holding surfaces. Possibly the holding device includes one or more actuators for moving the or each second holding surface to and from the holding position.

Possibly the holding device includes one actuator for moving each holding surface.

Possibly, in the holding position, each holding surface applies a contact force to the work piece. Possibly the contact force is less than 30 N. Possibly each actuator includes an hydraulic cylinder. Possibly each actuator includes a clamp for locking the respective holding surface in the holding position. Possibly the clamp is actuated by hydraulic pressure.

Possibly, the hydraulic pressure is greater than 200 bar.

Possibly the holding device includes a first support for supporting the first holding surfaces. Possibly the holding device includes a second support for holding the or each second holding surface.

Possibly the first and second supports are movable relative to each other, so that the holding device is movable between an open position, in which the first and second supports permit positioning of the work piece within the holding device, and a closed position.

Possibly the supports are arranged so that the direction of movement of each holding surface is an oblique angle to the normal of the face of the work piece at a point of contact. Possibly the oblique angle is less than 20°.

Possibly an oblique angle is substantially zero, so that the direction of movement of each holding surface is substantially normal to the face of the Possibly each holding surface is non planar, and may be convex and may be part spherical.

Possibly the holding device includes a plurality of datum projections which, in use, in the holding position may engage datum surfaces formed on the work piece. Possibly the holding device includes three datum projections which extend substantially vertically upward. Possibly the holding device includes one datum projection which extends substantially horizontally.

Possibly one of the first and second supports supports all of the datum projections.

Possibly the holding device includes position sensing means for sensing the engagement of the datum surfaces with the datum projections in use.

Possibly the holding device includes control means to control operation of the actuators. Possibly the position sensing means are arranged to provide a signal to the control means. Possibly the control means are arranged to permit the first holding surfaces to be moved separately to the or each second holding surface.

Possibly the work piece includes a part which is to be worked in use, and a part which is not to be worked in use, and the first non planar face is a face which is formed on the non-worked part and the second non planar face is a face formed on the non worked part.

Possibly the holding device includes one or more third holding surfaces which may movable between a holding position in which the or each surface is in contact with the worked part of the work piece and a retracted position in which the or each surface is spaced from the worked part.

Possibly the holding device includes one or more actuators for moving the or each third holding surface. Possibly the or each actuator includes an hydraulic cylinder, which may be a double acting hydraulic cylinder.

Possibly the holding device includes at least two fourth holding surfaces, which may be movable between a holding position in which each surface is in contact with a worked part of the work piece, and a retracted position in which each surface is spaced from the worked part.

Possibly, the holding device includes one actuator for moving each fourth holding surface. Each actuator may be manually powered.

Possibly the holding device is for a machine tool, and may be for holding a work piece for a machining operation.

Possibly the work piece is part of a gas turbine engine. The non worked part of the work piece may be an aerofoil.

According to a second aspect of the present invention, there is provided a method of holding a work piece, the method including the step of providing a holding device for holding a work piece, the work piece including a first non planar face and a second non planar face, the holding device including a plurality of first holding surfaces which are movable in use to a holding position in contact with a first non planar face and at least one second holding surface which is movable in use to a holding position in contact with the second non planar face.

The holding device may include any of the features described in any of

the preceding statements.

Possibly the method includes the steps of positioning the holding device in the open position, setting the work piece against the datum projections, moving the fourth holding surfaces to their respective holding positions, positioning the holding device in the closed position, moving the first and second holding surfaces to their respective holding positions, and then moving the fourth holding surfaces away from their respective holding positions.

Embodiments of the present invention will now be described, by way of example, and with reference to the accompanying drawings, in which:-Fig. 1 is a perspective view of a work piece; Fig. 2 is a simplified perspective view of a holding device in use which has been partly cut away; Fig. 3 is a simplified cross sectional schematic view of the holding device of Fig. 2; Fig. 4 is a schematic diagram of a work support; Fig. 5 is a side cross sectional view of position sensing means in use; Fig. 6 is a perspective view of another holding device in use; Fig. 7 is another perspective view of the holding device of Fig. 6; Fig. 8 is a side cross sectional view of the holding device of Figs. 6 and 7; Fig. 9 is a perspective view from below of part of the holding device of Figs. 6 -8 in use; Fig. 10 is a perspective view from above of part of the holding device of Figs. 6 to 9 in use.

Fig. I shows a work piece 10 which includes a body part 14 which is not to be worked and a pair of end parts 12 which are to be worked. In one example, the work piece 10 could be formed by forging of a material such as titanium. The body part 14 includes a first, upper, non planar face 16 and an opposed second, lower, non planar face 18. In one example, the body part 14 could be an aerofoil. The work piece finished product could be a blade or a vane for a gas turbine engine.

Fig. 2 shows in a simplified perspective view a holding device 20 which has been partly cut away. The holding device 20 includes a first, upper support 22 and a second, lower support 24. In the example shown in Figs. 2 and 3, the first and second supports 22, 24 are solid blocks, which could be formed of a metal material. Each of the supports 22, 24 define a plurality of cavities 58 (not all of which are provided with reference numerals for the sake of clarity).

The holding device 20 includes a plurality of first work supports 25.

Each first work support 25 locates in one cavity 58 of the first support 22.

Each first work support 25 includes a first actuator 26 in the form of an hydraulic cylinder and a first holding surface 28.

The holding device 20 includes a plurality of second work supports 33.

Each second work support 33 locates in one cavity 58 of the second support 24. Each second work support 33 includes a second actuator 34 in the form of an hydraulic cylinder and a second holding surface 36.

The first and second work supports 25, 33 are similar, and are shown in more detail in Fig. 4. Each actuator 26, 34 includes a body 48 which is in the form of a hollow cylinder, a piston 68 which is movable along the interior of the body 48, a piston rod 52 extending from the piston 68 at one end to the holding surface 28, 36 which is formed at the opposite end of the piston rod 52. The body 48 defines an hydraulic oil port 50 through which hydraulic oil flows as indicated by arrow A. The piston 68 is movable from a retracted to an extended position by hydraulic oil pressure, and from the extended to the retracted position by a return spring 80.

The holding surface 28, 36 is mounted to the piston rod 52 via a contact spring 56. In use, the hydraulic oil pressure moves the piston 68 from the retracted to the extended position, so that the holding surface 28, 36 comes into contact with the work piece 10. When contact is made, the piston 68 continues to extend, compressing the contact spring 56. Compression of the contact spring 56 switches the hydraulic oil supply from the piston 68 to clamps 54 which lock onto the piston rod 52, clamping it into position. Thus, the contact force exerted by the holding surface 28, 36 on the work piece 10 is equivalent to the compression force required to compress the contact spring 56.

In one example, the compression force to compress the spring 56 is less than 30 N. In one example, the hydraulic pressure actuating the clamps 54 could be greater than 200 bar. In one example, the clamps 54 apply a force of 3.6KN to the rod 52, at 240 bar hydraulic pressure. Thus, the work supports 25,33can be securely locked in their respective holding positions against the work piece 10, while exerting a relatively small contact force upon the work piece 10. Advantageously, each holding surface 28,36 is able to find its own holding position against the work piece 10, thus accommodating variations in the surface of the work piece 10 at the point of contact with the holding surfaces 28,36.

In one example, the maximum stroke of the work supports 25,33, that is the maximum distance of movement of the piston 68 from the retracted to the extended position could be between 5 and 15mm, and optimally could be 10mm.

As shown in Fig. 4, the holding surfaces 28, 36 are, in one example, non planar, and could be convex, and optimally could be part spherical. This shape permits an amount of offset of alignment of the direction of motion of the piston rod 52 which moves along the longitudinal axis 90 of the actuator 26, 34 from the normal 92 to the work surface 10 at the point of contact. In one example, the longitudinal axis 90 could subtend an angIe 88 with the normal 92. The angle 88 could be an oblique angle and could be less than or equal to 20°. Optimally the angle 88 is 0°.

The shape of the non planar, part spherical holding surface 28, 36 also helps prevent point loading which otherwise might occur at the point of contact with the non planar faces 16, 18.

Referring again to Fig 2, each of the supports 22, 24 defines a plurality of supply passages 30 which permit communication of hydraulic oil between the work supports 25, 33, feed supply pipes 32A, return supply pipes 32B and an hydraulic pump 42. The holding device 20 includes control means including a controller 44 and a plurality of valves 40 which control the flow of hydraulic oil between the hydraulic pump 42 and the work supports 25, 33.

The holding device 20 includes control lines 46 for communicating control signals between the controller 44 and the valves 40.

The holding device 20 includes one or more pressure sensors 106.

In the example shown in Fig. 2, the holding device 20 includes two pressure sensors 106 in the return supply pipes 32B, which are in signal communication with the controller 44 via control lines 46.

The holding device 20 includes a plurality of datum projections 38, which in the example shown in Fig. 3 extend upwardly in the case of datum projections 38A from the second support 24 and laterally outwardly in the case of datum projection 38B from the second support 24.

The arrangement of the datum projections 38 is shown in more detail in Fig. 5. Referring to Fig. 5, the holding device 20 includes position sensing means in the form of a position sensor 60, the position sensor 60 including a position sensor body 61 which defines a cavity 66, the position sensor 60 including a datum body 39 which is captive but movable within the cavity 66, the datum projection 38 extending from the datum body 39, projecting out of the cavity 66 and outwardly from the position sensor body 61. The position sensorbody 61 defines a pair of air passages 63, one passage 63 extending from an air inlet port 62 to the cavity 66, the other passage 63 extending from the cavity 66 to an air outlet port 64. The air outlet port 64 communicates with a detector 98 which is in signal communication via the control line 46 with the controller 44.

In use, when no work piece 10 is present on the datum projection 38, air is provided through the air inlet port 62 as indicated by arrow B, which lifts the datum body 39 within the cavity 66, permitting air to flow from the air inlet port 62 to the air outlet port 64 and then to the detector 98. The detector provides a signal to the controller 44 indicating that no work piece 10 is present on the datum projection 38. When the work piece 10 is located on the datum projection 38 so that a datum surface 104 formed on the work piece 10 locates against the datum projection 38, the datum body 39 is depressed within the cavity 66, blocking the air flow between the air inlet port 62 and the air outlet port 64, so that no airflow flows to the detector 98, which thus provides a signal to the controller 44 indicating that the work piece 10 is present on the datum projection 38. Thus, the position sensor 60 detects when the work piece 10 is correctly seated on or against the datum projections 38.

In one example, the holding device 20 could include three upwardly extending datum projections 38A and one laterally extending datum projection 38B.

Referring back to Fig 2, each of the supports 22, 24 could include a support surface 82, 84 respectively which corresponds in shape to the shape of the respective non planar face 16, 18 against which, or near to which, it is to be located. The longitudinal axis 90 of each of the work supports 25, 33 could substantially extend normally from the respective support surface 82, 84, or as previously described could subtend an angle 88 to the normal.

The supports 22, 24 also include side faces 94 which are also provided with work supports 25, 33 which act against internal faces 96 of the end parts 14. The internal faces 96 could also be non planar.

The first and second supports 22, 24 are movable between an open position, in which the first support 22 is spaced away from the second support 24 permitting positioning of the work piece 10 onto the datum projections 38, and a closed position.

In use, initially the holding device 20 is in the open position. The work piece 10 is positioned on the datum projections 38, so that the controller 44 receives a signal from the detector 98 that the datum projections 38 are depressed. This indicates that the datum projections 38 are correctly positioned against the datum surfaces 104 formed on the work piece 10. Any movement of the work piece 10 away from the datum projections 38 during the initial set up will be sensed by the position sensor 60 which will send a signal via the detector 98 to the controller 44, which can then output an alarm signal.

The holding device 20 is then moved to the closed position, in which gaps 86 are defined between the first support surface 82 and the first non planar face 16, the second support surface 84 and the second non planar face 18, and the side faces 94 and the internal faces 96. The holding device 20 is arranged so that the gaps 86 are less than the distance between the position of each holding surface 28, 36 when the piston 68 is in the retracted position and the position of each holding surface 28,36 when the piston 68 is in the extended position. In other words, the gaps 86 are less than the maximum stroke of the work supports 25,33. The holding device 20 is arranged so that the gaps 86 can accommodate any variation in the dimensions or shape of the body part 14, which results from the method of manufacture and the tolerances associated therewith. For example, forging results in variations in the shape and dimensions of the work piece 10. In one example, the gaps 86 are between 2 and 6mm.

With the holding device 20 in the closed position, the controller 44 operates control valves 40, moving the first and second holding surfaces 28, 36 to a holding position, in which the holding surfaces 28, 36 are in contact with the first and second non planar faces 16, 18 respectively. As described above, the contact is a relatively gentle spring pressure contact, so that the risk of damaging the work piece 10 through contact pressure is minimised.

Once the holding position is established, the clamps 54 of each actuator 26, 34 are actuated, locking the holding surfaces 28, 36 in the holding position against the first and second non planar faces 16, 18 and the internal faces 96.

With the holding surfaces 28, 36 locked in the holding position, machining operations can then be carried out on the end parts 12. Any variation in the hydraulic pressure locking the holding surfaces 28, 36 in the holding position is sensed by the pressure sensors 106 and communicated to the controller 44, which can then output an alarm signal and/or stop the machining operation, as appropriate.

In one example, part way during the machining operation, the hydraulic pressure to the clamps 54 can be relaxed, permitting movement of each piston 68 to the retracted position, so that the holding surfaces 28 move away from the holding position. This could permit stress relieving of the work piece 10 before a final machining operation. After a relatively short period of time, which could be approximately 10 seconds, the holding surfaces 28, 36 could be moved again into the holding position and the clamps 54 actuated to lock the holding surfaces 28, 36 in the holding position.

There is thus provided a holding device which securely holds a work piece 10 in position for working. The holding device accommodates non planar faces, and can accommodate a variation in the non planar faces which may result from manufacturing tolerances, such as those tolerances which arise during forging. In comparison with conventional holding devices, the holding device of the present invention permits much faster set up and turnaround of work pieces, reducing the machine time required for each work piece, and thus reducing cost.

Figs. 6 to 10 show another embodiment of the present invention, many features of which are similar to those described for the embodiments shown in Figs. 2 to 5. Where features are the same or similar, the same reference numerals have been used, and these features will not be described again in detail for the sake of brevity.

Figs. 6 to 10 show a holding device 120 including a first, upper support 22 and a second, lower support 24. The holding device 120 includes a plurality of first work supports 25 including first holding surfaces 28 which are located within cavities 58 defined by the first support 22 and a plurality of second work supports 33 including second holding surfaces 36 which are located within cavities 58 defined by the second support 24.

The holding device 120 includes clamps 100 for clamping the supports 22, 24 in the closed position.

The holding device 120 includes a plurality of third holding surfaces 72 each of which is movable by means of an actuator 70 between a holding position in which each third holding surface 72 is in contact with one end part 12 of the work piece 10 and a retracted position in which each third holding surface 72 is spaced from the respective end part 12. The third actuators 70 are double acting hydraulic cylinders.

The holding device 120 includes fourth holding surfaces 76 each of which is movable by means of a fourth actuator 74 between a holding position in which each fourth holding surface 76 is in contact with a holding projection 78 projecting laterally from the end part 12 and a retracted position in which each fourth holding surface 76 is spaced from the holding projection 78. Each fourth actuator 74 could be manually actuated and could include, for example, a screw drive.

Figs. 9 and 10 show a work piece 10 within the holding device 120, with many of the parts of the holding device 120 removed for clarity. As shown in Fig. 9, the holding device 120 includes three datum projections 38A which extend upwardly from the second support 24. As shown in Fig. 10, the holding device 120 includes one laterally extending datum projection 38B, which extends laterally from the second support 24.

In use, the holding device 120 is initially positioned in the open position, permitting location of the work piece 10 onto the upwardly extending datum projections 38A and against the laterally extending datum projection 38B. As described previously, the holding device 120 could include position sensing means, which ensure that the work piece 10 is correctly positioned on the datum projections 38A, 38B.

When correctly located, a stop 102 could be positioned to prevent further lateral movement of the work piece 10 relative to the holding device 120. The stop 102 could be manually actuated, and could be a screw stop.

The stop 102 could provide an aid to setup of subsequent similar work pieces.

The manually actuated fourth holding surfaces 76 can then be positioned in the holding position against the holding projections 78, to hold the work piece securely relative to the second support 24. Up to this stage, no hydraulic power has been used, so that operators are able to work safely within a safety enclosure (not shown in the figures) around the holding device 120.

The operators now leave the safety enclosure and the holding device is then positioned in the closed position, and the clamps 100 operated to secure the holding device 120 in the closed position.

In the closed position, the first and second holding surfaces 28,36 are moved to the holding position against the first and second non planar faces 16, 18 respectively and locked in the holding position by operation of the clamps 54. With the holding surfaces 28, 36 locked in position, the operators can re-enter the safety enclosure, and the manually actuated fourth holding surfaces 76 can be moved to the retracted position, away from the holding position, and away from the end parts 12 of the work piece 10. The fourth holding surfaces 76 thus provide secure holding of the work piece 10 during the set up operation. The manual actuation of the fourth holding surfaces 76 permits an operator to enter a safety cabinet around the work piece 10 to move the fourth holding surfaces 76 out of the holding position safely. At the same time, the operator can also move the stop 102 away from the end part 12.

The operators then leave the safety enclosure and the third holding surfaces 72 can be actuated to move to their respective holding position against the end parts 12, to provide additional support to the end parts 12 during a machining operation. During the machining operation, the double acting cylinders of the third actuators 70 can move the third holding surfaces 72 away from the end parts 12 to the retracted position to permit operation of a tool (not shown) on the end parts 12.

As described previously, part way during the machining process, the first and second holding surfaces 28, 36 can be moved away from the holding position to permit stress relieving of the work piece 10 before final machining.

Various other modifications could be made without departing from the scope of the invention. The supports could be formed in any suitable way.

For example, the supports could be formed by fabrication. The holding surfaces could be moved in any suitable way. The holding device could include any suitable number of holding surfaces which could be of any suitable size and shape. The work supports could be controlled in any suitable way. For example, the work supports could be operated to move the respective holding surfaces in a predetermined sequence. In one example, the first work supports could operate separately to the second work supports.

In another example each work support could be operated independently to the other work supports.

There is thus provided a holding device for a work piece which can securely hold a work piece by gripping a non worked body part which includes non planar faces. The holding device can accommodate variations in the size and shape of the body part which is gripped. The holding device only exerts a relatively low contact pressure upon the work piece, while holding the work piece securely during machining operations. The holding device permits reduced set up times in comparison with conventional arrangements while providing more accurate machining since there is less possibility of movement during the machining operation. The amount of scrap is thus reduced during machining. In comparison with conventional arrangements, in which non worked parts are encapsulated in wax, neither the holding device, the work piece, nor the machine tool suffers contamination by wax. The steps of encapsulation and removal of wax from the work piece are not required, reducing processing time and cost.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS1. A holding device for holding a work piece, the work piece including a first non planar face and a second non planar face, the holding device including a plurality of first holding surfaces which are movable in use to a holding position in contact with the first non planar face and at least one second holding surface which is movable in use to a holding position in contact with the second non planar face.
2. A holding device according to claim 1, in which the holding device includes a plurality of second holding surfaces.
3. A holding device according to claims 1 or 2, in which in the holding position, each holding surface applies a contact force to the work piece.
4. A holding device according to any of the preceding claims, in which the holding device includes one or more actuators for moving the first holding surfaces to and from the holding position.
5. A holding device according to any of the preceding claims, in which the holding device includes one or more actuators for moving the or each second holding surface to and from the holding position.
6. A holding device according to claims 4 or 5, in which the holding device includes one actuator for moving each holding surface.
7. A holding device according to any of claims 4 to 6, in which each actuator includes an hydraulic cylinder.
8. A holding device according to any of claims 5 to 7, in which each actuator includes a clamp for locking the respective holding surface in the holding position.
9. A holding device according to claim 8, in which the clamp is actuated by hydraulic pressure.
10. A holding device according to claim 9, in which the hydraulic pressure is greater than 200 bar.
11. A holding device according to any of the preceding claims, in which the holding device includes a first support for supporting the first holding surfaces.
12. A holding device according to any of the preceding claims, in which the holding device includes a second support for holding the or each second holding surface.
13. A holding device according to claim 12 when dependent on claim 11, in which the first and second supports are movable relative to each other, so that the holding device is movable between an open position, in which the first and second supports permit positioning of the work piece within the holding device, and a closed position.
14. A holding device according to any of claims 1110 13, in which the supports are arranged so that the direction of movement of each holding surface is an oblique angle to the normal of the face of the work piece at a point of contact.
15. A holding device according to claim 14, in which the oblique angle is less than 20°.
16. A holding device according to claim 15, in which the oblique angle is substantially zero, so that the direction of movement of each holding surface is substantially normal to the face of the work piece at the point of contact.
17. A holding device according to any of the preceding claims, in which each holding surface is non planar.
18. A holding device according to claim 17, in which each holding surface is convex.
19. A holding device according to claims 17 or 18, in which each holding surface is part spherical.
20. A holding device according to any of the preceding claims, in which the holding device includes a plurality of datum projections which, in use, in the holding position engage datum surfaces formed on the work piece.
21. A holding device according to claim 20, in which the holding device includes three datum projections which extend substantially vertically upward.
22. A holding device according to claims 20 or 21, in which the holding device includes one datum projection which extends substantially horizontally.
23. A holding device according to any of claims 20 to 22 when dependent on claims 11 or 12 or any claim dependent thereon, in which one of the first and second supports supports all of the datum projections.
24. A holding device according to any of claims 20 to 23, in which the holding device includes position sensing means for sensing the engagement of the datum surfaces with the datum projections in use.
25. A holding device according to claims 4 or 5 or any claim dependent thereon, in which the holding device includes control means to control operation of the actuators.
26. A holding device according to claim 25 when dependent on claim 24, in which the position sensing means are arranged to provide a signal to the control means.
27. A holding device according to claims 25 or 26 when dependent on claim 4, in which the control means are arranged to permit the first holding surfaces to be moved separately to the or each second holding surface.
28. A holding device according to any of the preceding claims, in which the work piece includes a part which is to be worked in use, and a part which is not to be worked in use, and the first non planar face is a face which is formed on the non-worked part and the second non planar face is a face formed on the non worked part.
29. A holding device according to claim 28, in which the holding device includes one or more third holding surfaces which are movable between a holding position in which the or each third holding surface is in contact with the worked part of the work piece and a retracted position in which the or each third holding surface is spaced from the worked part.
30. A holding device according to claim 29, in which the holding device includes one or more actuators for moving the or each third holding surface.
31. A holding device according to claim 30, in which the or each actuator includes an hydraulic cylinder.
32. A holding device according to claim 31, in which the hydraulic cylinder is a double acting hydraulic cylinder.
33. A holding device according to any of the preceding claims, in which the holding device includes at least two fourth holding surfaces.
34. A holding device according to claim 33 when dependent on claim 28 or any claim dependent thereon, in which each fourth holding surface is movable between a holding position in which each surface is in contact with a worked part of the work piece, and a retracted position in which each surface is spaced from the worked part.
35. A holding device according to claim 34, in which the holding device includes one actuator for moving each fourth holding surface.
36. A holding device according to claim 35, in which each actuator is manually powered.
37. A holding device according to any of the preceding claims, in which the holding device is for a machine tool.
38. A holding device according to any of the preceding claims, in which the holding device is for holding a work piece for a machining operation.
39. A holding device according to any of the preceding claims, in which the work piece is part of a gas turbine engine.
40. A holding device according to any of claims 28 to 39, in which the non worked part of the work piece is an aerofoil.
41. A method of holding a work piece, the method including the step of providing a holding device for holding a work piece, the work piece including a first non planar face and a second non planar face, the holding device including a plurality of first holding surfaces which are movable in use to a holding position in contact with a first non planar face and at least one second holding surface which is movable in use to a holding position in contact with the second non planar face.
42. A method according to claim 41, in which the holding device is according to any of claims I to 40.
43. A method according to claims 41 or 42, in which the method includes the steps of positioning the holding device in the open position, setting the work piece against the datum projections, moving the fourth holding surfaces to their respective holding positions, positioning the holding device in the closed position, moving the first and second holding surfaces to their respective holding positions, and then moving the fourth holding surfaces away from their respective holding positions.
44. A holding device as hereinbefore described and with reference to the drawings.
45. A method as hereinbefore described and with reference to the drawings.
46. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.