Abstract:
A method and apparatus for transferring a workpiece, such as a large wind turbine component, in datum alignment between tools during a manufacturing process is described. The workpiece is transferred from a first tool to a second tool, and the method comprises arranging a support structure in datum registration with the first tool; attaching the workpiece to the support structure; releasing the workpiece from the first tool; arranging the support structure in datum registration with the second tool and such that the workpiece is supported by the second tool; and releasing the workpiece from the support structure. Arranging the support structure in datum registration with the first and second tools ensures correct alignment of the workpiece with respect to the second tool. The invention eliminates the requirement to provide datum features on the workpiece itself. In preferred examples, the workpiece is attached to the support structure by means of vacuum.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to techniques used in the manufacture of large parts, such as component parts of modern utility-scale wind turbines. More specifically, the present invention relates to a method and apparatus for transferring a workpiece such as a large wind turbine component in datum alignment between tools in a manufacturing process. 
       BACKGROUND 
       [0002]    Many modern wind turbine components such as rotor blades and component parts thereof are created in large moulds. Once formed, the moulded parts are released from the moulds and transferred to another tool where subsequent manufacturing steps take place. For example, the moulded parts may be transferred to an assembly jig for assembly with other parts to form the finished component. The moulded part or finished component may also be transferred to tools for machining or finishing operations. 
         [0003]    It is generally important to ensure precise alignment between a workpiece, for example the moulded part referred to above, and the tool to which it is transferred. This ensures that assembly operations or subsequent machining or finishing steps are performed correctly, and within predefined acceptable tolerances. To facilitate this alignment, the workpiece is typically provided with a sacrificial portion at its periphery upon which a datum feature or equivalent reference point is provided. The datum feature on the workpiece engages with, or otherwise aligns with, a corresponding datum feature on the tool to which the workpiece is transferred to ensure correct alignment of the workpiece with the tool. The sacrificial portion of the workpiece is ultimately removed to form the finished component. 
         [0004]    An example of the above process is the production of a spar for a wind turbine blade. A spar is an elongate reinforcing member that extends inside the blade from root to tip to provide structural rigidity to the blade. Applicant&#39;s PCT patent application WO2009/153341A2 describes a spar formed from four parts: two spar caps and two shear webs. The spar caps each form part of a respective upper and lower aerodynamic shell of the wind turbine blade, whilst the shear webs comprise longitudinal beams that extend between the spar caps. The two spar caps and two shear webs are assembled to form a spar structure having a box-shaped cross section. 
         [0005]    The spar caps are made from carbon-fibre reinforced composite materials and are formed in female moulds. Once formed, the moulded spar caps are lifted from the moulds using a crane or other lifting equipment and transferred to an assembly jig for assembly with the two shear webs to form the spar box structure. 
         [0006]    During the initial moulding process, a sacrificial portion is created at an end of each spar cap. The sacrificial portion generally comprises a flange. A female datum feature is provided on the sacrificial portion. To ensure correct alignment of the spar cap on the assembly jig, the spar cap is arranged on the assembly jig such that the female datum feature mates with a corresponding male datum feature associated with the assembly jig. Once the spar structure has been assembled, the sacrificial portions of the spar caps are removed and discarded. 
         [0007]    Creating sacrificial portions on a workpiece ultimately leads to material wastage. The present invention aims to provide a more efficient and more cost-effective method and apparatus for ensuring the correct alignment of a workpiece with tools, which avoids the requirement for sacrificial portions to be provided on the workpiece. 
         [0008]    When lifting and transferring very large components such as part of a wind turbine rotor blade between tools, it is important to preserve the precise geometry of the component and prevent it from twisting or buckling. Many modern rotor blades are in fact designed to have a certain twist, and it is important to be able to support twisted components without altering this twist. At present, a substantial truss frame is attached to these components so that the components remain fully supported during transfer. The truss frame is very heavy, and positioning it relative to the component requires great care and the use of heavy-duty lifting equipment. 
         [0009]    The present invention also aims to provide an improved method and apparatus suitable for transferring large parts between tools, which may facilitate the manufacture of wind turbines and component parts thereof. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of the present invention there is provided a method for transferring a workpiece such as a large wind turbine component in datum alignment between tools during a manufacturing process, the method comprising: arranging a support structure in datum registration with a first tool, the first tool supporting a workpiece; attaching the workpiece to the support structure; releasing the workpiece from the first tool; arranging the support structure in datum registration with the second tool and such that the workpiece is supported by the second tool; and releasing the workpiece from the support structure, wherein arranging the support structure in datum registration with the first and second tools ensures correct alignment of the workpiece with respect to the second tool. 
         [0011]    The method according to the present invention eliminates the need to provide datum features on the workpiece itself. This is because the support structure can be precisely aligned with the tools, i.e. arranged in datum registration with the tools, and the workpiece remains attached to the support structure during transfer between the tools. The present invention therefore eliminates the need for datum-carrying sacrificial portions on the workpiece, thereby reducing material wastage and cost, both in terms of material costs, and process costs because removal of sacrificial portions requires additional process steps. 
         [0012]    Arranging the support structure in datum registration with the or each tool preferably involves forming an interlock between the support structure and the respective tool. The interlock may serve to restrict or substantially prevent relative lateral movement between the support structure and the tool, thereby maintaining precise alignment of the support structure relative to the tool. In preferred embodiments of the invention, the support structure and the tool are arranged in mating engagement by means of male and female datum formations provided on the support structure and the respective tools. 
         [0013]    The support structure may attach to the workpiece via any suitable attachment means, for example by mechanical engagement or by adhesion. However, the method preferably involves attaching the workpiece to the support structure by means of vacuum. For example the method may include attaching one or more vacuum pads or cups associated with the support structure to a surface of the part. Vacuum attachment is particular advantageous because it allows the part to be attached and released from the support structure quickly and does not damage the surface of the part. 
         [0014]    Releasing the workpiece from the first tool may comprise decoupling the workpiece from the first tool. If the workpiece is not coupled to the first tool, releasing the workpiece may simply involve lifting the workpiece off the first tool, or otherwise separating the workpiece and the first tool. 
         [0015]    If the first tool is a mould, releasing the workpiece from the first tool may involve demoulding the workpiece from a mould surface of the first tool. The method may comprise peeling the workpiece away from the mould surface. Peeling the workpiece from the mould surface generally involves first releasing an edge-part or end-part of the workpiece from the mould surface before sequentially releasing successively-adjacent parts of the workpiece, working towards an opposite edge or end of the workpiece. 
         [0016]    Peeling the workpiece from a mould surface is advantageous because it requires significantly less force than would otherwise be required to release the entire workpiece from the mould surface simultaneously. Peeling is therefore particularly beneficial for large workpieces such as modern wind turbine blades or component parts thereof. 
         [0017]    Releasing the workpiece from the first tool may involve applying a release force to the workpiece. The release force is preferably applied in a direction substantially perpendicular to a surface of the first tool on which the workpiece is supported. This ensures that the workpiece is released cleanly from the surface of the tool and is not dragged along the surface. 
         [0018]    Preferably the method involves exerting the release force on the workpiece with the support structure in abutment with the first tool such that the release force is reacted through the first tool via the support structure. An alternative would be to use external lifting equipment such as a crane, in which case the force would be reacted against the factory floor for example. However, if the coupling between the workpiece and tool is strong, which may be the case when the tool is a mould, using external lifting equipment could inadvertently result in the tool being lifted together with the workpiece, or more likely it could result in an uncontrolled detachment of the workpiece from the tool, which is dangerous and may result in damage to the component. Reacting the release force through the tool provides greater control of the release process. It also enables the release process to be performed independently from a subsequent lifting step. 
         [0019]    The method may involve applying the release force to the workpiece via the attachment means. For example the attachment means are preferably associated with actuators capable of moving the attachment means with respect to the support structure. 
         [0020]    Preferably the workpiece is attached to the support structure at a plurality of locations, for example at a series of locations along the length of the workpiece. The method may involve applying the release force sequentially at the series of locations to peel the workpiece from a mould surface of the tool. 
         [0021]    The method may involve arranging a series of support structures across the first tool for example at intervals along the length of the first tool in the case of an elongate workpiece such as a component of a rotor blade. The method may include attaching each support structure to a respective part of the workpiece via attachment means associated with the respective support structure. This arrangement is convenient because it avoids having to use very large support structures to cater for very large components. Scaling may be achieved by varying the quantity of support structures rather than the size of the support structure, which lends itself to the design of a modular system comprising a universal support structure. In a modular system, the attachment means of the respective modules may be operated independently, and sequentially to peel the workpiece from a mould surface of the tool. 
         [0022]    The method may involve applying the release force in a first direction to a first part of the workpiece and in a second direction to a second part of the workpiece, wherein the first and second directions are different. The first and second parts of the workpiece may be spaced apart relative to each other along the length of the workpiece. The first direction may be substantially perpendicular to a local surface of the first part of the workpiece and the second direction may be substantially perpendicular to a local surface of the second part of the workpiece. This scheme may be used when the workpiece exhibits a twist along its length, which is typical for parts of modern wind turbine blades such as the blade shells or spars. By applying the release force perpendicular to the local surface of a twisted workpiece, the workpiece can be released cleanly from the tool, and twisted parts can be peeled from a mould surface. 
         [0023]    The method preferably involves lifting or otherwise moving the support structure and attached workpiece away from the first tool. A crane or other lifting equipment may be attached to the support structure and used to lift the support structure and attached workpiece away from the first tool. Alternatively, it will be appreciated that the first tool could be moved relative to the support structure and attached workpiece. 
         [0024]    The method may involve transporting the workpiece to the second tool. The second tool may be an assembly jig for example, where the workpiece is assembled with other parts to form a finished component. The second tool may be located remotely from the first tool, for example at another station in the factory. The crane or other lifting equipment referred to above may be used to transport the support structure and attached workpiece from the first tool to the second tool. Alternatively, the workpiece may be supported in a single location and the tools moved relative to the workpiece. 
         [0025]    The workpiece preferably remains rigidly attached to the or each support structure during lifting and transporting of the workpiece such that relative movement between the workpiece and the or each support structure is substantially prevented. This ensures that the workpiece will be precisely aligned with the second tool when the support structure is arranged in datum registration with the second tool. 
         [0026]    Within the same inventive concept, there is provided a method of manufacturing a component comprising transferring a workpiece in datum alignment between tools in accordance with the method described above. The method is particularly suitable for manufacturing large parts, such as component parts of a wind turbine. For example, the method may be used to manufacture a rotor blade or a component part thereof such as a spar or a spar cap. Hence, within the present inventive concept there is also provided a wind turbine component manufactured in accordance with the method described above, and a wind turbine having a component manufactured in accordance with the method described above. 
         [0027]    According to a second aspect of the present invention there is provided an apparatus for transferring a workpiece such as a large wind turbine component in datum alignment between tools, the apparatus comprising: a support structure for carrying the workpiece between the tools; and an attachment device for attaching the workpiece to the support structure, wherein the support structure comprises datum features for aligning with corresponding datum features of the first and second tools. 
         [0028]    Within the same inventive concept, the present invention also provides a tool for supporting a workpiece such as a large wind turbine component, the tool comprising datum features for aligning with datum features of an apparatus as defined above. The first and second tools referred to above may be any tool in which alignment of the workpiece on the tool is important. For example, one or both tools may be a mould tool, an assembly jig, or part of a machining or finishing station. In preferred embodiments of the invention, the first tool is a mould tool. In the detailed examples described later, the first tool is a mould tool and the second tool is an assembly jig. 
         [0029]    In preferred embodiments of the invention, the support structure includes datum features that mate with complementary datum features of the tools. It will be appreciated that other non-mating datum features could be used for aligning the support structure with the tools. Mating datum features are preferred because these may form an interlock between the support structure and the tool, which may restrict or substantially prevent relative movement between the tool and the support structure and hence preserve the datum alignment. 
         [0030]    The datum features are preferably configured substantially to prevent relative lateral movement between the support structure and the tool. Relative vertical movement between the support structure and the tool may be restricted by the weight of the support structure bearing against the tool. If required, latches or other clamping means may be provided between the support structure and the tool to restrict relative vertical movement. 
         [0031]    The datum features may include projecting formations associated with one of the support structure and the tool, which are received in corresponding recesses provided in the other of the support structure and the tool. Alternatively, the support structure may have both projections and recesses for engaging with corresponding recesses and projections of the tool. 
         [0032]    The support structure may have a first set of datum features for aligning with datum features of the first tool and a second set of datum features for aligning with a second set of datum features of the second tool. Preferably, however, the support structure has a single set of datum features that align with corresponding datum features of both the first and second tools; in this regard the datum features of the first tool may match the datum features of the second tool. 
         [0033]    The support structure may include an abutment surface that abuts an abutment surface of the tool when the support structure is positioned in datum registration with the tool. The datum features of the support structure may be provided on or otherwise mounted to the abutment surface of the support structure, whilst the datum features of the tool may be provided on the abutment surface of the tool. In the case of a mould tool, the abutment surface of the tool may be a flange surrounding a mould surface of the tool. A female mould generally has a substantially concave mould surface with a flange around an upper periphery of the mould surface. A male mould generally has a substantially concave mould surface, and may have a flange surrounding a lower periphery of the mould surface. 
         [0034]    The datum features are preferably provided on the abutment surface of the support structure. To accommodate male mould tools and twisted mould tools for example, one or more datum features may be located on a spacer provided between the support structure and the tool. For example, a datum feature may be provided on the end of a spacer mounted to the support structure. In the detailed examples described later, a datum feature is provided on the end of a spacer in the form of a column, which is mounted to the abutment surface of the support structure. The spacer may alternatively be associated with the tool. In the case of a male mould tool, the spacers are suitably sized to accommodate the convex mould surface. In the case of a twisted workpiece, the spacers may be suitably sized to ensure that the support structure is substantially horizontal when it is placed against the tool. 
         [0035]    The apparatus is preferably configured such that relative movement between the workpiece and the support structure is prevented when the workpiece is attached to the support structure. This ensures that the workpiece will be precisely aligned with the second tool when the support structure is positioned in datum registration with the second tool. It is conceivable that the apparatus could be configured to allow some relative movement between the support structure and the workpiece, however additional means of referencing the position of the workpiece with respect to the support structure would then be required, making this option less preferable. 
         [0036]    The attachment device may comprise any suitable means for coupling the workpiece to the support structure. However, in preferred embodiments of the invention, the attachment device is part of a vacuum assembly which includes a vacuum generator and a vacuum attachment such as a vacuum pad, vacuum cup or equivalent, which is arranged to seal against a surface of the workpiece. A vacuum coupling is preferred because it allows the workpiece to be attached and released from the support structure quickly and without damaging the surface of the workpiece. 
         [0037]    The attachment device may include one or more actuators provided between the vacuum attachment and the support structure. Suitable actuators include hydraulic or pneumatic rams or equivalent. The actuators may be configured to move the vacuum attachment towards or away from the support structure. For example, once the support structure has been placed against the tool, the actuators may be extended to press the vacuum attachment against the workpiece. 
         [0038]    The apparatus is preferably configured to exert a release force upon the workpiece, causing the workpiece to release from the first tool. For example, once the vacuum attachment has been attached to the workpiece, the release force may be exerted on the workpiece by contracting the actuators such that the vacuum pad is moved towards the support structure. The apparatus is preferably configured such that the release force is reacted by the tool. This may be achieved by exerting the release force with the support structure remaining in contact with the tool. The benefits of reacting the release force through the tool have already been outlined above. 
         [0039]    The apparatus may include a plurality of attachment devices. For example a plurality of vacuum pads may be mounted on the support structure. Each attachment device may be arranged to attach to a local surface portion of the workpiece. The apparatus may be computer controlled. The apparatus is preferably configured such that the plurality of attachment devices are operable independently of one another. In certain embodiments, the apparatus may be configured such that the plurality of attachment devices are operated sequentially. This allows the workpiece to be peeled away from a mould surface of the tool. 
         [0040]    Modular embodiments of the apparatus are envisaged. For example, the apparatus may include a plurality of modules each comprising a support structures and an attachment device. This arrangement is particularly suitable for very large components, such as wind turbine blades and component parts thereof, which are formed in correspondingly-large moulds. The plurality of modules may be arranged at intervals along the length of the tool, with the supporting structures of the respective modules each attaching to a local surface of the workpiece. As mentioned above, the attachment devices of the respective modules may be activated sequentially to peel the workpiece from a surface of the tool. In this regard, and as mentioned above, the apparatus may be automated under computer control. 
         [0041]    The or each support structure preferably includes a mounting point for attaching to a crane or other lifting equipment. Once the workpiece has been released from the first tool, the lifting equipment may be operated to lift the or each support structure and attached workpiece away from the first tool and transfer the support structure and attached workpiece to the second tool. As already described above in relation to the first aspect of the invention, the or each support structure is positioned against the second tool such that the workpiece is supported by the second tool, and the workpiece is released from the or each support structure onto the second tool. By positioning the support structure in datum registration with the second tool, precise alignment of the workpiece with the second tool is ensured. 
         [0042]    The or each support structure is preferably sufficiently rigid to avoid buckling or twisting when supporting the weight of the workpiece. This ensures that the workpiece is fully supported and its geometry is preserved during transfer between tools. The rigid support structures facilitate transporting the workpiece between tools. This is particularly beneficial when working with very large workpieces such as wind turbine blades and component parts thereof because it eliminates the need to mount the workpiece to a substantial truss frame structure during transport. Instead, the crane or other lifting equipment may attach to a single mounting point of the support structure. Consequently, the speed and ease of the manufacturing process is increased. 
         [0043]    The present invention is particularly suitable for transferring very large parts between tools, for example parts of a wind turbine. Accordingly, the workpiece referred to above is preferably a wind turbine component such as a rotor blade or a component part thereof, for example a spar or part of a spar assembly such as a spar cap. Within the inventive concept the present invention provides a wind turbine component manufactured using the method or apparatus described above, and a wind turbine comprising said component. 
         [0044]    It will be appreciated that preferred or optional features described in relation to one aspect of the invention are equally applicable to other aspects of the invention and repetition has been avoided purely for reasons of conciseness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    In order that the present invention may be more readily understood, embodiments of the invention will now be described in detail, by way of example only, with reference to the following figures, in which: 
           [0046]      FIGS. 1 to 5  relate to a first embodiment of the present invention and show a sequence of steps in which a transfer assembly is used to transfer a workpiece from a female mould tool to an assembly jig, wherein 
           [0047]      FIG. 1  shows the transfer assembly positioned above the female mould tool; 
           [0048]      FIG. 2  shows the transfer assembly positioned in datum registration with the female mould tool and attached to the workpiece; 
           [0049]      FIG. 3  shows the transfer assembly releasing the workpiece from the female mould tool; 
           [0050]      FIG. 4  shows the transfer assembly and attached workpiece lifted from the female mould tool; and 
           [0051]      FIG. 5  shows the transfer assembly and attached workpiece arranged above an assembly jig to which the workpiece is to be transferred; 
           [0052]      FIG. 6  illustrates a second embodiment of the present invention in which the transfer assembly is adapted for use with a male mould tool; and 
           [0053]      FIG. 7  illustrates how the transfer assembly may be used in connection with a workpiece exhibiting a twist along its length. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]      FIG. 1  shows a workpiece  10  supported in a female mould tool  12 . In this example, the workpiece  10  is a spar cap for a spar structure of a wind turbine blade as described in applicant&#39;s PCT patent application WO2009/153341A2. The spar cap  10  is moulded from carbon-fibre reinforced composite material. 
         [0055]    The female mould tool  12 , which is shown in cross-section in  FIG. 1 , comprises a generally concave mould surface  14  and an upper shoulder portion or flange  16 . The flange  16  is essentially a flat surface that surrounds and extends outwards from a periphery  18  of the concave mould surface  14 . 
         [0056]    A plurality of female datum features  20  are provided in the flange. A pair of female datum features  20  is shown in  FIG. 1 . The female datum features  20  are located opposite one another on respective sides of the mould surface  14 . In this example, the female datum features  20  each comprise a substantially cylindrical recess. 
         [0057]    A transfer assembly  22  is shown in  FIG. 1 . The transfer assembly  22  comprises a support structure in the form of a frame  24  and a vacuum assembly. The vacuum assembly comprises a vacuum generator and a battery for powering the vacuum generator, which are carried on an upper surface  26  of the frame  24 , as represented by box  28  in  FIG. 1 . The vacuum assembly also includes an attachment device  30  in the form of a vacuum pad, which is mounted to an underside  32  of the frame  24  via a pair of actuators  34 . The vacuum pad  30  includes a high lift seal  36  for sealing against an inner surface  38  of the spar cap  10 . Whilst only a single vacuum pad  30  is shown in  FIG. 1 , in practice the vacuum assembly may include a plurality of vacuum pads  30  and associated actuators  34 . 
         [0058]    The transfer assembly  22  also includes a plurality of male datum features  40 , which are provided on an abutment surface  42  of the frame  24 . The abutment surface  42  is located in a peripheral region of the underside  32  of the frame  24 , and abuts the flange  16  of the mould tool  12  when the transfer assembly  22  is placed upon the tool  12 . A pair of male datum features  40  is shown in  FIG. 1 . The male datum features  40  are located opposite one another on respective sides of the vacuum assembly. In this example, the male datum features  40  each comprise a conical pin. 
         [0059]    The transfer assembly  22  is used to de-mould the spar cap  10  from the female mould  12  and transfer the spar cap  10  together with the datum from the female mould  12  to an assembly jig as will now be described below with reference to  FIGS. 2 to 5 . 
         [0060]    Referring to  FIG. 2 , in use the transfer assembly  22  is placed against the female mould  12  such that the abutment surface  42  of the frame  24  abuts the flange  16  of the mould  12  with the male datum pins  40  of the frame  24  being received inside the female recesses  20  of the mould flange  16 . In this configuration the frame  24  and mould  12  are effectively locked together in datum registration, such that relative lateral movement between the mould  12  and the frame  24  is substantially prevented. 
         [0061]    The actuators  34  are operated to lower the vacuum pad  30  towards the inner surface  38  of the spar cap  10  until the high-lift seal  36  is pressed firmly against said surface  38 . The vacuum generator  28  is then activated to withdraw air from the region within the seal  36 , between the vacuum pad  30  and the inner surface  38  of the spar cap  10 . This creates an effective vacuum between the vacuum pad  30  and the spar cap  10 , which serves to attach the spar cap  10  to the vacuum pad  30 , and hence to the frame  24  of the transfer assembly  22 . 
         [0062]    Referring to  FIG. 3 , once the vacuum pad  30  has been attached to the spar cap  10 , the actuators  34  are operated to lift the vacuum pad  30  towards the frame  24 . The upward force provided by the actuators  34 , which is represented by the arrows  44  in  FIG. 3 , releases or ‘demoulds’ the spar cap  10  from the female mould  12 . The demoulding load  44  is applied in a direction substantially perpendicular to the part of the mould surface  14  opposite the actuators  34 . The demoulding load  44  applied by the actuators  34  is reacted downwards through the flange  16  of the mould  12  via the frame  24 , as represented by the arrow  46  in  FIG. 3 . 
         [0063]    Referring to  FIG. 4 , with the spar cap  10  attached to the frame  24  of the transfer assembly  22  and demoulded from the mould tool  12 , the frame  24  is attached to a crane (not shown). The crane lifts the transfer assembly  22  and attached spar cap  10  away from the mould  12  in the direction of arrow  48  in  FIG. 4 . The crane carriers the transfer assembly  22  and attached spar cap  10  to an assembly jig  50  ( FIG. 5 ), where the spar cap  10  is assembled with a corresponding spar cap and shear webs to form the spar structure. 
         [0064]    Referring to  FIG. 5 , the assembly jig  50  comprises an assembly fixture  52  for supporting the spar cap  10 , which is located between a pair of sidewalls  54 . The sidewalls  54  each include respective female datum features  56 . A pair of female datum features  56  in the form of cylindrical recesses is shown in  FIG. 5 . The crane positions the transfer assembly  22  above the assembly jig  50  such that the male datum pins  40  are substantially aligned above the female datum features  56  of the jig  50 , as represented by the dashed lines  58  in  FIG. 5 . 
         [0065]    The transfer assembly  22  is then lowered towards the jig  50  until the male datum pins  40  are received within the female recesses  56  of the jig  50 , and the frame  24  abuts the sidewalls  54  of the jig  50 . A human operator may guide the transfer assembly  22  as it is lowered towards the jig  50 . Once the spar cap  10  has been positioned on the assembly fixture  52 , the vacuum generator  28  is deactivated and the vacuum pad  30  is decoupled from the inner surface  38  of the spar cap  10 . The transfer assembly  22  is then lifted away from the assembly jig  50 . 
         [0066]    By way of the above techniques, the requirement for providing datum features on the spar cap  10  itself is eliminated. This is because the datum features  40  are provided on the frame  24  of the transfer assembly  22  and the spar cap  10  remains rigidly coupled to the frame  24  throughout the demoulding, lifting and transfer operations. Precise alignment of the spar cap  10  with the assembly fixture  52  is ensured by the provision of corresponding datum features  56  on the assembly jig  50 . 
         [0067]    Whilst the cross-sectional views of  FIGS. 1 to 5  necessarily show only a single transfer assembly  22 , a plurality of transfer assemblies  22  or ‘modules’ are used in practice because a spar cap is very long—typically 50 to 80 metres in length. The modules are arranged at intervals along the length of the mould tool  12 . Once the or each vacuum pad  30  of the respective modules has been attached to the spar cap  10 , the actuators  34  of the respective modules are operated sequentially to demould the spar cap  10 , starting with the actuators  34  of the module located closest to one end of the spar cap  10 . This action effectively peels the spar cap  10  away from the mould surface  14 , which requires considerably less force than would be required if the actuators  34  were operated simultaneously. Standardising the datum features across tools enables a universal frame to be used for all tools. 
         [0068]    The transfer assembly  22  is also suitable for use with male mould tools, as will now be explained below with reference to  FIG. 6 . 
         [0069]      FIG. 6  shows a spar cap  60  similar to the spar cap described above in relation to  FIGS. 1 to 5 , however the spar cap  60  in  FIG. 6  is formed using a male mould tool  62 . 
         [0070]    The male mould tool  62  is essentially ‘top-hat’ shaped in cross-section, and comprises a generally convex mould surface  64  and a lower base or flange  66  that surrounds and extends outwards from a periphery  68  of the convex mould surface  64 . 
         [0071]    A plurality of female datum features  70  are provided in the flange  66 . A pair of female datum features  70  is shown in  FIG. 6 . The female datum features  70  are located opposite one another on respective sides of the mould surface  64 . In this example, the female datum features  70  each comprise a substantially cylindrical recess. 
         [0072]    A transfer assembly  72  is also shown in  FIG. 6 . The transfer assembly  72  is similar in most respects to the transfer assembly  22  described previously with reference to  FIGS. 1 to 5 . However, the transfer assembly  72  of  FIG. 6  further includes columns  74 , which are mounted to an underside  76  of the frame  78 . The columns  74  are mounted in pairs, on respective sides of the vacuum assembly. Two columns  74  are shown in the cross-sectional view of  FIG. 6 . 
         [0073]    Male datum features  80 , in the form of conical pins, are mounted at the ends  82  of the respective columns  74 . When the transfer assembly  72  is placed upon the male mould tool  62 , the pins  80  are received in the recesses  70  defined in the flange  66  of the male mould tool  62  and the ends  82  of the columns  74  abut the flange  66  of the mould tool  62 . The mating pins  80  and recesses  70  effectively lock the frame  78  in position relative to the mould tool  62  and prevent the frame  78  from moving laterally with respect to the mould tool  62 . 
         [0074]    The length of the respective columns  74  is selected so that the convex mould surface  64  of the male mould tool  62  is accommodated in a region  84  defined between the flange  66  and the frame  78  when the transfer assembly  72  is placed on the mould tool  62 . Once the transfer assembly  72  has been positioned in mating engagement with the mould tool  62 , the spar cap  60  is demoulded and transferred to an assembly jig in substantially the same way as already described above for the spar cap with reference to  FIGS. 1 to 5 . 
         [0075]    The geometry of long wind turbine components generally varies substantially along the length of the component. For example, and as mentioned by way of background above, it is common for components such as spar caps and blade shells to be designed with a twist along their length. Twisted components may be formed in twisted moulds, and the transfer assembly may be configured for use with such moulds as will now be described with reference to  FIG. 7 . 
         [0076]      FIG. 7  is a cross-section taken through a transfer assembly  85  in datum registration with a twisted female mould tool  86 . The female mould tool  86  may be the same mould tool as shown in  FIG. 1 , but with the respective cross-sections of  FIGS. 1 and 7  being taken at different locations along the length of the mould tool. 
         [0077]    The part of the mould tool  86  shown in  FIG. 7  has a mould surface  88  that is generally inclined relative to, for example, the plane of a factory floor on which the mould tool  86  is supported. Consequently, a first part  90  of a flange  91  on one side of the mould surface  88  is located higher, with respect to the factory floor, than a second part  92  of the flange  91  on the other side of the mould surface  88 . 
         [0078]    As shown, the first part  90  of the flange  91  includes a first female datum feature  93 , and the second part  92  of the flange includes a second female datum feature  94 . The datum features  93 ,  94  each comprise a substantially cylindrical recess. Consequently, the opposed female datum features  93 ,  94  are located at different respective heights in this section of the mould tool  86 . Whilst not evident from the cross-sectional view of  FIG. 7 , a series of female datum features is provided in the flange  91  on each side of the mould surface  88 , the datum features being provided at intervals along the length of the mould tool  86 . 
         [0079]    The cross-sectional view of  FIG. 7  also shows a pair of male datum features  96 ,  97 , each comprising a substantially conical pin, provided on the underside  98  of the frame  100 . The pin  96  for mating with the recess  93  in the relatively high part  90  of the flange  91  is mounted directly to the underside  98  of the frame  100 , whilst the pin  97  for engaging with the recess  94  in the relative low part  92  of the flange  91  is provided on the end of a column  102  mounted to the underside  98  of the frame  100 . When the frame  100  is placed upon the mould tool  86 , the pins  96 ,  97  are received in the respective recesses  93 ,  94  and the underside  98  of the frame  100  abuts the relatively high part  90  of the flange  91 , whilst the end  104  of the column  102  abuts the relatively low part  92  of the flange  91 . The length of the column  102  is selected so that the frame  100  sits substantially horizontally on the mould tool  86  (i.e. parallel to the factory floor) when the male and female datum features mate. 
         [0080]    To ensure that the vacuum pad  106  sits flush against the inner surface  108  of the spar cap  110 , the vacuum pad  106  is inclined relative to the frame  100  and relative to the factory floor by the same angle as the mould surface  88 . The inclination of the vacuum pad  106  is facilitated by the actuators  112 ,  113  that couple the vacuum pad  106  to the frame  100 , which are also inclined relative to the frame  100 . In this configuration, the actuators  112 ,  113  are necessarily extended by different amounts to hold the vacuum pad  106  flush against the inner surface  108  of the spar cap  110 , with the actuator  112  closest to the relatively high part  90  of the flange  91  being shorter than the actuator  113  closest to the relatively low part  92  of the flange  91 . The actuators  112 ,  113  are inclined such that a longitudinal axis of the actuators  112 ,  113  is substantially perpendicular to the plane of the mould surface  88  in the region of the mould surface  88  opposite the actuators  112 ,  113 . This ensures that the demoulding load, as represented by the arrows  114 , is applied to the spar cap  110  in a direction substantially normal to (i.e. perpendicular to) the plane of that part of the mould surface  88 . 
         [0081]    Whilst the above examples relate to wind turbine components, it will be appreciated that the present invention is equally suitable for use in manufacturing other parts, for example components for aeroplanes or indeed any other parts that are transported between manufacturing stations where maintaining precise alignment of the part is important.