Abstract:
A fixture having a foot clamp for clamping a foot of an aerofoil and a tip clamp for clamping a tip of the aerofoil is provided with an array of supports between the foot clamp and the tip clamp. Each support within the array is independently addressable to one of the aerofoil flanks when the aerofoil is secured to the fixture. The support is driven by pneumatics till it contacts the aerofoil when it is locked by an alternative mechanism.

Description:
TECHNICAL FIELD OF INVENTION 
     The invention relates to the manufacture of components and particularly the manufacture of aerofoils for gas turbine engines and even more particularly the manufacture of fan blades. 
     BACKGROUND OF INVENTION 
     Aerofoils such as fan blades for a gas turbine engine are often manufactured as what is commonly known in the art as “wide chord blades”. The blades, typically formed of titanium, have a leading edge, a trailing edge and pressure and suction flanks extending between the leading and trailing edges and are manufactured from a series of plates which are laminated together before a gas injected between the sheets causes the blade to inflate. The blade is subsequently twisted and the external surfaces machined and polished to its finished form. 
     The flanks, leading and trailing edges may need to be machined and it is an object of the present invention to seek to provide an improved method of treating an aerofoil. 
     STATEMENTS OF INVENTION 
     According to a first aspect of the invention there is provided a fixture for securing an aerofoil having a foot, a tip, a first flank and an opposing second flank, the aerofoil, when secured on the fixture presenting the second flank to a machine tool, the fixture having a base supporting a foot clamp for clamping the foot of an aerofoil and a tip clamp for clamping the tip of the aerofoil, and further having an array of supports between the foot clamp and the tip clamp, each support within the array configured to be independently addressable to the first flank when the aerofoil is secured to the fixture. 
     Preferably the tip of the aerofoil has a surplus of material with the tip clamp clamping the surplus material. 
     Preferably the tip clamp comprises at least one protrusion which engages a recess in the surplus material. 
     Each support may be functionally connected to pneumatic or hydraulic actuator. 
     Each support may have a locking device which locks the support against the aerofoil flank. 
     Preferably the fixture has at least three alignment pillar that is located beyond the array of support pillars. Preferably each of the three alignment pillars has an exposed reference surface. 
     According to a second aspect of the invention there is provided a method of securing an aerofoil having a foot, a tip, a first flank and an opposing second flank in a fixture, the fixture having a foot clamp for clamping the foot of an aerofoil and a tip clamp for clamping the tip of the aerofoil, and further having an array of supports between the foot clamp and the tip clamp, each support within the array being independently addressable to one of the aerofoil flanks, the method comprising the step of securing the aerofoil in the fixture using the tip clamp and the foot clamp and addressing each of the supports to a flank of the aerofoil, and locking the respective support once contact of the support to the flank is determined. 
     Preferably the contact is determined by a contact sensor. 
     The support may be addressable using pneumatics and contact is determined by the pneumatic air pressure. 
     The fixture may further comprise at least three pillars that are located on the base beyond the array of support pillars, each pillar providing a calibration feature, the method comprising the step of determining the position of each calibration feature relative to the first flank. 
     Preferably the position of each calibration feature relative to the first flank before the supports are addressed to the second flank and after the supports are addressed to the second flank, and comparing the positions to identify differences for indicating distortion in the first flank. 
     Each calibration feature may be an exposed reference surface having a photogrammetry marker. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts a body of material and the profile of a nominal machined aerofoil; 
         FIG. 2  depicts a datum feature 
         FIG. 3  depicts a cross-section through the datum feature of  FIG. 2 ; 
         FIG. 4  depicts an article mounted to a fixture through datum feature; 
         FIG. 5  is a perspective view of a fixture in accordance with an embodiment of the invention 
         FIG. 6  is an alternative perspective view of the fixture of  FIG. 5 . 
         FIG. 7  is a schematic of the process steps for the preferred method of determining the machining process. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  depicts a finished fan blade  2  within a body of material  4 . The body of material is indicative of the stock material which is the condition of supply of the component prior to the machining operation. Additional material is provided around the blade to permit the external surface to be machined to a desired tolerance and which leaves at least a minimum material between the inner volume and the external surface. 
     The stock material  4  is provided as a laminate of layers and two test pieces  6  are provided in the foot portion of the stock material and which are used to test the weld integrity of the stock. These lie outside the form of the finished blade and are removed during the machining of the stock material to the finished blade form. 
     The stock material is also provided with a body, or letterbox, of material  8  beyond the tip of the blade which is machined to assist with the support and alignment of the stock material so that the sides of the blade  4  may be machined in separate operations. The Figure shows the letterbox in partially machined form. Beneficially the letterbox can provide both an alignment and clamping location 
     The letterbox  8  is shown in more detail in  FIG. 2  and is provided with a series of datum features provided in a top surface  12  and the bottom surface  14 . A first set of datum features  18  are provided which are formed in the top surface and have a first portion  18   a  which is a circular cut-out with a first diameter in the top surface and a second portion  18   b  which is an aperture extending from the base of the circular cut-out to the bottom surface  14 . The diameter of the second portion is less than the diameter of the first portion. 
     A second set of datum features  16  are provided which are formed in the bottom surface  14  and have a first portion  16   a  which is a circular cut-out with a first diameter in the top surface and a second portion  16   b  which is an aperture extending from the base of the circular cut-out to the top surface  12 . The diameter of the second portion is less than the diameter of the first portion. 
     The first and second datum sets are manufactured sequentially preferably at the stage in the process at which each of the sides are machined. The datum features assist in the manufacture of the blade in a number of discrete steps and the first set of datum features  18  are used to determine the location of the second set of datum features  16 . 
     In the first step of the exemplary method the foot  20  of the blade is machined in a fixture along with the test pieces  6  and the first side of the letterbox  8 . During this step the first set of datum features  18  are also machined with the depth of the first portion  18   a  from the surface  12  being carefully controlled to provide an accurate datum. The location of the datum features is controlled relative to the position of the machined foot. 
     The blade with the machined foot is then addressed to a further fixture  24  as shown in  FIG. 4  using the datum features  18 . Two datum features  18  have a depth selected to locate the blade in a desired orientation on the fixture. The depths may provide two co-planar surfaces on which the blade tip can rest on the fixture. The fixture can also be provided with corresponding pads  22  the height of which can be adjusted to ensure that each pad rest on its co-planar surface. 
     The second portion  18   b  of the datum features  18  extends through the letterbox  8  to provide an aperture which is aligned with an aperture  28  in the pad  22 . A screw  26  is inserted through the aperture  18   a  and tightened to the pad using a screw thread and a corresponding screw thread on the internal surface of the aperture  28  in the pad. The screw securely bonds the letterbox and hence the blade to fixture  24 . 
     As the location of the datum features  18  relative to the foot is known the surface  4  of the blade to be machined is relatively accurately positioned ready for machining, but fine alignment may be used to further improve the positional accuracy of the blade. The surface is then machined. As part of the machining step the second set of datum features  16  are formed in the letterbox  8 . As the position of the first datum features  18  is known through the positioning of the screw securing the letterbox to the fixture it is possible to accurately align the second datum features to both the first datum feature and the foot  20 . 
     Following completion of the first side machining the blade can be removed from the fixture, turned over and then secured again using a similar method and pads as per the first side. Beneficially the known location of the second datum features  16  relative to the location of the first datum features and the foot permits the second side to be known relatively accurately to reduce alignment time of the machine tool to the second surface  2 . The second surface is machined and then the letterbox is removed to provide the finished blade. 
     The blade surfaces  2 ,  4  are relatively thin and may be subject to machining chatter or distortion caused by tool pressure. To minimise distortion and improve clamping the fixture is provided, as shown in  FIG. 5 , with a plurality of support pins which are addressable to the surface opposite the surface being machined. 
     Following secure clamping of the blade at the foot and letterbox the pins are driven up to touch the underside of the blade using hydraulic actuation at low pressure, or preferably pneumatic actuation. Preferably air pressure is used with the pressure used being insufficient to cause permanent distortion of the blade even if pressure is applied following first contact. The pins are then locked in position using an alternative clamping force, preferably hydraulic, in a direction orthogonal to that required to address the pin to the surface. Beneficially a large clamping force can be applied in this second direction as there is no risk of the force distorting the blade. 
     An array of pins  30  are used so as provide a uniform area of support directly opposite to the machining forces. 
     One of the benefits of the described clamping system is that the blade can be rigidly secured to the fixture independently of the tool which machines the surfaces. This permits accurate alignment, or determination of the position of the surface to be machined, relative to a datum on the fixture. This measurement can be time consuming which, if performed in the machining tool, can slow production rates below the minimum acceptable. 
     The fixture is provided with a plurality of calibration features  40  which are visible to the alignment device after the blade is secured to the fixture. The position of the blade on the fixture is controlled to reasonable tolerance due to the datum features  18 ,  16  and  22  but exact alignment is not required. The clamps being provided to grip the component rather than accurately set its position. 
     A schematic of the process steps for the preferred method of determining the machining process is shown in  FIG. 7 . In Step  1  a digital model of the component is determined using an optical digitising system using reference point alignment techniques known in the art. The model, preferably determined before the blade is attached to the fixture a reference point system combines individual measurements into a single co-ordinate system. The reference points may be non-contact points and which can be fluorescent, reflective or radiation absorbent features are applied to the component and optionally around the component. Alternatively the individual measurements may be determined by contact techniques such as CMM. The reference points may be temporarily secured to the component or may be a permanent feature e.g. a corner, edge or other identifiable feature. 
     Sensors are used to digitise an image of the component and the surface data is aligned to the reference points. Where reference points are applied to the component they are captured and digitised with the surface data and become an inherent element of the resultant 3D model. 
     The fixture is provided with at least one machine target reference point, Step  2  but preferably at least three are provided to enable accurate triangulation. In Step  3 , the component, or blade is loaded to the fixture using the method described above and clamped. A photogrammetry system is used to monitor the distortion of the blade as it is secured, to the fixture. The distortion can be measured as a deformation vector and the clamping adjusted dynamically to minimise the distortion to below defined limits, preferably 80 microns. 
     Output from the photogrammetry system can be used to determine the relationship between the clamped blade and the fixture and identify any change or movement in the pattern of reference points as the blade is clamped or following clamping. 
     Fixed photogrammetric reference points  40  which are mounted on pillars are pre-measured using a touch probe on the machine tool. Their coordinates are known with respect to the machine co-ordinate system. As the fixture uses a zero point location system it can be repeatedly loaded to the machine tool in a known position. Since the location of the fixture is accurately known because of the zero point location and the location of the component to the fixture is accurately known it is possible to determine the position of the component relative to the machine tool before the fixture is mounted thereto. 
     Once the blade has been fully clamped the photogrammetry system is used to measure the assembled set of reference points on the blade as well as the fixed photogrammetric reference points on the fixture to provide mobile reference points, Step  4 . 
     The fixture and blade is then transferred to the machine tool and the file describing the machine co-ordinates of the fixed photogrammetric reference points of the fixture loaded to provide a number of target reference points. Each fixture may be slightly different so it is beneficial to provide a specific file for each fixture. 
     The mobile reference points are aligned to the target reference points which are the true machine co-ordinates, Step  5 , and the digitised image subsequently aligned to the mobile reference points using the reference points that were captured with it, Step  6 . 
     Beneficially, this aligns the digitised image to the machine co-ordinates and represents the true location of the blade with respect to the machine co-ordinates. Once this is known an alignment may be calculated which adjusts the co-ordinates of the toolpaths to the true position of the component on the machine tool, Step  7 . The component may then be machined using the adjusted tool paths, Step  8 . 
     Automatic check of integrity of the fixture points by comparison. Can then touch probe to confirm/check etc. 
     One of the many advantages is that the point to point alignment is more accurate and robust than point-to-surface alignment. In addition there is significant time saving since once the clamping has been completed and a measurement of the full set of assembled reference points has been made, the alignment can be calculated and the tool paths specified, adjusted and loaded into the machine tool before the fixture and the component is loaded onto the machine tool. As a further advantage any discrepancies above a predetermined threshold between aligned component and the expected location provides an automatic check of the integrity of the fixture points by comparison. Where the discrepancy is above the threshold touch or other contact probes may be used to further check values. The approach also supports a more flexible approach to high volume manufacture as no further probing may be needed and a shorter turnaround is possible. Fully automatic machining of the surface may be possible and robot controlled. 
     Although the invention has been described with respect to fan blades it is equally applicable to other aerofoil types such as compressor blades and vane, turbine blades and vanes, aircraft wings and ailerons.