referring to fig1 a and 2 b , an additive manufacturing apparatus according to an embodiment of the invention comprises a build chamber 101 having therein partitions 114 , 115 that define a build volume 116 and a surface onto which powder can be deposited . a build support 102 defines a working area in which an object 103 is built by selective laser melting powder 104 . the build support 102 is capable of being lowered within the build volume 116 using mechanism 117 as successive layers of the object 103 are formed . a build volume available is defined by the extent to which the build support 102 can be lowered into the build volume 116 . layers of powder 104 are formed as the object 103 is built by dispensing apparatus comprising powder hopper 125 , metering device 127 and a wiper 126 . for example , the metering device 127 may be as described in wo2010 / 007396 . a laser module 105 generates a laser for melting the powder 104 , the laser directed onto the powder bed 104 as required by optical module 106 under the control of a computer 118 . the laser enters the chamber 101 via a window 107 . computer 118 comprises a processor unit 119 and memory 120 and a data connection to modules of the laser melting apparatus , such as optical module 106 , laser module 105 and motors ( not shown ) that drive movement of the dispensing apparatus and build support 102 . the computer 118 controls the laser unit 105 , optical unit 106 and movement of build platform 102 based upon the scanning instructions stored in memory 120 . a door ( not shown ) is provided in the chamber 101 for removing the object therefrom . a build plate 108 is removably mounted on the build support 102 . the build support 102 and build plate 108 comprise complementary mounting formations 109 and 110 , respectively , for locating the build plate 108 in a set position on the build support 102 . in this embodiment , the mounting formations 109 and 110 form a kinematic mount . in particular , the build plate 108 comprises three spaced apart balls 110 a , 110 b and 110 c arranged for engaging complementary grooves in the build support 102 , each formed by a pair of parallel cylinders 109 a , 109 b and 109 c . engagement of the balls in the grooves constrains the position of the build plate 108 in six degrees of freedom . magnets 111 , 112 are provided on the build plate 108 and build support 102 such that the build plate 108 is urged towards the build support by the attractive forces of the magnets 111 , 112 . the method of the invention will now be described with reference to fig3 to 7 . a build plate 108 is mounted onto the support plate 102 such that the mounting formations 109 , 110 are engaged to locate the build plate 108 in a set position relative to the build support 102 . the orientation of the build support 102 may be adjusted to ensure that a surface of the build plate 108 is parallel to a plane in which powder is spread across the build plate 108 . alternatively , the wiper may be adjusted such that the wiper is parallel with a plane of the base plate 108 . a fixture 200 is built on the base plate 108 using additive manufacturing based on a geometric model 700 of the fixture . the fixture 200 comprises a slot 203 for receiving a part , in this embodiment a blade , and means for securing the part in the fixture in the form of threaded holes 202 a , 202 b , 202 c arranged to receive bolts . once the fixture 200 is formed , powder is removed from the build plate 108 and the part 300 is mounted in the fixture 200 within the additive manufacturing machine . in the case of repair of a part , such as the blade 300 shown in the drawings , the blade 300 may first be machined into an appropriate shape for repair . for example , the part may be modified by wire electrical discharge machining ( edm ), to provide a straight upper surface of the part 300 for alignment with the plane of the layers of material formed using the wiper when mounted in the fixture 200 . the machining may be carried out within the additive manufacturing apparatus , with the build plate remaining mounted in the set position . alternatively , the base plate 108 may be removed from the additive manufacturing apparatus for machining and / or measurement of the part 300 , the kinematic mounting formations 109 , 110 allowing the base plate 108 to be remounted on the build support 102 to locate the base plate 102 in substantially the same position on the build support in which the fixture 200 was formed . the part 300 and fixture 200 are measured using a measurement probe 400 , such as touch trigger or scanning contact probe . the probe may be mounted on an articulating head , such as a revo 5 - axis head as sold by renishaw plc , and / or coordinate measuring machine ( cmm ) for moving the probe 300 around the part and fixture 200 . a cloud of measurement points 500 are obtained ( see fig5 ) and a cad model 600 of the part and fixture is produced from the measurement points 500 and the pre - existing cad model 700 of the fixture . an alignment of the part model with the fixture model can be achieved from measurement data of datum points on the fixture 200 . the combined cad model 600 of the fixture and part is then imported into software for determining a scan path for the laser beam of the additive manufacturing apparatus to take to repair the part . for example , the model determined from the measurement data may be compared to an ideal / nominal model of the part to determine portions of the part that should be built / rebuilt . once these portions have been determined , at least these portions are sectioned to determine layers to be formed in the additive manufacturing process and scan paths are determined for each layer . the part mounted in the fixture is located in the additive manufacturing apparatus . the build support 102 is lowered such that the straight upper surface of the part is level with the top of the build volume 116 . powder is dumped into the build volume 116 to fill the build volume . the additive manufacturing apparatus is then activated to consolidate material onto the part based upon the determined scan paths . with this method , the fixture grown in the additive manufacturing apparatus provides a means for aligning the part in a known orientation within the apparatus . accordingly , as long as the relative positions between the part 300 and fixture 200 are known material can be consolidated on the part as desired . if the part was located within the additive manufacturing apparatus by a fixture not manufactured using the apparatus , a separate alignment process would have to be carried out for determining a location of the part relative to a coordinate system of the optical module which directs the laser beam . such an alignment process would be complex and time consuming . furthermore , if multiple parts are to be repaired / modified in a single build , multiple fixtures are required . if an alignment process had to be carried out , it would have to be carried out on each part separately . by carrying out the method of the invention , multiple fixtures can be grown on a single base plate such that multiple parts can be repaired / modified in a single build . modifications and alterations to the above described embodiment may be made without departing from the invention as defined herein . for example , the fixture may be arranged to secure the part in a known position relative to the fixture . with such an arrangement , it may not be necessary to measure the fixture and part in order to determine their relative positions . the part may be machined such that the machined part has a known shape . accordingly , it may not be necessary to compare a measured shape of the machined part to a nominal shape to identify differences as the portion that needs to be built has been predetermined from the machining step . thus , a scan path may be determined for repairing / modifying the part from the nominal model of the part and based on a known location on the model down to which the part has been machined . in a further arrangement , in which the base plate cannot be mounted in the build chamber in a repeatable manner , a sacrificial fixture may be manufactured on the base plate and the base plate removed from the additive manufacturing apparatus . a part may then be mounted in the fixture and the fixture and part measured . a data model of the sacrificial fixture and part may then be formed from the measurements . a further fixture may then be built in the additive manufacturing apparatus , identical to the sacrificial fixture . without removing the base plate with further fixture attached from the additive manufacturing apparatus , the part is swapped from the sacrificial fixture to the further fixture . material may then be consolidated on the part based on the assumption that the combination of the part and the further fixture is substantially the same as the measured combination of the part and the sacrificial fixture . multiple parts may be measured in a single sacrificial fixture and multiple further fixtures , identical to the sacrificial fixture , grown on a single base plate such that material can be consolidated on the multiple parts in a single build . in another embodiment , the fixture may be manufactured with kinematic mounting features to allow the part to be mounted in a repeatable position on the fixture . the fixture may be built of a different material to the part , for example to allow for the different requirements of the fixture and the part . the fixture and / or part may be manufactured with features that can be optically recognised such that the location of the part relative to the fixture may be determined from the location of these optically recognisable features . for example , the additive manufacturing apparatus may comprise one or more cameras for imaging the fixture and the part , the location of the fixture and the part determinable from the images .