Method of forming a cast component

Abstract

A method of forming a turbine nozzle guide vane 2 for a gas turbine engine, the guide vane 2 comprising an internal cavity 14 and having internal features 16 , comprises investment casting metal around a core member 30 that defines the internal cavity 14 and internal features 16 of the guide vane, removing the core member 30 , and machining the guide vane 2 . The process of investment casting around the core member 30 includes the step of casting at least one feature 40 external to the guide vane 2 . The external feature 40 is used as a reference point to indicate the location of the internal features 16 of the guide vane 2 during the machining of the guide vane 2.

1 . A method of forming a hollow, cast component in which the exact position of internal features of an internal cavity must be determined accurately for the purposes of a post-casting machining operation, the method comprising: 
 investment casting metal around an internal core member that defines the internal cavity and internal features of the component,    removing the internal core member, and    performing a machining operation on the component, the orientation of which is to be relative to the internal features of the internal cavity, characterised in that    the internal core member includes at least one part thereof which produces in the cast component at least one externally accessible feature having a known relationship to the said internal features in the cast component, and in that    the at least one externally accessible feature is used in the post-casting machining operation as a reference to indicate the position and orientation of the internal features of the component so as to determine the required orientation of the machining operation.    
   
   
       2 . A method as claimed in  claim 1 , wherein the at least one externally accessible feature is cast using an aperture that passes through a portion of the core member that extends outside the guide vane.  
   
   
       3 . A method as claimed in  claim 1 , wherein the at least one externally accessible feature is a protrusion.  
   
   
       4 . A method as claimed in  claim 1 , wherein the process of investment casting around the core member includes the step of casting three externally accessible features.  
   
   
       5 . A method as claimed in  claim 1 , wherein the externally accessible feature or features define three separate faces in mutually perpendicular planes.  
   
   
       6 . A method as claimed in  claim 5  wherein the three separate faces are spaced apart.  
   
   
       7 . A method as claimed in  claim 1 , wherein the cast component comprises a guide vane platform having a double skin construction in which the internal cavity is formed between first and second skins and a post casting machining operation is carried out on one of the skins relative to internal features formed in the internal cavity.  
   
   
       8 . A method as claimed in  claim 7 , wherein the internal features formed in the internal cavity comprise columns that extend between the first and second skins and the machining operation comprises drilling holes through a skin into the spaces around the said columns.  
   
   
       9 . A method as claimed in  claim 1 , further comprising removing the externally accessible feature after the component has been machined.  
   
   
       10 . (canceled)  
   
   
       11 . An internally cooled aerofoil for a gas turbine engine, the aerofoil having internal cooling passages, internal features and at least one externally accessible feature, characterised in that the cooling passages, internal features and externally accessible features are formed by a common core component whereby the position of the at least one externally accessible feature is indicative of the position of the internal features formed by the common core.