Abstract:
A method of manufacturing a forging for forming a split casing comprising the steps of creating a forging, for example, by a ring rolling process and then deforming the forging by compressing it along a first axis and/or extending it along a second axis.

Description:
This is a continuation of International Application Number PCT/GB2004/002989 filed Jul. 9, 2004, designating the United States. 

   FIELD OF THE INVENTION 
   The present invention relates to a method of manufacturing a forging for forming a split casing. It particularly relates to forgings which are used to form the casing for a gas turbine engine. 
   BACKGROUND OF THE INVENTION 
   The current process for forming a gas turbine engine casing is illustrated in  FIGS. 1A ,  1 B,  1 C and  1 D.  FIG. 1A  illustrates a forging  10  created using a ring rolling process. The forging  10  has a cylindrical shape, where the axis of the cylinder extends into the page. The cross-section of the forging  10  illustrated in  FIG. 1A  is annular. The inner surface  12  of the forging  10  and the outer surface  14  of the forging  10  are concentric cylinders. 
   The forging  10  is used to form an axial split casing  20  for a gas turbine engine. It may be formed from corrosive resistant steel, titanium or nickel alloy. 
     FIG. 1B  illustrates a rough machining which is carried out on the forging  10  of  FIG. 1A  before the casing  10  is split to form the casing  20 . If an orthogonal coordinate system (X, Y) is defined with the origin on the axis of the cylindrical forging  10 , then the forging is rough machined in the following way: 
   a. A lathe is used to remove material from the interior of the forging  10 . The lathe cuts a cylindrical tube of radius R 1  centred at (−X 1 ,  0 ). 
   b. A lathe is used to remove material from the interior of the forging  10 . The lathe cuts a cylindrical tube of radius R 1  centred at (X 1 ,  0 ). 
   c. A lathe is used to remove material from the exterior of the forging  10 . The lathe cuts a cylindrical tube of radius R 2  centred at ( 0 , Y 2 ). 
   d. A lathe is used to remove material from the exterior of the forging  10 . The lathe cuts a cylindrical tube of radius R 2  centred at ( 0 ,−Y 2 ). 
   In  FIG. 1B , the circular dashed lines indicate the original boundaries of the forging  10  before rough machining. The solid lines indicate the boundaries of the forging after rough machining. The dotted lines indicate lines at Y=−X 1  and Y=+X 1 . 
   The forging is then axially split by removing the material between −X 1  and +X 1  as shown in  FIG. 1C  to form first  20   a  and second  20   b  portions of the axial split casing  20 . Fine machining of the portions  20   a ,  20   b  is then carried out for example, by milling the interior and exterior of the portions. 
   The two portions  20   a ,  20   b  are then joined as shown in  FIG. 1D , to form the axial split casing  20 . It will be appreciated, that the axial split casing has a substantially cylindrical shape. 
   The rough machining of the interior and exterior of the forging illustrated in  FIG. 1B  is required to compensate for the removal of material between −X 1  and +X 1  when the forging is split axially so that the finished product, the axial split casing  20  is substantially cylindrical. 
   There are several problems associated with the above mentioned process. A considerable amount of material may need to be removed from the interior and the exterior of the forging during the rough machining process. This is a waste of material and also results in excessive wear to the lathes used to perform the rough machining. 
   It would therefore be desirable to provide an improved process for forming an axial split casing. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a method of manufacturing a forging for forming a split casing comprising the steps of: creating a forging; and deforming the forging to compress it along a first axis and/or extend it along a second axis perpendicular to the first axis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention reference will now be made by way of example only to the following Figs. in which: 
       FIGS. 1A to 1D  illustrate the prior art process for forming a split casing; and, 
       FIGS. 2A to 2D  illustrate a new method of forming a split casing according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The forging  110  illustrated in  FIG. 2A  is formed by a ring rolling process and it may, for example, be made from corrosion resistant steel, titanium or a nickel alloy. The forging  110  has a cylindrical tubular shape, where the axis of the cylindrical tube extends into the page. The cross-section of the cylindrical tubular forging  110  illustrated in  FIG. 2A  is annular. The inner surface  112  of the forging  110  and the outer surface  114  of the forging  110  are concentric cylinders. 
   The forging  110  of  FIG. 2A  is deformed to form the deformed forging  118  illustrated in  FIG. 2B . The deformation may be achieved by compressing the forging along the axis A, which passes through the axis of the cylindrical forging  110 , dividing it into two equal portions. Alternatively, or in addition, the cylindrical forging may be deformed by extending the cylindrical forging of  FIG. 2A  along the axis B which extends through the axis of the cylindrical forging and divides the cylindrical forging into two equal portions. The first axis A and the second axis B are orthogonal to each other and to the axis of the cylindrical forging  110 . The deformation is achieved by using mandrels to apply force to the forging  110 . For example, to compress the forging along the axis A, a first mandrel is used to apply a force F 1  along the first axis towards the axis of the cylindrical forging  110  at position  111  and a second mandrel is used to apply an opposing force F 2  at position  113  along the axis A towards the axis of the cylindrical forging  110 . In order to extend the forging along the second axis B, a third mandrel applies a force F 3  to the interior surface of the cylindrical forging at a position  115  along the axis B away from the axis of the cylindrical forging  110  and a fourth mandrel applies a force F 4  to the interior surface of the cylindrical forging at a position  117  along the axis B away from the axis of the cylindrical forging  110 . 
   The deformed forging  110  is then split along the first axis A. The splitting of the deformed cylindrical forging produces first  120   a  and second  120   b  portions of an axial split casing  120 . When joined the portions  120   a ,  120   b  create a substantially cylindrical tubular casing  120  as shown in  FIG. 2D . 
   Thus, the deformation of the cylindrical forging reduces or obviates the need to perform off-centre rough machining to ensure that portions of the forging, when split and rejoined, form a substantially cylindrical tubular casing. Consequently, the original forging provided in  FIG. 2A  will be smaller and cheaper than that provided in  FIG. 1A  as less material will need to be removed. In addition, there will be substantially less wear on the tools used for rough machining. 
   Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without the parting from the scope of the invention as claimed. For example, although the deformation of a forging has been described with relation to a cylindrical forging only, it is possible to apply the principle of the invention to other shapes of forging. 
   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.