Abstract:
A method and apparatus for draw forming a flat sheet blank into a 3-dimensional-nacelle nose cap for an aircraft engine in two stages; the blank having an outer annular portion and an inner annular portion. Draw forming the inner annular portion into a shape of an arcuate circular inner wall and then draw forming the outer annular portion into an arcuate outer wall adjacent the inner wall with the combination of walls being U-shaped in cross section.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and apparatus for deep drawing a metal blank and in particular to the forming process used to form nacelle nose caps for jet engines using aluminum alloy and steel alloy sheet metal. 
   BACKGROUND OF THE INVENTION 
   Traditionally, jet engine nose caps or nose cap rings are draw formed in multiple stages with intermediate heat treatments on a draw die or a hydro press or a combination of both. The tools represent the final part geometry in cross-section. The intermediate stages are formed by withdrawing the punch when it&#39;s partly immersed in the die. The punch travel is restricted by temporarily filling the die cavity with a filler material such as rubber, to reduce its depth. Generally sheets of flat rubber or custom fit resin plugs are used for this purpose. This process is non-optimal as the staging of the process is arbitrary and the process has a very high scrap rate between 50%-70% and, therefore, very low yield. The choice and number of intermediate forming stages and heat treatments depends on the individual forming the parts and the depth to diameter ratio of the nose caps. 
   SUMMARY OF THE INVENTION 
   The process of present invention employs a minimum strain path to deform the metal in forming the nose cap. The process employs the optimal/minimum number of stages required to form nose caps. The nose caps are formed in two stages with or without one intermediate heat treatment. In the first stage, the blank forms an inner arcuate wall of the nose cap ring into a final shape with a single draw of a punch and mating die. In the second stage, the outer arcuate wall of the nose cap ring is formed with a single draw. The selvage edge and selvage center are cut away leaving the nose cap in final shape. The stages can be reversed so the second stage is formed before the first. The invention has equal application where ever a three dimensional annular ring is draw formed. The process can also be used in a hydroforming press where the mating shaped die is a layer of rubber, which deforms around the male punch as they are brought together. 
   The principal object of the present invention is to provide a draw formed method for a nose cap having less thinning of the metal sheet and therefore more strength than the prior art nose caps. 
   Another object of the present invention is to provide a method utilizing less time and stages in forming and a decreased scrap rate. 
   A further object of the present invention is to provide a method of draw forming with only a minimum of forming stages rendering the process less arbitrary as to the intermediate forming stages and less heat treatments 
   Another object of the present invention is to provide a nacelle nose cap having a thinner width than the prior art methods. 
   Other objects and advantages of the present invention will become apparent as the description therefore proceeds. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which are incorporated in and constitute a part of the specification illustrate the embodiment of the prior art technology and the current invention showing the various method steps necessary to practice the present invention. 
       FIG. 1  is the prior art multi-stage forming process for forming the engine nacelle nose caps. 
       FIG. 2  is a sectional view of the first stage forming process of the present invention prior to forming. 
       FIG. 3  is a sectional view of a blank as it is removed from the first stage. 
       FIG. 4  is a sectional view of the second stage process prior to forming. 
       FIG. 5  is a sectional view of the blank after it is formed in the second stage. 
       FIG. 6  is a sectional view of an annular blank after removal from the first stage with the second stage forming of the blank shown in dotted lines. 
       FIG. 7  is an elevational view of a jet engine nacelle nose cap with variable cross-sections. 
       FIG. 8  is a sectional view taken along lines  8 - 8  of  FIG. 7 . 
       FIG. 9  is a sectional view taken along lines  9 - 9  of  FIG. 7 . 
       FIG. 10  is an elevational view of a non-circular nose cap with a constant cross-section;
         and  FIG. 11  is a sectional view taken along lines  11 - 11  of  FIG. 10 .       
       FIG. 12  is a perspective view of a typical jet engine nacelle with its nose cap. 
       FIG. 13  is a sectional view of a first stage hydroform. 
       FIG. 14  is a sectional view of a second stage hydroform. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates the traditional prior art method of draw forming engine nacelle nose caps in multiple stages with intermediate heat treatments between stages. In stage  1  the die  48  is partially filled with a filling material such as rubber  42  to reduce its depth so that the punch  43  is only partially inserted in the die  48 . In the stages  2 ,  3 , etc. up to the final stage the amount of filling material is progressively reduced until there is no filler in the final stage  44  with heat treatments provided between stages. The number of stages and heat treatments can vary upwardly depending upon the part geometry and the operator forming the nose caps. Stages  1  through  3  are in a sense a hydroforming press in that there is no mating shaped female die but rather a flat layer of rubber that deforms to the shape of the male punch  43 . The final stage is conventional draw forming where there are a pair of mating ridged dies which form the blank. 
   The present invention forming process involves only two stages and the first stage is set up as shown in  FIG. 2 . A blank  10 , which is a circular sheet of aluminum or steel in an annealed condition, is shown positioned between a punch  12  and a female die  16 . The blank  10  could also be annular in shape as shown in  FIG. 6 . The blank is held in place by a holder ring  18 , which applies pressure against the die  16  through blank  10  during the forming process. Punch  12  includes an arcuate circular surface  14  around its lower periphery while die  16  includes a similar shaped surface  17 . The blank or sheet  10  of aluminum is shown positioned between punch  12  and die  16  and are brought together by a hydraulic ram  30  or any other type of linear actuator means. Either the punch  12  or a die  16  can be stationary while the other moves. An alternate shaped punch  12 ′ is shown in dotted line in  FIG. 2  wherein the inner wall  22  of the nose cap is formed by stretching blank  10  over arcuate surface  17  of die  16 . 
   With the first stage completed the blank appears as a wide flanged cup  20  as shown in  FIG. 3 . The blank  20  has an inner circular portion A surrounded by an outer circular portion B. The inner portion A is formed into a curved or arcuate circular inner wall  22  during the first stage of forming while outer circular portion B in the second stage of forming is drawn into a similar arcuate outer wall  36  as shown in  FIGS. 5 and 6 . 
   In the second stage of forming, blank  20  is placed in die  32  as shown in  FIG. 4  with arcuate inner walls  22  resting on the die surface  34 . Punch  13 , which has a similar shape to punch  12  of the first stage, is brought into contact with cup  20  with cup  20  tightly held between the punch  13  and die  32 . Second stage female die  33  is lowered against the selvage edge  24  of the blank  20  while holder ring  28  holds the selvage edge  24  of the blank  20  against female die  33 . Female die  33  and holding ring  28  are lowered forming the outer circular portion of the blank B into an arcuate outer wall  36  as shown in  FIG. 5 . As female die  33  and ring  28  are lowered, the blank  20  is stretched over the outside surface  35  of female die forming an arcuate outer wall  36 . When the two die surface  46  and  35  are brought together in contacting relation with blank  20  all the wrinkles and irregularities in the blank  20  are removed from the outer wall  36 . Forming wax or any other suitable lubricant is used to lubricate the blank  20  and dies during the process. 
   An alternate design for the tooling in stage  2  would be to eliminate the mating arcuate surface  46  on the female die  33  as shown by dotted line  54  whereby wall  36  of the blank was formed strictly from stretching the blank  20  over convex surface  35  of die  32 . Another alternate design would be shape the punch  13  as shown by dotted lines  13 ′ in  FIG. 4 . A further alternate design for stage  2  would be a hydroforming stage where female die  33  was replaced with a layer of rubber  60  which deforms the outer portion of blank  20  against arcuate surface  35  as shown in  FIG. 14 . 
   In both stage  1  of  FIG. 2  and stage  2  of  FIG. 4  requires that the selvage edge  24  of the blank  20  be clamped between the tooling and the holder ring  18  or  28  during both forming stages of the nose cap. After stage  2  is complete, the center of the blank  26  is cut away along with the outer selvage edge  24  as shown by cut lines  48  in  FIG. 5 , thereby forming a fully formed nose cap ring  40 . 
   Instead of using a circular sheet metal blank  10  as shown in  FIG. 2 , an annular blank as shown in  FIG. 6  could be used having a center opening  38 . The first stage tooling with an annular blank would be identical to that shown in  FIG. 2  and the second stage tooling would also be identical to that shown in  FIG. 4 . 
   The completely formed nose cap ring  40  mounts on the front of engine nacelle  42  as shown in  FIG. 12 . Nose cap of  40 ′ as shown in  FIG. 7 , can have varying radial cross-sections as illustrated in  FIGS. 8 and 9 . The cross sectional shape of the engine nacelle  42  varies with the particular engine. It can be circular or oblong. The intake opening can be concentric or nonconcentric. In  FIG. 10 , the nose cap  40 ″, is shown having equal radial cross-sections as shown in  FIG. 11 , but with a generally oval nacelle. 
   One advantage with the forming method of the present invention allows the nose cap to be formed with a very narrow width or radial cross-section, which the prior art method is incapable of achieving. 
   When applying the current invention by hydro-forming, only the female die  58  and deformable rubber sheet  60  would be required in stage one, as shown in  FIG. 13 . In stage  2  hydroforming ( FIG. 14 ) only the female die  62  would be required and deformable rubber sheet  60 . 
   While this invention has been described as having a preferred design, it is understood that the invention is capable of further modifications, uses, and/or adaptations which follow in general the principal of the present invention and includes such departures from the present disclosure as come within known or customary practice in the forming art and fall within the scope of the limits of the appended claims.