Page 56 - MY STORY
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DIMPLING TITANIUM

            First, let me explain “dimpling.”  Much of an airframe is

            constructed of sheet metal held together by rivets.  For
            high-speed aircraft, the rivets holding pieces of external

            sheet metal together are flush with the metal surface to
            lower aerodynamic drag from the aircraft skin.  In order
            to accept flush-head rivets, the surfaces where holes are
            drilled to accept the rivets have  to be  “dented” so that

            rivet  heads  after  installation  are  indeed  flush  with  the
            surface.  Sheet  metal  is  too  thin  to  countersink.  The

            radius  of  curvature  of  the  bent  (dented)  surface  is
            carefully  controlled  by  engineering  drawings  and
            manufacturing  process  documents.  This  dent  in  the

            surface is called a “dimple.” The manufacturing floor has
            a full set of dimpling tools depending on hole diameter
            and skin thickness.


            The  propeller-driven  DC-7C  was  the  first  Douglas
            commercial transport to use titanium in place of the 40%

            heavier stainless steel for elevated temperature use.  The
            titanium was essentially a pure form of the element but
            did  not  take  kindly  to  being  “dimpled”  at  room

            temperature.  The result  was  a  second  set  of  dimpling

            tools  in  the  shop  that  operated  at  approximately  400
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