Page 56 - MY STORY
P. 56
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