Smoother skins for aircraft
A
well-known jet-fighter manufacturer had a major machining dilemma.
Because the company was drilling and countersinking wing skins
with different fixtures at different times, the skins suffered
from hole misalignment and fraying. This caused minute gaps and
stresses on the surface of the finished wing where the skin was
forced to compensate to align holes.
The wing skin also suffered damage of its
aluminum underside because the drill bit punctured through the
final few thousandths of an inch of wing skin during machining.
Additionally, as that same drill bit retracted, the edges would
contact the edge of the hole and pull at the carbon fibers causing
delamination. Not only were the flaws problematic to the wing
skins, but the accuracy discrepancies meant that each skin had
to be custom fit to an aircraft. A custom, gantry-type machine
in a vertical orientation, designed and built by Applied International
Motion LLC of La Verne, Calif., proved the solution for these
accuracy and repeatability problems.
The wing-skin machining process begins when
a filler material is applied to the wing frame where the skin
and the frame make contact. Then the skin is placed over the frame
and secured in a jig. The fastener holes were drilled first using
a special set of about 50 to 100 drills configured in a group
called a "bonnet." Another bonnet then countersunk the holes.
The new system simultaneously drills and countersinks each hole
so no separate bonnet is needed for countersinking, which solves
the alignment problem.
At the heart of the new gantry machine is a SKF Precision Technologies, a unit of SKF USA Inc., belt-driven, computer-controlled
spindle. By slowing down the feed immediately before the drill
penetrates the skin, the spindle solves the problem of the drill
punching through and distorting the aluminum surface. Having a
'pressure foot' come forward and contact the wing skin as the
drill is being retracted prevents the delamination of the carbon
fibers. In addition to increasing accuracy, the new system has
reduced downtime and increased ease-of-operation by running directly
from the manufacturer's Catera design program, eliminating the need to convert programming to traditional machine tool G-code.
