Aluminium Design and Construction John Dwight

Figure 3.1 Alternative designs for a gusset attachment: (a) machined; (b) welded.
3.1.5 Machining
Because of the high metal removal rate that is possible with fully-heattreated
material in the strong alloys, a valid technique for the manufacture
of large structural components is to machine them. For the aircraft of
World War II, this was the standard way of producing spar-booms
(extending the length of a wing), these being machined out of thick
extrusions. Since then the method has been extended to the milling of
wide stiffened panels out of thick plate. Such panels, forming large parts
of a wing or fuselage, typically comprise a skin of varying thickness with
integral stiffeners, and are machined in the flat before curving. Large
computer-controlled milling machines have been developed for such
manufacture, in which 80% or more of the original plate may be removed.
Two operations must be carried out on such material before machining
begins. One is to stress-relieve it and so prevent distortion as metal is
removed. This is done by stretching, the required force in the case of
thick plates being several thousand tonnes. The other essential operation
is ultrasonic inspection, to check for the possible presence of small
inclusions in the aluminium.
It is not only in the aero-industry that machining is a valid form of
fabrication for structural components. A simple example would be the chord
section shown in Figure 3.1 to which a cross-member has to be bolted. In
design (a) an extrusion is used, with an integral flange that is machined
away over most of its length. The alternative method of welding on a local
gusset (b) would reduce the strength of the member, especially in fatigue.
3.2 MECHANICAL JOINTS
3.2.1 Bolting and screwing
Bolts in aluminium structures can be close-fitting, in reamed holes, or
clear-ance. For the latter, BS.8118 originally limited the clearance to 0.4
mm or 0.8 mm respectively, for diameters below and above 13 mm.
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