Aluminium Design and Construction John Dwight

material, it is significantly greater; while for 2xxx material, and also for
the stronger versions of 7xxx and 5xxx material, it is considerably so.
2.3.8 Tolerances
When one sees a batch of gleaming aluminium extrusions, one is tempted
to think of them as a precision product, although in fact they are subject
to tolerances in the same way as steel. Such tolerances cover width,
thickness, section shape, straightness and twist. When a particular
dimension is critical, it is usually possible to negotiate with the extruder
a tighter figure for the tolerance concerned. An example would be when
two sections have to mate together. It is beyond our scope to cover
tolerances in detail, and the following is a rough guide.
The normal tolerance on section width w may be estimated from the
following approximate expression:
(2.4)
where the width w is in mm. This agrees reasonably with the British
Standard BSEN.755 requirements when w > 100 mm, erring on the high
side for low values of w. An equivalent expression for the tolerance on
thickness t is:
(2.5)
where w and t are in mm. This refers to the thickness of the main
elements of the section, and does not apply to small fins etc. Again it
tends to be pessimistic at low w.
Expression (2.5) can give disturbing results, if taken at face value.
For example, if w=300 and t=3 mm, the estimated tolerance would be
±0.7 mm. Applied over the whole section this could lead to an area
some 20–25% above nominal, with a corresponding increase in the weight.
If the section was being bought at a fixed rate per kilogram, the effect
on the economy of the design would be disastrous. In fact, the tolerance
value given by equation (2.5) mainly covers local variations in thickness
around the section. The reason it increases so much with w is to allow
for flexing of the die in a wide thin section. Such flexing affects the
thickness at the middle of a wide thin element. At the edges the variation
will be much less, and can be estimated by ignoring the second term in
expression (2.5). Therefore the possible variation in cost per metre will
be much less than might at first be supposed. But it is still significant.
An important factor governing variation in thickness is die wear. On
a long run, the die has to be taken out of service when it gets too worn
and replaced by a fresh one. A tighter tolerance can be achieved by
limiting the interval between such replacements.
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