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starts at the end and propagates
towards the centre of the extrusion.
The primary bending
moment is generated by the
rotating dies, and is typically constant
along the workpiece. The
process is characterized by very
low transversal (shear) forces and
thus also low contact forces
between extrusion and dies.
Rotary stretch bending can be
implemented in a dedicated
press tool or in a stand-alone
bending machine.
3.3.8. Three-dimensional
stretch-bending
Over fixed or rotating dies (lasts),
the extrusion si gripped at its
ends, and stretched into a threedimensional
shape (“out of the
plane”). This may be done with
tools where the movements are
defined mechanically, or in programmable
tools or machines.
3.3.9. Manipulation
of the cross section
This is usually performed by
indenting or pressing in a dedicated
tool in a press.
3.3.10. Mechanical calibration
of parts of the extrusion
This is usually performed by compression-
stretching or expansionstretching
in a dedicated tool in a
press.
3.3.11. Achievable bending
radius
The achievable radii for bent
extrusions are highly dependent
on the geometry of the profiles
and are difficult to predict.
Therefore it is advisable to carry
out tests on specimen. Table VII.1
gives guidance for bending of
hollow circular tubes. If smaller
radii are needed, filling of the
tubes with sand before bending
is helpful.
D: Outside diameter t: Thickness
BENDING HOLLOW TUBES (D ≤ 90mm) BENDING RADIUS AS A FUNCTION OF RATIO D/t
Alloy Temper
Ratio D/t
5 10 15 20 25 30
5754 H111 1 to 1.5 D 2.5 to 3 D 3.5 to 4 D 4.5 to 5 D 6 to 7 D 8 to 9 D
6060 O 1 to 1.5 D 2.5 to 3 D 3.5 to 4 D 4.5 to 5 D 5 to 6 D 7 to 9 D
T5 2 to 2.5 D 3 to 4 D 4 to 5 D 6 to 7 D 8 to 10 D 12 to 15 D
TABLE VII.1
101
EUROPEAN ALUMINIUM ASSOCIATION
Drilling aluminium alloys is a simple
operation but calls for careful
sharpening and polishing of the
drills given the relative softness
of the aluminium alloys used in
the manufacture of commercial
vehicles. If inadequate sharpening
causes the drill to bend or
buckle, it will tear the metal
around the part of the hole that
is already drilled.
The following types of drill can be
used for drilling aluminium alloys3:
• the standard twist drill - the
most common type,
• the straight flute drill,
• the gun drill,
• the half-round or three quarter
round drill.
4.1. Twist drill
To have a substantial sharpening
gradient, the helix angle must be
40° while the point angle varies
from 120° to 140° according to
the shape of the neck, with a
clearance angle of 8°. The other
characteristics of the twist drill
are as follows:
• cutting speed 30 to 80 m/min
depending on rating and the
desired quality - for very accurate
holes the ideal speed is 30 m/min,
• penetration rate determined by
the drill diameter: 0.05 mm/rev
for a 2 mm diameter drill, to 0.3
mm/rev for a 30 mm diameter
drill,
• soluble oil cooling,
• point height : this must
exceed the thickness of the
drilled material.
4.2. Straight flute drill
This drill facilitates rapid chip
removal and is more efficient for
aluminium alloys of medium
hardness than the twist drill. The
four cylindrical witnesses also
prevent "triangulation" of the
hole and provide drill guidance.
4.3. Gun drill
This type of drill is excellent for
large diameter holes of 20 mm
and over, also for drilling stacked
sheets. The drilling conditions are
the same as for the standard
twist drill.
4.4. Half-round or three
quarter round drill
These drills are used mainly in
boring operations. The accuracy
of the bore diameter achieved
with these tools is of the order of
0.02 mm:
• cutting speeds are between 10
and 15 m/min,
• rate of advance is 0.05 mm/rev,
• cooling is with cutting oil.
4. Drilling
3. Steel twist drills can be used for small
runs with mainly manual tools.
102
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VII FABRICATION 102 | 103
Threads in aluminium may be
made, when other joining techniques
are not applicable4.
If threads are made in aluminium,
care should be taken to
ensure that the thread length is
sufficient for the purpose. The
thread length may be between 1
　
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