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and 2 times the major diameter
of the threads, and must depend
on the application, the alloy as
well as the temper of the material.
For example, the necessary
thread length of a high strength
6000 alloy in T6 may be 1.2 times
the major thread diameter.
Conversely a softer alloy demands
a longer thread length.
There are two methods of tapping:
• by chip removal,
• by upsetting.
5. Tapping
DIAMETER OF PILOT HOLES FOR THREAD TAPPING
∅ nominal 4 5 6 8 10 12 14 16 18 20
Pitch 0.70 0.80 1 1.25 1.50 1.75 2 2 2.5 2.5
Diameter 3.2 4.2 4.9 6.6 8.3 10 11.7 13.7 15 17
TABLE VII.2
4. In a connection, where continuous
joining methods such as welding or
bonding are used, no additional fasteners
should be applied.
Threaded holes in aluminium should
only be used where no other possibility
exists and the yield strength of the metal
exceeds 200 N/mm2. The bearing length
of the bolt should be 1,5 x diameter of
the bolt. If the bolts must be used for
repeated loosening and tightening,
inserts should be applied e.g. Helicoils.
5.1. Chip removal
Only taps with straight threads
should be used to avoid seizing
the metal at the flanks. Table
VII.2 gives diameters for pilot
holes for tapping aluminium
alloys in the 5000 and 6000
series. Pilot holes for these alloys
in the annealed condition must
be some 3-5% bigger than in
Table VII.3 and for castings with
12 and more % silicon content
some 2% smaller.
The cutting speed varies from 10
to 50 m/min depending on the
machine and method of clamping
the tap, whether floating or
in a chuck. Cooling is done with
cutting oil.
103
EUROPEAN ALUMINIUM ASSOCIATION
5.2. Upsetting
The thread is achieved by plastic
deformation of the metal using a
tap with- a rounded polygon section
that has no cutting wedge.
The diameter of the pilot hole
will depend on the desired
thread depth, and must be
drilled accurately. Upsetting
speeds can attain 50 m/min,
cooling is done with cutting oil.
Tapping by upsetting offers a
number of advantages with aluminium
alloys:
• the tap has a long life,
• it increases the hardness of the
thread, its tearing resistance and
fatigue strength,
• no chips.
5.3. Threaded inserts
It is usual to use threaded inserts
– available in diameters M2 to
M68 (Figure VII.9) – when
screwed aluminium alloy assemblies
are required to be frequently
dismantled. The inserts are in
the form of a spring made from
rolled wire or are a diamond section
made of stainless steel.
Captive threads can also be used.
These have one or two flights
that grip the flanks of the screw
thread and counteract the loosening
effects of dynamic stresses,
vibrations or thermal shock.
Boring is done using a standard
twist drill, but tapping must be
done with a special tap. All chips
and cutting fluid must be
removed from the bore before
the insert is fitted.
Threaded inserts are fitted with
pneumatic hand tools which
hold them by a driver at the top
end of the thread. This can then
be broken off after fitting.
Threaded inserts can also be
used to repair a tap in aluminium
that is worn or has been rejected
during manufacture.
Deep drawing is mainly used in
the automotive sector.
For more information on that
technique, please refer to the
Aluminium Automotive Manual:
http://www.eaa.net/aam/
HELI-COIL INSERTED THREAD
FIGURE VII.9
Notch
Driver
6. Deep drawing
104
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER VII FABRICATION 104 | 105
Spinning is a forming technique
used in the commercial vehicle
industry to make some components
such as tank-ends.
7.1. Advantages of
spinning
The tools used in spinning are
very simple, being basically the
internal shape of the required
form. However production times
can be up to 20 longer than for
deep drawing.
Calculations combining the cost
of tooling and production costs
show that spinning is competitive
for short runs.
7. Spinning
DIAMETER OF SPINNING BLANKS
FIGURE VII.10
D d D
H
h
∅ blank = D × Π
2 ∅ blank = 4 h + D
∅ blank = H + d 3
7.2. Diameter of spinning
blanks
Three formulae are used to
quickly determine the diameter
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