aluminium in commercial vehicles - European Aluminium Association

EUROPEAN ALUMINIUM ASSOCIATION ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER VII 103
5. Tapping
Threads in aluminium may be
made, when other joining techniques
are not applicable6 .
6. 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.
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
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.
TABLE VII.2
There are two methods of tapping:
• by chip removal,
• by upsetting.
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
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.2 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.