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ALUMINIUM EXTRUSION INDUSTRY
Fig. 6: Feed force during flow-drilling of continuously reinforced aluminium profiles
a thread which is true to gauge can
be produced in any investigated position
relative to the reinforcing element.
The number of usable threads
is increased compared to the original
wall thickness by means of the flowdrilling
operation. As shown in Fig.
7, the shape of the flanks differs with
the threading operation and the position
of the hole relative to the reinforcing
elements.
Thread forming in non-reinforced
aluminium material results in regular
shaped thread flanks with typical
split crests [9]. The upper part of
the collar is widened due to radial
forces. As a consequence, the teeth
are not completely formed in this
area. The machining of reinforced
profiles leads to irregularities in the
area where the wires are located after
the flow-drilling operation. Due to
the different flow characteristics of
the two materials, there are unequal
crests of threads, however the roots
of the threads are well shaped. When
using a cutting operation to produce
threads like tapping or thread milling,
the reinforcing elements are well
integrated into the flanks, regardless
of the position of the hole relative to
the reinforcement. The thickness of
the collar decreases with the height
of the collar. In the upper part of the
collar, the depth of cut exceeds the
wall thickness. Therefore parts of the
collar can be cut off.
For quantitative analysis of the
machining results, tensile tests were
conducted on a universal testing machine.
A threaded stud was screwed
into the workpiece at one end and
fixed to a clamping device at its other
end. During testing, the workpiece is
moved relative to the stud in the direction
of the central axis of the hole
with a defined speed while forces
are recorded. The maximum tensile
force is charted for a sample of five
threads in each case. Fig. 8 shows the
mean values as well as the margin
of error. In addition to tensile tests
Fig. 7: Qualitative analysis of threads produced by thread forming, tapping and thread milling
with threads that were produced by
flow-drilling in combination with a
subsequent threading operation, tests
with samples that have no collar were
conducted to see the effect of the additional
lateral area on the maximum
tensile force. The holes of these samples
were produced by circular milling.
The benefit of the collar is obvious
in most cases and particularly high for
a subsequent thread forming operation.
The position of the hole relative
to the reinforcing wires is a major
influencing factor on the maximum
tensile force.
While the increase in transferable
tensile forces due to the collar is about
ΔFz = 5 - 10 kN for every threading
operation without wire or with two
consecutive wires, the benefit of the
flow-drilling operation is significantly
smaller for a position of the holes
with one reinforcing element in the
middle. The displacement of the wire
enhances the cracking within the collar.
As seen in Fig. 7, this reduces the
40 ALUMINIUM · 4/2010