EuroWire January 2016

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Technical article
January 2016 82
Technique for precise
concentricity measurement
of a conductor in the
insulation and relevant
parameters of cables
and wires
By Harald Sikora and Katja Giersch, Sikora, Germany
Introduction
Wire and cable manufacturers of
automotive and control wires as well as
coax and communication cables are using
measuring and control technologies in
their extrusion lines to maintain specified
cable parameters. This is important
for process optimisation and quality
assurance but is also demanded when
products are assembled automatically.
In these cases, assembly failure rates
are recorded statistically, registered and
assigned to the manufacturer. For this
reason, cable plants are aiming to supply
cables with reliable high quality standards.
Challenges in
cable extrusion
A challenge in the cable extrusion process
is maintaining measurement values
with tight tolerances of the conductor
concentricity in the insulation, the
conductor diameter, the outer diameter
of the cable and the ovality of the
outer diameter. During the extrusion
process deviations of the measuring
values can occur, which would remain
undetected without special equipment for
visualisation.
Lumps, neckdowns, faults or bare spots in
the cable can lead to malfunctions during
assembly.
Early in the assembly process, the cable or
wire is cut to lengths and stripped at the
ends in order to be subsequently fitted
with plugs. The pull-off forces used for the
stripping are exactly defined. Deviations
of the pull-off forces within the conductor
insulation can cause interferences during
the stripping. The use of innovative
measuring and control technology allow
the cables to meet the high demand
requirements and also identify potentials
for process optimisation of the extrusion
line.
Rotating/oscillating
conductor
The requirements for measuring and
control technologies in cable extrusion
lines are very precise where tolerances
within a few micrometres have to be
maintained. While the mean value of
the diameter or the wall thickness of the
insulation is sufficient for controlling the
extrusion line, the reliable recording and
visualisation of short-term variations,
especially of the eccentricity values,
is additionally necessary under the
aspect of the minimum wall thickness.
Slight oscillations of the conductor
directly before the crosshead can lead to
significant deviations of the concentricity,
especially if the conductor is oscillating in
one plane or rotating. Such eccentricities
cannot be shown with a conventional
graphical visualisation. A fast rotating
vector of a constant eccentricity value can
only be displayed to the operator by a
visualisation beyond the standard display.
Angle position of
a conductor
During the extrusion process it is possible
that a cable, with or without guide rollers,
has a slightly angled position when
running through the measuring plane.
Eccentricity measuring systems have to
be able to generate accurate measuring
values even when such a position occurs
(see Picture 6).
A sag or bend of
the conductor
Even on the assumption that the
conductor is stretched when running
through an extrusion line, it may still show
some bends, which have to be detected
when entering the gauge head of an
eccentricity system. Without any special
measures, even invisible curve radii in
the range of 5 to 20 metres can lead to
eccentricity errors of up to 40 micrometres.
Only in an ideal case, guide rollers can
compensate this imperfect stretching of
the conductor, or vice versa, even increase
the error. Therefore, the aim should be to
eliminate guide rollers completely.