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for all comparisons in this study. The overall lay lengths
measured fall in to a rather tight band. The variation is on
the order of 1% or less, and the mode of the distribution
can be determined with somewhat additional precision.
2.1.2 Individual Pair Measurements
In one experiment, a single twinning machine was set to a
number of process set points, but the lay length set point
was unchanged. A series of pairs were made with no two
set points in immediate succession, and the samples were
made in replicate.
Figure 2
shows the comparison of the
same lay set points measured at two different times from
the same twinner, but with all other machine set points
identical. This result shows high repeatability of both the
twinner and the measuring technology when pairs with
the same twinner, lay set point and process conditions are
measured.
In another experiment, the process set points of one
twinner were changed, but the lay set point was left
constant. In this case, pretwist ratio and twinner bow
speeds were changed.
Figure 3
shows distinct differences
in the resultant lay as a result of process set points.
With conventional manual or visual
measurement techniques, changes this
small would be quite difficult to discern.
In another experiment, two twinning
machines were set to the same lay set
point, and the pairs from each machine
measured.
Figure 4
is an example from
that experiment and shows the shift in
nominal from one machine to another.
In several other cases, changes on the
order of a millimetre or two were readily
discernable in the histogram.
2.1.3 FFT of individual pair lays
The ability of the equipment to sample
at a high data rate allows lay variations
to be seen on a scale of about each lay
length. A surprising outcome was that
a number of other variation signatures
were also captured in addition to the
primary lay length signature.
Waterfall analysis is a way to capture
frequency signatures over time and
display in an understandable format.
For instance, waterfall displays are
commonly used in measuring and
troubleshooting SRL on primary
extrusion lines. For this analysis, we also
used waterfall analysis tools to better
capture the variation of the pair lay over
the length of the spool.
Figure 5
is an example of a waterfall
FFT, with time on the horizontal axis,
frequency on the vertical axis, and
shading as the ‘z’ axis. For the ‘z’ axis,
the lighter the colour the more intense
the frequency signature is. An expected
outcome was the frequency signature
from the primary lay length. A surprising
outcome was the additional signature
content found in the FFT waterfall. A
signature is clearly visible from both
the twinner bow RPM (2x lay length,
½ frequency) as well as the pretwist
(at pretwist %) imparted on the wire before twinning. There
are also a number of other unexpected signatures found
within the FFT waterfall that indicate other mechanical
patterns of the manufacturing equipment.
Figure 6
shows the difference in the FFT pattern when
a change in the pretwist ratio is implemented. In this
diagram, the signature of the primary lay and the bow
speed is the same as
Figure 5
. However, the signature for
the pretwist has a shift that is consistent with the changes
in the set points on the twinning machine.
2.1.4 Relationship of Process Set Points to Nominal
Lay Length
Using DOE techniques with the nominal lay length as
an output and a 2x2 matrix of process set points as the
inputs, the relationship of set points to nominal lay could
be determined. The main effect plots in
Figure 7
show that
there is a strong relationship between the pretwist ratio
and the lay length. The bow speed relationship is not as
strong. Interestingly there is also a sign of an interaction
effect of bow speed and pretwist ratio on lay length as
shown by the interaction plot in
Figure 8
.
❍
❍
Figure 6
: FFT showing same lay as above, but with a different
pretwist ratio and the resultant shift in the FFT pattern
Primary Lay Signature
Bow Signature
Pretwist Signature
Lay Length Signature
Bow Signature –
1
/
2
Lay
Pretwist Signature % of lay
❍
❍
Figure 5
: FFT showing measured frequency pattern at lay, bow
speed, and pretwist ratio