WCA March 2012

(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 .

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

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

Primary Lay Signature

Bow Signature

Pretwist Signature

❍ ❍ Figure 6 : FFT showing same lay as above, but with a different pretwist ratio and the resultant shift in the FFT pattern

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