## 3 Results and Discussion
### Verification of Correlation Usage and Health Threshold Identification  
The correlations of the force profiles and of the force SRS profiles were successful in detecting anomalies associated with the common faults--quality deterioration, non-die component damage, and die damage--addressed during maintenance. The two correlations detected the weakening in the force exerted on the workpiece due to die faults that reduced the die's contact area with the workpiece and to non-die component movement causing rotational misalignment. In addition, the analyses detected the loosening of the tolerance screw, which increased the moulds' dimension and changed the force distribution. Both correlations allowed the determination of empirical health thresholds for the 4P and 9P dies, below which deviations were indicative of faulty performance or condi  
Figure 7: 9 March force SRS correlation of machine start-up to subsequent force SRS signal profiles for (**a**) 4P and (**b**) 9P. tion. Furthermore, all empirical thresholds except the 9P force profile empirical threshold coincided with the lower limits derived from the one-tailed T-distribution confidence interval test at the 90% level of confidence. The upper limit would be 1, representing perfect correlation. In other words, there is a 90% certainty that all values falling at or above this threshold represented healthy or good machine condition. Conversely, fluctuations falling below the threshold represented faulty machine condition. For each type of correlation, Table 2 provides the range of values for each die for the days analysed and the empirical health range using data from the two manufacturing days. As shown in Table 2, the fluctuation in correlation values and, consequently, the health range is much larger for the 9P die under either approach, as this die exerts the largest force on the workpiece to remove its centre material. The health thresholds for each type of correlation and each die were tested on a longer time period that spanned the last half of February, March and the first half of April, during which quality deterioration, non-die replacements, and die replacements occurred. The thresholds were reliable over the longer time period. For ease of presentation, just five manufacturing days were selected to show the reliability of the health thresholds. Only the top portion of the graphs with the health threshold is shown. For 4P, Figure 8a showed the threshold for the force signal correlation, while Figure 8b presented the health threshold for the force SRS correlation. Sustained fluctuations below the thresholds in Figure 8 were all maintenance related. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Die** & **Date** & **Correlation** & \multicolumn{2}{c}{**Range of Correlation**} & **Health Range** \\ \hline \multirow{4}{*}{4P} & 24–25 February & \multirow{2}{*}{Force Profile} & 1.0000–0.9950 & 1.00000–0.9983 \\ \cline{3-5}  & 9 March & & 1.0000–0.9974 & 0.99983 \\ \cline{2-5}  & 24–25 February & \multirow{2}{*}{Force SRS Profile} & 1.0000–0.9890 & 1.00000–0.99985 \\ \cline{3-5}  & 9 March & & 1.0000–0.9990 & 0.99985 \\ \hline \multirow{4}{*}{9P} & 24–25 February & \multirow{2}{*}{Force Profile} & 1.0000–0.9950 & 1.00000–0.99960 \\ \cline{3-5}  & 9 March & & 1.0000–0.9680 & 1.00000–0.99934 \\ \cline{1-1} \cline{2-5}  & 24–25 February & \multirow{2}{*}{Force SRS Profile} & 1.0000–0.9680 & 1.00000–0.99934 \\ \cline{1-1} \cline{2-5}  & 9 March & & 1.0000–0.7199 & 0.99934 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Correlation ranges and health ranges for 4P and 9P force and force SRS profiles. Figure 8: Enlarged area showing February and March correlation values for machine start-up to subsequent 4P profiles with empirical threshold for (**a**) force signal and (**b**) force SRS signal profiles. In all cases, values for the force signal and force SRS signal correlations that remained below the health threshold for a minimum of 20 min were fault indicators.