## 5 Implementation
### Phase 3--Selected Methodology Application
#### 5.3.1 Data Preparation and Modeling/Analyze/Evaluation  
The root cause of the problem in milling operation 2 and process stability was analyzed in this phase. The data on tool wear and drift were obtained from a newly developed CNC-CMM line and it was disorganized because the size of the medical implants being machined, and its production demand was different for each batch. The data preparation step in CRISP-DM was employed to examine the raw data and preprocess it for analysis. The dimensions of the parts were referred to as DIM on the CMM report and each DIM was machined by a particular tool. The validity of the measurement was assessed by conducting the normality  
Figure 10: Ishikawa diagram of manual actions in milling operation 2.  
test [34]. Statistical strategies, such as capability studies and Design of Experiments (DoE), have been used in literature to reduce variation [81]. Capability studies are used to analyze process performance with respect to its specifications, and DoEs provide more information on process performance by emphasizing on how different factors affect the process significantly [81,82]. Since the number of sensors installed were limited, designing a DoE using different factors in this study was unfeasible. The stability of milling operation 2 was analyzed in this phase using the Six Sigma capability analysis tool on Minitab to emphasize on the need for first-off inspection. Process capability index (Cpk) and process performance index (Ppk) are the two indexes that can be used to conduct capability study. Cpk determines how well the output of a process lies within the customer's specification limit. A capable process has less variation, and it is well within the specification limit. It is given as,  
\[\text{Cp}=\frac{\text{Tolerance band}}{6\sigma} \tag{1}\]  
\[\text{Cpk}=\text{min}\left(\frac{\text{USL}-\frac{\overline{\text{x}}}{3 \sigma}}{\sigma},\frac{\overline{\text{x}}-\text{LSL}}{3\sigma}\right) \tag{2}\]  
where,  
\(\sigma\)--within standard deviation. USL--upper specification limits. LSL--lower specification limits. \(\overline{\text{x}}\)--mean [83]. Ppk gives information about how the process varies from the specification limits. Ppk evaluates the performance of the actual process. It is given as,  
\[\text{Pp}=\frac{\text{Tolerance band}}{6\text{s}} \tag{3}\]  
\[\text{Ppk}=\text{min}\left(\frac{\text{USL}-\frac{\overline{\text{x}}}{3 \text{s}}}{\sigma},\frac{\overline{\text{x}}-\text{LSL}}{3\text{s}}\right) \tag{4}\]  
where,  
s--overall standard deviation. USL--upper specification limits. LSL--lower specification limits. \(\overline{\text{x}}\)--mean [83]. Cpk and Ppk values of higher than or equal to 1.67 indicates that the process is in statistical control whereas the process might be within specification limits, but not centered, which would indicate that the performance is low. * Low capability is when \(\text{C}_{\text{P}}\leq 1\), \(\text{C}_{\text{pk}}\leq 1\), \(\text{P}_{\text{P}}\leq 1\) and \(\text{P}_{\text{pk}}\leq 1\). * Good capability is when \(1<\text{C}_{\text{P}}\leq 1.67\), \(1<\text{C}_{\text{pk}}\leq 1.67\), \(1<\text{P}_{\text{P}}\leq 1.67\) and \(1<\text{P}_{\text{pk}}\leq 1.67\). * High capability is when \(\text{C}_{\text{P}}>1.67\), \(\text{C}_{\text{pk}}>1.67\), \(\text{P}_{\text{P}}>1.67\) and \(\text{P}_{\text{pk}}>1.67\)[82,83]. The variables for CNC milling operation 2 from the CMM were measured for 330 sample parts and then collected for analysis via MTConnect. In statistics, a population is defined as a collection of people about whom certain information must be gathered. A statistical population does not only have to be made up entirely of people. There can be a population of heights, weights, BMIs, or any other set of data points [84]. A sample consists of one or more observations drawn from the population. A sample may have fewer observations than the population, the same number of observations as the population, or more observations depending on the sampling process. From the same population, several samples can be obtained.