# Preassembly Cryogenic Drilling in Carbon Fiber Sandwich Sheets with Inner Foam Core
## 3 Results
### Experimental Results  
Thrust forces, Fz, required to drill laminates are low, according to Table 2, perhaps due to the small thickness of the carbon fiber laminates and the low density of the foamed polyvinyl chloride; this parameter was already found to be conclusive in the required thrust forces [35]. These forces agree with values obtained in studies focused on cryogenic drilling of CFRP, although lower due to different cutting conditions and drill bits [36]. The average values for cutting conditions and foam thickness are shown in Figure 2, which clarifies the trend of Fz, in particular, Figure 2a-c. Figure 2a shows that Fz decreased as FT increased, except for F = 400 mm/min. As the thickness increases, the forces are reduced as N increases, except for N = 4000 rpm (see Figure 2b). Figure 2c shows that there is no clear relationship between N and F. This clear lack of trend usually occurs in composite materials when more than two values of the same factor, Fz, are evaluated, for example, in CFRP [26]. Consequently, it is necessary to analyze the interactions among the factors, as discussed in Section 3.2.  
\begin{table}
\begin{tabular}{c c c} \hline
**Factors** & **Levels (Dimensionless)** & **Values** \\ \hline Foam Thickness (FT) & 1 & 4 mm \\  & 2 & 10 mm \\ \hline Rotation Speed (N) & 1 & 2000 rpm \\  & 2 & 4000 rpm \\  & 3 & 6000 rpm \\ \hline Feed Rate (F) & 1 & 200 mm/min \\  & 2 & 400 mm/min \\  & 3 & 600 mm/min \\ \hline \end{tabular}
\end{table}
Table 1: Values of factors and levels.  
Figure 2: Mean values of experimental tests: (**a**) Fz for each FT and F; (**b**) Fz for each FT and N; (**c**) Fz for each N and F.  
As expected, the difference in Di (\(\Delta\)Di) is similar in the two types of laminates (see Table 2); in general, \(\Delta\)Di increases with N and decreases with F. Note that the differences between 2000 and 4000 rpm and between 400 and 600 mm/min are minimal. More clearly, Figure 3a shows the mean values of \(\Delta\)Di. Their values allow these composites to be kept within H7 tolerances, according to ISO 286-2 standard [37], and they have been found in other plates of different thickness. Avoidable cutting conditions are identified with high cutting speeds and low feed rates.  
The difference in Do (\(\Delta\)Do) is less than \(\Delta\)Di, as expected. The best outlet diameter results were achieved at low feed rates but, when the FT was increased, the feed rate could also be increased up to 400 mm/min. Considering the ISO 286-2 standard [37], these external diameters are less than H8 tolerance. In very strict industries, fiber-reinforced polymers usually require tolerances between 0 and 30 \(\upmu\)m [1]. Therefore, the results obtained are very acceptable, and all cutting conditions may be recommended for FT = 4 mm and, excluding  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Number of Tests**} & \multicolumn{2}{c}{**Foam**} & \multicolumn{2}{c}{**F**} & \multirow{2}{*}{**\(\Delta\)Di (\(\upmu\)m)**} & \multirow{2}{*}{**\(\Delta\)Di (\(\upmu\)m)**} & \multirow{2}{*}{**\(\Delta\)Do (\(\upmu\)m)**} \\  & **Thickness** & & & & & \\  & **(mm)** & & & & & \\ \hline
1 & 4 & 2000 & 200 & 24 & 3 & 1 \\
2 & 4 & 2000 & 400 & 23 & 3.3 & 6 \\
3 & 4 & 2000 & 600 & 22 & 10 & 7 \\
4 & 4 & 400  
the conditions N = 6000 rpm, F = 200 mm/min, and N = 6000 rpm, F = 400 mm/min for FT = 10 mm (see test 7 and 8 in Table 2). Figure 3b shows the variation in the \(\Delta\)Do values when different parameters were varied.  
Table 2 lists the values close to zero for both diameter differences. However, these results are an average of four values (see Section 2.1); thus, some of these measurements are negative, and the diameter found is less than the nominal diameter.