## 3 Results and Discussions  
### Machining Ability  
In the implemented tests, the obtained hole depth is equal to the displacement of the workpiece and was captured by an LVDT every 0.001 seconds. All experiments were conducted with  
\begin{table}
\begin{tabular}{l l} \hline \hline Spindle speed & 1500 rpm \\ Axial force & 6, 9, 12 kgf \\ Cutting condition & Dry \\ Drill diameter & 3 mm \\ Hole depth & Through, 40 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: The parameters for experimental tests.  
a programmed hole depth of 40 mm. The readings were noted until a through hole had been produced or when the tool was unable to penetrate the material. Figure 4 presents an example of the time history of the drilling depth obtained when applying an axial force of 6 kgf for both cases of CD and UAD. As shown in Figure 4, the holes were either through drilled (with UAD) or blind (in the case of CD). The time when the tool continuously penetrated the workpiece was named as \"Feed in\". If a through hole was successful, a \"Feed out\" time was taken. If the tool was unable to penetrate for more than 10 seconds, the tool was retracted, and a blind hole was made. For cases of both through and blind holes, a short \"Retract\" time was then taken for drawing back the workpiece, and the test was finished.  
In order to compare the machining abilities of UAD and CD, the maximum values of the hole depths obtained from all 30 tests were taken and are shown in Figure 5.  
Figure 5 shows that all holes of number 1 were successful as the tool was totally new. For the remaining holes, all UAD tests completed through holes except for hole number 5 under 9 kgf of axial force. However, only one through hole was made with CD; this was at hole 2 under 12 kgf axial force. In CD, 11 out of 12 holes were blind holes. As observed, the maximum tool life was 5 holes in UAD, as opposed to 1 or 2 holes in CD. It can be seen that UAD prolonged the tool life, as well as improving the machinability.  
One reason for this phenomenon could be found by looking at SEM images taken at the cutting edge of tool bits after drilling 5 holes, shown in Figure 6. It can be seen that phenomena of adhesion of chips in flutes appeared in UAD using 9 kgf axial force and in all CD tests. It should be noted that the  
Figure 4: Time histories of the hole depth obtained with conventional drilling (CD) (red solid line) and UAD (black dot) when an axial force of 6 kgf was applied.  
Figure 5: Maximum hole depth of all tests using (**a**) CD and (**b**) UAD.  
tool bit used in those cases were unable to cut after a certain time, as mentioned above. As shown in Figure 6, chips in those tests adhered on cutting edges and also flocked in tool flutes, preventing both the cutting process and chip evacuation. This phenomenon did not appear in UAD with 6 kgf or 12 kgf axial force, where all holes were produced successfully. To conclude, the cutting ability of the tool bit quickly dropped because of chip adhesion on the cutter and chip flock in the flutes. Superimposed ultrasonic vibration would be a major cause of the elimination of those phenomena. The two factors found to reflect this effect are torque and temperature, as shown in Section 3.3.