# The Machining of Hard Mold Steel by Ultrasonic Assisted End Milling
## 3 Results and Discussion  
Figure 2 shows the detailed procedure flow chart, each experiment was repeated three times to ensure reproducibility. Figure 3 shows the variation in amplitude with input voltage for different stretch lengths of the cutter (25 and 40 mm). The amplitude for the stretch length of 40 mm increased with input voltage from 2.1 \(\upmu\)m at 100 V to 9.2 \(\upmu\)m at 450 V. In addition, the amplitude for the stretch length of 25 mm increased with input voltage from 0.6 \(\upmu\)m at 100 V to 3.7 \(\upmu\)m at 450 V. It was found that the amplitude of the ultrasonic displacement increased with increasing stretch length and input voltage. Although increasing the stretch length of the cutter can increase the amplitude, it leads to less rigidity. Figure 4 also shows the variation in the amplitude with input voltage for different cutter holding force (10, 15, 20 and 25 N). The amplitude increases with an increase in the input voltage for the holding force of 15, 20 and 25 N. However, for a holding force of 10 N, the amplitude increased with the input voltage up to 250 V, and was then reduced with further increase in voltage. A holding force of 15 N or more is recommended because the amplitude will then remain constant. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{} & **Parameters** \\ \hline Cutting parameters & Cutting speed 113 m/min, Feed rate 0.004 mm/tooth, depth of cut 100 \(\upmu\)m. \\ Ultrasonic vibration conditions & Frequency 50 and 20 kHz, amplitude 2–10 \(\upmu\)m \\ Workpiece & Stavax (modified AISI 420) HRC 52 stainless mold steel block 50 \(\times\) 50 \(\times\) 50 mm \\ Milling Cutters & Tool diameter 6 mm, rake angle \(-6^{\circ}\) and \(6^{\circ}\), helix angles \(25^{\circ}\), \(35^{\circ}\) and \(45^{\circ}\). \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental conditions. Figure 1: The ultrasonic-assisted milling system. Figure 4: Variation in amplitude with input voltage for cutter holding force of 10, 15, 20 and 25 N.  
Figure 3: Variation in amplitude with input voltage for cutter stretch length of 25 and 40 mm. Figure 2: Detailed procedure flow chart. Figure 5 shows the variation in surface roughness (\(Ra\)) of the workpiece for unassisted milling and ultrasonic-assisted milling (amplitude of 2.20 and 3.68 \(\upmu\)m) at two different frequencies (20 and 50 kHz). The purpose of the experiment was an evaluation of the effect of ultrasonically assisted milling and whether it improved the roughness of the machined surface or not. Three samples were processed using the same machining parameters, and the surface roughness was assessed as an average of the roughness of the three. For unassisted milling, the \(Ra\) value was approximately 0.236 \(\upmu\)m. For assisted milling at an ultrasonic amplitude of 2.20 \(\upmu\)m, the \(Ra\) values were 0.136 and 0.110 \(\upmu\)m at 20 and 50 kHz, respectively. At an amplitude of 3.68 \(\upmu\)m, \(Ra\) values were 0.151 and 0.174 \(\upmu\)m at 20 and 50 kHz, respectively. It is clear from these \(Ra\) figures that a better finish can be achieved with ultrasonic-assisted milling. It also became clear that there is an optimal value for the amplitude of the ultrasonic vibration used which will give the best finish under the conditions of these experiments. This is believed to be a result of the \"hammer effect\" induced by ultrasonic vibration. Because of the high-frequency vibration, the cutter teeth will hammer the workpiece constantly. This means the quality of the processed surface can be improved by the choice of an appropriate ultrasonic amplitude. Simply raising the amplitude does not work because the ideal amplitude depends on other cutting conditions such a feed, depth of cut and so on. This finding agrees with that of [PERSON] et al. [4]. Figure 6 shows optical micrographs (400\(\times\)) of the surface of a sample machined by unassisted milling, compared to that of samples milled with ultrasonic assistance at frequencies of 20 and 50 kHz.