# The Machining of Hard Mold Steel by Ultrasonic Assisted End Milling
## 3 Results and Discussion
In Figure 6(a) there are prominent machining marks on the Stavax (modified AISI 420) sample surface. In Figure 6(b),c the machining marks are less evident and there are dark lines present associated with ultrasonic assistance. The surfaces were also much finer and more uniform than that processed by unassisted milling. Figure 5: Variations in surface roughness (\(Ra\)) for conventional milling and ultrasonic-assisted milling (amplitudes 2.20 and 3.68 \(\upmu\)m) at frequencies of 20 and 50 kHz. Figure 6: \(Cont.\)  
The helix angle of the tool is the angle formed between a tangent to the helix and the tool axial center line. If the flutes were straight, the whole tooth, up to the depth of the cut, would contact the material at the same time. The forces involved would be greater and there would be more vibration. The helix allows the tooth to make gradual contact with the material and softens the impact between the cutter and the material giving a smoother finish. The helix also helps move chips up and out of the cutting zone and a 45\({}^{\circ}\) helix generates an excellent surface finish. The bar chart in Figure 6 shows variations in the average surface roughness (\(R\)a) of Stavax (modified AISI 420) samples, machined with and without ultrasonic vibration, using cutters with helix angle of 25\({}^{\circ}\), 35\({}^{\circ}\) and 45\({}^{\circ}\), all other experimental parameters remaining identical. Each experiment was repeated three times to ensure reproducibility. It can be seen from the figure that the surface roughness remains better for ultrasonic-assisted milling. The \(R\)a for helix angles of 25\({}^{\circ}\), 35\({}^{\circ}\) and 45\({}^{\circ}\) were 0.181, 0.170 and 0.135 \(\upmu\)m, respectively. The best \(R\)a was obtained using a cutter with a 45\({}^{\circ}\) helix. The optical micrographs (500\(\times\)) in Figure 7 show the milled surfaces of the Stavax (modified AISI 420) samples machined with and without ultrasonic assistance using cutters of different helix angle. Quite prominent cutter feed lines can be seen on the conventionally milled surface, see Figure 8a, while the surfaces machined using ultrasonic assistance have much less obvious lines. The surface shows micro-chipping which is characteristic of ultrasonic vibration and is caused by the high frequency hammering, see Figure 8b-d.  
Figure 6: Optical micrographs (400\(\times\)) of milled sample surfaces: (**a**) Conventional milling; (**b**) with ultrasonic assistance at 20 kHz and (**c**) at 50 kHz. Figure 7: Variations in the average surface roughness (\(R\)a) of Stavax (modified AISI 420) milled with and without ultrasonic vibration using cutters with a helix angle of 25\({}^{\circ}\), 35\({}^{\circ}\) and 45\({}^{\circ}\). Tool wear is a most important parameter that must be taken into account with respect to the accuracy of a machined component. Accuracy is also reflected by the degree to which the machined surface corresponds to that of the cutting tool with which it was in contact during the process. Optical micrographs of the cutting edges of tools with helix angles of \(25^{\circ}\), \(35^{\circ}\) and \(45^{\circ}\), used with and without ultrasonics, are shown in Figures 9-11. It was found that tool wear with ultrasonic vibration was less than in conventional milling due to the axial movement of the cutter which periodically increases the gap between the tool and the workpiece which promotes interfacial lubrication. This is similar to the findings of [PERSON] et al. [14] and [PERSON] et al. [15]. The conclusion is that tools used in ultrasonic-assisted machining last longer. Figure 8: Milled surface quality: (**a**) conventional milling; (**b**) helix angle of \(25^{\circ}\); (**c**) helix angle of \(35^{\circ}\) and (**d**) helix angle of \(45^{\circ}\) (all \(500\times\)). Figure 1:0 \(25^{\circ}\) helix cutter surface: (**a**) unused tool; (**b**) tool after conventional milling; and (**c**) tool after ultrasonic-assisted milling (all \(500\times\)).