## 3 Experimental Results and Analysis
### Influence of Feed on Axial Drilling Force  
Under the conditions of a workpiece overhang size of 70 mm \(\times\) 70 mm, a thickness of 1.5 mm, 2-side constraint (left and right opposite sides fixed), a cutting speed of \(v=18.8\) m/min (spindle speed \(n=750\) r/min), a feed of \(f=0.01\)-\(0.1\) mm/r, and dry cutting conditions, the variation in axial force with time under different feeds is shown in Figure 11.  
Figure 10: Effect of cutting speed on convex deformation of the workpiece before drilling into the drill bit. Figure 9: Effect of cutting speed on the value of maximum axial force during drilling and on the maximum value of axial force in the phase of linear change at the initial stage of cutting. For the same reason as mentioned above, the axial force at the beginning of drilling increases linearly under different feeds; after the drill bit is drilled into the workpiece, the axial force increases with the increase in the cutting length of the main cutting edge of the drill bit, and the slope of its increase is smaller than that at the beginning of the drilling; however, the position point of the maximum axial force and the characteristics of the axial force waveforms are different under different feeds. The trend of the axial force waveforms is basically the same in the case of \(f\) = 0.01 mm/r and \(f\geq\) 0.05 mm/r, and the latter part of the axial force waveform in the case of \(f\) = 0.01 mm/r is characterized by more obvious plastic deformation of the drill cap. Figure 11: Waveform of drilling axial force with time under different feed rate conditions. (**a**) \(f\) = 0.01 mm/r. (**b**) \(f\) = 0.025 mm/r. (**c**) \(f\) = 0.05 mm/r. (**d**) \(f\) = 0.075 mm/r. (**e**) \(f\) = 0.1 mm/r. When \(f\) = 0.01 mm/r, the feed amount is too small; due to the influence of cutting edge radius, in the process of drilling, the extrusion friction effect is larger, the temperature of the bottom surface below the drill tip is higher (Figure 12), and the contact area between the drill tip and the workpiece is more prone to plastic deformation, and in the intersection point of the main and secondary cutting edges of the drill bit when it is about to be drilled out of the bottom surface of the workpiece, the plastic outward flap occurs so that the axial force increases rapidly, and the plastic deformation resistance decreases rapidly after the maximal value is reached. When \(f\) = 0.025 mm/r, as mentioned in Section 3.2, the process of drilling cap formation is dominated by cutting action, and the contact area between the main cutting edge and the workpiece machining surface undergoes plastic deformation in the small thickness to form the drilling cap. \(f\) = 0.05 mm/r, the axial force in the process of drilling cap formation shows a small increase, and no significant sudden change in axial force occurs due to the larger plastic deformation. There is no significant change in axial force due to large outward plastic deformation. With the further increase in feed, after \(f\) \(\geq\) 0.075 mm/r, the slope of the increase in the axial force in the second stage and the increase in the axial force in the third stage converge. With the increase in the feed volume, the drilling speed of the drill bit is fast, and the axial force increase rate after the main cutting edge is drilled in becomes larger, at the same time, from the temperature field of the bottom surface of the workpiece at the time of the maximum drilling force (Figure 12), the maximum temperature of the bottom  
Figure 12: Morphology of drill caps formed by drilling thin-walled AF1410 high-strength steel workpieces and their bottom surface temperature fields under different feed conditions. (**a**) \(f\) = 0.01 mm/r. (**b**) \(f\) = 0.025 mm/r. (**c**) \(f\) = 0.05 mm/r. (**d**) \(f\) = 0.075 mm/r. (**e**) \(f\) = 0.1 mm/r.