Influence of included drill angle on cutting forces during drilling of aluminum alloy

The paper presents the analysis of the influence of the twist drill geometry, in particular the 2κr angle, on the cutting forces during holes drilling in the element made of aluminum alloy for plastic forming EN AW-2024. In addition, the impact of the application of the drill geometry correction on the values of cutting forces was also investigated. During the tests, the values of components Fx, Fy and Fz of the cutting force for tools with different geometry with the set, variable technological parameters, i.e.: feed f and cutting speed vc were measured. The changes in the value of the axial component Fz were analyzed in detail. It was found that the use of drills with an angle of 2 κr < 100°, due to the significant value of the axial force Fz and high amplitude is unfavorable. Also, the lack of drill correction translates into an increase in the value of the axial force Fz and its amplitude.

Hole machining is one of the most common technological treatments and therefore twist drills are popular cutting tools.The drilled holes have diameters ranging from hundredths of parts to even several hundred millimeters, with holes up to 100 mm [4,6,10] usually made using twist drills.It should be emphasized that drilling in comparison with other methods of processing takes place in relatively difficult conditions, because [11,13]:  load on the cutting edge is high;  bits have relatively low stiffness for bending and twisting;  cutting speed varies from the largest on the outside diameter to zero in the drill axis;  chip evacuation, especially in long openings, is difficult;  machining fluid has difficult access to the cutting zone.
Value of the cutting force during drilling depends on many factors, including from [1,2]:  properties of the work material,  technological parameters used,  length of the transverse cutting edge (shear),  size of the apex angle 2κr,  helix angle λ's.
In the literature, the apex angle 2κr impact on the value of the axial force is not clearly defined.According to some authors and the results of research [1,3], increasing this angle causes an increase in the value of axial force.The results of other works [5,12] are opposite.It seems that the type of material being processed is of decisive importance [1,11].Increasing the angle λ's causes an increase in the angle of attack, which translates into a reduction in cutting resistance and thus an axial force.
The value of the axial force is also influenced by the shape of the flank [14] and the geometry of the transverse cutting edge [1,11], which is characterized by negative values of the rake angle with very large absolute values (up to -60°), which translates into the formation of a very high axial force.Therefore, the drill geometry correction is often applied, which involves shortening or removing the cut (tab.I) [7,8,11].

TABLE I. Types of sharpening drill bits (in accordance with DIN 1412) [7]
Form  Angle values of 2κr were changed in the range of 90÷135°.To obtain the required geometry, the drill was ground on a Cormak sharpener and then measured.The measured values of the apex angle are shown in fig. 2. For drills with a standard angle of 2Kma = 118°, the length of the transition edge (shear) was also measured, with correction (fig.3a) and without correction (fig.3b).The use of correction allowed more than fourfold reduction in shear length, which translated into lower cutting force.
During the drilling tests, variable machining parameters were usedtheir values are listed in tab.II.
The holes were drilled in a plate with a thickness of 10 mm, made of aluminum alloy for plastic forming EN AW-2024 T351the chemical composition and selected mechanical properties of this material are given in the tab.III.
The tests were carried out on a vertical milling center FV-580A.The Kistler set consisting of:  piezoelectric dynamometer 9257B,  four-channel 5070 load amplifier,  DAQ module with built-in A/D card,  DynoWare software, allowing data acquisition and analysis.
The cutting force measurements were repeated four times.In addition, the wear status of the cutting blades was monitored.

Results
The measurements made allowed to determine the time courses of components Fx, Fy, Fz cutting forces.An example of such a waveform for vc = 60 m/min and f = 0.16 mm/rev is shown in fig. 4.After analyzing the measurement results it was found that the values of Fx and Fy constituted approx.3÷10% of the axial component Fz, therefore at subsequent stages of research, the focus was on the assessment of this strength.In some cases, a slight increase in the values of the Fx and Fy components was observed in the final phase of the borehole, i.e. when the drill bit exited from the material.Fig. 5 shows the time courses of the Fz component at vc = 60 m/min and f = 0.16 mm/rev for four drills with an angle of 2κr from 90° to 135°.

TABLE II . Technological parameters used Sample
number Cutting speed vc, m/min Feedrate f, mm/rev