An Investigation of the Effect of Parameters and Chip Slenderness Ratio on Drilling Process Quality of AISI 1050 Steel

.e chip slenderness ratio is a vital parameter in theoretical and applicable machining operations. In predrilled drilling operations of AISI 1050 steel alloy, HSS drills were employed, and the effect of the selected parameters on the chip slenderness ratio and also the effect of the chip slenderness ratio on the thrust force, surface roughness, drilled hole delamination, tool wear, and chip morphology were investigated. .e major parameters, influential on the chip slenderness ratio, were feed rate and point angle, while spindle speed was too small to be negligible. With increasing the chip slenderness ratio, the thrust force and the tool wear decreased, which resulted in appropriate chip morphology, but there were increases in surface roughness. However, the chip slenderness ratio had no effect on the drilled hole delamination.


Introduction
Drilling process is a metal cutting method, which processes metal pieces in required geometry and dimension, in manufacturing area [1]. is manufacturing method has an important place as it is used about 33% among traditional manufacturing methods [2].In the drilling process, researchers conducted many works after having completed complicated operations and manufacturing processes in order to obtain the required highest sensibility and best quality [3].A lot of parameters such as drill point angle, material used for drilling, and properties of drilling equipment affect the drilling process.e tool equipment affects the demented of the drilled hole, the tool wear, the surface quality, and the chip length and thus the chip morphology [4].As the feed rate increases, the surface quality is influenced inversely, and in other words, the surface roughness increases.Because the thrust force and the tool wear decrease at high spindle speeds and drill point angles, the surface roughness values decrease; thus, the surface quality improves.However, the process temperature and drilled hole delamination increase at high spindle speeds and drill point angles [5].e delamination size of the drilled hole depends on the thrust force and feed rate in drilling processes [6,7].Besides, thrust force increases the drilled hole delamination and the tool wear [8].While the numbers of the drilled hole increase, the tool wear ratio increases as well [9].As the chip height and thrust force increase due to increase in the feed rate, the chip height and the thrust force decrease with increase in spindle speed [10,11].
Furthermore, the feed rate is a significant parameter, affecting the process quality and the surface roughness, more than the spindle speed in the drilling process [12][13][14][15][16][17][18][19][20].With sufficient increase in the thrust force in the drilling process, the web thickness of the drill and the relief angle increase; otherwise, the drill point angle and the helical angle decrease [21,22].However, with increase in the feed rate and point angle, the thrust force and surface roughness increase [23].In the drilling process, vibration, tool wear, and dry process conditions cause changes in the diameter of the drilled hole [24].At low cutting speeds and feed rates, due to the requested chip morphology availability, a better performance is obtainable in drilling processes.In addition, the tool point angle does not significantly affect the process temperature [25].During the process, discontinuous and short shaped chips are obtained, due to the intermittent plastic deformation at low spindle speeds.With increasing the drill point angle and the spindle speed, the thrust force, resulting from the applied pressure on workpiece material by the drill point, decreases.Chip thickness varies considerably on all along the cutting edge during a cycle of the drill point [26].With increasing the cutting conditions, the morphology of chip structure changes into a brittle shape from ductile.With enhancing the feed rate, the chip height increases, and also the thrust force, the drilled hole delamination, the surface roughness, and the tool wear increase as well [11].Conical cutting edge, starting from the drill point, causes changes in cutting force all along the cutting edge of the drill [27].Because the conical geometry is formed by the point angle of drill, the main feature of the drilling process shows differences from all along the cutting edge.While the feed rate gets the maximum ratio at the outmost diameter of the drill, it approaches to zero value toward the point axis of the drill where it constitutes core of the drill because of decrease in the diameter [28].
e chip slenderness ratio is the proportion of cutting depth to feed rate, which is the vital parameter to affect the outcomes, in machining.erefore, the investigation of the chip slenderness ratio is the main phenomenon.Furthermore, the goal of this study is to investigate the effect of the chip slenderness ratio on the predrilled drilling process.In the literature, it is identified as a ratio of the depth of cutting to the feed rate, constituting an important parameter in machining operations.In drilling processes, due to the fact that the depth of the chip depends on the diameter of the drilled hole and the drill point, the chip slenderness ratio was calculated by the ratio of the square of the feed rate to the tool chip contact area.Considering the geometry of the drill point, the chip area was calculated mathematically depending on the drill point angle.
e chip areas were calculated by crossing these two measured geometric dimensions.e effects of the drill point angle, spindle speed, and feed rate on the chip slenderness ratio were investigated during the study.In addition, with the help of the calculated chip area from this study, theoretical chip slenderness ratios were calculated.

Experimental Procedure
e experiments were conducted on Mazak Vertical Center CNC Milling Machine, which is the NEXUS 510 C-II model as shown in Figure 1.AISI 1050 alloy steel samples were prepared as 100 × 100 × 15 mm 3 dimensions.e samples were predrilled 5 mm in diameter before the experiment.
e distance was arranged as 20 mm between the centers of  With the help of the depth micrometer, the drilled hole delamination was measured as µm unit at 4 different points, which is 5 mm in depth, and the arithmetic average of these values was taken into account.Wear areas of the drill points were monitored with Stereo LEICA Z16 APO microscope, and images were recorded.Surface roughness values were measured as 0.8 mm by using TR 200 TIME SIRF, a surface roughness value device.
During the experimental study, 4 different drill point angle values were selected: 100 °, 118 °, 136 °, and 154 °. e drill points were grinded at selected point angles by using the tool grinding machine, as shown in Figure 2(a).Pictures of the grinded drill points can also be seen in Figure 2(b).e geometrical properties of HSS drill points are shown in Table 2.

Geometry of Chip Analyses.
e length of the cutting edge is formed depending on the angle of the drill point during the drilling process.erefore, the geometry of drill is vital for drilling processes because of changing the cutting speed and showing differences throughout the cutting edge.Cutting speed gets the maximum ratio at the outmost diameter of the drill, and it decreases and approaches to zero toward the axis of the drill where the drill constitutes core [28].e angle of the drill point has an important influence on the length of the cutting edge in the drilling process.
e geometry of the drill point is shown in Figure 3.In this figure, Ød o is the diameter of the predrilled hole of the workpiece sample before experimental processes.Effective forces on 1st and 2nd cutting edges of the drill were shown, respectively, as F t1 and F t2 tangential forces, F z1 and F z2 thrust forces, and F y1 and F y2 radial forces in the revolving direction.F zc-1 and F zc-2 are the effective thrust forces on the core of the drill point.Φ/2 � φ is half of the value of the drill point angle.Besides, the values of the point angles used in this study are Φ 1 , Φ 2 , Φ 3 , and Φ 4 , respectively, and they are equal to 100 °, 118 °, 136 °, and 154 °, respectively.
In this experimental study, in order to eliminate the effects of the drill core, holes were predrilled 5 mm in diameter.e dimensions of the cutting edges of the drills, used in experimental, drilling processes, in which the surplus were not predrilled, are calculable as seen in ( 1), depending on the geometry of the tip of the drill, as shown in Figure 4 according to predrilling diameters and drill point angle: e uncut chip height (h D ) can be calculated based on the feed rate (f z ) and point angle (φ) by using (2), in accordance with the geometry of the drill point, as shown in Figure 4. Since the height of the chip does not change depending on the predrilling diameter, the following equation is valid for both predrilled and without predrilled drilling processes: Figure 3: Geometry of the drill point [29].
Advances in Materials Science and Engineering In this experimental study, as surplus were predrilled 5 mm in diameter, the influential chip width and height values were demonstrated with b De and h De , respectively.
In these circumstances, the effective chip area (A e ) and chip width (b De ), depending on the chip height (h De ), can be calculated in accordance with the following equation: In this experimental study, 4 different drill point angles were selected.ese were represented with Φ 1 , Φ 2 , Φ 3 , and Φ 4 symbols, and they were selected as 100 °, 118 °, 136 °, and 154 °, respectively.
According to the geometry of the drill point, the length of the cutting edge without predrilled hole (l), and the length of cutting edge with predrilled hole (l e ), in other words, the uncut chip width values (b D ) and (b De ), showed differences depending on the change in the drill point angle.
In calculating the chip slenderness ratio, the chip width (b De ), the chip height (h De ), and the chip area (A ce ) values were taken into consideration in predrilled drilling operations, depending on the drill point angles by using (1)-(4).

Analysis of the Chip Slenderness Ratio.
e chip slenderness ratio is an important parameter when the feed rate of cutting depth in expanded cutting speed law is considered. is ratio is named as "chip slenderness factor" or "chip morphology."For the same chip contact area, the chip slenderness ratio is high at higher selected chip cutting depths, whereas it is low at smaller selected chip cutting depths.Manufacturers investigate the effect of the cutting depth and feed rate on the chip slenderness ratio simultaneously in real applicable machining operations.
e optimum chip slenderness ratio is 5 : 1 according to the results in the literature.
However, this value (5 : 1) is not valid for the turning operations because the selected feed rates are bigger than the selected cutting depths.Measuring of the chip dimensions, obtained from the experiments, such as chip thickness and chip width, is too difficult.erefore, the calculated chip slenderness ratio derived from ( 5)-( 7) was taken into consideration.
e chip contact area can be written as demonstrated in the following equations in machining operations: As seen in the following equation, the chip slenderness ratio (G) can be identified as the rate of the depth of cutting (a) to the feed rate (f): When the common solution of ( 5) and ( 6) is considered, the chip slenderness ratio (G) can be calculated by the ratio of the chip contact area (A) to the square of the feed rate (f 2 ):

Effects of the Selected Parameters on the Chip Slenderness
Ratio. e chip slenderness ratio is an operational parameter accepted by researchers, working in machining manufacturing area. is parameter is demonstrated as the ratio of selected chip depth (a) to the feed rate (f) [30].Feed rate is an important result in machining manufacturing operation, and it is the most important parameter determining the quality of the operation.Due to the fact that 4 Advances in Materials Science and Engineering the chip slenderness ratio is expressed as the ratio of the chip depth to the feed rate ( 6), the effect of this ratio for operation is important.As the chip depth can be selected by the operator in milling and turning operations, this ratio can be directly considered as the ratio of the chip depth to the feed rate.However, in drilling processes, the chip depth is based directly on the diameter of hole to be drilled; in short, it is based on the diameter of the selected drill.So, this ratio should be considered as the ratio of the chip area to the feed rate square (7), and it should also be considered for conducting investigations and analysis in this study.In this study, chip areas were investigated as calculated (theoretical) chip areas.
e calculated (theoretical) chip areas were calculated using ( 2)-( 4) and depending on the geometry of the drill point as shown in Figure 3.
It was investigated that the effects of parameters should be calculated by chip slenderness ratios in Figure 4. e calculated chip slenderness ratio was between 7.72 and 84.00.Differences between theoretical chip slenderness ratios were numerically high, but it was determined that chip slenderness ratios are directly proportional to the change in accordance with the selected parameters.e most effective parameters on the chip slenderness ratio were feed rate and angle of the drill point.When both feed rate and point angle increase, the chip slenderness ratio decreases linearly as shown in Figure 4.However, the effect of spindle speed on the chip slenderness ratio is too little.
Effects of the spindle speed on the theoretical chip slenderness ratio should be zero according to ( 6) and (7).
In the circumstance, it is concluded that geometrical and mathematical equations providing the trust results, theoretically, depend on the selected parameters.
e highest value of the theoretical chip slenderness ratio was 84 and the lowest 7.72.

Effects of the Chip Slenderness Ratio on rust Force.
During drilling operations, thrust force is one of the important factors determining the quality of process and supplying information about the results.If the thrust force is low during the operation, this means that operation quality is high.In Figure 5, it is seen that the thrust force decreased when the chip slenderness ratio increased.On the other hand, it was also seen that the chip slenderness ratio decreased collaterally with the thrust force when the drill point angle increased for the theoretical chip slenderness ratio.
However, with increase in the theoretical chip slenderness ratio, decrease in thrust force was observed.
As demonstrated in Figure 5, the lowest thrust force values were obtained as 274.70 and 292.09N at the theoretical chip slenderness ratio of 84.However, the spindle speed had no effects on the theoretical chip slenderness ratio depending on the geometry of the drill point.On the contrary, the change resulted in only depending on the feed rate and the drill point angle.

Effects of the Chip Slenderness Ratio on Surface Roughness.
Because of the chip slenderness ratio directly depends on the feed rate, the most effective parameter on surface roughness, thus on the chip slenderness ratio is the most influential parameter on surface roughness.According to the graph in Figure 6, when the chip slenderness ratio increased, the values of surface roughness also increased, but surface quality decreased.
At the highest (60-80) and the lowest (7-20) chip slenderness ratio values, the maximum and the minimum surface roughness values were 4.50 µm and 1.6 µm, respectively, as shown in Figure 6.

Effect of the Chip Slenderness Ratio on Drilled Hole
Delamination.Due to the fact that the drilled hole delamination in drilling operations mostly depends on the motions of the tool and the interactions between the tool and the workpiece, it is not much affected by the chip slenderness ratio.e effect of the theoretical chip slenderness ratio on the delamination of the drilled hole is demonstrated in Figure 7.According to the results, the effect of the chip slenderness ratio on the delamination was unclear, and change in delamination values was determined depending on the changes in the chip slenderness ratio.Hole delamination cannot be investigated depending on the chip slenderness factor.Advances in Materials Science and Engineering

e Effect of the Calculated Chip Slenderness Ratio on Chip
Morphology.In drilling operations, morphology is the most important marker, affecting the operation quality.In this study, the effect of the chip slenderness ratio on the chip morphology was demonstrated, as shown in Figures 8(a)-8(c), for selecting 0.025, 0.050, and 0.075 mm/rev feed rates, respectively.
As shown in Figure 8(a), at 0.025 mm/rev feed rate, with increasing the drill point angle, the chip morphology changed from brittle and short forms to continuous.Furthermore, at lower point angles, especially at 100 °and 118 °, the serrating chip form resulted in the obtained chips, while the serrating chip formation decreased at higher point angles, as 136 °and 154 °. e serrated chip formation demonstrates the quantity of the vibrations, undesired phenomenon during the machining operations because of causing the severe wear and shorter tool life.However, at 0.05 mm/rev feed rate, as shown in Figure 8(b), at higher point angles, especially at 136 °and 154 °, and at collateral spindle speed, 1600 rpm spindle speed, the serrated chip formation decreased.At 0.075 mm/rev feed rate, as shown in Figure 8 (c), at lower point angles, 100 °and 118 °and at lower spindle speeds, 800 and 1200 rpm, the serrated chip formation increased linearly.However, at 136 °and 154 °point angles, the serrated formation in the removed chip decreased as shown in Figure 8(c), definitely.
While the requested ductile form was obtained, long and continuous chip morphology with increasing point angle, at 0.025 mm/rev feed rate conditions in machining operation, also the tool wear decreased.In addition, with increasing the drill point angle, the chip slenderness ratio decreased.
Continuous, long, and ductile chip morphology with decreasing chip slenderness ratios was obtained with the selected drill point angles 136 °and 154 °.As shown in Figure 8(b), decrease in chip slenderness ratio, at 0.050 mm/rev feed rate, was only obtained at the drill point angle 154 °.But the continuous, long, and ductile chip morphology was also obtained with other drill point angles.In Figure 8(c), while chip slenderness ratio increased under 0.075 mm/rev feed rate and 800 rpm spindle speed conditions, the value of this ratio decreased because of increasing the spindle speed.
Here, positive effects were observed when the feed rate and the spindle speed collaterally changed with each other.Also at 0.075 mm/rev feed rate, the lowest chip slenderness ratios were obtained with selective drill point angles 136 °and 154 °.Moreover, chips were obtained in the requested morphology, and tool wear decreased at these angles.Burning marks on the cutting edge of the tool and chips were seen because of high process temperature during the drilling operations.

e Effect of the Calculated Chip Slenderness Ratio on Tool
Wear.Tool wear is an important identifier for the tool life in machining operations.Less tool wear provides longer tool life.e effect of the drill point angle, spindle speed, and feed rate on the tool wear is shown in Figure 9 in predrilled drilling operations.
At 0.025 mm/rev feed rate, with increasing drill point angle, calculated chip slenderness ratio decreased linearly.Moreover, the tool wear quantity decreased as shown in Figure 9(a).
At 0.050 mm/rev feed rate, the tool wear increased at higher spindle speeds, as 1200 and 1600 rpm, but at lower drill point angles, it decreased as 100 °and 118 °.However, at 500 rpm spindle speed and 100 °drill point angle, the tool wear decreased.At 136 °and 154 °point angles, the tool wear decreased for all selected spindle speeds.However, at higher spindle speeds, due to the higher process temperature, there were burning marks on the drill cutting edges as shown in Figure 9(c).
With increasing the feed rate to 0.075 mm/rev, the tool wear increased only at lower spindle speed and 100 °point angle; with increasing the point angle, the wear quantity of the tool cutting edge increased at 1600 rpm spindle speed.

Conclusions
In predrilled drilling operations of AISI 1050 steel alloy, HSS drills were employed, and the effect of the selected parameters on the chip slenderness ratio and also the effect of the chip slenderness ratio on the thrust force, the surface roughness, the drilled hole delamination, the tool wear, and the chip morphology were investigated.e derived results in this study were listed below.
e major parameters influential on the chip slenderness ratio were the feed rate and the drill point angle, but the effect of the spindle speed was too small to be negligible; with decreasing the chip slenderness ratio, the thrust force and tool wear decreased, and also the removed chip morphology changed from shorter, brittle, and serrated to continuous, ductile, and free of serrated form.Besides, with increasing the calculated chip slenderness ratio, the surface roughness increased collaterally.However, the chip slenderness ratio did not have any important effect to be taken into consideration on the hole delamination.
Furthermore, with changing point angles from 100 °to 154 °, the chip slenderness ratio decreased from 84 to 7, the   Advances in Materials Science and Engineering quantity of the tool wear decreased, and the removed chip morphology was changed from brittle, shorter, and serrated forms, undesired chip forms, to continuous, longer ductile, and free of serrated form, the most desired chip form in machining operations.

Figure 2 :
Figure 2: (a) Tool grinding machine and (b) drills with different point angles.

Figure 4 :
Figure 4: Effects of the selected parameters on the calculated chip slenderness ratios.

Figure 5 :
Figure 5: Effects of the experimental and theoretical chip slenderness ratios on the thrust force.

Figure 6 :
Figure 6: e effect of the calculated chip slenderness ratios on the surface roughness.

Figure 7 :
Figure 7: Effect of the calculated chip slenderness ratios on the drilled hole delamination.

TABLE 1 :
Mechanical characteristics and chemical components of AISI 1050 alloy steel.

Table 2 :
Characteristics of HSS drill points.