1 Characteristics of twist drill and drilling test conditions

Twist drill is one of the cutting tools with large consumption. The semi-enclosed cutting environment and special geometric structure of the twist drill make the drilling temperature higher than that of lathe and milling under the same cutting conditions. The cutting speed of the drilling edge at different parts and the friction speed between the cutting surface are distributed in a triangular gradient along the radius. Affected by this, the drilling edge has different wear processes and characteristics in different areas, and the difference in this wear characteristic will be aggravated with the increase of the twist drill speed or diameter. Obviously, the wear law of the overall drilling edge of the twist drill is more complicated. If the traditional theory and method of wear of lathe or milling cutter are simply followed to deal with the wear problem of the twist drill(such as only the size of the corner wear is used to represent the degree of wear of the drill bit and the wear criterion of the drill bit is formulated according to this), on the one hand, due to the lack of a comprehensive theoretical exposition and experimental basis for the wear law of the drilling edge, on the other hand, it is difficult to select the drilling quantity reasonably in actual processing, and it is difficult to give full play to the overall cutting effect of the twist drill. Especially in the automatic processing environment, it is difficult to implement and has little significance to use a variety of sizes of twist drills for simultaneous processing, and the standard of service life is difficult to implement. However, based on the analysis and test of the cutting failure characteristics of the twist drill, the life management of the twist drill with forced tool change is more simple and reasonable. For this reason, this paper selects the twist drill with more drilling processing application of automotive normalized structure steel, and carries out drilling tests in the common range of automatic drilling quantity, and attempts to reveal the overall wear law of the twist drill by analyzing the wear pattern of different areas of the drilling edge and its change trend with the drilling speed, and then studies the influence of the drilling speed on the cutting life of the twist drill. The test conditions are shown in Table1.

2 Drilling blade wear pattern characteristics

The shape characteristics of the wear zone and the law of change with cutting time and cutting conditions are the basic content of the tool wear theory. Unlike turning and milling tools, all the edges of the drill bit's front cone participate in cutting, and its back knife face wear area is large, and it is three-dimensional distributed on the front cone surface and the corner zone intersecting with the edge band. To facilitate observation, on theDIMILANO119966 tester, a specially made three-way clamp is used to level the main edge, so that the wear zones of the corner zone, the main back knife face, and a part of the cross edge are clearly displayed in one view, and then the wear pattern is measured and drawn or directly photographed. Figure1 is a typical wear pattern of the twist drill obtained from the experiment, the wear zone of the corner is an irregular triangle, its height(along the axial direction of the drill bit) is the maximum widthVB_c of the wear zone of the drill bit, while the wear zone of the main edge and the cross edge is a strip-shaped zone with a more uniform shape and a smaller width sizeVB. The experiment shows that no matter how high the speed of the drill bit is or at any moment in the wear process, the wear pattern with the maximum width value and the fastest expansion rate is always maintained. More importantly, when the speed is high, there is a greater difference in the wear nature of the corner zone and the adjacent small part of the main edge wear zone and the rest of the main edge and cross edge wear zones, the former appears with obvious burning color and the proportion of regular groove shape decreases, because the gradient of the cutting speed along the radial direction of the edge increases, and the temperature and friction speed at the outer edge of the drill bit increase sharply, and thermal wear(oxidation, diffusion wear) becomes the main wear form.

It is obvious that the overall wear degree of the drill bit and whether it can continue to cut or not not only depend on the corner wearVB_c, but also on the wear of the main and cross edgesVB, and are affected by the drilling speed. In the drilling test of changing the drilling speedn, the corner wear and the wear of the main edge of the drill bit when it cannot drill normally are measured, and the results are shown in Figure2. The values of the corner wear of the drill bit are significantly different at different speeds, and theVB_c value increases with the increase of the speed. For example, whenn=1125r/min, the drill bit can still cut normally whenVB_c=0.90mm is measured; while the same drill bit fails whenn=600rpm is drilled, and whenVB_c=0.76mm it has already been worn out and cannot continue to drill. On the contrary, the change of speed has on the wear zone of the main edge and the cross edge. When the speed is low, theVB value corresponding to the failure of the drill bit is large; when the speed is high, theVB value corresponding to the failure of the drill bit is small, but the difference between the two is not large. Therefore, the improvement of the drilling speed has little effect on the wear characteristics of the main edge and the cross edge, which is the difference between drilling and turning. Table2 lists the ratio of the width value of the wear band of the two wear zonesVB_c／VBThe change of the drilling speed. With the increase of the drilling speed, the ratio of the width of the two wear bands increases significantly, that is, in the lower speed range, both the main cutting edge and the corner wear area affect the overall cutting performance of the drill bit, while at higher speeds, the thermal wear of the corner area is the main influencing factor. Therefore, the wear pattern and its change characteristics of the drill bit reflect that the wear process of different cutting areas is not linear, and it is difficult to completely determine the overall wear degree of the drill bit only based on theVB_cvalue, and it is also meaningless to stipulate aVB_cvalue as the dulling standard of the drill bit without considering the influence of the drilling speed.

3.The cutting life of twist drillTis related to drilling speedV

As mentioned before, the cutting theory often considers the drill bit as a complex tool, requiring the assurance of a larger durability and imposing certain restrictions on the amount of corner wear as a criterion for blunting. However, due to the large wear area, special and irregular pattern of the twist drill, and the complexity of the wear mechanism, it is difficult to implement the blunting criterion and to study the relationship between durability and cutting speed. Therefore, in production, the total drilling time until the drill bit cannot cut(or the cutting action fails) is often used as the cutting life of the drill bit, and on the automatic production line, the maximum number of holes that can be drilled is often used to indicate the cutting performance of the drill bit, and the drill bit life is managed based on this. Therefore, the experiment uses the same drill bit to bore steel plates of equal thickness at different speeds, and studies the effect of cutting speed on the number of holes and the cutting life of the drill bit, the results are shown in Figure3. With the increase of cutting speed, the cutting life of the drill bit decreases, especially in the speed range of28.3～35.3m/min, the decrease is the largest. If the tool life formula(Taylor formula)is used to regress the influence of speed on the cutting life of the drill bit, the results show that the linearity on the logarithmic coordinate is not significant, which indicates that different speed ranges have a large difference in the degree of influence on the life of the drill bit, that is, the speed range with Taylor characteristics is very narrow. Therefore, the drill bit should operate in a speed range that has a relatively small influence on the cutting life, while being able to obtain a higher processing efficiency, and the number of holes test can conveniently solve this problem. From Figure3b, it can be seen that when the speed increases from600r/minto900r/min, the number of holes decreases from117to109, with a decrease of only7%, and the cutting efficiency increased by50%. When the speed continues to increase, the number of holes drilled decreases sharply, and only5 holes can be drilled at a speed of1350r/min. Therefore, from the comprehensive consideration of the cutting performance of the drill bit and the processing efficiency, it is more reasonable to select a speed of900r/min. At this time, the ratio of the width of the two wear bands is small, about5.

4.Conclusion

1.   The twist drill has two wear pattern features and wear mechanisms that are different, and the difference between the two increases with the increase in drilling speed, which is expressed as the significant increase in the ratio of the width of the wear zoneVB_c/VB.

2.   The ability of the twist drill to continue cutting is related to both wear zones, and the drilling speed has a significant impact on the limit value of the wear zone, it is meaningless to formulate the criterion for the wear of the twist drill corner away from the drilling speed;

At different ranges of drilling speed(revolutions per minute)the degree of influence of speed on the cutting life of the drill bit is different. A reasonable selection of drilling parameters should result in a small value ofVB_c/VBand obtain a larger number of holes.