On Cutting Forces in Drilling Steels with Internally Cooled Drills

Yu. V. Shchepetilnikov, D. V. Kozhevnikov

(Presented by the scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

Drilling is characterized by a closed course of the cutting process. As the drill penetrates into the hole, access of the coolant to the cutting zone worsens, which leads to an undesirable increase in cutting temperature. Replacing flood cooling with the delivery of coolant directly into the cutting zone, while substantially reducing the cutting temperature [1], should change the nature and magnitude of the other parameters of the metal-cutting process during drilling. This article presents the results of investigations of the effect of supplying coolant directly into the cutting zone through holes in the drills on the nature and magnitude of changes in cutting forces when drilling steels.

The experiments were carried out on a modernized 2A150 vertical drilling machine with drills made of R9 high-speed steel, Ø23 mm, and with Ø20 mm drills fitted with VK8 carbide inserts and having rolled holes. Coolant was supplied to the rotating drills under a pressure of 25 kg/cm² from a pumping station located outside the machine, through a special chuck mounted on the machine spindle. A 5% emulsion was used as the coolant. All drills were ground on the flank surfaces in a plane; and, in order to reduce runout of the cutting edges, the drills equipped with carbide inserts also had, in addition, flat grinding on the rake faces \((\gamma = 0)\). The remaining geometrical parameters of the drills were: point angle \(2\varphi = 120^\circ\), clearance angle \(\alpha = 12^\circ\). Drilling was performed only with sharply ground drills. Blunting of drills in the form of a wear land along the periphery of the flank face greater than 0.1 mm was not permitted.

Measurement of the cutting forces was carried out with a two-component dynamometer, the basic layout of which is similar to the layout of the dynamometer presented in [2]. Foil strain gauges FKPA 20×200 were used as the sensing element in the dynamometer. Recording of signals from the dynamometer was performed on RF-3 X-ray film by means of H135-1.5 vibrators of a loop oscillograph Sh102.

Investigations carried out in drilling four grades of steels—St. 45, 45G17YuZ, EI-316, 1Kh18N9T—showed that the nature of the change

Fig. 1

…the character of the dependences of the cutting forces on speed is similar for all steels. Therefore, in the present work only the dependences obtained when drilling steel 45 are presented; because of the larger permissible feeds, these dependences are most clearly expressed for this steel.

The dependence of the change in cutting forces on speed for high-speed-steel drills is shown in Fig. 1. For drilling without the use of cooling and with flood cooling, the dependences obtained are similar to those obtained earlier by other researchers [3]. Therefore, the dependence obtained when the cooling fluid is supplied directly into the cutting zone through holes in the drills, at a pressure of \(25 \ \mathrm{kg/cm^2}\), is of greatest interest. In this case, at first (the region of low cutting speeds), an increase in cutting speed leads to an increase in the axial force and torque, which is explained by an increase in the coefficient of friction and by the unchanged magnitude of the rake angles along the length of the cutting edge due to the absence of a built-up edge. With the appearance of a built-up edge, the increase in cutting forces ceases, and as soon as the built-up edge acquires a stable character capable of changing the actual cutting angle, the cutting forces begin to decrease. In comparison with flood cooling and dry drilling, the region of built-up-edge formation is shifted significantly into the zone of higher cutting speeds. Thus, for the attained level of cutting speeds, in the case of internal cooling only a decrease in the cutting forces is observed, which is explained by the continuing increase in the volume of the built-up edge with further growth of the cutting speed; for, as the special investigations carried out by us have shown, the cutting temperature in the case where the cooling fluid is supplied directly into the cutting zone at a pressure of \(25 \ \mathrm{kg/cm^2}\) is much lower than with flood cooling and dry drilling. Moreover, this decrease in cutting temperature increases with increasing cutting speed.

Fig. 2

With a decrease in the feed rate, a shift of the extremal points of the dependences into the region of higher cutting speeds is observed.

The measurement of the axial force and torque as functions of cutting speed for drills tipped with carbide is presented in Fig. 2. In this case, the cutting forces were measured during dry drilling and with coolant supplied directly into the cutting zone under a pressure of 25 kg/cm². When these drills were cooled by flooding, the experiment could not be carried out because of chipping of the carbide inserts due to unstable access of the coolant to the cutting zone. The character of the dependences shown in Fig. 2 is similar to the character of the dependences obtained when drilling with high-speed-steel drills. However, with internal pressure cooling of drills equipped with carbide inserts, as the cutting speed increases above 55 m/min (at the same feed rate of 0.4 mm/rev as for the high-speed-steel drills), there is a tendency toward a certain increase in the cutting forces, which can be explained by a decrease in the rake angles as the size of the built-up edge decreases. But even in carbide drills, which permit significantly higher cutting speeds in comparison with high-speed-steel drills, part of the cutting edges operates in the built-up-edge region. This is explained by the variation of cutting speed along the length

Fig. 3

of the drill cutting edges and the existence of the chisel cutting edge, where cutting, even at a substantial magnitude of the cutting speed at the periphery of the drill, is always carried out at microspeeds. The dynamometer, however, measures the cutting forces summed along the length of the cutting edges. This also determines the specific character of the dependencies shown in Figs. 1 and 2.

In the case of reaming holes (Fig. 3), when the chisel edge and part of the main cutting edges do not participate in the work, the change in cutting forces during drilling with increasing cutting speed follows the law of the typical curve obtained in turning with lathe tools [4]. But in this case as well, with internal pressure cooling, a significant shift of the extreme points of the dependencies into the region of higher cutting speeds is observed.

Conclusions

1. Internal pressure cooling leads to a significant shift of the extreme points of the dependencies of cutting forces during drilling into the region of higher cutting speeds.

2. In the case of reaming holes, the dependence of the change in cutting forces on speed coincides completely with analogous dependencies in turning.

3. When working with carbide-tipped drills, despite a significant increase in cutting speed, the chisel cutting edge and the portions of the main cutting edges adjoining it operate under conditions of built-up-edge formation, which determines the character of the dependence obtained.

References

1. E. G. Kuzovenko. High-speed, carbide-tipped drills with central cooling. In: Twist Drills, NIIMASH, Moscow, 1966.

2. Pahlitsch, Spur. Einrichtungen zum Messen der Schnittkräfte beim Bohren. “Werkstatttechnik,” 1959, No. 6.

3. Metal Cutting and Tools. Collection of articles edited by Prof. Dr. Rosenberg A. M. “Mashinostroenie,” Moscow, 1964.

4. A. N. Eremin. The Physical Nature of Phenomena in Steel Cutting. Mashgiz, Moscow, 1951.