ON THE NATURE OF PROCESSES IN HIGH-SPEED CUTTING OF STEEL

V. M. MOSTOVOI

In all areas of our socialist industry, great attention is being devoted to high-speed methods of work. In the field of machine building, extensive creative work has unfolded on the introduction and further development of methods of high-speed cutting of metals. During the period from 1935 to the present, industrial workers and our scientists have accumulated extensive experience in high-speed cutting, on the basis of which certain theoretical premises have already been created for explaining the phenomena occurring in high-speed cutting. At the same time, however, the physicomechanical phenomena occurring in high-speed machining of metals as a whole are still poorly studied. The further spread of high-speed machining methods and the increase of cutting speed are matters of primary importance. Therefore, clarification of the nature of high-speed cutting is absolutely necessary. To answer this question means to elucidate the causes under whose action favorable conditions are created for the implementation and further development of high-speed machining methods.

Before proceeding to resolve the question posed, we consider it necessary to dwell on the work of E. I. Feldshtein, “On the Nature of the Processes of High-Speed Cutting of Steels.” The author of this work, more fully and more precisely than other researchers, has given an answer to this question.

First of all, E. I. Feldshtein, in our opinion, correctly orients the matter toward the fact that the further development and improvement of methods of high-speed machining “is impossible without a correct understanding of the essence of the processes of high-speed cutting of metals, without clarifying the nature of the phenomena that determine the character of the interaction of the work material and the cutting tool during high-speed cutting” [1].

In his conclusions, the author arrives at the fundamentally correct conclusion that the transfer of mechanical characteristics obtained in the rupture of a specimen to cutting characteristics is erroneous.

At the same time, however, we cannot agree with other assertions in the conclusions of E. I. Feldshtein, namely:

1. In the author’s conclusions, during high-speed cutting the material being machined affects tool life only through mechanical actions: the flowing chip abrades the front face of the cutter (mechanical wear of the front face). In our opinion, the material plays a more active role: the heat released as a result of the plastic deformations occurring at the moment of chip formation actively affects the resultant force—$R$, which shrinks the chip, through a change in the coefficient of friction—$\mu$ and the friction force $F$ (Fig. 1).

2. Further, the work notes that the effectiveness of high-speed cutting is determined primarily by the red-hardness and wear resistance of titanium-tungsten alloys. One cannot but disagree with this conclusion, since

one likewise cannot but agree that, in high-speed turning, the temperature in the cutting zone reaches 700–800°C. But when the author denies, in essence, the influence of the temperature arising in the cutting zone on the subsequent cutting process, we cannot agree with this. According to the author, the chip on the front face of the cutter does not have time to be heated through its entire thickness, since its heating takes place from the contact surface over a very short interval of time. Here the author has not sufficiently fully revealed the physical

Fig. 1

essence of the phenomena taking place between the shear plane and the plane parallel to it and passing through the place of contact (the end) of the chip with the cutter. Having confined himself to establishing the fact that high temperatures are present in the cutting zone, the author did not show that the high temperature arising in the cutting zone has a substantial influence on the formation

Fig. 1a

of the forces acting on the front face of the cutter, which, in turn, (in particular, the resultant force—\(R\)) actively act on the cutting zone through a change in the angle of inclination of the shear plane—\(\beta_1\) (Fig. 1a).

In this work we make an attempt to give a more complete explanation of the nature of high-speed cutting. Basing ourselves on the latest achievements

in the field of metal physics and experimental data in the field of metal cutting [2, 3], we believe that the nature of high-speed cutting of metals can be explained by the following factors:

a) the red-hardness and wear resistance of the hard alloy of the cutting tool;

b) a reduction in the coefficient of friction between the contact surfaces of the chip and the cutter;

c) a change in the magnitude and direction of the resultant force \(R\) under the action of the coefficient of friction. To determine the influence of temperature on chip formation, the metal ahead of the cutter and in the cutting zone was subjected by us to artificial preliminary heating. This made it possible to compare the results obtained with those obtained in chip removal under ordinary turning conditions.

At a high cutting speed, the cutter, penetrating into the metal, displaces the chip at the moment of its formation relative to the base metal in the direction of the shear plane. As a result of plastic deformations in the cutting zone and of the relative displacement of the forming chip in the shear plane, the metal particles throughout the entire thickness of the chip are subjected to intense heating to a high temperature. In addition, the chip, being in contact with the front face of the cutter, is additionally heated as a result of the action of the frictional force and the presence of deformations in the contact layer of the chip; here the heating of the chip can extend only to an insignificant depth, owing to the short duration of contact. For our intensive experimental conditions, i.e., at \(V = 218\ \text{m/min.}\), \(\zeta_1 = 1.5\), \(l = 1\ \text{mm}\), the time of contact of the chip with the cutter will be:

\[ t=\frac{60\cdot l\cdot \zeta_1}{1000V} =\frac{60\cdot 1\cdot 1.5}{1000\cdot 218} =0.0004\ \text{sec.} \]

In such an insignificant time the chip, from the side of the contact surface, will be heated only to an insignificant depth. In practice it may be considered that the chip throughout its entire thickness will have the temperature acquired by it in the cutting zone. The change in the temperature of the contact layer is of interest to us because it causes a change in the coefficient of friction \(\mu\) and, consequently, in the friction force \(F\) and the resultant \(R\).

Fig. 2

Under ordinary machining conditions, i.e., without artificial heating of the metal in the cutting zone, with an increase in cutting speed from \(V = 50\ \text{m/min}\) and higher, the coefficient of friction decreases (Fig. 2). With the use of artificial heating, an analogous decrease is observed at \(T = 600^\circ\text{C}\) (Fig. 3).

With a decrease in the coefficient of friction, the friction force decreases, and the resultant force—\(R\)—also changes its magnitude and direction (Fig. 1).

With a change in the direction of the resultant force, the angle of inclination of the shear plane \(\beta_1\) increases, and with an increase in the angle \(\beta_1\) the position of the shear plane relative to the cutting plane changes, i.e., the position of the base of the chip being formed changes relative to the parent metal of the workpiece. An increase in the angle \(\beta_1\) leads to a decrease in chip compression, which also follows from Fig. 1a and from the expression:

\[ \tg \beta_1^\circ = \frac{\cos \gamma^\circ}{\zeta_a - \sin \gamma^\circ}, \tag{1} \]

where \(\beta_1^\circ\) is the angle determining the position of the shear plane, \(\gamma^\circ\) is the rake angle, and \(\zeta_a\) is the chip compression in thickness.

Thus, from our reasoning it follows that the chip acquires one temperature or another only at the moment of its formation in the cutting zone. Along the path of its removal, i.e., between planes I—I and II—II (Fig. 1), the chip retains the temperature it has acquired. The contact layer of the chip is subjected to additional heating. This, in our opinion, is how heat generation occurs in the process of cutting metal without preheating.

Fig. 3

What will happen in the cutting process if we artificially heat the metal in the cutting zone and remove the chip at the same cutting speed as in the first case? By preheating we reduce the mechanical properties of the metal to be removed. Consequently, in the cutting zone, before the cutter begins to penetrate into the metal, we have created some elevated temperature. Further, in the cutting process, along the path of its motion in the region between planes I—I and II—II (Fig. 1), the chip will be additionally heated from the side of the contact surface. And we have grounds to suppose that the temperature on the rake face of the cutter will be higher than in the preceding case.

An increase in temperature between the contact surfaces, as is known, reduces the coefficient of friction—\(\mu\), the resultant force—\(R\), increases the angle of inclination of the shear plane—\(\beta_1^\circ\), and, consequently, reduces chip compression¹. On the basis of these considerations, we believe:

1) The coefficient of friction—\(\mu\), chip compression—\(\zeta_1\), cutting forces—\(P_z\), and stress in the shear plane—\(\sigma_s\)—will have lower values when cutting a chip from a workpiece preheated in the cutting zone, in comparison with the values that would be obtained under

removal of the chip from the blank without preliminary heating of it in the cutting zone at the same cutting speed.

2) If equality of temperatures on the front face of the cutter is maintained in both cases, i.e., with artificial heating and without heating of the metal in the cutting zone, equality of chip shrinkages and of the coefficients of friction between the contact surfaces of the chip and the cutter should be preserved.

We shall prove these assumptions on the basis of our experimental data.

Fig. 4

In carrying out the experiments, in one case we removed the chip from the workpiece at variable speeds. In this case the thermal regime in the cutting process was determined by the speed regime. The temperature obtained by changing the speed regime we denoted by \(T_v^\circ\). In the other case the metal in the cutting zone was preheated. The temperature of preliminary heating we denoted by \(T^\circ\).

Fig. 5

Having adopted this method, we were able to determine the influence of the temperature of preliminary heating and to establish the share of its effect on the cutting process.

When the metal in the cutting zone is heated to \(T = 550\text{--}600^\circ\mathrm{C}\) and the cutting speed is such as to exclude the influence of the cutting temperature on chip shrinkage, i.e., at \(V = 3\ \mathrm{m/min}\), the chip shrinkage is \(\xi_l = 3.5\) (Fig. 5).

In the other case the cutting temperature was changed by changing the cutting speed. From the curve (Fig. 4) it follows that at a cutting speed \(V = 50\ \mathrm{m/min}\) the chip shrinkage \(\xi_l = 3.5\).

On the basis that regimes with identical temperatures on the front face of the cutter are accompanied by identical geometry of the built-up edge, identical

coefficients of friction and chip shrinkages, irrespective of the source of heat generation, and also on our experimental data, we believe that at a cutting speed \(V = 50\ \text{m/min}\) and \(T = 0^\circ\) the temperature on the front face of the tool was the same, but higher than the temperature in the cutting zone. Thus, on the basis of the experimental data

Fig. 6

we arrive at the conclusion that the chip shrinkage at a cutting speed \(V = 50\ \text{m/min}\) and at the temperature on the front face of the tool corresponding to this speed, \(T_v = 550\text{–}600^\circ\) at \(T = 0^\circ\), is equal to the chip shrinkage at a cutting speed \(V = 3\ \text{m/min}\) \((T_v = 0^\circ)\) and \(T = 550\text{–}600^\circ\).

Fig. 7

Next we carried out experiments with variable cutting speeds and temperatures of preliminary heating of the metal ahead of the tool and in the cutting zone, and obtained the following results at \(V = 50\ \text{m/min}\): with heating to \(T = 125^\circ\) (Fig. 6), the chip shrinkage decreased to \(\zeta_1 = 3.2\).

With a further increase in temperature (Figs. 6, 7, 8), the shrinkage correspondingly decreases:

As is seen from the experimental data, chip shrinkage invariably decreases with an increase in the temperature of preheating (Figs. 6, 7, 8) and in the cutting temperature (Figs. 9, 10, 11, 12). Equality of shrinkages is preserved when the temperatures on the front face of the tool are equal, regardless of the way in which this temperature was obtained. From the experimental graph (Fig. 13) it is seen that the force \(P_z\) at \(T = 500\text{–}600^\circ\) and \(V = 3\ \text{m/min}\) has a smaller value in comparison with the force \(P_z\) at \(V = 50\ \text{m/min}\) and the temperature on the front face corresponding to this speed (Fig. 14).

Fig. 8

Fig. 9

Thus, at a cutting speed \(V = 50\ \text{m/min}\), but without preliminary heating of the metal in the cutting zone, i.e., at \(T = 0^\circ\), the force \(P_z = 171\ \text{kg}\). In another case, chip removal was carried out at a cutting speed \(V = 3\ \text{m/min}\), i.e., at \(T_v = 0^\circ\), but with preliminary artificial heating of the metal.

Fig. 10

ahead of the cutter and in the cutting zone to \(T = 550\text{–}600^\circ\). Under these conditions we obtained a cutting force \(P_z = 156\) kg. Thus, at identical temperatures on the rake face of the cutter, in the one case and in the other we obtained

Fig. 11

different cutting forces. We then cut chips at variable cutting speeds and temperatures of preliminary heating of the metal ahead of the cutter and in the cutting zone, and at \(V = 50\) m/min obtained the following values.

Fig. 12

At \(T = 225^\circ\) (Figs. 15, 16), the force \(P_z = 142\) kg and, respectively:

\[ \begin{aligned} \text{at } T &= 300^\circ \qquad & P_z &= 132\ \text{kg},\\ \text{at } T &= 400^\circ \qquad & P_z &= 124\ \text{kg},\\ \text{at } T &= 500^\circ \qquad & P_z &= 114\ \text{kg},\\ \text{at } T &= 600^\circ \qquad & P_z &= 106\ \text{kg},\\ \text{at } T &= 700^\circ \qquad & P_z &= 95\ \text{kg}. \end{aligned} \]

Fig. 13

As can be seen, the cutting force \(P_z\), with an increase in temperature due to artificial heating, decreases steadily. The value of the cutting force \(P\) also decreases with increasing cutting speed.

Fig. 14

Thus, we have arrived at an extremely important conclusion: when the temperatures on the front face of the cutter are equal, the cutting force \(P_z\) does not remain equal.

Fig. 15

Let us note that the cutting force \(P_z\), like shrinkage, decreases steadily with an increase in the temperature of preliminary artificial heating \(T^\circ\) and the cutting temperature \(T_v^\circ\).

Fig. 16

The same is observed with the change in stress in the shear plane. From the graphs (Figs. 17, 18) it follows that the maximum values of the stress \(\sigma_s\) correspond to the cutting speed \(V = 50\ \text{m/min}\) and to a preheating temperature \(T = 550\text{—}600^\circ\); that is, at identical temperatures on the front face of the cutter, the stress in the shear plane during chip removal without preliminary heating of the material in the cutting zone is greater than with heating.

Fig. 17

How, then, is the inequality of the cutting forces and the stress in the shear plane to be explained when the shrinkages and friction coefficients are equal, if the temperatures in both cases on the front face are the same?

In the first case, i.e., with artificial heating of the metal in the cutting zone, the work expended on converting mechanical energy into heat is supplied by an external source outside the experimental setup.

Fig. 18

In the second case, in order to provide the corresponding heating temperature of the metal in the cutting zone, it is necessary to perform additional mechanical work. On this basis, one may write the equality:

\[ A_n = (A_v - A_T) - A_p, \tag{2} \]

where \(A_n\) is the work expended outside the experimental setup on converting electrical energy into heat; \(A_v\) is the work expended on converting mechanical energy into heat during chip removal without preliminary heating of the metals in the cutting zone; \(A_T\) is the work expended on converting mechanical energy into heat during chip removal from a preliminarily heated blank in the cutting zone; \(A_p\) is the work associated with losses along the path from the source of electrical energy to the heating ...

device (inductor) inclusive. Consequently, in order to bring the temperature in the cutting zone up to the temperature of preliminary heating, we would have had, on the basis of the law of conservation of energy, to expend additional work on converting mechanical energy into heat; moreover, in magnitude this work must correspond to the work \((A n)\) that was expended on converting electrical energy into heat during the preliminary heating of the metal in the cutting zone.

Conclusions

We explain the essence of high-speed cutting as follows:

1. By a considerable reduction in the absolute value of the resultant force \(R\) and by a change in its direction under the action of the coefficient of friction \(\mu\), as well as by a lower value of the normal force, which acts on the force \(R\) in the direction of reducing it.

As a result of the shearing of the elements of the chip being formed relative to the base metal at the moment of its formation, intense heating takes place. The chip, between the plane of shear and the plane parallel to it and passing through the terminal point of contact of the chip with the cutter, retains in its mass the temperature acquired in the cutting zone. But at the same time the chip, moving along the rake face, is additionally heated to an insignificant depth and, in this way, a very thin layer of more softened chip metal is created between the chip and the cutter.

The presence of such a layer between the chip and the cutter changes, through the coefficient of friction, the direction of the resultant force \(R\) that compresses the chip.

Summarizing what has been set forth, one may say that in the cutting zone the mechanical properties of the chip metal change at the moment of its formation. And on the rake face conditions are created that determine the magnitude and direction of the resultant force \(R\) compressing the chip and determining the position of the shear plane relative to the cutting plane.

1. By the red-hardness and wear resistance of the hard alloy of the cutting tool.

2. With an increase in cutting speed above \(V = 50\ \text{m/min}\), or in the temperature of preliminary heating of the metal in the cutting zone above \(T = 550\text{—}600^\circ\), all cutting parameters—the chip shrinkage in length \(\zeta_l\), in cut width \(\zeta_a\), the cutting force \(P_z\), the coefficient of friction \(\mu\), and the stress in the shear plane \(\sigma_s\)—decrease sharply.

In conclusion, it may be said that the struggle for the further development of high-speed cutting is a struggle:

a) to reduce the coefficient of friction between the chip and the rake face of the cutter,

b) to reduce the normal force \(N\) acting on the rake face of the cutter,

c) to create new grades of alloys for cutting tools possessing higher red-hardness,

d) to improve the design and geometry of cutting tools.

Literature

1. E. M. Fel’dshtein. On the question of the essence of steel cutting. Stanki i instrument, No. 12, 1950.
2. A. M. Rozenberg. Stanki i instrument, No. 10, 1949.
3. A. M. Rozenberg. Stanki i instrument, No. 9, 1951.

1. Specific temperature zones are meant, within which the coefficient of friction decreases. ↩