INVESTIGATION OF CONTACT PROCESSES ON THE RAKE FACE OF A TOOL DURING CUTTING OF LS-59-1 BRASS

V. I. Karnov

(Presented by the scientific seminar of the departments of machine tools and metal cutting and mechanical-engineering technology)

The chip formed during metal cutting is, over a certain length, in contact with the rake face of the tool. The question of the influence of various factors and cutting conditions on the size of the contact areas is of fundamental importance in studying the dynamics of the cutting process, and therefore is an object of investigation by many authors.

In the metal-cutting laboratory of TPI, dynamic experiments were carried out in cutting low-ductility LS-59-1 brass. Cutting was performed with VK-8 carbide cutters of various geometry \((\gamma=-13^\circ,\ -2^\circ,\ 16^\circ,\ 30^\circ)\) over broad ranges of variation of the cutting conditions:

cutting speed — \(V=0.2\text{—}550\ \text{m/min}\);
feed — \(S=0.07\text{—}0.61\ \text{mm/rev}\);
depth — \(t=1\text{—}5\ \text{mm}\).

Direct observation through a microscope of the cutting edges of the tool made it possible to establish the width of the contact between the chip and the rake face of the cutting tool. The contact area of the chip with the rake face of the tool was determined as the product of the contact length by its width. The contact length was taken as equal to the width of the layer of metal being cut; the contact width was measured from the traces of adhesion of the work material in the grooves of the scratches on the rake face of the tool. In individual cases, for clearer identification of the contour of the contact area, a thin layer of ink was applied to the rake face of the tool. When working with such carbide inserts, the flowing chip quickly removed the applied ink layer on the contact portion of the rake face, and its contour became clearly revealed. Simultaneously with the contact width, the length of the sliding line \(AB\) was read off, and the ratios

\[ \frac{c}{a} \quad \text{and} \quad \frac{AB}{c}, \]

where \(a\) is the chip thickness, were determined.

Figure 1 presents curves of the variation in the width of contact of the chip with the rake face, as well as curves of variation of the ratios \(\frac{c}{a}\) and \(\frac{AB}{c}\), as functions of feed and the rake angle of the tool. From the arrangement of the curves it is evident that, with increasing feed, the width of chip contact ...

Fig. 1

...with the rake face of the tool increases almost in proportion to the feed. In this case the ratio \(\frac{c}{a}\) decreases, while \(\frac{AB}{c}\) increases, which indicates a slower change in the contact width than in the length of the slip line and the thickness of the layer being cut. With an increase in the tool rake angle, the contact width \(c\) decreases, and more strongly than the length of the slip line \(AB\), as a result of which the ratio \(\frac{AB}{c}\) increases. At the same time, for a constant cutting thickness the ratio \(\frac{c}{a}\) decreases.

Having from the experiments the cutting forces measured by a dynamometer, one can calculate the magnitude of the friction force \(F\) and the normal force \(N\) [1]. Having determined the forces \(F\) and \(N\) and the area \(f\) of contact of the chip with the rake face of the tool, one can determine the magnitudes of the tangential and normal stresses:

\[ \tau_F=\frac{F}{f}, \qquad \sigma_N=\frac{N}{f}. \]

Thus, the values of the coefficient of friction can be expressed as the geometric ratios of the tangential and normal forces, or of the corresponding stresses in the friction zone on the rake face of the cutting tool,

\[ \mu=\frac{F}{N} \quad \text{or} \quad \mu=\frac{\tau_F}{\sigma_N}, \]

whence

\[ \tau_F=\mu\cdot\sigma_N. \]

The physical meaning of the expressions given, as well as the nature of the change in the coefficient of friction under various cutting conditions and regimes, can be explained if the causes and nature of the change in the forces \(F\) and \(N\), or in the stresses \(\tau_F\) and \(\sigma_N\), under the influence of various factors of the cutting process are established. Friction in the cutting process takes place under conditions of very high pressures and temperatures on the rubbing surfaces. A specific feature of the friction process in metal cutting is also that the normal stresses on the rake face are distributed nonuniformly, decreasing sharply with distance from the cutting edge of the tool. The friction force arising between the chip and the rake face of the tool causes additional, secondary deformation of the chip in the near-edge region and affects the course of the cutting process as a whole.

Table 1

Fig. 2

Experiments in cutting LS-59-1 brass, carried out over a wide range of changes in cutting conditions and tool geometry, yielded quite definite results that make it possible to speak about the values of the coefficient of friction during cutting and about the nature of its change with changes in the cutting conditions.

According to the experimental data, Table 1 gives the values of the tangential and normal stresses \(\tau_F\) and \(\sigma_N\) and of the coefficient of friction \(\mu\). The dependence of \(F\) and \(N\) on the imprint width \(c\) and the rake angle \(\gamma\) is shown in Fig. 2. With an increase in feed (contact width), both the friction force \(F\) and the normal force \(N\) increase in direct proportion to the chip thickness. With an increase in the rake angle, the friction force increases, while the normal force decreases. The changes in the values of \(\tau_F\) and \(\sigma_N\) have the same character as a function of the rake angle.

As a result, the ratio of the forces \(\frac{F}{N}\) and of the stresses \(\frac{\tau_F}{\sigma_N}\), which represents the coefficient of friction, increases with increasing rake angle. The fact that the mean normal pressure \(\sigma_N\) increases with increasing chip thickness and decreasing rake angle is indirectly confirmed in the work of M. I. Klushin and M. B. Gordon [2]. The average co-

Fig. 3

The coefficient of friction when cutting brass is practically independent of the thickness and width of the layer being cut. According to N. N. Zorev [3], the coefficient of friction usually decreases with an increase in chip thickness in the presence of a stagnant zone. The constancy of the average coefficient of friction when the chip thickness changes is observed in the absence of a built-up edge on the rake face, which is what we have in cutting brass.

The coefficient of friction depends on the normal pressure and on the relative sliding velocity of the rubbing surfaces. In this connection, at constant velocity the coefficient of friction is in a one-to-one relationship with the normal pressure in the contact zone of the chip with the tool. Figure 3 presents the relationship between the coefficient of friction and the normal stress, from which it is evident that $\mu$ is completely determined by the magnitude of the average normal stress over the area of contact of the chip with the rake face of the tool.

LITERATURE

1. A. M. Rosenberg, A. N. Eremin. Elements of the Theory of the Process of Cutting Metals. Mashgiz, Sverdlovsk, 1956.

2. M. I. Klyushin. Cutting of Metals. Mashgiz, Moscow, 1958.

3. N. N. Zorev. Problems in the Mechanics of the Process of Cutting Metals. Mashgiz, Moscow, 1956.