FEATURES OF THE CUTTING PROCESS OF PRE-DEFORMED LS-59-1 BRASS

V. I. Karnyuk

(Presented by the Joint Scientific Seminar of the Departments of Machine Tools and Metal Cutting and Mechanical-Engineering Technology)

The influence of prior deformation of a metal on the principal—indeed, the most important—characteristics of the cutting process is a new question in the theory of metal cutting and is of unquestionable practical and theoretical interest. Indeed, while in sections concerning the “universal” problems of the cutting process quite clear and unshakable propositions have been developed, very little has so far been done on this question.

In any form of machining metals by cutting, the cutting tool converts into chips metal that has previously undergone some degree of deformation. This circumstance is not taken into account in most cases, although a number of researchers [1, 2] point out that the cutting tool always removes a previously deformed, work-hardened layer of metal located directly beneath the layer being cut off. Not only the layer of metal being removed is subjected to severe plastic deformation, but also the layer located below the cutting line. At first this layer undergoes tensile deformation, and then, as the cutter passes over it under pressure, compressive deformation. In this process strengthening, or work hardening, of the surface layer of the metal being machined occurs.

The work-hardened layer differs in its physicomechanical properties from the properties of the base metal being machined; the properties and structure of the work-hardened metal layer change. The change in structure amounts to the fact that the metal grains are crushed into finer particles, internal stresses arise in the grains, the crystal lattice is distorted, and, finally, a certain definite orientation of the crystals, or texture, is formed. The physicomechanical properties of the metal being machined and its condition largely determine the chip-formation process, the deformations accompanying it, and, consequently, the resistance forces.

In our investigations we set ourselves the task of determining how prior deformation of the metal affects the cutting process of low-ductility brass. In our experiments, both ordinary and preliminarily compressed LS-59-1 brass were subjected to turning. Two degrees of compression were carried out. At the first degree of compression, a specimen of height \(h_0 = 90.5\) mm was compressed to the size \(h = 72.5\) mm, which corresponds to the ratio

\[ \frac{h_0}{h} = 1.25 \]

and to the value of the relative

shear strain \(\varepsilon = 1.5 \ln \dfrac{h_0}{h} = 0.335\). The second time, the specimen was compressed to a height of \(h = 60\) mm; the ratio \(\dfrac{h_0}{h}\) was then equal to 1.5, and the relative shear was \(\varepsilon = 0.62\). Cutting of ordinary and preliminarily deformed brass was carried out with a single setting of the cutter, in order to eliminate the influence of random factors in measuring the cutting forces. Simultaneously with measurement of the cutting forces, the contact area of the chip with the rake face of the cutting tool, the path length of formation of one element, and the shear angle \(\beta_1\) were measured; the distribution of microhardness in the cutting zone and in the removed chip was also studied.

As a result of the investigation, a quite definite regularity was obtained: the cutting forces decrease with an increase in the degree of preliminary deformation. Measurement of the shear angle in the cutting process showed that the magnitude of this angle increases with increasing degree of deformation of the metal, while the length of contact of the forming chip with the rake face of the tool decreases. The values of the angles \(\omega\) and \(\eta\), of the tangential and normal stresses, and of the coefficient of friction on the rake face of the tool, calculated from known formulas, are given in the following table.

Table 1

As can be seen from the table presented, with an increase in the degree of preliminary deformation of the brass, with an increase in the shear angle \(\beta_1\), the values of the inclination angle of the resultant \(\omega\), the friction angle \(\eta\), and the coefficient of friction \(\mu\) decrease. The tangential and normal stresses on the rake face of the cutting tool also decrease. Calculation of the tangential stresses in the shear plane showed that their magnitude does not depend on the degree of preliminary deformation of the metal and is numerically equal to the tangential stress in the shear plane when cutting ordinary brass [3]. Since the tangential stresses remain constant, it is obvious that, for preliminarily compressed brass, the increase in the shear angle \(\beta_1\) leads to a decrease in the cutting forces.

Physically, this phenomenon can apparently be explained as follows. Deformed brass has an increased hardness in comparison with the original metal, and, as is known, a change in hardness is associated with a change in the mechanical properties of the metal. The greater the degree of preliminary deformation, the less ductile the metal becomes. In a less ductile metal, the deformation in the process of formation of an element is smaller, and the shape of that element approaches a triangular one more closely than a trapezoidal one (accordingly, the deformation zone decreases). Because the shear angle \(\beta_1\) increases with increasing degree of preliminary compression while the tangential stress in the shear plane remains unchanged, the work of shear deformation decreases, and this leads to a reduction in the cutting forces.

The reduction of the cutting forces with increasing degree of preliminary deformation (a decrease in the shear area at constant tangential

stresses) can also be explained in the following way. As is known, every process of plastic deformation is shear in nature, and therefore it is characterized by lines of maximum tangential stresses, that is, by shear lines. From the boundary conditions

Fig. 1.

on the free surface of the chip, where external loads are absent, it follows that all shear lines must make identical angles with the tangents to this surface, equal to

\[ \frac{\pi}{4}. \]

To satisfy this condition, Fig. 1 gives a qualitative explanation of the increase in the shear angle \(\beta_1\) with an increase in the degree of preliminary deformation. In the indicated figure, the numeral 1 denotes the contour of the free surface of the chip of ordinary brass, the numeral 2 denotes that for brass having the first degree of compression, and the numeral 3 denotes that for the next stage, when the metal becomes most brittle. The shear lines (for simplicity they are taken as straight) must be inclined at different angles \(\beta_i\) to the cutting speed, so that with the tangents to the surfaces of their contours they form an angle of \(45^\circ\). From Fig. 1 it is clearly seen that the angle \(\beta_1\) increases with increasing plastic deformation from specimen 3 (the most deformed) to specimen 1 for ordinary brass:

\[ \beta_1^3 > \beta_1^2 > \beta_1^1. \]

When cutting ordinary brass, the length of the path for the formation of one element (the pitch of the elements) does not depend on the cutting speed, but increases sharply with increasing feed and less intensively with increasing tool rake angle (Fig. 2). As experiments have shown, when cutting preliminarily compressed brass, the pitch of the elements decreases with increasing degree of deformation (Fig. 3). Shown here are the dependences of the element pitch on the degree of preliminary deformation for a tool with rake angle \(\gamma = 13^\circ\). This situation is in complete agreement with the data obtained by M. E. Itkin [4], who established that ductile metals give a greater increase in element pitch with increasing feed. Thus, for steel EI 437, M. E. Itkin obtained an element pitch 2.5 times smaller in size than for steel 45.

As for the hardness of the chips and of the lateral surface of the cutting zone, their measurement shows that the shape and dimensions of the stressed zone during cutting of preliminarily deformed brass completely coincide with those for ordinary brass. In this case as well, the stressed zone, revealed by measuring microhardness on an instru-

Figure 2.

Figure 3.

PMT-3 indenter, encompasses a volume of metal that is located not only beyond the shear line, but also below the cutting line. The maximum hardness occurs in the zone of final shear of the element, and it is equal to the hardness of the deformed specimen in compression tests at the moment of fracture of that specimen.

All this once again confirms that, when low-plasticity LS-59-1 brass is cut, it acquires in the zone of final shear its limiting strain hardening, independent of the cutting conditions, the tool geometry, or the preliminary treatment of the metals.

REFERENCES

1. V. S. Kudryavtsev. The Influence of Lubrication on the Process of Cutting Copper. Mashgiz, 1951.

2. A. N. Eremin. The Physical Nature of Phenomena in the Cutting of Steels. Mashgiz, 1951.

3. V. I. Karnov, Yu. A. Rozenberg. Investigation of the process of cutting brass. Izv. TPI, vol. 133, 1965.

4. M. E. Itkin. Investigation of the influence of cutting conditions and tool geometry on the pitch of chip elements. Proceedings of the Kazan Aviation Institute, issue 52, 1960.