CHIP FORMATION PROCESS DURING CUTTING OF LOW-DUCTILITY BRASS

V. I. Karnov

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

Chip formation during the cutting of metals is a complex form of plastic deformation that includes both elements of shear deformation and elements of compression deformation. Most of the works carried out on the subject of chip formation concern the machining of ductile metals, in the cutting of which continuous chips are formed. At the present time, a large group of low-ductility metals is subjected to machining by cutting (cast irons, bronzes, brasses, titanium alloys), in the cutting of which continuous chips are not formed. Under certain conditions it is not possible to obtain continuous chips when cutting some grades of steels either, and also when machining ductile metals in cases where a layer of considerable thickness is removed at relatively low cutting speeds and small tool rake angles.

If, in the formation of continuous chips, the process of successive plastic shears is steady and continuous, then in the formation of segmented chips this process is periodic, cyclic. The duration of each cycle corresponds to the formation of one segment. As is known, the type of chip formed is influenced most strongly by the tool rake angle, the cutting speed, and the thickness of cut.

In our experiments, chips were obtained when cutting low-ductility brass over wide ranges of variation in cutting conditions:

cutting speed \(V = 0.2—550\ \text{m/min}\);

feed \(S = 0.07—0.61\ \text{mm/rev}\);

depth of cut \(t = 1—5\ \text{mm}\);

rake angle \(\gamma = -10 \div 45^\circ\).

In most cases, when cutting brass, segmented chips of trapezoidal or triangular form are produced. During the formation of a segment, its plastic deformation occurs, encompassing a certain volume of metal located both beyond the surface of the final shear of the segment and below the cutting line.

Plastic deformations in the segment do not lead to its destruction; it is an integral solid body, making it possible to measure its hardness. A study of the stress state in the cutting zone [1] shows that, in the general case, the deformation in the segment is plane, although in the initial period of segment formation, in front of the cutting edge of the tool, there is a small closed zone in which the deformation is in-

the second is volumetric, in which we have the greatest strengthening of the metal. Under the action of the friction force of the cut layer against the rake face, as well as of the normal force against the rake face, neighboring elements may adhere to one another. The side facing the rake face of the tool is smooth, polished. The chip is formed either in the form of an apparently continuous ribbon consisting of individual elements (Fig. 1), or as completely separated elements.

Fig. 1.

Owing to the periodicity of the process of formation of elemental chips, as experiments with circular imprints [1] have shown, the deformation in the formed element is distributed nonuniformly. In elemental chips, apparently, initial slip lines occur; however, in view of the considerable deformation of each element, the slip lines within it do not retain a constant direction.

An important task is to determine the conditions under which continuous chips are formed. With an increase in the rake angle and cutting speed, and also with a decrease in the thickness of the cut, the sliding of the elements becomes incomplete; neighboring elements remain connected to one another and form a more or less strong ribbon. However, in the resulting metal ribbon the individual elements are sharply delineated, and the entire chip has the appearance of a bent saw, as it were (Fig. 2). Further, the stepped form becomes less and less distinct, and the chip changes into a continuous ribbon possessing considerable rigidity. In such chips (Fig. 3) traces of the elements remain in the form of striations or small teeth on the outer surface of the chip.

Fig. 2.

Fig. 3.

A particularly great influence on the magnitude of plastic deformation, and hence also on the type of chip produced, is exerted by the rake angle of the tool. As the rake angle is increased, a gradual transition from elemental chips to continuous chips can be obtained. At a small rake angle, chip formation is accompanied by severe plastic deformation. With an increase in the rake angle, the chip element is deformed less, its shear is facilitated, and the removed layer

comes off in the form of a continuous ribbon. The phenomenon of transition from continuous chip to elemental chip with an increase in the thickness of cut is explained by a decrease in the ductility of the material being machined. With increasing thickness of cut, the comparatively “soft” conditions of deformation of the layer being cut change to more “severe” ones. As the investigations of S. I. Gubkin and O. V. Bridgman have shown, transition to more severe deformation conditions is always accompanied by a decrease in the ductility of the metal being machined. And if the reserve of ductility of the metal is insufficient, then at some stage of deformation, under the action of increasing stresses, it will begin to fail (to shear).

On the basis of the experiments carried out, a diagram is given (Fig. 4) for determining the type of chip formed as a function

Fig. 4.

of the tool rake angle and the feed. When cutting low-ductility brass, no substantial influence of cutting speed on the chip-formation process was found: a change in cutting speed by more than a factor of 25 does not affect the character of the chip formed.

It should also be noted that, in our opinion, the following circumstance is of interest: the pitch of the elements (the distance between the shear planes of two neighboring elements) also does not depend on cutting speed. A quite regular, sharp increase in the size of the element pitch is observed only as a function of feed; the element pitch increases with increasing tool rake angle. This is apparently explained by the more constrained state of the cutting zone for a negative rake angle than for a positive one, as a result of which, in the former case, less time is required to reach the limit at which brittle shearing of the element occurs.

LITERATURE

1. Metal Cutting and Tools. Edited by Prof. A. M. Rozenberg. Mashinostroenie, 1964.