ON THE CONDITIONS FOR THE FORMATION OF SHEAR CHIPS

G. L. KUFAREV, M. G. GOLDSHMIDT, N. I. KALABUKHOV

(Presented by the scientific seminar of the Department of Machine Tools and Metal Cutting)

Cutting of almost all materials, depending on the conditions of the process, as is known, may be accompanied by the formation of both continuous chips and shear chips.

The principal external factor determining the type of chip formed is the friction force on the rake face. Thus, for example, the cutting of brass LS59-1, at constant rake angle and depth of cut, is accompanied—independently of cutting speed—by the formation of shear chips [1]. This is not difficult to explain if one takes into account that the coefficient of friction under these conditions remains constant.

When cutting soft transformer iron, an artificial reduction in the length of the contact area leads to a decrease in friction forces, as a result of which the so-called pseudo-segmented chip becomes continuous [2]. Even at a depth of cut \(a = 0.06\) mm, cutting steel 1Kh18N9T at microspeed \((\gamma = +8^\circ)\) is accompanied by the formation of shear chips. By using carbon tetrachloride as a lubricant, it is possible to obtain a continuous chip up to a depth of cut of \(0.35\) mm, which is due to a reduction in the friction force (from \(77\) kg to \(34.5\) kg).

The causes and conditions that promote destruction of the chip material are described in different ways. Some authors believe that shear chips are formed as a result of a decrease, under a corresponding change in cutting conditions, of the normal stresses in the shear plane and the consequent reduction in the plastic properties of the metal [3]. Others explain this phenomenon by the fact that “the metal in the zone of the final shear receives limiting strain hardening” [4], or that the energy of shear deformation reaches a critical value. Still others associate the destruction of the chip material with the formation, as a result of tensile stresses acting in the shear plane near the tool tip, of an advancing crack [5].

Numerous experiments on cutting materials differing in their properties show that brittle fracture (shearing) of the chip element is preceded by the appearance of an advancing crack. This phenomenon is expressed the more clearly, the higher the plastic properties of the metal being machined. An analysis of the stress state as a function of the depth of cut [6] showed that the opinion set forth in [3] cannot be regarded as sufficiently substantiated.

Because slipping begins with the formation of a crack, while the presence of a stress concentrator may lead to fracture not necessarily of a metal “strengthened to the limit,” the explanation of this phenomenon in [4] may be regarded, at least, as insufficiently complete. This is all the more so because, in compression tests of LS59-1 brass specimens, the maximum shear stress recorded before fracture reached \(34\text{—}35\ \mathrm{kg/mm^2}\), whereas during cutting the shear stress in the shear plane at the moment preceding its separation did not exceed \(27\text{—}28\ \mathrm{kg/mm^2}\).

Let us consider several characteristic phenomena established experimentally.

1. When zinc is cut at microspeeds, an increase in the chip thickness or a decrease in the rake angle leads to the formation of a leading crack, which is directed into the depth of the metal at an angle of \(15\text{—}30^\circ\) to the direction of motion of the tool. Subsequently it propagates along a definite trajectory, and chip elements are formed in the form of slightly deformed segments. With a chip thickness \(a = 1\ \mathrm{mm}\) and a rake angle \(\gamma = +35^\circ\), beginning with a speed \(V = 18\ \mathrm{m/min}\), an analogous picture is observed. Chips of the same type are also formed when plastics are cut.

2. When plastic metals are cut at microspeeds, it is possible to select such a regime in which a continuous chip is formed; however, small “scuff marks,” resembling “abandoned” cracks in shape, are visible on the machined surface. A similar picture was also observed under conditions of built-up-edge formation [5], but it cannot be regarded solely as a consequence of the influence of the latter.

3. At a certain stage in the formation of a shear-type chip element, when steel ShKh15 is cut at microspeeds, a crack arises which, as in cutting zinc, is directed into the depth of the specimen. However, fracture does not occur; the tool deforms a new volume of metal, leaving a serration on the machined surface. Then a new, deeper crack is formed, and so on, until finally one of them leads to the separation of a chip element. The profilogram of the machined surface, obtained on an instrument designed by M. F. Poletika and V. A. Naumov, very clearly illustrates the process described.

The interrelated phenomena described above can, in all probability, be explained as follows.

Under certain conditions of the cutting process, chiefly as a result of the influence of the friction force, such a stressed state is created at the tool tip that a crack arises. Here, apparently, tensile stresses play an important role, since the crack is usually directed into the depth of the material and has a clearly pronounced rupture character. It is reasonable to suppose that the known scheme of the stress field in the subsurface layer is thereby deformed in such a way that the boundary line between the zones of compressive and tensile stresses approaches the direction of the emerging crack. At the same time, the normal stresses at the tool tip are compressive [6]. Consequently, the conditions for crack initiation—the result of the action of tensile stresses—do not depend on the stressed state in the shear plane.

As noted, the shear plane and the direction of the crack that arises usually do not coincide. Therefore it can be shown that the presence of compressive normal stresses in the first case and tensile normal stresses in the second, near the tool tip, is not a contradiction.

According to N. N. Zorev’s chip-formation scheme, the cutter tip

is a singular point [7, 8]. This means that the hydrostatic pressure, and consequently the normal stresses as well, undergo a discontinuity at it, i.e., they are different for different segments of slip lines passing through the apex.

In practice this means that, if the cutter edge is traversed in a direction away from the rake face, the hydrostatic pressure at the apex changes abruptly from compressive to tensile. Therefore it seems possible that the ray coinciding with the direction of the crack corresponds to a positive hydrostatic stress.

The crack that has arisen may affect the chip-formation process in the following way.

1. If the resistance of the material to the action of tensile stresses in the presence of stress concentration is small, it fractures along a geometry predetermined by the stress state in the deformation zone. This can be observed in cutting zinc and plastics.

2. If, however, the stresses near the apex do not exceed the strength of the material and the latter, in the vicinity of the crack, is sufficiently plastic to “extinguish” its influence, the deformation of chip formation continues. Burrs remain on the machined surface.

3. In the case where the combined influence of the stress concentrator and the comparatively large degree of deformation of the chip exceeds the capacity of the strain-hardened material for further plastic deformation, fracture occurs—the element shears off. The crack then changes its direction, orienting itself in accordance with the stress state, in the direction of the shear surface.

REFERENCES

1. V. I. Karnov. Investigation of the cutting process of low-ductility brass. Dissertation, Tomsk, 1965.

2. K. I. Trigger, B. F. Turkovich. Chip formation in high-speed cutting of copper and aluminum. Design and Technology of Machine Building, IL, series B, No. 4, 1963.

3. M. Field and M. E. Merchant. Mechanica of Formation of the Discontinuous chip in Metal Cutting. Trans. ASME, vol. 41, 1949.

4. Metal Cutting and Tools, ed. by A. M. Rozenberg. Mashinostroenie, 1964.

5. M. Chandiramani, N. H. Cook. Investigation of the character of surface finish and its dependence on cutting speed. Design and Technology of Machine Building. “Mir,” vol. 86, series 8, No. 2, 1964.

6. M. G. Goldshmidt, G. D. Del, G. L. Kufarev. The stress state during the formation of a shear chip. Izv. TPI, vol. 139, 1965.

7. N. N. Zorev. Problems in the mechanics of the metal-cutting process. Mashgiz, 1956.

8. M. G. Goldshmidt. Investigation of the stress-strain state in the cutting zone. Author’s abstract of dissertation, Tomsk, 1966.