INVESTIGATION OF THE METAL CUTTING PROCESS USING CINEMATOGRAPHY

Yu. A. Rozenberg, I. B. Filipchenko

(Presented by Prof. Dr. A. M. Rozenberg)

A number of works carried out recently on the investigation of the process of cutting brittle metals (for example, gray cast iron) [1, 2, 3, 4, 5] and others have shown that this process has certain regularities: during cutting, shear-type chips are formed. The cutting process is a process of brittle shear deformation; a built-up edge forms on the tool; the cutting forces, friction coefficients, geometry of the built-up edge, the angle determining the direction of the shear plane, etc., are in a definite dependence on the parameters of the cutting regime, and so on. All this has been determined by measuring cutting forces, observing the external form of the chip, observing under a microscope the zone of chip formation, etc. At the same time, it would be of interest to carry out an investigation of the process of formation of individual chip elements, their deformation and fracture, using the method of cinematography, which would make it possible to record the individual stages, their sequence, and the regularity of the development of an element and, accordingly, to introduce corrections into the theory of this process. Such an investigation is being conducted in the metal-cutting laboratory of the Tomsk Polytechnic Institute. Filming was carried out both when cutting “brittle” metals (gray cast iron, malleable cast iron, brass LS-59-1) and when cutting “ductile” metals (steel 45, steel 20Kh, copper, aluminum alloy). The investigation is divided into two parts. The first part includes experiments at medium cutting speeds from 5 to 60 m/min. Cutting was performed both with high-speed steel tools and with tools fitted with VK-8 hard-alloy inserts. When cutting copper and aluminum alloy, tools with a rake angle of \(+25^\circ\) were used (such geometry was necessary to obtain continuous chips). All the other metals were machined with tools having rake angles of \(+10^\circ\)—\(11^\circ\). The thickness of the cut varied within the range from 0.22 to 0.67 mm/rev. This series of experiments was carried out on a rigid lathe using an SKS-1 high-speed motion-picture camera, which permits filming at a rate of up to 5300 frames per second. Filming was performed with a 3–9-fold magnification on the film. The workpiece, a round blank, was fastened in the chuck of the machine. At the end of this blank a rib 3 mm thick and not more than 10 mm high was turned. The tool with a holder of \(20 \times 30\) mm was clamped with minimum overhang in the carriage, and cutting was carried out with transverse feed. To ensure joint

displacement of the cutter and the motion-picture camera; the latter was also mounted on the carriage. Illumination was provided by point-type projectors used in IKT vertical optimeters. Three such projectors, with 50-watt, 12-volt lamps inserted in them and supplied with a voltage of 14 volts, made it possible to film on panchromatic DN-type film at a speed of 5000–5300 frames per second and with a corresponding exposure time of 0.00004–0.000038 s.

The second part of the experiments was carried out at microspeeds from 0.0093 to 0.76 m/min on a universal milling machine by the method of free planing. Filming was performed with an operator’s motion-picture camera at a frequency of 8 to 32 frames per second on 35-mm film. Simultaneously with the filming of the cutting process, the cutting forces were recorded on an oscillograph film. The oscillograph was switched on, through a special electrical circuit, together with the illumination of the filming zone, while the motion-picture camera was switched on with a certain lead. In this way the synchronism of the recording of the forces and the filming of the cutting process was ensured.

For measuring the cutting forces, a two-component elastic dynamometer with inductive sensors was used.

The cutting zone was illuminated by the same projectors as in the first part of the experiments; however, their number was reduced to two, and the voltage supplied to the lamps was lowered to 10 V. In the experiments, high-speed steel cutters with rake angles from \(+30^\circ\) to \(-11^\circ\) were used. The chip thickness was varied within the range from 0.1 to 1 mm.

As a result of the experiments carried out, a considerable number of motion-picture films were obtained with frames recorded on them of the chip-formation process when cutting various metals under various conditions. Figs. 1–6 show frames of the chip-formation process in cutting LS-59-1 brass, malleable cast iron, and gray cast iron, which belong to the category of “brittle” metals. An analysis of these figures permits the following observations.

1. When cutting brass, malleable cast iron, and gray cast iron, the chip-formation process proceeds identically both at microspeeds and at medium speeds. The chip has an elemental structure. From the side, an element has the shape of a trapezoid or a convex triangle. One side of the element is in contact with the rake face of the tool, while the other is formed during the final shear of the element.

2. During the formation of an element, its plastic deformation takes place and its shape changes. The plastic deformations occurring inside the element do not end in the destruction of the element. Chip elements of gray and malleable cast iron and of brass have sufficient strength. In order to destroy them, considerable forces must be applied. Elements from all these metals permit hardness measurements to be made on them with a Vickers instrument under a load of 5–10 kg.

The optical system used in the investigation made it possible to film with a very small depth of field (0.2–0.5 mm); nevertheless, an actually equally sharp image was obtained both of the workpiece being machined and of the chip. This fact indicates that the plastic deformations during the formation of a chip element proceed in planes parallel to the cutting plane.

1. When a certain degree of deformation is reached, the deformed volume lying ahead of the cutter separates along a surface (the shear of the element occurs), which even at low cutting speeds may be taken, with a small approximation, as a plane. This plane

Fig. 1. Motion-picture frames of the cutting process for malleable cast iron at \(V = 25\ \text{m/min}\),
\(a = 0.6\ \text{mm}\), \(\gamma = +10^\circ\).

Fig. 2. Motion-picture frames of the cutting process for malleable cast iron at \(V = 13\ \text{m/min}\), \(S = 0.35\ \text{mm/rev}\), \(\gamma = 0^\circ\).

Fig. 3. Motion-picture frames of the cutting process of gray cast iron at \(V = 98\ \text{m/min}\), \(a = 0.2\ \text{mm}\), \(\gamma = +11^\circ\).

Fig. 4. Motion-picture frames of the cutting process of gray cast iron at \(V = 30.5\ \text{m/min}\), \(S = 0.35\ \text{mm/rev}\), \(\gamma = +10^\circ\).

Fig. 5. Motion-picture frames of the cutting process of brass at \(V = 9\ \text{m/min}\), \(a = 0.4\ \text{mm}\), \(\gamma = 6^\circ\).

goes at a certain angle $\beta_1$ to the direction of the cutting speed and, as follows from the frames presented above, occurs in the cutting of all the metals investigated. The angle at which the final fractures of the elements pass, in cutting a given metal and under the given unchanged cutting conditions, has a definite constant value. When the cutting conditions are changed (cutting speed, rake angle, etc.), the angle $\beta_1$ changes.

1. The near-tool layer of the chip, owing to its braking by the rake face of the tool, lags behind the overlying layers. This fact is not consistent with the data of S. P. Tambovtsev [6], who asserts that the near-tool layers of the chip flow considerably faster than the upper ones, and explains the curling of the chip by this. It must be noted that the authors observed the braking of the near-tool chip layers in the cutting of all the metals investigated.

Fig. 6. Motion-picture frames of the process of cutting brass at
$V = 10\ \text{m/min},\ S = 0.35\ \text{mm/rev},\ \gamma = 0^\circ$.

Thanks to this braking, an additional deformation is created in the near-tool layer, which under certain conditions leads to the formation of cracks in this layer (Fig. 7).

Summarizing all the above, one may conclude that the process of chip formation in cutting brittle metals has certain regularities, and that this process is a process of plastic shear deformation. S. V. Egorov [7], studying the process of cutting steel 45, heat-resistant steel, and gray cast iron with the aid of an SKS-1 high-speed motion-picture camera, came to the conclusion that the cutting process is a process of constrained plastic compression. One cannot agree with this assertion, because the deformation of plastic compression is a three-dimensional deformation, whereas the deformation in the cutting process, as is evident from our experiments, is a plane deformation. Plane deformation can only be simple shear. Exactly the same result was also obtained by G. L. Kufarev [8], but using another method.

S. S. Nekrasov [9] has studied the cutting process by the method of high-speed cinematography. On the basis of these investigations he asserts that, in cutting gray cast iron, under the action of the rake

...of the cutter edge, compression of the layer of metal lying ahead takes place. When the compressive stresses reach the maximum permissible value, a chip element of a very irregular shape breaks out. The separation of the chip in gray cast iron occurs predominantly along carbon inclusions, i.e., along the most weakened places in the metal. This also explains the very irregular shape of the chip elements and the unevenness of the machined surface. At the same time, the author points out that plastic deformation is absent when cutting gray cast iron. Summarizing these propositions of S. S. Nekrasov, it may apparently be said that there are no regularities in cutting gray cast iron. And indeed, if the chip breaks off along weakened places, then one cannot speak of a plane of maximum tangential stresses; consequently, there are no regularities in the change of cutting forces with changes in cutting conditions, tool geometry, or the mechanical properties of the metal being machined. However, as is evident from the material presented above and from a number of other works [1], [2], [3], [4], when cutting gray cast iron there is plastic shear deformation, and the cleavage of elements occurs along a plane directed at a definite angle to the direction of cutting speed.

Fig. 7. Motion-picture frames of the process of cutting steel 45 at \(V = 9\) mm/min, \(a = 0.4\), \(\gamma = +30^\circ\).

Of course, graphite inclusions in cast iron make this metal nonuniform, disrupt its continuity, and to some extent distort the regularities of the process of its cutting, but only to a certain extent. On the whole, on the basis of a large number of tests, these regularities are clearly revealed.

When considering the results obtained in our investigations, doubts may arise as to whether frames of the chip-formation process taken from the side surface of the metal reflect the actual phenomena occurring inside the metal. However, experiments carried out with simultaneous filming of the process and recording of the cutting forces make it possible to consider that the phenomena observed on the side surface generally coincide with the phenomena occurring inside the metal. Thus, when the filmed frames of the chip-formation process are compared with the curves of the change in cutting forces, it is evident (Fig. 8) that during the formation of an element the force \(P_z\) increases, reaches a maximum, and at the moment of the observed cleavage drops sharply.

In addition, during high-speed filming with the SKS-1 motion-picture camera, a time marker was used, which made it possible to determine the path of formation of a single chip element. This path of formation of a single element can be related to the magnitude of the final shear angle \(\beta_1\). The smaller the angle \(\beta_1\), the greater the path of formation of the element must be. This proposition was confirmed by the experiments carried out.

In conclusion, it is necessary to note that the method of motion-picture filming in investigating the metal-cutting process does indeed make it possible to reveal a number of features of this process. Thus, for example, it is known that when machi...

formation on the rake face of the tool, the effective rake angle of the tool increases and the cutting forces decrease. However, V. B. Livshits, investigating the dynamics of turning an aluminum alloy, found that during built-up-edge formation the cutting forces increased. Cinematographic recording of the cutting process of an aluminum alloy made it possible to determine that in this case the built-up edge on the cutter has a peculiar

Fig. 8. Oscillograms of forces during cutting
a) malleable cast iron: \(V = 9\ \text{mm/min}\), \(a = 0.7\ \text{mm}\),
recording speed \(4\ \text{mm/sec}\);
b) gray cast iron: \(V = 9\ \text{mm/min}\), \(a = 0.5\ \text{mm}\), \(\gamma = +11^\circ\), recording speed
\(10\ \text{mm/sec}\).

shape and reduces the rake angle of the tool (Fig. 9), as a result of which the cutting forces also increase.

As was also the case for a number of other investigators [7], [6], [10], we succeeded in filming the process of built-up-edge formation during the cutting of metals. Along

Fig. 9. Motion-picture frames of the aluminum-cutting process.

with this, it is nevertheless necessary to note that the results obtained in investigating the cutting process by the method of cinematography always require careful verification, since phenomena occurring on the side surfaces of the chip, the tool, etc., are recorded here.

S. P. Tambovtsev, on the basis of his investigations [6], drew the general conclusion that “the process of formation of each chip element

is not a monotonic process; on the contrary, it proceeds nonuniformly. A chip element, regardless of whether the chip is continuous, segmented, or elemental, is formed periodically from separate crystal blocks (grains)—“subelements.” The cutting of comparatively soft metals (steel 10–15, high-strength cast iron, etc.) is accompanied by the formation of a pressing lump, replacing the cutting edges. As it grows, the lump is positioned on the front face of the cutter in the form of a “cantilever.”

Fig. 10. Motion-picture frames of the copper-cutting process.

It is impossible to agree with all these propositions of the author. First, in all the frames we obtained of the chip-formation process in metal cutting, no “subelements” were found. And in the records obtained of the forces during cutting at microspeeds, where any change in force is clearly registered, it can be seen (Fig. 8) that the forces during the formation of a single element change monotonically, without jumps. Second, the terms “lump” and “cantilever” are not clear to us. If by “lump” one means a built-up edge, a buildup, or, in general, a retarded zone, then these phenomena can be observed in motion-picture filming. We were unable to detect any other “lumps” in our frames. And in the formation of a continuous chip, the cutting process proceeds altogether monotonically. This is clearly seen from Fig. 10, which presents a photographed frame of the copper-cutting process.

REFERENCES

1. Yu. A. Rosenberg. Investigation of the cutting process of gray cast iron. Author’s abstract of dissertation, Tomsk, 1952.

2. A. M. Rosenberg, A. N. Eremin. Elements of the theory of the process of cutting metals. Mashgiz, 1956.

3. P. F. Kiselev. Investigation of deformation during the cutting of cast iron. Author’s abstract of dissertation, Moscow, 1949.

4. A. Ya. Artamonov. Investigation of the machinability of high-strength cast iron, Mashgiz, 1955.

5. I. N. Zorev. Investigation of elements of the mechanics of the cutting process, Mashgiz, 1952.

6. S. P. Tambovtsev. Investigation of the process of cutting metals by high-speed motion-picture filming. Stanki i instrumenty, No. 2, 1957.

7. S. V. Egorov. Investigation of the chip-formation process by the method of high-frequency motion-picture filming. Vestnik mashinostroeniya, No. 11, 1953.

8. G. L. Kufarev. Experimental study of plastic deformation during the cutting of metals. Investigations in Solid-State Physics, Academy of Sciences of the USSR, 1957.

9. S. S. Nekrasov. Formation of chips during the cutting of materials. Scientific Transactions of the Moscow Mining Institute, Collection No. 21, issue 1, 1957.

10. K. F. Romanov. Investigation of the chip-formation process at high cutting speeds by means of high-speed motion-picture filming. Abstracts of papers of the interuniversity scientific conference on contemporary problems in metal cutting, Tbilisi, 1958.