Bulletin of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 147　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1966

Effect of the Hardness of ShKh-15 Steel on the Chip-Formation Process

N. I. Khovakh

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

At the present time, the turning of hardened steels with carbide tools has found rather wide application in place of the low-productivity grinding process. The present work is devoted to the study of the regularities of the cutting process for ShKh-15 steel hardened to different hardnesses.

The investigation was carried out on a model 163 engine lathe. Bushings were used as blanks for machining; their outside diameter, inside diameter, and length were selected so that they would be through-hardened throughout the entire volume of the blank. By appropriate heat treatment, bushings with hardness \(H_B = 200;\ 250;\ 300;\ 350;\ 400;\ 500\) were obtained. Hardness was measured with a Brinell press after every 2–3 passes. The blanks were mounted on the machine on a mandrel, which in turn was mounted in the chuck and in the built-in rotating center of the tailstock.

In the course of the experiment, all three components of the cutting force, the cutting temperature, and the length of the chip contact area with the tool were recorded, and chip shrinkage was calculated.

The cutting forces were measured by means of a low-inertia three-component elastic-electric dynamometer with inductive pickups. The force values were recorded by BB-662 self-recorders manufactured by the Kalibr plant; their electrical circuit had been slightly modified, which ensured the absence of zero drift and high stability of the records.

The dynamometer (Fig. 1) is mounted on the cross slide of the machine carriage and is a welded structure consisting of three main parts. The elastic elements of the dynamometer are ribs 1, 3 and crosspieces 2, whose deformation by the cutting-force components (respectively \(P_z, P_x, P_y\)) was recorded by pickups installed in a definite manner.

The cutting temperature was measured by the natural-thermocouple method. The length of the chip contact area with the tool was measured with a Brinell magnifier at 24× magnification and a scale division value of \(0.05\) mm. The experiments were carried out over a wide range of speeds from \(5\ \text{m/min}\) to \(180\ \text{m/min}\). Feeds varied from \(0.056\ \text{mm/rev}\) to \(0.708\ \text{mm/rev}\). The depth of cut remained constant and equal to \(2\) mm. Cutters with mechanically clamped T15K6 carbide inserts were used as the cutting tool. The tool geometry remained constant, with the exception of the rake angle \(\gamma\), whose values were taken as \(+10^\circ;\ 0;\)

—10°. The remaining geometric parameters of the tool were as follows: the main and auxiliary clearance angles \(\alpha=\alpha_1=8^\circ\), the main plan angle \(\varphi=75^\circ\), the auxiliary plan angle \(\varphi=15^\circ\), the inclination angle of the cutting edge \(\lambda=0^\circ\), and the radius of rounding of the tool nose \(r=0.3\) mm. Cutting was carried out with a sharply ground tool, with the tool dressed using car—

Fig. 1

Fig. 2.

bide boron after each experiment. The quality control of the cutting edges of the tool was performed with a Brinell magnifier.

In the experiments described below, chip shrinkage was determined by the weight method. Fig. 2 presents the dependence of chip shrinkage on cutting speed, obtained when turning ShKh-15 steel hardened to different hardnesses with a tool having rake angle \(\gamma=0^\circ\). The change in shrinkage

the chip shrinkage with increasing cutting speed proceeds along typical curves characteristic of the cutting of other metals that have not undergone heat treatment. This dependence of chip shrinkage on cutting speed is retained for all hardnesses, including the tested blanks having a rather high hardness, \(H_B = 500\). Substantial changes are undergone by the chip shrinkage depending on the hardness of the steel being machined. With increasing hardness the shrinkage decreases regularly, and this regularity is not disturbed over the entire range of change in cutting speed. The character of the change in chip shrinkage with hardness remains the same for all feed values within the range from \(S = 0.056\) mm/rev to \(S = 0.708\) mm/rev and does not change when cutting with tools having rake angles \(\gamma = 10^\circ\) and \(\gamma = -10^\circ\). Although chip shrinkage is connected with the plasticity of the material only indirectly, the decrease in shrinkage can nevertheless be explained by the decrease in the plasticity of ShKh-15 steel as its hardness increases.

Figure 3 gives the dependence of the length of the contact area between the chip and the tool on the cutting speed. As follows from the graph, the length of the contact area monotonically decreases with increasing cutting speed and increasing hardness of the steel.

Fig. 3

A comparison of the dependences \(\xi = f(v)\) and \(c = f(v)\) makes it possible to conclude that changes in chip shrinkage and in the length of the contact area are related to one another, and that this dependence and its character are preserved for all specimens having different hardnesses.

L. M. Reznitskii, in his work [2], speaking of the change in cutting forces with speed when turning an alloy chromium-nickel-molybdenum steel hardened to hardness \(H_{RC} = 49\) with a tool having rake angle \(\gamma = -5^\circ\), at \(t = 2.41\) mm and \(S' = 0.395\) mm/rev, writes that with an increase in cutting speed within the range from 5 to 50 m/min, “the force \(P_z\) decreases regularly, while at the same time the forces \(P_x\) and \(P_y\) do not change with the change in cutting speed.”

In our experiments (Figs. 4, 5, 6) we observed a regular change in all three components of the cutting forces \(P_z, P_x, P_y\) with speed

Fig. 4

Fig. 5

Fig. 6

cutting; moreover, the character of the dependence of the forces on speed does not change with a change in the hardness of the steel and does not differ from that known for turning thermally untreated steels.

The indicated regularity remains valid within the above-mentioned ranges of feeds and front rake angles of the tool.

Especially interesting, in our view, is the fact that a change in the hardness of the steel over such a wide range does not exert any substantial influence on the cutting forces.

In the above-mentioned work by L. M. Reznitskii, when the hardness of the steel changes from \(H_B = 146\) to \(H_B = 600\), the components of the cutting force \(P_x, P_y, P_z\) increase by factors of 9.9, 8.8, and 2.55, respectively. Since the cutting of annealed steels was carried out with a tool having a front rake angle \(\gamma = +20^\circ\), and that of hardened steels with an angle \(\gamma = -5^\circ\), the results are not generally comparable; however, the author explains such a large increase in force not only by the decrease in the front rake angle, but also by the higher hardness of the material being machined.

As our experiments have shown, the change in all components of the cutting forces with a change in hardness does not exceed 15%.

In the works of many researchers [3, 4, 5, 6] it has been shown that the cutting force depends not only on the dimensions of the chip cross section, the geometry of the cutting tool, and the strength characteristics of the material being machined, but is also a function of chip deformation, i.e., chip compression.

An increase in the hardness of steel obtained as a result of its heat treatment, and, consequently, an increase in its ultimate strength \(\sigma_B\), should influence the value of the cutting forces in the direction of increasing them, while the compression decreasing with increasing hardness of the material being machined will have the opposite effect on the forces. The mutual compensation of these factors, which have opposite effects on the cutting force, is the reason for the absence of any increase in forces with increasing hardness of the machined specimens, as revealed by our experiments.

The results of our experiments agree with the viewpoint of N. N. Zorev [4], who believes that, for cutting speeds characteristic of hard alloys, the influence of the properties of the material being machined on the specific work of chip formation and on the cutting force is very small.

When turning hardened steels, L. M. Reznitskii [2] points out, the relative value of \(P_x\) and \(P_y\) increases significantly in comparison with the machining of ordinary steel. Thus, according to L. M. Reznitskii, in turning steel of \(H_{Rc} = 59\), the ratios of the cutting-force components for different depths and feeds vary within the limits

\[ P_x : P_z = 0.51 \div 0.81; \]

\[ P_y : P_z = 0.76 \div 2.42. \]

As our investigations have shown, the ratios of the axial and radial components of the cutting force to the main component (taking into account the angle in the plan of the tool) do not go beyond the ratios characteristic of the machining of unhardened steels and, for the experiments described above, are equal to

\[ P_x : P_z = 0.42 \div 0.45; \qquad P_y : P_z = 0.22 \div 0.25. \]

When cutting steels hardened to high hardness, wear of the tool on the flank face proceeds more intensively and manifests itself even with a comparatively short duration of the dynamic experiment. The influence of tool wear, first and foremost and most of all, leads to an increase in the radial and axial components of the cutting force, and, owing to the inertia of the instruments recording the cutting forces

(hydraulic dynamometer), L. M. Reznikov apparently did not succeed in recording the forces before the onset of wear, the magnitude of which was sufficient for a substantial increase in \(P_x\) and especially \(P_y\).

Relatively large values of the force \(P_y\) were obtained in machining hardened steels also in the experiments of N. N. Zorev [4]. But these experiments do not refute our conclusions, since N. N. Zorev’s experiments were carried out with a blunted cutter.

Figures 7 and 8 show the dependences of the normal force \(N_1\) and tangential force \(F_1\), acting on the rear surface of the tool, on chip shrinkage, found by the method of extrapolating the force dependences to zero cut thickness. The data of our experiments pertain to turning with a sharp cutter.

If the cutting forces measured with a dynamometer, as shown above, change hardly at all as a function of the hardness of the material being machined, then the forces acting on the rear face of the tool, as follows from the graphs presented, increase continuously as the hardness increases. Thus, for example, at a chip shrinkage \(\xi = 2.5\), the normal force increases by a factor of 2.5, and the tangential force by a factor of 4.5. With increasing chip shrinkage, the forces \(N_1\) and \(F_1\) increase regularly, and this regularity

Fig. 7

Fig. 8

remains valid for all hardnesses of the material being machined.

In conclusion, it should be noted that all the dependencies described above are also preserved when cutting with tools having rake angles \(\gamma = +10^\circ\) and \(\gamma = -10^\circ\).

Increasing the hardness of the material being machined from \(H_{\mathrm{B}} = 197\) to \(H_{\mathrm{B}} = 500\) does not change the nature of the influence of the rake angle itself on the parameters of the cutting process.

REFERENCES

1. A. M. Rozenberg, N. I. Khovakh, V. I. Livshits. A three-component lathe dynamometer for heavy machine tools. Collection of Rationalization Proposals, No. 12 TsNIITelektroprom, Moscow, 1963.

2. L. M. Reznikov. Turning of Hardened Steels. Lenizdat, 1951.

3. A. M. Rozenberg, A. N. Eremin. Elements of the Theory of the Metal-Cutting Process. Mashgiz, 1956.

4. N. N. Zorev. Problems in the Mechanics of the Metal-Cutting Process. Mashgiz, 1956.

5. T. N. Loladze. Chip Formation in Metal Cutting. Mashgiz, 1952.

6. T. N. Loladze. On certain phenomena in chip formation. Proceedings of the Georgian Polytechnic Institute, Tbilisi, 1949.