Nature of Tool Wear and Its Influence on the Main Parameters of the Cutting Process During Turning of Hardened ShKh-15 Steel

G. L. Kufarev, N. I. Khovakh

(Submitted by the scientific seminar of the Department of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

One of the ways of ensuring high strength, hardness, and wear resistance of parts of modern machines operating under steadily increasing speeds and loads is the use of hardened structural steels.

The desire to replace the low-productivity grinding process by machining hardened parts with a carbide cutting tool has aroused interest in this problem among many researchers and production engineers. It should also be added that turning hardened steel with cutters yields a surface quality which, in a considerable number of cases, is not inferior to machining with an abrasive tool.

The overwhelming majority of researchers concerned with the machining of hardened steels have devoted their work to the selection of the tool sharpening geometry and the grade of carbide, as well as cutting regimes that ensure acceptable tool life.

The aim of the present work was to determine the nature of wear and its influence on the cutting forces, chip shrinkage, cutting temperature, and the length of the contact area between the chip and the cutter, depending on cutting speed, feed, and the hardness of ShKh-15 steel obtained by the corresponding heat treatment.

The experiments were carried out on a screw-cutting lathe, using tool holders 163 with a mechanically clamped insert of T15K6 carbide. The tool geometry selected was as follows: rake angle \(\gamma = 0^\circ\), clearance angles \(\alpha = \alpha_1 = 10^\circ\), principal cutting-edge angle in plan \(\varphi = 45^\circ\), auxiliary cutting-edge angle in plan \(\varphi_1 = 15^\circ\), cutting-edge inclination angle \(\lambda = 0^\circ\), and cutter nose radius \(r = 0.6\) mm.

The studies of N. S. Logak [1] showed that, when turning hardened steels, there is no single optimum cutter geometry common to the entire hardness range \((30 \div 60\ \mathrm{HRC})\). In all our experiments, the geometry indicated above remained constant, which made it possible to eliminate its influence on the quantities under investigation.

The cutting forces were measured with a three-component elastic-electric dynamometer, with the forces recorded by self-recording instruments of type BV-662 manufactured by the Kalibr plant [2].

The cutting temperature was measured by the natural thermocouple method. The amount of cutter wear on the flank face, “\(\delta\),” and the dimensions of the contact area between the chip and the rake face of the tool, “\(l_k\)” and “\(f_k\),” were measured with a Mir-2 microscope.

Annealed ShKh-15 steel, with a hardness of \(HB = 192\ \text{kg/mm}^2\), and bushings of the same steel, hardened to a hardness of \(HB = 300,\ 400,\ 500\ \text{kg/mm}^2\), were used as the work material.

Tool wear. A characteristic feature of tool wear when turning ShKh-15 steel is intensive wear on the rake face, with the formation of a crater. Wear on the flank face proceeds with considerably lower intensity.

The dimensions of the crater and of the wear land on the flank face of the tool are determined by the duration of tool operation, the cutting conditions, and the hardness of the work material.

Tool breakage was most often observed at the moment when the crater reached the cutting edge of the tool, at a comparatively small amount of flank wear that, in other cases, would still permit further operation of the tool. In the next moment, after chipping of the cutting edge, flank wear increases sharply and almost instantaneously (Fig. 1).

As the hardness of the hardened steel increases, the intensity of flank wear increases (Fig. 1). However, it should be noted that when turning unhardened ShKh-15 steel the intensity of flank wear of the tool is higher than when machining steel of hardness \(HB = 300\ \text{kg/mm}^2\). When turning this steel, flank wear reached a value of \(0.15\ \text{mm}\) during twenty minutes of tool operation. With further operation, the increase in wear with time is extremely small and, over a time \(T = 68\ \text{min}\), reaches \(0.17\ \text{mm}\).

When turning steel of hardness \(HB = 500\ \text{kg/mm}^2\) under the same cutting conditions \((V = 110\ \text{m/min},\ S = 0.222\ \text{mm/rev},\ t = 2\ \text{mm})\), flank wear occurs rapidly and reaches \(0.15\ \text{mm}\) within \(0.4\ \text{min}\) (Fig. 1). With further operation, the intensity of wear decreases, but to a much smaller extent than when machining the softer steel.

The dependence of tool flank wear on cutting speed and feed when turning hardened ShKh-15 steel does not differ from the analogous dependences for thermally untreated steels. An increase in cutting speed and feed increases the intensity and magnitude of flank wear of the tool.

As indicated above, wear on the rake face of the tool proceeds with the formation of a crater. The crater forms at a certain distance from the cutting edge, producing a land, the width of which \(f_k\), together with the length of the contact area \((l_k)\), characterizes tool wear on the rake face.

The influence of the hardness of the work material on the values of \(l_k\) and \(f_k\) is shown in Fig. 2. The length of chip contact with the tool increases continuously as flank wear of the tool increases. The intensity of the increase in contact length is greater, the higher the hardness of the work material.

When machining unhardened ShKh-15 steel, the increase in \(l_k\) is insignificant in magnitude and begins only after a fairly long period of tool operation. The initial length of the contact area, corresponding to operation with a sharp tool, is smaller the higher the hardness of the steel [3].

The initial contact length, corresponding to operation with a freshly sharpened tool, when turning hardened steels at cutting speeds \(V = 80\ \text{m/min},\ 110\ \text{m/min},\ 140\ \text{m/min}\), almost coincides, while retaining the tendency for the contact length to decrease as the cutting speed increases.

As flank wear increases, this regularity remains valid for cutting speeds \(V = 80\ \text{m/min}\) and \(V = 110\ \text{m/min}\). For the cutting speed \(V = 140\ \text{m/min}\), in the case of cutting steel

Fig. 1. Dependence of cutter wear on the flank face on operating time, feed, and steel hardness.

Left part of the graph:

● \(S = 0.022\ \mathrm{mm/rev}\);
\(+\ S = 0.22\ \mathrm{mm/rev}\);
\(\Delta\ S = 0.07\ \mathrm{mm/rev}\)

\(H_b = 400,\quad v = 110\ \mathrm{m/min},\quad t = 2\ \mathrm{mm}\)

\(\gamma = 0^\circ,\quad \varphi = 45^\circ,\quad \varphi_1 = 15^\circ,\quad \alpha = \alpha_1 = 10^\circ,\quad \lambda = 0^\circ,\quad r = 0.6\ \mathrm{mm}\)

Right part of the graph:

● \(H_b = 190\), × \(H_b = 300\), \(\Delta\ H_b = 400\), \(+\ H_b = 500\)

\(v = 110\ \mathrm{m/min},\quad S = 0.222\ \mathrm{mm/rev}\)

\(t = 2\ \mathrm{mm},\quad \gamma = 0^\circ,\quad \varphi = 45^\circ,\quad \varphi_1 = 15^\circ\)

\(\alpha = \alpha_1 = 10^\circ,\quad \lambda = 0^\circ,\quad r = 0.6\ \mathrm{mm}\)

Vertical axis: \(\delta,\ \mathrm{mm}\)

Horizontal axis: \(T,\ \mathrm{min}\)

\(HB = 400\ \mathrm{kg/mm^2}\), with \(\delta > 0.15\ \mathrm{mm}\), the length of the contact area proves to be greater than at lower cutting-speed values, owing to the intensity of the growth of wear on the flank and rake surfaces, which increases as the cutting speed increases.

The magnitude of the land \(f_k\) on the rake surface of the tool, as the duration of cutter operation increases, decreases sharply in the first period and then more slowly.

Fig. 2. Dependence of the length of the chip contact area with the rake face of the cutter and of the land on the rake face on operating time and tool wear on the flank face during turning of steel hardened to different hardnesses.

The softer the material being machined, the more gently sloping is the curve expressing the dependence of the land size on time (Fig. 2).

Thus, when turning steel of hardness \(HB = 190\ \mathrm{kg/mm^2}\), after thirty minutes of tool operation (correspondingly \(\delta = 0.25\ \mathrm{mm}\)) the decrease in land size ceases. When turning steel \(HB = 500\ \mathrm{kg/mm^2}\), the decrease in \(f_k\) occurs continuously and rapidly. The initial magnitude of the land decreases with increasing cutting speed and increases with increasing feed. This dependence is valid for the entire hardness range of the materials being machined. Formation of the wear crater occurs the faster, the harder the steel being machined, and the greater the feed and cutting speed.

When the land size reaches \(f_k \ll 0.03 \div 0.04\), chipping of the cutting edge occurs.

It is interesting to note that, when turning hardened steel, the character of the departing chip changes over the operating time of the cutter. The change in the form of the departing chip is associated with the change in the geometry of the cutter’s rake surface as it wears.

In the initial period of operation the chip comes off as a ribbon; as the crater on the rake surface of the tool increases, it changes into a helical spiral, then into a flat spiral, and finally the chip

breaks up into separate small coils. The type of chip produced changes as the tool wears when turning ShKh-15 steel hardened to a hardness of \(H_B = 300, 400, 500\ \text{kg/mm}^2\).

Fig. 3. Dependence of the cutting forces, chip shrinkage, and cutting temperature on tool flank wear, on the hardness of the material being machined, and on the cutting speed.

Figure annotations: \(m(t')\); \(\xi\); \(P_z\), kg; \(P_x\), kg; \(P_y\), kg; \(\delta\), mm. Cutting conditions shown: \(V = 110\ \text{m/min}\), \(S = 0.222\ \text{mm/rev}\); \(t = 2\ \text{mm}\), \(\gamma = 0^\circ\), \(\varphi = 45^\circ\); \(\varphi_1 = 15^\circ\), \(\alpha = \alpha_1 = 10^\circ\), \(\lambda = 0\), \(r = 0.6\). Legend: \(\bullet\)—\(H_B = 190\); \(\times\)—\(H_B = 300\); \(\Delta\)—\(H_B = 400\); \(+\)—\(H_B = 500\). Additional legend: \(\bullet\)—\(V = 80\ \text{m/min}\); \(\times\)—\(V = 110\ \text{m/min}\); \(\Delta\)—\(V = 140\ \text{m/min}\). Additional cutting conditions shown: \(H_B = 400\), \(S = 0.222\ \text{mm/rev}\); \(t = 2\ \text{mm}\), \(\gamma = 0^\circ\), \(\varphi = 45^\circ\), \(\varphi_1 = 15^\circ\), \(\alpha = \alpha_1 = 10^\circ\), \(\lambda = 0\), \(r = 0.6\).

Influence of hardness on the cutting forces and chip shrinkage. When turning hardened steels, the change in all components of the cutting forces, chip shrinkage, and cutting temperature as a function of tool flank wear follows hump-shaped curves (Fig. 3).

In the initial period of operation, tool wear proceeds mainly only along the flank face of the cutter, which also causes an increase in all components of the cutting forces. The increase in tool wear on the flank face causes a much greater increase in the radial \(P_y\) and axial \(P_x\) components, since the normal specific contact loads on the flank surfaces are always higher than the tangential ones [4], while the absolute values of the forces \(P_y\) and \(P_x\) are considerably smaller than the main component \(P_z\).

With an increase in the hardness of the steel, the growth of the horizontal components as cutter wear increases occurs more intensively, and their absolute values also increase. Hardness has a smaller effect on the main component of the cutting force, \(P_z\).

The subsequent decrease in the cutting forces is caused by the character of tool wear. During further operation of the cutter, the intensity of wear on the flank face decreases; in turn, the crater that forms changes the geometry of the tool in the direction of increasing the rake angle.

The increase in the rake angle and the stagnant zone, which probably forms on the chamfer, lead to a decrease in the cutting forces. The stagnant zone that forms reduces the intensity of the growth of wear on the flank face, and in some cases even leads to its stabilization (Fig. 1, \(HB = 300\ \text{kg/mm}^2\)).

As our experiments [3], carried out with a sharp cutter, have shown, changing the hardness of ShKh-15 steel from \(HB = 200\ \text{kg/mm}^2\) to \(HB = 500\ \text{kg/mm}^2\) does not exert any appreciable influence on the cutting forces. The change in all components of the cutting force with the change in hardness does not exceed 15%.

The change in chip shrinkage and cutting temperature with wear proceeds along analogous curves and is determined by the reasons set forth above. The break in the curves of forces, shrinkage, and temperature occurs, with respect to cutter operating time, the sooner the higher the hardness of the material being machined, which is caused by the different intensity of wear. The absence of humps on the curves expressing the dependence of the forces on wear when turning unhardened steel is explained by the considerably smaller wear of the cutter on the rake face.

A. Ya. Malkin, in his work [5], adopts as the wear criterion for cutters when turning hardened steels the wear on the flank face equal to \(0.8 \div 1.0\ \text{mm}\). Such wear, in the author’s opinion, does not cause a noticeable increase in the radial component of the cutting force.

The experiments we conducted showed that the growth of the horizontal components already becomes significant when cutter wear on the flank face is equal to \(0.15 \div 0.20\ \text{mm}\).

Influence of cutting speed on the main parameters of the cutting process at different cutter wear on the flank face.

When turning hardened steels, as the cutting speed decreases, the cutting forces increase (Fig. 3).

The growth of all components, especially the horizontal ones, is manifested more strongly (in the region of predominant wear on the flank face) at large values of the cutting speed. The increasing intensity of wear on the rake face with increasing cutting speed is the reason for the shift of the maximum of the curves toward a smaller value of wear on the flank face; i.e., the higher the cutting speed, the sooner the inflection of the curves occurs with a shorter duration of cutter operation.

The change in chip shrinkage and cutting temperature with the growth of tool wear on the flank face follows curves analogous to the change in cutting forces. Moreover, the maximum values of shrinkage and temperature correspond to the same value of cutter wear \(\delta\) at which the cutting forces also have their maximum values.

When turning unhardened steel, a hump-shaped curve occurs only at a cutting speed \(V = 175\) m/min. At lower cutting-speed values, only the horizontal components of the cutting force show substantial growth. The nature of the curves shows that in this case tool wear on the flank face is of predominant importance.

The influence of cutting speed on the cutting forces in the presence of tool wear does not have the clear dependence characteristic of work with a sharp tool.

The cutting temperature increases with increasing wear, but when there is a maximum on the curves \(P_z, P_x, P_y = f(\delta)\) (\(V = 175\) m/min), the dependence \(t^\circ = f(\delta)\) also has a maximum at the same value of flank wear.

The influence of feed on the main parameters of the cutting process for hardened steel. When turning hardened steels, the hump-shaped character of the curves showing the dependence of the main parameters of the cutting process on wear occurs at all investigated feed values.

As the feed increases, the maxima of the curves shift into the region of larger values of wear \(\delta\), corresponding to the increase in wear intensity with increasing feed.

When working with a worn tool, the radial component of the cutting force \(P_y\) exceeds the axial component \(P_x\).

In the case of turning with a feed \(S = 0.522\) mm/rev, at tool wear \(\delta = 0.4\) mm the cutting edge chipped, which led to an instantaneous increase in tool flank wear from \(\delta = 0.4\) mm to \(\delta = 1.1\) mm. Since the chipping of the cutting blade occurred at the moment the cutting forces were being recorded, it was possible to record the latter at tool wear \(\delta = 1.1\) mm. At such wear, both horizontal components of the cutting force proved to be greater than the main component \(P_z\).

Brief Conclusions

1. When turning hardened ShKh-15 steel, tool wear occurs on both the flank and rake surfaces. In the initial period, wear is more intense on the flank surface; subsequently, it is more intense on the rake face.

2. Tool failure occurs as a result of the wear crater formed on the rake surface reaching the cutting edge, with comparatively small tool wear on the flank face.

3. The change in all components of the cutting forces, chip shrinkage, and cutting temperature with increasing tool flank wear occurs along hump-shaped curves. The increase in the horizontal components \(P_x\) and \(P_y\) is predominant in comparison with the increase in the main component \(P_z\). A significant increase in the components \(P_x\) and \(P_y\) is already observed at \(\delta = 0.15 \div 0.2\) mm.

REFERENCES

1. N. S. Logak. Fine turning of hardened steels. In: Finish Machining of Structural Metals, Mashgiz, 1951.

2. A. M. Rozenberg, N. I. Khovakh, V. I. Livshits. A lathe dynamometer for measuring forces up to 2 tons. Stanki i instrument, No. 9, 1964.

3. N. I. Khovakh. Influence of the hardness of ShKh-15 steel on the chip-formation process. Izv. TPI, vol. 147, 1966.

4. N. N. Zorev. Cutting forces in turning hardened steels. ITEI AS USSR, 1955.

5. A. Ya. Malkin. High-speed turning of hardened steels. In: High-Speed Machining of Metals by Cutting, Oborongiz, 1951.