EFFECT OF ARTIFICIALLY CHANGING THE CHIP–TOOL CONTACT LENGTH ON THE CUTTING PROCESS

M. F. Poletika, V. S. Kushner

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

The study of contact phenomena on the rake face of a cutter is of great practical interest, since it not only contributes to the development of the theory of chip formation, but at the same time is one of those connecting bridges that make it possible to pass from the theory of the chip-formation process to questions of wear and life of the cutting tool and the machinability of metals by cutting.

One method of studying the processes occurring on the rake face of a cutter is to conduct experiments with cutters having a shortened rake face. Such a cutter is shown in Fig. 1. When cutting with an ordinary cutter, the contact length $c$—in this case it is appropriate to call it the natural contact length—is established automatically, since it is a function of the cutting conditions. By using the cutter shown in Fig. 1, we artificially reduce the contact length. It will now be determined by the length of the chamfer on the cutter. If the natural contact length changes with a change in the cutting conditions, the chamfer length remains constant. Thus, it becomes possible to separate the influence of the contact length on the cutting process from the influence of the other factors.

Fig. 1.

The application of the described method to the study of the cutting process entails the possibility of one significant error, which researchers, as a rule, either do not take into account or take into account insufficiently [1, 2]. The point is that a cutter with a shortened rake face creates favorable conditions for the formation of a built-up edge. Our numerous experiments [3, 4] show that, when cutting steel, even with a zero rake angle on the chamfer, a built-up edge can exist on such a cutter up to very high cutting speeds, and is characterized by great stability. The same conclusions can be drawn from the experiments of M. N. Klushin [5] on steel and copper.

Naturally, the appearance of a built-up edge fundamentally changes the chip-formation process. The chip then flows not along the built-up edge, and moreover

a change in cutting speed and feed under these conditions entails a change in the geometry of the built-up edge and, as a result, a change in the chip-flow angle, which is not taken into account by the investigators.

In order to eliminate this error, we first set up experiments to determine the limits of existence of the built-up edge. The experiments were carried out on 12KhN3 steel and consisted of observing, through a microscope, the lateral side of the cutting zone during turning. The method of such observations, as well as the instrument intended for them, has already been described in the literature [6]. The tool was made and mounted as a tangential tool, which facilitated observation of it during the cutting process. The plan angle was provided by rotating the tool holder of the machine. The rake angle on the chamfer was varied from \(-10^\circ\) to \(20^\circ\). The chamfer length, depending on the feed, was varied from 0.2 to 1.5 mm.

It had previously been established [1] that the chip-flow angle (\(\eta_0\) in Fig. 1) depends mainly on the ratio between the thickness of cut and the chamfer length of the tool, \(a/f\), increasing as this ratio increases, and on the cutting speed, with an increase in which it decreases somewhat. Consequently, the cutting conditions we require, ensuring the absence of a built-up edge on a tool with a chamfer, will correspond to relatively small feeds and high cutting speeds.

The experiments confirmed our assumptions. As an example, Fig. 2 gives the results of experiments for a rake angle of \(20^\circ\). The graph is plotted in coordinates (cutting speed—feed), and the lines drawn on it connect the points at which the built-up edge disappears. Each of these lines divides the region of the parameters \(s\) and \(v\) into two halves: the built-up-edge formation zone (on the left) and the zone of absence of a built-up edge (on the right).

Fig. 2.

After this, on the basis of the data obtained and selecting the chamfer length and cutting regime so that the built-up edge was always absent, we carried out experiments measuring all the parameters of the cutting process: chip shrinkage, cutting temperature, and the components of the cutting force. For comparison, experiments were conducted with an ordinary tool providing the natural contact length.

The investigation was carried out with four rake angles \(\gamma = -10^\circ, 0^\circ, 10^\circ, 20^\circ\) and with unchanged values of the remaining geometri-

geometric parameters of the tool \((\varphi = 70^\circ,\ \alpha = 10^\circ,\ \lambda = 0^\circ)\), with the exception of the land length, which was varied in accordance with the objective of the study.

Figure 3 gives the results of experiments with a rake angle of \(10^\circ\). In analyzing this graph, we must first of all note that the length of the rake face has a marked effect on chip compression and, perhaps even more significantly, on the components of the cutting force. At the same time, whereas in ordinary cutting with a naturally established contact length \((f = \infty)\), as the cutting speed increases the chip compression and the cutting force decrease, with an artificially stabilized contact length these parameters are practically independent of speed. The bends in the curves on their left-hand side at \(f = 0.5\) are explained by the appearance of built-up edge under these conditions. Thus, it follows from Fig. 3 that, under the experimental conditions, the length of the rake face is the principal parameter governing the chip-formation process.

Fig. 3.

A more detailed analysis of the phenomena in the cutting process with a tool having a shortened rake face required separating the forces acting on the rake and clearance faces of the tool. For conditions that provide the natural contact length, we did this using the usual-

by extrapolating to zero feed the lines of dependence of the components of the cutting forces on feed. For a tool with a shortened rake face, additional assumptions were required.

The usual method of separating the forces on the flank face of the tool, proposed by Prof. Dr. A. M. Rozenberg, is based on the hypothesis that, at constant cutting temperature, chip shrinkage is constant, and that for a given tool the forces acting on the rake face are proportional to the chip thickness, while the forces on the flank face are constant. The magnitude of the forces on the flank face is determined primarily by the strength properties and, consequently, by the degree of hardening of the surface layers under the tool. On this basis, and also from an analysis of the curves of the dependence of the forces on the flank face on cutting speed, it may be concluded that, for the given work material, these forces should depend chiefly on chip shrinkage (this proposition was subsequently confirmed by experiments).

By changing the length of the rake face, as is seen from Fig. 3, we change the chip shrinkage and may therefore expect a change in the forces on the flank face. Therefore, when calculating the forces acting on the flank face of a chamfered tool, we used the results of determining these forces on an ordinary tool, taking into account the change in chip shrinkage.

Having separated the forces on the rake face, we calculated the coefficient of friction, as well as the normal and tangential stresses on the chamfer. The results of this calculation are given in Fig. 4. We see that the tangential

Fig. 4.

stress on the chamfer (specific friction force) depends neither on the length of the chamfer nor on the rake angle, remaining a constant value, somewhat greater than on an ordinary tool under the same conditions. The normal stress increases sharply as the length of the rake face decreases; as a result, we observe a decrease in the coefficient of friction.

As has been shown by the photoelastic method, the tangential stress on the rake face is constant over the plastic portion of contact and decreases continuously (to zero at the point where the chip separates from the tool) in the region of elastic contact. By reducing the length of the rake face to the length of the plastic portion, we somewhat increase the average tangential stress. With further reduction of its length, this stress should remain constant, which is also confirmed by our ...

experiments. On this basis it may be considered that, when a cutter with a shortened rake face is used under conditions in which no built-up edge is present, the plastic contact region occupies all or almost all of the chamfer area.

REFERENCES

1. P. B. Oxley. Analysis of orthogonal cutting with restricted tool contact with the chip. “Trans. ASME,” 1962, No. 4 (Russian translation).

2. Chao B. T., Trigger K. J. — Controlled contact cutting tools. — “Trans. ASME,” 81, No. 2 (1959).

3. M. F. Poletika. Features of the cutting process with a chamfered tool in high-speed turning. Izvestiya TPI, vol. 75, 1954.

4. M. F. Poletika. On built-up-edge formation on a cutter with a shortened rake face. Izvestiya TPI, vol. 107, 1963.

5. M. I. Klushin. Metal Cutting. Mashgiz, 1958.

6. A. N. Eremin. The Physical Nature of Phenomena in Steel Cutting. Mashgiz, 1951.