THE INFLUENCE OF TOOL MATERIAL ON THE CHIP-FORMATION PROCESS

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

In the cutting process, the material of the cutting tool and the material being machined are the two elements of the rubbing pair chip–tool. As is well known, the friction conditions in cutting are highly specific. They are characterized by a high cleanliness of the contacting surfaces, high contact pressures, and a high temperature in the contact zone. Therefore, friction in cutting is always accompanied by deep plastic deformations in the contact layer of the chip.

Under such conditions, one should expect that resistance to friction must be determined chiefly by the mechanical characteristics of the material being machined—more precisely, of the chip material. How, then, will the tool material manifest itself? A partial answer to this question is found in N. N. Zorev [1], according to whose experiments the tool material has little influence on the chip-formation process.

From the standpoint of the physics of the friction process, N. N. Zorev’s experiments are insufficient to answer the question of the role of the tool material in friction, if only because they were carried out with a very limited number of materials close to one another in their chemical composition. With the aim of obtaining a more complete solution of this problem, we carried out cutting experiments with a wide range of variation in the properties of the tool material. Some results of these experiments are presented below.

In order to reduce the number of factors through which the tool material affects the cutting process, lead was first chosen as the material being machined. Cutting lead is characterized by a relatively large chip shrinkage, which is a favorable factor, since it increases the sensitivity of the chosen method of investigation. At the same time, the cutting temperature in machining lead is very low, which makes it possible to vary the properties of the tool material over an extremely wide range without concern for its cutting qualities.

Finally, lead as the material being machined has one more advantage. According to existing views [2, 3], the tendency of rubbing pairs toward seizure and the anti-scuffing properties of a metal working in a pair with another metal (or nonmetallic material) depend to a large extent on their chemical affinity and, in particular, on their mutual solubility in the solid state. From this point of view, lead is distinguished by the fact that it forms no solid solutions with any of the elements included in cutting materials, with the exception of titanium. The latter can dissolve in lead

(in the solid state), as well as form chemical compounds with it.

The results of experiments on cutting lead with tools made of 5 different materials are presented in Fig. 1. We see that the tool material noticeably affects chip shrinkage; moreover, this effect is not in any relation to its tendency to seize. Otherwise one would expect the greatest chip shrinkage to occur when using a tool material with a high titanium content.

Fig. 1.

Meanwhile, the chip-shrinkage curve for this alloy lies somewhere in the middle.

How, then, in this case is the influence of the tool material on chip shrinkage in cutting lead to be explained?

A convincing answer to this question is provided by the graph in Fig. 2, which reproduces the dependence of chip shrinkage (at constant cutting speed) on the thermal conductivity of the tool material. The higher the thermal conductivity, the lower the cutting temperature should be, and the greater the chip shrinkage turns out to be. And the fact that the VT-2 alloy falls outside this regularity does not contradict it, but rather confirms it, since it is undoubtedly the result of this metal’s tendency to seize with lead.

The dashed curve on the same graph is plotted according to the data of N. N. Zorev and corresponds to the case of machining steel. We see that here, too, the mechanism of the influence of the tool material on chip shrinkage consists in a change in the cutting temperature as a function of the tool’s thermal conductivity. This testifies to the general character of the regularity we have observed.

It should be pointed out that the character of the influence of cutting temperature on chip shrinkage in the present case is somewhat different from that usually observed when the temperature is changed by altering the cutting conditions. As is known, a change in feed or depth of cut causes only a displacement of the curve \(\zeta = f(v)\) along the velocity axis, while the following law is maintained: equal temperatures correspond to equal chip shrinkages. It is not difficult to see that in the present case this law is no longer applicable. This follows at least from the mismatch in the heights of the humps of the curves in Fig. 1.

To determine how substantially the tendency of the tool material to seize with the material being machined may affect the cutting process, experiments were carried out on the cutting of red copper. Unlike lead, copper forms solid solutions and is chemically active with respect to most metals, and choosing a cutting material “inert” in this respect proved rather difficult. Preliminary experiments were conducted which made it possible, from the chip shrinkage, to estimate approximately the influence of different tool materials on the cutting process. After this we proceeded

Fig. 2.

to experiments with variation of all the characteristics of the chip-formation process, using two cutters: one made of VT-2 alloy and one of beryllium bronze BrB2. The latter contains about 98% copper and, consequently, should exhibit the maximum tendency to seize with copper chips.

Both cutters were heat-treated to the same and sufficiently high hardness \((H_B = 360)\), which made it possible to vary the cutting speed up to 130 m/min. The rake angle was equal to \(25^\circ\). At smaller rake angles the cutting process became unstable, which prevented the obtaining of reliable results.

The results of the copper-cutting experiments are presented in Figs. 3, 4, and 5. They are very indicative. We see that the change from titanium alloy to beryllium bronze as the tool material caused a threefold increase in the length of contact of the chip with the cutter, and a little more than a twofold increase in chip shrinkage. The cutting forces increased as follows: \(P_z\) by a factor of 2, and \(R_{xy}\) by more than a factor of 4. The coefficient of friction increased from 1 to 3.

Such an abrupt change in all the characteristics of the cutting process is observed in this case because, compared with titanium alloy, bronze BrB2 possesses not only an increased tendency to seize, but also higher thermal conductivity.

Figure 5 also shows the results of calculating the contact stresses on the rake face. It is characteristic that the specific friction force, despite the jump in all the characteristics, changes only slightly when the tool is changed. This occurs because the material being machined is the same in both cases, while the tangential stress on the contact-

Fig. 3.

Fig. 4.

of the rake face is a function of the deformation of the surface layer of the chip. The normal stress, by contrast, when cutting with a bronze tool, is much lower than for a titanium tool, which is associated with a sharp change in the size of the contact area.

Thus, the results of the investigation of the influence of the tool material on the cutting process, set forth above, show that the nature of this influence is twofold. First, the tool affects the characteristics of the cutting process through its thermal conductivity, owing to changes in which the cutting temperature changes.

Fig. 5.

Second, the tool’s capacity to seize with the chip material may also have an effect, and a very substantial one. This property manifests itself through a change in the length of contact between the chip and the tool and, in a certain sense, is identical to the effect of the medium on the cutting process.

REFERENCES

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

2. A. P. Semenov. Problems in the theory of seizure of metals. In: Wear and Wear Resistance. Antifriction Materials. Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.

3. Roach A. E., Goodzeit C. L., Hunnicutt R. P.—Frictional characteristics and surface damage of thirty-nine different elemental metals in sliding contact with iron.—Trans. ASME, 78 (1956), 8.