INFLUENCE OF THE ADHESIVE PROPERTIES OF TOOL MATERIAL ON THE FORCES ACTING ON A CUTTER DURING FREE PLANING

M. F. POLETIKA, A. M. TURTYAKOV

(Presented by the scientific seminar of the departments: Machine Tools and Metal Cutting, and Mechanical-Engineering Technology)

As previously performed studies have shown [1, 2, 3], among the properties of the tool material, a noticeable—and at times significant—influence on the characteristics of the cutting process is exerted by its thermal conductivity and its adhesive capacity with respect to the material being machined. Since a change in the tool material entails a change in its entire complex of properties, assessing the influence on the cutting process of each property separately presents great difficulties. In [3], to solve this problem, a method of galvanic coating of the cutter was proposed and used. The cutter was coated with a thin layer of metal having, with respect to the material being machined, adhesive properties different from those of the material of the cutter itself. With a small coating thickness, this made it possible to sharply change the adhesive capacity of the cutter without noticeably changing the conditions of heat removal into the cutter.

In the present work, the method of galvanic coating was applied to investigate the influence of the adhesive capacity of the tool material on the cutting process at microspeeds, i.e., under such conditions when heat generation may be neglected and the adhesive properties of the cutter material appear in a “pure” form. In particular, this completely eliminates also those small errors in estimating the adhesive capacity of the cutter material that may be caused by the influence of the thermal conductivity of the coating layer on the conditions of heat removal into the cutter.

The investigations were carried out during free planing of specimens made of beryllium bronze of hardness HB110, which were plates 3 mm thick. The cutter material was beryllium bronze BrB2, heat-treated to hardness HB365. The coating material was chromium. Chromium plating was carried out in an electrolytic bath. The coating thickness averaged about 0.015 mm.

In the cutting process there are two contact zones, the course of phenomena in which is connected with the mutual adhesive capacity of the machined and tool materials. These are the zone of contact of the chip with the cutter and the zone of contact of the cutter with the cutting surface. For a differentiated determination of the effect of the coating in one zone and the other, along with chromium plating of the entire cutter, separate chromium coating of the flank and rake surfaces was also carried out. Unfortunately, chromium plating only the flank surface did not yield reliable results. Because of the high contact pressures on the rake surface (near the re—

coating on the cutting edge) the coating was damaged immediately after the cutter entered beneath the chip. The cutting edge lost its integrity, which distorted the results of measuring the cutting forces and did not make it possible to determine the forces applied to the rear surface of the cutter.

Thus, three series of experiments were carried out: 1) with an uncoated cutter; 2) with a cutter in which the front surface was chromium-plated; 3) with a fully chromium-plated cutter; the geometrical parameters of the cutter in all three series were taken to be identical: \(\gamma = 20^\circ\), \(\alpha = 8^\circ\), \(\lambda = 0^\circ\).

Planing was carried out on a “Dzerzhinets” planing machine. The specimen being machined was clamped in special vises on the machine table. The cutter was mounted in a two-component piezoelectric dynamometer with inductive sensors, which was attached to the ram of the machine. The dynamometer readings were recorded with an N-700 magnetoelectric oscillograph. The dynamometer was calibrated for both measured components (\(P_z\) and \(P_y\)) in the usual manner, with calibration being performed both before and after the experiments. Along with determination of the cutting force, chip shrinkage was measured.

The experiments were carried out at a cutting speed of \(V = 0.06\) m/min. The depth of cut varied from 0.05 to 0.2 mm.

Fig. 1. Effect of depth of cut on the components of cutting force and chip shrinkage: cutter 1—uncoated; cutter 2—only the front surface coated with chromium; cutter 3—front and rear surfaces coated with chromium

Figure 1 presents the results of measuring the components of the cutting force and chip shrinkage as functions of the depth of cut. Both force components, as well as the shrinkage, take their greatest values when working with a cutter made of BrB2 bronze, i.e., of the same material as the plate being machined. In this case the mutual adhesive capacity of the tool and work materials (which differed only in hardness) was the highest.

Chromium-plating the cutter leads to a considerable decrease both in shrinkage and in the force components. At the same time, the cutter in which only the front surface is chromium-plated gives intermediate values of the force \(P_y\) and of the shrinkage. The magnitude of the force \(P_z\) is almost the same for both cases. It is characteristic that, when working with the non-chromium-plated cutter, the shrinkage decreases strongly with increasing depth of cut. When using a cutter with a galvanic coating, the dependence of shrinkage on depth of cut is almost imperceptible.

To separate the forces recorded by the dynamometer into the forces applied to the front surface and the forces applied to the rear

surface, the method of extrapolating the linear dependence of the cutting force to zero chip thickness was used. As is known, this method requires obtaining the indicated dependence under conditions of constant chip shrinkage (or constant mean contact temperature, when these conditions are adequate). Meanwhile, at low cutting speeds (microspeeds), it is difficult to maintain the condition of constant shrinkage, since the chip thickness has an independent effect on shrinkage that cannot be compensated by a small change in cutting speed (for example, in Fig. 1, the shrinkage curve for an unchromed cutter).

To overcome the indicated difficulties, we used a special method developed earlier [4]. This method is based on the fact that at low speeds the contact conditions on the rake face, as well as the characteristics of the chip-formation process, stabilize relatively slowly, as a result of which the chip shrinkage proves to be variable along the cutting path. In order to use this circumstance, the cutting path (the length of the specimen plate) was divided by marks into several sections, and the chip shrinkage and the components of the cutting force were determined separately in each section. Then “shrinkage—force” curves were plotted (for each of the measured components of the cutting force and for each chip thickness), and from them points corresponding to a certain constant chip shrinkage were selected. After this, for the given shrinkage, the dependence “chip thickness—cutting force” was plotted, which was then used to determine the forces on the flank face by extrapolation to zero chip thickness.

In this way the forces on the flank face of the cutter were determined for several values of chip shrinkage. The results of these calculations are given in Fig. 2. From examination of it it follows that chromium plating of the flank face reduces the friction force \(F_1\) acting on it by more than a factor of 1.5. The values of the force \(F_1\) for cutters with a chromed and an unchromed rake face at shrinkage 3.5 are close to one another. The normal force on the flank face \(N_1\) proved to be greatest in the case when only the rake face of the cutter is coated with chromium. For the unchromed cutter and for the fully chromed cutter, the force \(N_1\) is the same.

Fig. 2. Effect of chip shrinkage on the normal and tangential force on the flank face

At small chip shrinkages, the ratio of the forces \(N_1\) for different cutters changes somewhat, while the difference in the forces \(F_1\) is almost leveled out.

Figure 3 presents the results of determining the friction force \(F\) and the normal force \(N\) on the rake face (again as functions of chip contraction). The friction force on the chromium-plated rake face remains practically constant regardless of whether the flank face is chromium-plated or not. It is approximately one and a half times smaller than on the tool without coating. If, however, one takes into account that in Fig. 3 the comparison is made not at constant contraction, but under identical cutting conditions, the contraction for the non-chromium-plated tool is higher than for the chromium-plated one (Fig. 1), then the difference in friction forces will be still more substantial.

Fig. 3. Influence of chip contraction on the normal and tangential force on the rake face

The normal forces on the rake face for the different tools differ considerably less than the friction forces; moreover, for the non-chromium-plated tool the force \(N\) is the smallest. Such relationships between the normal forces and the friction forces predetermine an even sharper difference between the mean coefficients of friction when working with chromium-plated and non-chromium-plated tools.

The results of determining the forces on the rake face are highly indicative. They fully confirm the data obtained earlier at high cutting speeds [1, 2], and agree well with the established concepts of the mechanism by which the adhesive properties of the tool material influence the cutting process.

Chromium plating of the cutting surfaces of the tool sharply changes its adhesive properties with respect to the work material, since chromium practically does not dissolve in copper and its adhesive capacity with respect to copper is very low. This is what causes the sharp reduction in the friction force even when the chip contraction remains constant.

The convincing nature of the results obtained for the forces on the rake face is an indirect confirmation of the reliability of the data in Fig. 2. Chromium plating of the flank face also leads to a significant reduction in the friction force \(F_1\). The reduction in the difference between the forces \(F_1\) found for the different tools at small contractions is apparently due to the fact that, because of the particular features of the method for separating the forces on the flank face, small chip contractions correspond to the initial portions of the cutting path, when the process has not yet become established.

The normal force \(N_1\) on the flank face is greatest in the case when the friction and adhesion conditions on the rake and flank faces differ sharply. Under identical conditions these forces are approximately the same, regardless of whether the tool is chromium-plated or not.

References

1. M. F. Poletika. Influence of the tool material on the chip-formation process. Izv. TPI, vol. 133, Tomsk, 1965.

2. M. F. Poletika, V. D. Suvorov. On the influence of the thermal conductivity of the cutting-tool material on the characteristics of chip formation. Scientific Transactions of the Tyumen Industrial Institute, collection No. 3, Tyumen, 1967.

3. M. F. Poletika, V. D. Suvorov. On the influence of the chemical properties of the tool material on chip formation. Scientific Transactions of the Tyumen Industrial Institute, collection No. 3, Tyumen, 1967.

4. M. F. Poletika, V. V. Melikhov. Forces on the rear surface during free planing in various media. Izvestiya Vuzov, “Mechanical Engineering,” No. 3, Moscow, 1968.