INFLUENCE OF WEAR ON CUTTING FORCES IN TURNING

G. L. KUFAREV, V. A. NAUMOV

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

The study of wear and its consequences in the cutting of various metals is currently proceeding in several directions:

1. Clarification of the physical causes of wear, i.e., the physical processes occurring in the contact of the pair cutter—machined metal. The purpose of these investigations is to reveal the causes that produce cutter wear and to eliminate or weaken their influence on the wear process.

2. Establishment of the regularities of the mechanics of wear: the type of wear (on the front or rear face), tool life, the most favorable geometry, etc. (tool-life investigations). On the basis of these investigations, standard data for metalworking are developed.

3. Study of the influence of wear on the characteristics of the cutting process: chip shrinkage, cutting temperature, cutting forces. The data from these investigations are necessary for technological calculations of machine–fixture–tool–workpiece systems, for the design of metal-cutting machine tools and tools, fixtures, etc.

These directions are closely connected and intertwined with one another; therefore, the proposed systematization of investigations is to a certain extent conventional.

The least studied is the third direction. Few works have been published on this question. We know of works [1, 2] which, in our opinion, have a different orientation and do not fully illuminate the essence of the matter. This situation is explained by the complexity of the question being studied and by the following difficulties in setting up the experiment.

In the process of wear, the front, principal, and auxiliary relief angles of the cutter change, as do the nose radius and the radius of rounding of the main cutting edge. It is very difficult to record and determine these changes quantitatively. For this purpose it is necessary, after each test, to remove the cutter and record the changes in its profile on a profilograph. But in this case, in a repeated test, it will not be possible to set the cutter exactly in its initial position, and the character of the wear will change. In addition, to record the worn profile of the front or rear faces of the cutter, an instrument is required that satisfies the following requirements:

1. The instrument must have a sufficiently high degree of magnification.

2. It must have a large travel of the measuring tip.

1. To have the capability of recording the profile in precisely fixed spatial coordinates.

2. To have small dimensions of the sensing tip and a small sensing force.

At present, the industry of the USSR does not produce instruments possessing all these qualities.

Recording the profile of a worn tool directly, without removing it from the machine tool, appears to be quite impossible for technical reasons (insufficient dimensions of the cutting zone, the bulkiness of all the recording apparatus, etc.).

Finally, for the investigation a lathe is required, equipped with the appropriate dynamometric and recording apparatus, which in itself requires fairly complex preparatory work.

Research Methodology

Taking into account the difficulties set forth above, in the metal-cutting laboratory of the TPI a method was developed for investigating changes in cutting forces when turning with a worn tool. Its essence is as follows.

A heavy lathe of type 163 was equipped with a three-component dynamometer designed by A. M. Rozenberg, N. I. Khovakh, and V. I. Livshits. To record the components of the cutting force, re-equipped electric self-recorders of the “Kalibr” plant, type BV-662, were used. To record the cutting temperature, an electronic balanced self-recorder KVT 6/EN manufactured by MAW (GDR) was used.

The carbide insert (cutter) was mounted in a special holder and insulated from the body of the latter by mica plates. Before mounting, on the rake face of the carbide insert, parallel to the main cutting edge, a scratch was made with a diamond crystal, and the distance from the scratch to the main cutting edge was measured with a microscope. The scratch served as a base for determining the displacement of the cutting edge during wear. After mounting the holder in the dynamometer, a plumb bob was suspended so that its thread touched the main cutting edge, as shown in Fig. 1.

1 insert T15K6

2 plumb bob

Fig. 1. Diagram for determining the true relief and rake angles of the cutter

On the opposite side, along the line of centers of the machine, a mirror camera was installed, and photography was carried out with a high degree of magnification. This is necessary for the precise fixation of the true relief angle $\alpha_d$ after mounting and clamping the cutter. After developing and fixing the film, the image was placed in a “microphoto” apparatus or a large projector, where $\alpha_d$ was measured with sufficient accuracy.

Experiments to study the influence of cutting speed and feed on the intensity of

and the geometry of wear, as well as the influence of the latter on the dynamic and temperature characteristics of the process. The tool had the geometry \(\varphi=70^\circ\); \(\varphi_1=20^\circ\); \(\alpha_{\mathrm{d}}=9^\circ 50'\); \(\gamma_{\mathrm{d}}=10^\circ 10'\); \(\alpha_1=10^\circ\); \(\lambda=0^\circ\). The investigations were carried out at four speeds \(V=193;\ 140;\ 117;\ 73.5\ \text{m/min}\) with a feed of \(0.3\ \text{mm/rev}\), and at four feeds \(S=0.2;\ 0.3;\ 0.4;\ 0.47\ \text{mm/rev}\) at a speed of \(140\ \text{m/min}\). The work material was steel ShKh15, HB-197—207.

To record the change in the geometry of the tool, the imprint method was used, first applied by Palenov, Davydov, and Martirosov [3]. After each experiment an imprint was taken from the rake face, the main flank face, and the auxiliary flank face. For this purpose, molten Wood’s alloy was poured into a special mold placed against the corresponding face, without removing the tool from the machine. Wood’s alloy was chosen for the following reasons: it has a low melting point (\(68^\circ\)C), has very good copying properties, has (according to reference data) no shrinkage, and has very low adhesion.

The imprints obtained in this way are a reverse copy of the worn surface. They can be stored for any length of time, processed, and studied under conditions convenient for this purpose. This is the chief advantage of the method used.

The profile was recorded on an instrument specially designed and manufactured for this purpose, described in the article by M. F. Poletika and V. A. Naumov, “An Instrument for Investigating Wear of a Cutting Tool,” included in this collection. Recording was carried out as follows: the imprint was placed on the table in a horizontal position, and at the initial moment the needle was aligned with the trace of the scratch imprinted on the imprint of the rake face. In this way it was possible to determine the true position of the tool tip and the wear crater at the moment of the experiment relative to the tip and the rake face of the sharply ground tool. When profiling the flank faces of the tool, such orientation was not provided for. If one takes into account that the actual relief angle \(\alpha_{\mathrm{d}}\) was recorded photographically, it may be asserted that the geometry of the tool in space was established with sufficient accuracy.

Experimental results

The results of the experiments performed to determine the influence of the tool operating time (or its path) on the cutting forces and the actual relief angle at various speeds are presented in Fig. 2. The feed was kept constant, \(S=0.3\ \text{mm/rev}\).

As can be seen from the graphs, the components \(P_z\), \(P_x\), \(P_y\) decrease slowly in the initial portion of the path and then rise sharply to a certain limit. For \(P_z\) this limit is small and amounts to 10–20% of the value for a sharp tool. For \(P_x\) and \(P_y\) these values are very large: their magnitude may reach 2–2.5 times the value for a sharp tool, which, of course, cannot be neglected in any case. A similar character of the curves is also retained when working with other feeds (the graphs are not presented in order to save space).

The cutting process was accompanied by wear on the rake face, wear on the flank faces, and also wear at the tool tip, which made the analysis of the curves obtained more difficult.

The actual relief angle \(\alpha_{\mathrm{d}}\) changed in the direction of decrease and then assumed negative values; at the same time the tool continued to remain serviceable and the quality of the machined surface was satisfactory.

In the figure:

\[ v = 73.5 \ \text{m/min} \]

\[ v = 117 \ \text{m/min} \]

\[ v = 140 \ \text{m/min} \]

\[ v = 193 \ \text{m/min} \]

\[ \text{ShKh15--T15K6}; \quad t = 3 \ \text{mm}, \quad S = 0.3 \ \text{mm/rev} \]

\[ \varphi = 70^\circ, \quad \varphi_1 = 20^\circ, \quad \alpha = 10^\circ \]

\[ \alpha_d = 8^\circ 50', \quad \gamma_d = 11^\circ 10' \]

Fig. 2. Graph of \(P_x\), \(P_y\), \(P_z\), \(\alpha_d\) as functions of the tool path \(L\) and cutting speed.

Top labels: 18 m; 54 m; 124 m; 194 m; 334 m; 439 m.

Scale: 100 : 1; 400 : 1.

a)

Rake face

Scale: 100 : 1; 400 : 1.

Cutting path: 0; 18 m; 54 m; 124 m; 334 m; 439 m.

b)

ShKh15–T15K6
\(\gamma = 11^\circ 10'\); \(\alpha_g = 8^\circ 50'\)
\(\varphi = 70^\circ\); \(\varphi_1 = 20^\circ\); \(\alpha_1 = 10^\circ\)
\(\lambda = 0^\circ\)
\(V = 140\ \mathrm{m/min}\)
\(t = 3\ \mathrm{mm}\)
\(S = 0.3\ \mathrm{mm/rev}\)

Fig. 3. Profilograms of tool wear

In Figs. 3,a and 3,v, profilograms are presented of the change in the front and rear faces in the principal cutting plane. For clarity, the profilograms in Fig. 3,a are superimposed and the length of the tool path over which one or another profile was formed is indicated. We draw the readers’ attention to the fact that, when recording the profile, different magnification scales were used: horizontal 100:1 and vertical 400:1, which distorted the true values of the angles.

Such recording is provided by the design of the instrument. The need to use different scales was caused by the large difference in the order of magnitude of the quantities being measured.

In Fig. 3,v, profilograms of the rear face in the principal cutting plane are presented. The wear land is shaded. The plastic deformation of the cutting wedge is noteworthy, manifested in the bulging of the portion of the rear face adjacent to the wear land. Deformation of the cutting wedge was observed at all speeds and feeds; moreover, it is already noticeable in the initial period of the tool path. Fig. 3,v is also made at different magnification scales.

Conclusions

1. The influence of tool wear on cutting forces in turning is substantial, and it should be taken into account in calculations of technological systems and cutting conditions, in the design of metal-cutting machine tools, etc.

2. The forces \(P_z\), \(P_x\), \(P_y\) in the initial portion of the path decrease somewhat, and then increase to a certain value, after which, up to complete wear, they either do not increase or even decrease slightly.

3. Basically, the influence of wear is reflected in an increase in the components \(P_x\) and \(P_y\), whose values often increase by a factor of 1.5–2.5.

4. Increasing the speed causes an increase in the magnitude of the cutting force, in the limit to which this force increases, and also accelerates the onset of the moment at which the forces begin to increase.

5. The actual clearance angle varies over wide limits and may take negative values. This phenomenon is not reflected in the finish of the machined surface, and the tool continues to retain its serviceability.

6. At the initial moment of cutting, the tool tip undergoes plastic deformation, expressed in a certain settling of the front face and bulging of the tool material on the rear face. In the course of cutting, deformation of the tool tip increases.

LITERATURE

1. Yu. A. Tolochkov. Change in cutting forces with tool wear. Journal “Stanki i instrument,” No. 10, 1960.

2. M. I. Saakyan. Wear of cutters in metal cutting. Proceedings of the Yerevan Polytechnic Institute. Vol. 22, 1964.

3. G. M. Palenov, V. G. Davydov, G. B. Martirosov. Character of wear of a roughing turning tool made of high-speed steel. NIIMASH, No. 5, Moscow, 1934.