STUDY OF THE CUTTING PROCESS OF COMMERCIALLY PURE TITANIUM

M. F. Poletika, A. I. Afonasov

(Presented by Professor, Doctor A. M. Rozenberg)

One of the characteristic features of the development of metalworking over the last two decades has been the sharp increase in the range of materials being machined, caused by the ever wider use in engineering of various nonferrous metals, including such metals as titanium, zirconium, niobium, beryllium, and others that until recently were considered rare.

Titanium and its alloys occupy a rather important place among these metals. The combination of low specific weight with high strength, exceptionally high corrosion resistance, and certain other valuable properties has made them indispensable in aircraft engineering, chemical machine building, shipbuilding, and other branches of industry.

In terms of machinability by cutting, titanium alloys occupy an intermediate position between stainless steels and heat-resistant alloys. The main reasons for the difficulties in machining them are the low thermal conductivity and relatively high hardness of these alloys.

The few publications on the machining of titanium alloys by cutting that have appeared in recent years in the specialized literature are mainly generalizations of practical recommendations.

The process of chip formation in cutting titanium and its alloys, and the dynamics of this process, are considered only in two or three works [1, 2, 3], and then only to a very limited extent. This gap is to be filled, to some degree, by research being conducted in the metal-cutting laboratory of TPI. Some of its results, concerning experiments with titanium alloy VT-2, were published earlier [4, 5].

The present work sets out the results of an experimental study of the cutting process of commercially pure titanium.

Commercially pure titanium (alloy VT-1), in contrast to most titanium alloys, is a relatively ductile metal. Its mechanical characteristics are: ultimate tensile strength \(\sigma_{\mathrm{v}} = 80\ \text{kg/mm}^2\), relative elongation at fracture \(\delta = 25\%\); hardness \(\mathrm{HB} = 200\ \text{kg/mm}^2\). Medium-hardness carbon steel has approximately the same properties.

Experiments on cutting commercially pure titanium were carried out on a model 1D63A lathe. The range of variation of cutting speed co-

ranged from 4 to 180 m/min. The feed varied from 0.03 to 0.47 mm/rev. The depth of cut in all experiments remained constant and equal to 2.5 mm.

As cutters, VK4 carbide inserts were used; they were sharpened and honed with the following geometrical parameters: rake angle—10°, clearance angle—10°, plan angle—70°, auxiliary plan angle—20°, inclination angle of the main cutting edge—0°.

In each experiment, chip shrinkage, the length of chip contact with the cutter, the cutting temperature (by the natural thermocouple method), and the cutting force were measured. To determine the cutting force, a three-component elastic-electric dynamometer with wire strain gauges was used [6]. This instrument was used together with a three-channel strain-gauge amplifier IDD-2. Readings were taken visually from panel microammeters of the M24 type, and also by photographing the panel with the instruments.

The results of measuring the cutting force are given in Figs. 1 and 2. The curves of the dependence of force on cutting speed were smooth, without the “humps” and “dips” characteristic of steel, which indicates the absence of built-up edge in machining titanium. The main component \(P_z\) depends rather noticeably on speed, especially in the range of low speeds. The effect of cutting speed on the forces \(P_x\) and \(P_y\) is manifested in a somewhat distinctive way.

Fig. 1. Dependence of the main component of the cutting force on speed. VK4 cutter \(\gamma = 10^\circ\), \(\varphi = 70^\circ\), depth of cut \(t = 2.5\) mm.

At first, as speed increases, both of them decrease, and the more sharply the larger the feed. At a feed \(S = 0.47\) mm/rev, \(P_x\) decreases approximately 4-fold, and \(P_y\)—by more than 4-fold. With a further increase in speed, the forces \(P_x\) and \(P_y\) no longer change and are the same for all feeds. In other words, the feed ceases to affect the horizontal—

cutting-force components. When the feed is increased 15-fold, the forces \(P_x\) and \(P_y\) remain constant.

This paradox found its explanation after the forces on the flank face were excluded from the total forces acting on the tool (for this purpose we used the method of extrapolating the lines \(P=f(S)\) to zero feed and calculated the coefficient of friction of the chip against the tool (Fig. 4)). It turned out that, beginning with a cutting speed of \(80 \div 100\ \text{m/min}\),

Fig. 2. Effect of cutting speed on the components \(P_x\) and \(P_y\).
Cutter VK4, \(\gamma = 10^\circ\), \(\varphi = 70^\circ\), depth of cut \(t = 2.5\ \text{mm}\).

the friction angle becomes equal to the rake angle \((\arctg 10^\circ = 0.176)\). In this case the resultant force on the rake face will be directed parallel to the cutting-speed vector, and its horizontal component will be equal to zero. Then the total horizontal force will be determined only by the force on the flank face of the tool. The latter, however, does not depend on the feed.

Indeed, the horizontal force \(R_{xy}\) can be expressed through its components on the rake and flank faces as follows:

\[ R_{xy}=F\cos\gamma - N\sin\gamma + N_1, \]

where \(N\) is the normal force on the rake face, \(F\) is the friction force of the chip against the tool, \(N_1\) is the normal force on the flank face, and \(\gamma\) is the rake angle. After simple transformations this expression becomes

\[ R_{xy}=N\cos\gamma(\mu-\tg\gamma)+N_1, \]

where \(\mu\) is the coefficient of friction of the chip against the tool.

It is not difficult to see that, when \(\mu=\tg\gamma\), we obtain \(R_{xy}=N_1\); that is, the horizontal component of the cutting force is equal to the normal force on the flank face, which does not depend on the feed.

Figure 3 shows the dependence of chip compression on the cutting temperature. In addition to the small absolute values of compression, which is characteristic of titanium and its alloys and is noted by all researchers, we find in this graph an unambiguous relationship between compression and temperature. Thus the feed affects chip compression only as a temperature factor, just as is observed in the cutting of steel.

The study of the structure of titanium chips showed that, with an increase in cutting speed, continuous chips change into segmented chips. An increase in feed also promotes the formation of segmented chips. This phenomenon had earlier been observed in turning the low-ductility titanium alloy VT-2 and was explained [5]. It appears to be characteristic of titanium alloys in general and, apparently, is the result of their increased sensitivity to the rate of deformation.

Fig. 3. Dependence of chip shrinkage \(\xi\) on cutting temperature. VK4 tool; \(\gamma = 10^\circ\), \(\varphi = 70^\circ\), depth of cut
\(t = 2.5\) mm.

Fig. 4. Effect of cutting speed on the specific friction force and the coefficient of friction on the rake face. VK4 tool; \(\gamma = 10^\circ\), \(\varphi = 70^\circ\), depth of cut
\(t = 2.5\) mm.

Investigating the reasons for the decrease in chip shrinkage with increasing cutting speed, we also calculated the specific friction force on the rake face, using the visible area of contact between the chip and the tool as a basis. The results of this calculation are plotted in Fig. 4. We find that, with an increa-

with an increase in cutting speed, the specific friction force \(q_F\) decreases. Evidently, this is what causes the reduction in the coefficient of friction and the decrease in chip shrinkage.

The absolute values of \(q_F\) and \(\mu\) when cutting commercially pure titanium, although higher than when cutting alloy VT-2, are still much lower than those obtained for steel with the same mechanical properties as titanium. Being the result of the special nature of the interaction of titanium with the air environment, these features of the titanium-cutting process also lead to the small value of chip shrinkage.

REFERENCES

1. Danelyan A. M. Thermal balance in cutting titanium alloy VT-2. Vestnik mashinostroeniya, No. 1, 1957.

2. Chubarov A. D., Novikov N. N. Features of the deformation of the surface layers of titanium and heat-resistant alloys during machining by cutting. Vestnik mashinostroeniya, No. 9, 1958.

3. Reznikov N. I. On “negative” shrinkage in the machining of titanium alloys. Proceedings of the Kuibyshev Aviation Institute, issue IX, 1959.

4. Poletika M. F. Friction in cutting titanium alloy at microspeeds. Izvestiya vuzov SSSR, Mashinostroenie, No. 9, 1960.

5. Poletika M. F. Investigation of the features of the cutting process of titanium alloy VT-2. Izvestiya vuzov SSSR, Mashinostroenie, No. 11, 1961.

6. Vasil’ev V. I. The use of wire transducers for measuring cutting forces in metal machining. “Periodicheskaya informatsiya,” topic No. 28, No. K-54-83, Moscow, 1953.