DEPENDENCE OF CUTTING TEMPERATURE ON THE METHOD OF SUPPLYING CUTTING FLUID

B. M. ORLOV

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

In the process of metal cutting, a large amount of heat is evolved. The principal source of heat generation is the plastic deformation of the metal being machined ahead of the cutter and beneath the cutter, the friction of the chip against the rake face of the cutter, and the friction of the flank surfaces of the cutter against the workpiece being machined. Depending on the amount of heat inflow and removal per unit time, the temperature on the cutter and in the chip changes. The high temperature arising during cutting affects the durability of the cutting tool and its wear, substantially influences the cleanliness of the machined surface and the coefficient of friction on the cutter. A change in the coefficient of friction, in turn, changes the magnitude and direction of the resultant cutting force, which leads to changes in chip shrinkage, built-up edge, etc. Thus, the temperature arising during cutting has a great influence on the course of the entire cutting process.

The study of the laws governing temperature variation in the cutting process is of great theoretical and practical importance.

The study of questions of tool life and of the entire cutting process is inconceivable without a detailed study of the laws governing temperature variation on the rake face of the tool as a function of various cutting factors.

One of the ways of reducing cutting temperature and increasing the life of the cutting tool is the use of cutting fluids during cutting.

At present, in machining metal by cutting, two methods of supplying cutting fluids are used.

1. The conventional method, in which the fluid is supplied from above to the place of chip formation in a copious stream, in an amount of 10–20 liters per minute.

2. The jet-pressure method of supplying cutting fluid. In this method, the cutting fluid is supplied to the cutting edge of the tool from the side of the flank or rake face in a thin jet, through a nozzle orifice 0.25–0.5 mm in diameter, under a pressure of 25–30 atm.

The conventional method of supplying cutting fluid is the older and more widespread one. In the conventional method of supply, the cutting fluid reaches the place of chip formation from above, removes heat and, in doing so, is heated and partially evap—

... evaporate, as a result of which cooling of the tool and chip takes place.

With the pressure method of cooling, the fluid, at high velocity, reaches the cutting edge directly; because of the high temperature it boils and evaporates intensively. This ensures better heat removal and better penetration of the lubricant onto the active friction surfaces, i.e., the lubricating-and-cooling properties of the fluid are utilized to the fullest extent.

Studies carried out in recent years \[1, 2, 3\] have established that, with the pressure method of cooling, the life of the cutting tool increases by a factor of 3–4 in comparison with the ordinary cooling method.

The purpose of the present work is to determine the dependence of cutting temperature on cutting speed when using the lubricating-cooling fluids most widely employed in production, with various methods of supplying them.

Method of Conducting the Work. Equipment.

Tool

The investigations were carried out on a lathe, machining carbon steel 30 and ball-bearing steel ShKh 15. Before the experiments, the steels were annealed.

The following lubricating-cooling fluids were adopted for the investigation: 1) water with the addition of 1.5% calcined soda; 2) a 10-percent emulsion of standard emulsol “B”; 3) sulfofresol. In addition, carbon tetrachloride was adopted as a lubricating-cooling fluid, as a chemically active fluid giving the greatest reduction in the dynamic characteristics of cutting at micro-speeds. The experiments were carried out both dry and with the use of lubricating-cooling fluids supplied by the ordinary and pressure methods. With the ordinary method of supplying coolant, the fluid was pumped from above to the place of chip formation in an amount of 12 liters per minute. With the pressure method of cooling, the fluid was supplied as a fine atomized jet through a nozzle orifice 0.3 mm in diameter, under a pressure of 28–30 atm, to the cutting edge from the side of the flank face.

Since the investigations were carried out over a wide range of cutting speeds (from 2 to 250 m/min), hard alloy T15K6 was adopted as the cutting-tool material.

Fig. 1

The hard-alloy insert 2 was insulated with mica and fastened mechanically in a special holder I (Fig. 1). When fixed in the dynamometer, the holder was insulated by fiber gaskets. The cutters had the following geometry: \(\gamma = 0^\circ\); \(\alpha = 10^\circ\); \(\varphi = 70^\circ\); \(\varphi_1 = 13^\circ\); \(\lambda = 0^\circ\); \(r = 0.3\) mm. In order to determine the influence of the method of supplying the lubricating-cooling fluid under various machining conditions (roughing and finishing), the experiments were carried out with feeds \(S = 0.43\) mm/rev and \(S = 0.131\) mm/rev. The depth of cut in the experiments was \(t = 2\) mm.

The cutting temperature was determined by the method of the natural thermocouple tool—workpiece. One link of the electrical circuit of the natural thermocouple was a mercury bath at the end of the spindle, the other was the carbide insert of the cutter. The circuit was closed through a galvanometer.

In order to avoid hot junctions and thereby exclude the possibility of parasitic thermocouples arising, the thermal current was led away from the carbide insert through carbide post 3 and long steel rod 5. This post and rod were insulated from the holder by means of a tube of insulating material 4 (Fig. 1). To prevent liquid from entering the groove with the carbide insert during cutting, the groove was covered from above with plasticine 6.

Calibration of the carbide—workpiece thermocouple was carried out in a lead bath using a control chromel-alumel thermocouple.

The cutting forces were determined by means of a hydraulic three-component lathe dynamometer.

Chip shrinkage was determined from the equation

\[ \zeta = \frac{a_1}{a} = \frac{l \cdot b}{l_1 \cdot b_1}, \]

where \(a_1, b_1, l_1\) are, respectively, the thickness, width, and length of the chip in mm,

\(a, b, l\) are, respectively, the thickness, width, and length of the cut in mm.

Results of the investigation

The results of experiments on cutting steel 30 dry and with all the investigated fluids, using the ordinary method of supply at a low feed \(S = 0.131\) mm/rev, are shown in Fig. 2, and for the pressure cooling method—in Fig. 3. From Fig. 2 it is evident that, as the cutting speed increases up to 10 m/min, the temperature both in dry cutting and with the use of various lubricating-cooling media rises sharply. Then, over a certain interval of increasing cutting speed (depending on the medium), the intensity of the temperature rise decreases. With further increase in cutting speed, the character of the temperature change repeats itself: the temperature again rises sharply—

Fig. 2

decreases, and then this increase becomes less intense. Thus, the course of the curve of the dependence of cutting temperature on cutting speed forms, as it were, a characteristic bend. This bend is especially pronounced on the temperature curve when the pressure method of cooling is used (Fig. 3). Thus, when cutting with pressure cooling by water or emulsol

Fig. 3

(liquids possessing good cooling capacity), with feed \(S = 0.131\ \mathrm{mm/rev}\), the temperature at the tool, within the range of cutting-speed variation from 10 to \(70\ \mathrm{m/min}\), remains almost constant, about \(200^\circ\). In dry cutting, however, the temperature at these speeds increased from \(240^\circ\) (at \(v = 10\ \mathrm{m/min}\)) to \(640^\circ\) (at \(v = 70\ \mathrm{m/min}\)).

From Figs. 2 and especially 3 it is evident that, when various cutting fluids are used in different ranges of cutting speeds, the reduction of cutting temperature, in comparison with dry cutting, is different.

A substantial reduction in cutting temperature due to the use of cutting fluids, which may influence the durability of the cutting tool and the entire cutting process, can be spoken of only up to cutting speeds of about \(100\ \mathrm{m/min}\).

Fig. 4

A similar character of the change in cutting temperature with cutting speed when various cooling methods are used also occurs in rough machining with \(S = 0.432\ \mathrm{mm/rev}\) (Fig. 4). The investigations carried out established that the magnitude of the reduction in cutting temperature (in the range of cutting speeds where the fluids exert the greatest cooling effect) is determined by the cooling capacity of the medium, the method of its delivery, and the machining conditions.

Fluids possessing the greatest latent heat of vaporization, heat capacity, and thermal conductivity (water, emulsol) give the greatest reduction in cutting temperature.

The effectiveness of the cutting fluid in reducing the cutting temperature is manifested to a greater degree with thin cuts. The pressure method of cooling gives greater effectiveness in reducing the cutting temperature than the ordinary method.

The cause of the formation of the deflection in the temperature curve, in our opinion, is the built-up edge that forms on the cutter both in dry cutting and when cutting fluids are used.

Our experiments have established that the greatest bend in the temperature curve is obtained at the maximum built-up edge.

It is known [4] that when cutting steel without the use of cutting fluids, the cutting force and chip shrinkage, with increasing cutting speed, vary along a curve having a maximum and a minimum. It has also been established that this character of the change in the chip-shrinkage and cutting-force curves in the cutting process is determined by the change in cutting temperature. The minimum of the curve corresponds to a cutting temperature of 200–300°, and the minimum is obtained owing to the presence of a built-up edge in this temperature range. The maximum of the curve, when the built-up edge disappears, corresponds to a cutting temperature of 500–600°. The same character of change in chip shrinkage and cutting force with increasing cutting speed (cutting temperature) also occurs when cutting with the use of cutting fluids, with ordinary and pressure cooling methods. This is evident from the graphs presented in Figs. 5 and 6.

Fig. 5

From comparison of Figs. 5 and 6 with Figs. 2 and 3 it is seen that, although the shift in cutting speed of the chip-shrinkage curves for different cutting fluids is different, all minima of the curves (regardless of the cooling medium) are located in one temperature range, 200–300°, and all maxima at 500–600°. From the graphs presented one may observe that the maximum deflection in the tempera—

Fig. 6

Fig. 8. Chip-formation zone when cutting steel 30 with pressure cooling by sulfufresol. \(S = 0.432\) mm/rev, \(\gamma = 0^\circ\), \(v = 15\) m/min \((\times 37)\)

Fig. 9. Chip-formation zone when cutting steel 30 with pressure cooling by water. \(\gamma = 0^\circ\), \(S = 0.432\) mm/rev, \(v = 42\) m/min \((\times 37)\)

the temperature curve corresponds to the minimum of the shrinkage curve; that is, the dip in the temperature curve is located in the range of cutting speeds at which a built-up edge exists.

When cutting without a built-up edge, the chip comes into contact with the rake face of the tool, and the cutting surface with the flank face. In these places of contact, electromotive forces arise during the cutting process. The magnitude of the e.m.f. recorded by the galvanometer is equal to the average sum of the e.m.f. of the microthermocouples arising on the rake and flank faces of the tool. If, however, a built-up edge is present during cutting, then the chip comes into contact with the front surface of the built-up edge and of the cutter, and also the rear surface of the built-up edge with the workpiece (Fig. 7). Since the material of the built-up edge and of the machined workpiece is one and the same as that of the chip, it is difficult to assume the possibility of an e.m.f. arising at the place of their contact, in regions 1 and 2 (Fig. 7). The e.m.f. will arise

Fig. 7

only in region IV, at the place of contact of the built-up edge with the rake face of the tool, and in region III, where the chip contacts the rake face. In this case the zones of greatest heat generation in regions I and II will be removed from the place where the e.m.f. arises. As a result, the galvanometer will record a lower e.m.f. (temperature), and on the temperature curve the points are located lower (a dip is obtained).

In addition, when there is a built-up edge on the cutter, the actual rake angle increases. An increase in the rake angle facilitates the cutting process (the deformation of the metal during cutting decreases). This, in turn, reduces the amount of heat generated. Consequently, the cutting process with a built-up edge in itself promotes a reduction in the heat released during cutting. When cutting in the built-up-edge region with the use of pressure cooling, the cutting fluid strikes in a strong jet against the flank face near the main cutting edge of the cutter, is atomized, evaporates because of the high temperature, and, washing the built-up edge from the flank face, cools it intensively. In this case, from the main sources of heat generation in the regions—

as I and II, the heat is intensively absorbed through the mass of the built-up edge by the sprayed liquid, and the temperature in region IV changes little with increasing cutting speed. Therefore, during cutting with pressure cooling, over the entire range of cutting speeds with a built-up edge, the temperature on the rake face of the tool changes mainly due to the intensity of the temperature rise in contact region III. In addition, intensive cooling of the built-up edge from the side of the flank face creates conditions for its more stable existence (the built-up edge breaks off less often). At the same time, the region in which the built-up edge exists is extended over a larger range of cutting speeds.

With a further increase in cutting speed, despite intensive cooling, the temperature rises so much that the built-up edge quickly disappears. In this case the chip is in complete contact with the rake face of the tool, and the machined surface with the flank face. Intensive cooling of the flank face of the tool and partial removal of heat from the rake face due to heat transfer take place. In this case the temperature recorded by the galvanometer will be somewhat lower than the temperature during dry cutting (Figs. 3 and 4).

Thus, during cutting with pressure cooling from the side of the flank face, owing to the stable position of the cooled built-up edge, the life of a high-speed-steel tool will be considerably higher than with ordinary cooling.

A number of investigators [2, 3] indicate that during cutting with the pressure method of supplying cutting fluid, no built-up edge forms on the tool.

To verify this proposition, as well as the proposition we advanced concerning the influence of the built-up edge on the cutting temperature, we conducted experiments at various cutting speeds with instantaneous interruption of the cutting process. Instantaneous interruption of the cutting process was carried out by means of a “falling tool” fixture with impact action, both during dry cutting and with the ordinary and pressure methods of supplying cooling with all the fluids investigated.

Figures 8 and 9 present microphotographs of sections of the chip-formation zone obtained during cutting of steel 30 with a feed of \(S = 0.432\) mm/rev in the region of the built-up edge, with pressure cooling by sulfofrezol and by water.

As can be seen from Fig. 8, despite the good lubricating ability of sulfofrezol, with the pressure method of supply a built-up edge forms on the tool, which affects the cutting process. And during cutting with pressure cooling by water at a cutting speed of 42 m/min, a built-up edge also forms on the tool. This is evident in Fig. 9.

The same was obtained with pressure cooling by other fluids and with ordinary cooling. Consequently, the assertion of some investigators that during cutting with pressure cooling a built-up edge does not form on the tool is without foundation.

Relationships analogous to those considered were also obtained by us in machining steel ShKh15.

Conclusions

1. During cutting at medium speeds (within the operating range of a high-speed-steel tool), a built-up edge forms on the tool. In this case the built-up edge forms both during dry cutting and with the use of the most widely used cutting fluids in production, with both the ordinary and the pressure method of supplying them.

1. The presence of a built-up edge on the cutter provides a certain reduction in the temperature on the rake face, which is reflected in the temperature curve in the form of a deflection. This reduction in temperature occurs owing to the facilitation of the cutting process in connection with an increase in the actual rake angle of the cutter due to the built-up edge, and also as a result of the removal from the rake face of the cutter of the principal sources of heat generation.

2. The use of the pressure method of cooling from the flank-face side provides intensive cooling of the built-up edge, as a result of which the built-up edge becomes more stable, the region of its existence expands over a larger range of cutting speeds, and a considerable reduction in temperature on the rake face is ensured.

3. The use of the pressure method of supplying cutting fluid during cutting in the region where a built-up edge exists makes it possible to increase the cutting speed while considerably increasing the durability of the cutting tool.

4. When cutting with carbide cutting tools at high speeds (beyond the maximum of the curve of shrinkage and cutting force), cutting fluids, both with the ordinary method and with the pressure method of supplying them, produce only a barely perceptible reduction in cutting temperature.

REFERENCES

1. P. P. Grudov and K. V. Dobronovich, Cooling of high-speed-steel lathe tools, Stanki i instrument, No. 9, 1952.

2. Increasing the durability of a cutting tool by applying the jet-pressure method of cooling, Referaty, Technical Information Bulletin, issue 8, 1953.

3. Stocker I. Three to twelve times more cutting tool life, The Machinist, vol. 96, No. 10, 1952.

4. A. N. Eremin, The Physical Essence of Phenomena in Steel Cutting, Mashgiz, Sverdlovsk, 1951.

5. A. M. Danielian, Heat and Wear of Tools in the Process of Cutting Metals, Mashgiz, Moscow, 1954.