FEATURES OF THE CUTTING PROCESS WITH A CHAMFERED TOOL IN HIGH-SPEED TURNING

A. M. Rozenberg and M. F. Poletika

One of the characteristic features of most designs of tools for high-speed machining is the presence of a chamfer (land) along the main cutting edge. This chamfer is usually ground with a small negative rake angle (the chamfer inclination angle) and with a width approximately equal to the feed. Behind the chamfer the rake angle is made positive \((10 \div 20^\circ)\), and the shape of the rake face is either flat or radiused (a tool with a groove). Such a geometry of the rake face of a tool for high-speed cutting has been successfully used by a number of Stakhanovite high-speed machinists (Bortkevich, Bykov, Krotov, Ionov, Biryukov, and others). At present it has also been recommended by a standard of the Ministry of Machine-Tool Building of the USSR, compiled on the basis of a generalization of the research work of scientific institutions and the experimental data of leading machine-building enterprises.

As is known, high-speed-steel and carbide tools with a chamfer were used long before the advent of high-speed cutting, and the rational geometry of such tools, developed under the direction of S. L. Kamenkovich, was approved in 1943 as an all-Union standard (GOST 2320—43). The increased durability of a chamfered tool and the reduced expenditure of energy during its operation at low and medium cutting speeds have been exhaustively explained by the high stability of the built-up edge, for which the presence of the chamfer creates a natural platform. The built-up edge completely covers the chamfer, which, on the one hand, increases the rake angle of the tool and, consequently, reduces the power required for cutting, and, on the other hand, protects the tool from rapid wear. The features of chip formation and the dynamics of the cutting process when a chamfered tool operates in the region of low and medium cutting speeds have been studied in detail in the works of A. N. Eremin [1] and M. F. Poletika [2].

The situation is different in the region of high speeds. In high-speed cutting the built-up edge on a tool without a chamfer does not remain, which is very often explained either by the softening of the built-up edge under the influence of high temperature, or by the fact that it does not have time to form because of the high velocity of chip motion over the tool. From the standpoint of either of these interpretations, which are fairly widespread in the literature, we have the right to assert that in high-speed cutting a built-up edge should not form on a chamfered tool either, although there more favorable conditions are created for its appearance. It remains unclear, then, how to explain the improved cutting qualities of carbide tools with a chamfer when they operate under high-speed conditions (with the exception, of course, of the case when the chamfer is used to strengthen the cutting edge of the tool, for example, in interrupted cutting).

Meanwhile, observations of the geometry and state of the built-up edge on a high-speed-steel tool with a chamfer, carried out at low and medium cutting speeds [1; 2], make it possible to assume the existence of a built-up edge also at high speeds. For example, on a tool without a chamfer the rake angle of the built-up edge, as

as A. N. Eremin [1] showed, in a certain range of speeds decreases sharply with increasing speed, as a result of which, at a certain quite definite temperature on the rake face (somewhat below \(600^\circ\)), the built-up edge disappears. On a tool with a chamfer, the rake angle of the built-up edge decreases only very slightly with increasing speed (Fig. 1), and at a cutting temperature close to \(600^\circ\) a stable built-up edge can be observed on such a tool.

In both cases the character of the change in the rake angle of the built-up edge becomes understandable if the appearance of the built-up edge is associated with the formation of a stagnation zone during the motion of the chip along the tool. The outline of the stagnation zone from which the built-up edge arises must apparently be determined by the nature of the stressed state of the chip material and, consequently, by the normal forces and friction forces on the rake face of the tool. It follows from this that the principal factor determining the rake angle of the built-up edge is the coefficient of friction of the chip on the tool. Since this coefficient changes with increasing cutting speed [1, 3, 4], the rake angle of the built-up edge must also change,

Fig. 1

which ultimately leads to the disappearance of the built-up edge on a tool without a chamfer. A chamfer creates especially favorable conditions for the formation of a stagnation zone. Therefore the built-up edge on a tool with a chamfer is very stable, and its rake angle depends little on the cutting speed. It is natural to suppose that on such a tool the built-up edge should also be retained at higher speeds than those permitted by a high-speed-steel tool.

In order to verify this assumption we investigated the process of chip formation when a carbide tool with a chamfer operated at high speeds. A tangential tool was made with a brazed insert of alloy T15K6, to which the following geometry was given: chamfer inclination angle \(\gamma_0 = 0^\circ\), rake angle behind the chamfer \(\gamma = 28.5^\circ\), plan angle \(\varphi = 90^\circ\), chamfer width \(f = 0.27\) mm. The tool was mounted in a special holder allowing unobstructed observation of the chip-formation process. Observations were made with an optical goniometer of A. N. Eremin’s design [1]. As the work material we used carbon steel of medium hardness \((H_B = 170\ \text{kg}/\text{mm}^2;\ \sigma_b = 40\ \text{kg}/\text{mm}^2;\ \delta = 24\%)\).

The experiments carried out showed that in high-speed cutting a built-up edge on a tool with a chamfer not only persists, but also rather stably retains its geometrical parameters. Having measured with the optical goniometer the rake angle of the built-up edge at various cutting speeds, we obtained the same dependence as for a high-speed-steel tool with a chamfer: with increasing speed the rake angle decreased insignificantly, starting from

from low speeds up to the very highest (Fig. 2). In doing so, we judged the magnitude of the front angle of the built-up edge mainly from the direction in which the chip flowed off. This proved possible because the chip came off the tool without curling, and the large front angle of the tool prevented the chip from contacting the rake face behind the chamfer. The chip was in contact only with the built-up edge and with the edge formed by the intersection of the chamfer plane and the rake face (Fig. 3).

Fig. 2

We were also able to observe that, at high cutting speeds, the built-up edge does not overhang the flank face of the tool. If, at low speed, in the presence of a built-up edge, a gap between the cutting edge of the tool and the machined surface is clearly visible, then at high speed this gap is no longer noticeable. As a result, the built-up edge loses some of its protective properties; this, incidentally, is also indicated by the observed wear of the tool on the flank face, which at lower speeds is not observed at all on a tool with a chamfer.

Fig. 3

Observations also showed that the radius of rounding of the apex of the built-up edge at high speed assumes a much smaller value than at low speed; that is, as the cutting speed increases, the built-up edge becomes sharper.

It is interesting to note that the quality of the surface machined by a tool with a chamfer, even at a speed on the order of 300 m/min, is noticeably worse than when machining under the same conditions with a tool without a chamfer. The surface retains

luster, but proves to be, as it were, overgrown with dark fibers which are undoubtedly particles of built-up edge pressed out by the chip beyond the auxiliary cutting edge of the tool.

In addition to observations of the built-up edge, we also measured the shrinkage of the chip and the components of the cutting force acting on the chamfered tool in high-speed cutting. The dependences of shrinkage and force on the cutting speed are presented in Figs. 4 and 5. Figure 6 shows the dependence of the front

Fig. 4

angle of the built-up edge on the depth of cut at constant chamfer width and constant speed, while Fig. 7 interprets the nature of the change in chip shrinkage under these conditions. If we compare these curves with Figs. 8 and 9, which present analogous dependences for a high-speed-steel chamfered tool at low cutting speed, we are convinced of their complete similarity. This once again confirms the fact that a built-up edge exists on a chamfered tool

Fig. 5

at high speeds, since the distinctive form of the curve of chip-shrinkage variation as a function of feed at constant chamfer width for a high-speed-steel tool (Fig. 9) finds its only correct explanation in the regularity of the change in built-up-edge geometry. The shrinkage at first decreases rapidly and then slowly, because the front angle of the built-up edge increases in this case (also at first rapidly, and then more slowly).

Chamfered tools with a radiused form of the rake face, in terms of the conditions for formation of a built-up edge, should not differ substantially from the tool with which our experiments were conducted. And indeed, when we esta-

Fig. 6

Fig. 7

Fig. 8

conducted an experiment with a tool having a groove (hollow) behind the chamfer, we became convinced that even on it, at high speeds, a built-up edge is present. This was evidenced both by direct observation through a microscope and by inspection of the surface of the part after machining. Indirectly, the presence of a built-up edge on the tool with a groove is confirmed by the character of the change in chip shrinkage when this tool is operating (Fig. 10).

Fig. 9

Analysis of the geometrical parameters of chamfered tools used by Stakhanovite high-speed machinists (Bortkevich, Bykov, and others) and recommended in the standards shows that they are very close to those under which the most favorable conditions are created for the existence of a stable built-up edge. Thus, for example, Stalin Prize laureate G. Bortkevich, when sharpening his tools, makes the chamfer width equal to \(f=(0.8 \div 1.5)S\), where \(S\) is the feed in mm/rev, with its inclination angle \(-2^\circ\). Stalin Prize laureate P. Bykov

Fig. 10

recommends sharpening tools with a chamfer width from \(0.2\) mm to \(1.5\) mm, depending on the material being machined. Comparing these figures with the feed values used by Bykov, we arrive at a ratio of the same order, \(f=(0.8 \div 1.5)S\), which, at a plan angle of \(45^\circ\), corresponds to \(f \simeq (1.0 \div 2.0)a\), where “\(a\)” is the chip thickness, or \(a/f=0.5 \div 1.0 \simeq 0.75\). Approximately the same result is obtained when analyzing the chamfer-width values recommended by the USSR Ministry of Machine-Tool Building standard.

If we now turn to Figs. 6 and 7, it is not difficult to see that it is precisely with this ratio between the chip thickness and the chamfer width \((a/f \simeq 0.75)\) that the built-up edge acquires a stable form. When \(a/f < 0.75\), the rake angle of the built-up edge decreases, its stability on the tool decreases, and the chip shrinkage begins to increase. At very large values of \(a/f\), the stability of the built-up edge is preserved, but the bridge between the groove and the flank face of the tool becomes too narrow in comparison with the chip thickness and heats up strongly, as a result of which tool life is reduced.

Tools with a groove on the rake face (a radiused form of the rake face with a chamfer) are readily used by Stakhanovite high-speed turners, since they ensure breaking and curling of the chip without special chip breakers or chip curlers. At the same time it is known that the ability of such a tool to break the chip depends on the choice of cutting conditions. When the cutting conditions (feed, depth, speed) are changed, the character of chip flow changes. Sometimes it separates in short curls, sometimes it twists into long spirals. Under some conditions breaking ceases altogether. It is not difficult to show that not only all these regularities in the change of the character of chip flow from the tool, but also the very fact of breaking or curling, are in large measure explained by the presence of a built-up edge on the tool with a groove. We carried out several series of experiments with a tool having a groove, in order to study the influence of cutting conditions on chip breaking. The material machined was ShKh-15 steel, and the tool was fitted with a plate of T5K10 alloy. Observations of the character of chip flow showed the following.

1. At small values of the ratio \(a/f\) of chip thickness to tool chamfer width \((a/f < 0.2)\), the chip leaves the tool as a continuous ribbon with a very large curling radius.

2. As the ratio \(a/f\) increases, the chip curling radius continuously decreases.

3. At a ratio \(a/f\) close to 0.5, the chip begins to come off in large curls with a radius of \(8 \div 12\) mm; these curls, striking the flank face of the tool, often break into large half-rings.

4. With a further increase in the ratio \(a/f\), the curling radius decreases still more, and the chip curls into large spirals that move along the tool groove.

5. When the ratio \(a/f\) reaches \(0.6 \div 0.7\), the chip begins to break into small half-rings; these pieces are the smaller, the greater the value of the ratio \(a/f\).

If we turn to Fig. 6, we shall see that with an increase in the ratio \(a/f\) (an increase in chip thickness at constant chamfer width), the rake angle of the built-up edge continuously increases. This is precisely the reason for the decrease in the chip curling radius. At large ratios \(a/f\), when the rake angle is large, the built-up edge forces the chip to curl so sharply that its temporary resistance to bending is exceeded and the chip begins to break into pieces.

Consequently, in order to ensure chip breaking when working with a tool having a groove, it is necessary to maintain a definite ratio between the chamfer width and the chip thickness, at which the rake angle of the built-up edge will be sufficiently large. Since this angle also depends on the cutting speed, decreasing as the latter increases, the speed too must influence the character of chip flow. Experiments showed that chip breaking occurs at the smaller values of the ratio \(a/f\), the lower the cutting speed.

It would be wrong to think that the geometry of the built-up edge is the only factor determining the character of chip flow from the tool. The intensity of breaking or curling of the chip also depends on the angle

the inclination of the main cutting edge of the tool, and on the depth and width of the groove. Therefore, a tool with a groove, as a means for chip breaking, proves to be very sensitive to all kinds of changes in the geometrical parameters of the tool and in the cutting conditions, including those changes that occur as the tool wears. The most intensive wear during operation of a tool with a groove is observed along the edge of intersection of the chamfer plane with the curvilinear surface of the rake face. The rake angle behind the chamfer is thereby reduced and the groove becomes flatter, which leads to an increase in the chip-curl radius and to a deterioration of the conditions for chip breaking.

Summarizing the results of the investigation of the operation of carbide tools with a chamfer at high cutting speeds, we arrive at the following conclusion:

1. On a tool with a chamfer, at high cutting speeds, a stable built-up edge is formed; this is never observed when a tool without a chamfer operates under these conditions.

2. The nature of the change in the geometry of the built-up edge as a function of cutting factors at high speeds is the same as at low speeds: the front angle of the built-up edge increases with an increase in the ratio $a/f$ and decreases only slightly as the cutting speed is increased.

3. At high speeds the built-up edge becomes sharper and overhangs the flank face of the tool less; therefore it affords only weak protection of this face against wear.

4. The presence of a built-up edge on a tool with a chamfer leads to a certain deterioration in the quality of the machined surface in comparison with that obtained when a tool without a chamfer is used.

5. The chip shrinkage and the cutting force on a tool with a chamfer decrease continuously, though only slightly, with increasing speed, up to the very highest speeds.

6. The chip shrinkage decreases as the ratio of the thickness of cut to the width of the chamfer increases.

7. The presence of a built-up edge on a tool with a groove promotes chip curling and breaking.

8. The cutting condition under which proper curling or breaking of the chip is ensured is determined by the combination of the geometrical parameters of the built-up edge and the groove on the tool.

References

1. Eremin, A. N. The Physical Nature of Phenomena in the Cutting of Steels. Mashgiz, 1951.

2. Poletika, M. F. Investigation of the Cutting Process with a Tool Having a Double Rake Face. Author’s abstract of dissertation. Tomsk, 1951.

3. Rozenberg, A. M., and Eremin, A. N. “Toward a Theory of the Process of Metal Cutting.” Stanki i instrument, No. 10, 1950.

4. Rozenberg, A. M., and Zinman, Yu. P. “Certain Questions of Friction in Metal Cutting.” In the collection Problems of Machine Science and Mechanical Engineering, Novosibirsk, ZSO AN, 1951.