STUDY OF THE INFLUENCE OF TURNING MODES ON THE CHIP-BREAKING PROCESS

K. A. NASSONOV, P. N. FALKIN

(Presented by the Department of Mechanical-Engineering Technology, TPI)

In the process of lathe machining, the question of chip formation and chip removal becomes of great importance.

A chip coming off in the form of a ribbon winds around the part, the cutting tool, and their clamping devices, thereby making it necessary to stop the machine in order to remove it. This type of chip creates a danger for the operator in the working zone.

Metalworking practice has by now provided a number of methods that ensure the breaking of ribbon chips or their self-curling [1, 2, 3, 4]. Some of the indicated methods are limited in use for the reason that they require either additional mechanisms mounted on the carriage [3], or a comparatively complex technology for manufacturing chip breakers. The tool geometry tested by us, proposed by V. Bernavsky [1], made it possible to obtain only spiral chips, and of considerable length; that is, here the problem of chip breaking is only partially solved.

Sufficiently satisfactory results were obtained in studying tools made on the basis of the geometry proposed by M. Markov and A. Sergeev [4].

Figure 1 gives the sharpening parameters of the tool used by us.

Fig. 1

A characteristic feature of this tool is the presence of special sharpened portions—chamfers—on the rake face, on the side of the main and auxiliary cutting edges.

As a result of such sharpening, a ridge is formed on the rake face of the tool, dividing the rake face into sections. The presence

this edge and the ribbon ensured, as will be shown below, chip breaking.

The tool rake angle behind the ribbon is 14°, the main clearance angle is 12°, the main cutting-edge angle in the plan view is 52°, and the auxiliary cutting-edge angle in the plan view is 20°.

The rake angle on the ribbon, on the side of the main cutting edge, is negative and amounts to 5–7°; the ribbon on the side of the auxiliary cutting edge was ground at the same angle. The experiments were carried out in turning specimens of steel 45, in the as-delivered condition, on a 1K62 lathe, without cooling and with cooling by a five-percent solution of emulsifying oil in water.

Before the experiments, the working surfaces of the tools were finished on a cast-iron disk with boron-carbide powder.

The investigations were carried out with three different ribbon widths: 0.4 mm, 0.8 mm, and 1.3 mm. Experiments performed without cooling established that use of the tool described above makes it possible to obtain broken chips in the form of rings, half-rings, or even smaller particles. Moreover, changing the cutting conditions within certain limits does not change the character of chip formation.

Only a substantial change in the cutting conditions worsens the favorable character of chip formation, leading to the appearance of spiral chips consisting of several turns (from 3 to 6 turns), or to the formation of ribbon chips.

In Fig. 2, in coordinates of feed—depth of cut, the experimental points are plotted for the study of a tool with a chamfer width equal to 0.8 mm. On this graph, crosses indicate the data of experiments in which

Fig. 2.

we observed good chip breaking, when the resulting chip had the form of half-rings or even smaller particles. Circles indicate cases in which the chip had the form of a continuous spiral or ribbon, while circles with crosses inside characterize those experiments in which chip formation was in an intermediate state: after two to six turns, fracture of the chip occurred. The arrangement of the experimental points on the graph makes it possible to distinguish a certain zone in which good chip breaking occurs. Outside the limits of the marked zone, the chip-breaking process ceases; this transition from one chip form to another occurs gradually.

Experiments carried out with tools having a different width of the chamfer lands on the rake face showed the same qualitative dependence of the shape of the outgoing chip on the turning conditions (depth and feed). However, as the width of the chamfer lands changes, the boundary of the chip-breaking zone changes noticeably. Thus, with a chamfer-land width of 0.4 mm (Fig. 3), this boundary is determined by significantly smaller limits.

Fig. 3.

Fig. 4.

with changes in depth and feed, whereas with a chamfer-land width of 1.3 mm (Fig. 4), the chip-breaking zone is noticeably expanded, and on the graph it was possible to distinguish only its lower boundary.

It should be noted that the experiments described above were carried out at an approximately constant cutting speed of 50–55 m/min and on specimens whose diameter varied only slightly (30–36 mm).

In order to identify the possible influence of the cutting speed and the diameter of the workpiece being machined on the chip-breaking process, special experiments were conducted in which, all other conditions being constant, only the cutting speed or the specimen diameter was varied, respectively. A tool was used whose land width on the rake face was 0.8 mm. The experiments to determine the influence of cutting speed on the chip-breaking process were carried out at a constant feed of 0.34 mm/rev.

In the first series of experiments, the specimens being machined had a diameter of 32 mm, and turning was performed at a depth of cut of 1 mm. In the second series of experiments, the specimen diameter was taken to be 29 mm, and the depth of cut was 0.25 mm. In the first case the cutting speed was varied within the range from 20 m/min to 200 m/min, and in the second—from 18.2 m/min to 145 m/min. However, over such a broad range of variation in cutting speed, in all cases we observed sufficiently good chip breaking.

This fact indicates that variation of the cutting speed within considerable limits does not affect the chip-breaking process. However, it would be erroneous to speak of the absence of an influence of cutting speed on the processes of chip formation and chip breaking in general. This will be shown below.

Investigations to determine the possible direct influence of the diameter of the workpiece being machined on the chip-breaking process were carried out at an approximately constant cutting speed (43–48 m/min), a constant depth of cut (1 mm), and feed (0.34 mm/rev). The diameter of the specimens being turned had the following values, in millimeters: 20, 25, 30, 41, 53, 66.

In all experiments of this series we observed sufficiently good chip breaking, and no noticeable influence of the diameter of the specimens being machined on the chip-breaking process was found.

Figures 5 and 6 present the results of experiments carried out when working with cooling. Here, on the graphs, in depth-of-cut—feed coordinates, with the same conventional designations as above, the data of experiments observing the character of the chip obtained under various cutting conditions are plotted.

On these graphs it also proved possible to distinguish a certain zone within which good chip breaking was observed.

However, comparing the results obtained when working without cooling and when working with cooling, it is not difficult to establish that cooling noticeably changes the boundaries of the chip-breaking zone.

For example, from a joint consideration of Figs. 2 and 5 it follows that cooling significantly narrows this zone, which, in all probability, is explained by the conditions of elastoplastic contact between the chip and the rake face of the tool changing as a result, and, above all, by the changing contact temperature.

In this respect, attention is deserved by experiments carried out with cooling at different cutting speeds using a tool whose land width was taken to be 0.8 mm. Table 1 gives the conditions of these experiments.

Considering the table given, we note that when working with cooling, at a depth of cut equal to 1 mm and a feed of 0.43 mm/rev, at cutting speeds of 55 m/min, 71 m/min, and 90 m/min, we observed the form of the emerging chip as a continuous spiral. But at a cutting speed of

55 m/min, when working without cooling, good chip breaking could be observed (Fig. 2). However, when working with cooling, as the cutting speed increased, and consequently the cutting temperature as well, the shape of the chip being removed changed noticeably. Thus, for the conditions under consideration, at a cutting speed equal to 113 m/min (and at higher speeds), the chip left the cutter finely broken.

Fig. 5.

Fig. 6.

Table 1

For the other conditions indicated in Table 1, there are corresponding critical cutting speeds; exceeding them is accompanied by a change in the shape of the chip being removed: thus, at a depth of cut of 0.6 mm and a feed of 0.34 mm/rev, the appearance of broken chip was observed at a speed equal to 71 m/min, and at a depth of cut of 0.25 mm and a feed of 0.34 mm/rev—130 m/min.

It may be assumed that the critical cutting speeds create, at the contact surfaces of the cutter and the chip, the favorable conditions for chip breaking that occurred in analogous cases when working without cooling at lower speeds, where we observed good chip fragmentation.

All experiments were performed with freshly sharpened cutters. It was undoubtedly of interest to determine what effect tool wear would have on the chip-fragmentation process. For this purpose we carried out an experiment in which the cutter was subjected to natural dulling, while observations were simultaneously made of the character of the chip obtained.

The conditions of the experiment were as follows: cutting speed 50 m/min, feed 0.34 mm/rev, depth of cut 1 mm, operation without cooling. Dulling of the cutter was continued until a wear land 0.5 mm wide was obtained on the rear surface. Throughout the entire experiment, at all stages of wear, we observed good chip fragmentation.

Thus, under the specified machining conditions, cutter wear did not substantially affect the shape of the chip obtained.

Conclusions

1. Cutters having special chamfered grinding facets on the rake face along the cutting edges (Fig. 1), over wide ranges of variation in turning conditions, can ensure reliable chip breaking, are characterized by a simple sharpening technology, and can be recommended for use.

2. When working without cooling, for feeds from 0.2 mm/rev to 1 mm/rev, the width of the chamfered grinding facets may be taken as not less than \((2–1.3)\,S\) mm, with the larger coefficient values corresponding to the smaller feed values in the indicated range, and vice versa.

3. No direct influence of the diameter of the workpieces being machined on the chip-fragmentation process was found.

4. The use of a cooling fluid has a noticeable influence on the shape of the issuing chip, changing the region of machining conditions within which good chip fragmentation is observed when working without cooling. We associate this influence with a change in the conditions of elastic-plastic contact and, above all, with a change in the cutting temperature.

5. A substantial change in cutting speed within certain limits does not change the chip-fragmentation process. However, when working with cooling, for specific machining conditions it is possible to indicate a certain threshold of cutting speeds, the exceeding of which leads to a substantial change in the chip-fragmentation process.

6. Wear of the cutting tool, characterized by a wear land on the rear surface up to 0.5 mm wide, has no noticeable effect on the chip-breaking process.

7. When using the cutters described above under conditions of multi-tool machining, measures should be provided that are aimed at increasing the rigidity of the elastic technological system.

Literature

1. V. Bernavskii. The process of chip utilization and a new cutter design. Machines and Tooling, No. 2, 1958.

2. M. I. Nodelman. An effective method of chip breaking. Machines and Tooling, No. 4, 1959.

3. A. Ya. Malkin and D. I. Semenchenko. Design and operation of automatic lines. Machines and Tooling, No. 8, 1959.

4. B. V. Vinogradov. A cutter with a groove-free chip breaker. Machine Builder, No. 6, 1959.