Possibilities of Natural Fragmentation of Large-Section Steel Chips

G. L. KUFAREV, V. P. PROKOPIEV

(Presented by the scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

One of the main tasks in the development of the country’s national economy for 1971–1975 in the field of labor is the task of “...ensuring further improvement of working conditions, and increasing the provision of enterprises with modern means of safety engineering and labor protection” [1]. This task is especially urgent under the conditions of heavy-engineering plants, where the cross section of the layer removed during turning and vertical-lathe operations reaches values of 40–60 mm², and therefore the resulting chips are sometimes the cause of very serious injuries to the operator. The unfavorable shape of the chips also leads to a reduction in productivity, since the machine often has to be stopped solely in order to remove chips that interfere with normal operation. Sometimes a machine-tool operator is assigned an auxiliary worker whose main duty is chip removal, which substantially lowers the economic indices of the operation.

It was shown in [2] that, by regulating the parameters of the cutting process and the geometry of the cutting part of the tool, it is possible to obtain curling and breaking of chips of medium cross sections by a natural method, without the use of special chip-curling or chip-breaking devices.

This article describes investigations carried out under the conditions of the Krasnoyarsk plant “Sibtyazhmash,” in which the possibility of curling and breaking chips during the machining of products made of steel 40 on vertical lathes was tested.

Machining was performed with a cutter made of T5K10 alloy, having the following geometric parameters: rake angle $\gamma = 0^\circ$, rake angle on the chamfer $\gamma_f = -5^\circ$, principal cutting-edge angle in plan $\varphi = 60^\circ$, principal clearance angle $\alpha = 8^\circ$, inclination angle of the main cutting edge $\lambda = 0^\circ$. The cutting speed varied from 25 to 134 m/min. The feed ranged from 1.03 to 1.64 mm/rev.

Cutting was carried out at three depths: 10, 15, and 20 mm. Thus, in the experiments the cross-sectional area of the removed layer reached values somewhat exceeding 30 mm².

The resulting chips obtained in the investigations described can be divided into three types: ribbon-like, practically non-curling chips; spiral chips, which curl in a plane perpendicular to the rake face of the tool and come off in the form of a spiral of indeterminate length; and broken chips, which, while curling in a plane perpendicular to the rake face, break into pieces equal to one turn of the spiral or part of it.

In Fig. 1 the experimental results are summarized in a graph that makes it possible to determine the regions of existence of chips of various types. Analysis of this graph makes it possible to assert that chip fragmentation is the more probable, the smaller the ratio of the width of the layer being cut to its thickness and the higher the cutting speed. At the same time it should be noted,

Fig. 1.

that, for medium cross sections of cut \((2\text{–}4\ \mathrm{mm}^2)\), an increase in cutting speed leads to an increase in the radius of chip curling, i.e., to the production of step-like chips.

The graph in Fig. 2 makes it possible to determine the boundary of transition of spiral chips into fragmented chips as a function of the parameter \(\dfrac{a \cdot v}{b}\), where \(a\) and \(b\) are, respectively, the thickness and width of the layer being cut, and \(v\) is the cutting speed.

It was found that, for each depth of cut, there exists a certain zone in which the appearance of chips of both types is possible. Thus, for example, at \(t = 20\ \mathrm{mm}\), the appearance of both types of chips is possible at \(\dfrac{a \cdot v}{b} = 3.4\text{–}4.05\ \mathrm{m/min}\).

The depth of cut affects the chip type not only through a change in the ratio, but also directly. This follows from the fact that in Fig. 2 the boundary of transition of spiral chips into step-like chips is the higher, the greater the depth of cut. It may be assumed that this is a consequence of the different stiffness of the chip at a constant ratio \(\dfrac{b}{a}\).

Since the cutting depth is most often a specified quantity, changes in the type of chip can be achieved by varying only the feed and the cutting speed.

It should be noted that, from the standpoint of productivity, it is immaterial which of these parameters is increased, since both an increase in feed and an increase in cutting speed lead to its growth.

Fig. 2

From the standpoint of attaining the greatest tool life, it is advisable to increase the product \(a \cdot v\) by means of the feed. However, it should be borne in mind that when working with large chip cross sections, an increase in feed may lead to a loss of strength of the cutting wedge. In this case, the transition from spiral chip to fragmented chip should be accomplished by increasing the cutting speed.

The introduction of the results of these studies at the Sibtiazhmash and Krasnoyarsk Cement plants made it possible to increase productivity in turning and vertical-lathe operations by a factor of 1.3, while maintaining the tool-life period and ensuring safe work in these operations.

REFERENCES

1. “Directives of the XXIV Congress of the CPSU on the Five-Year Plan for the Development of the National Economy of the USSR for 1971–1975.”

2. G. L. Kufarev, K. B. Okenov, V. A. Govorukhin. Chip Formation and Quality of the Machined Surface in Non-Free Cutting. Frunze, “Mektep,” 1970.