Study of the Cadmium Cutting Process
Abstract Generated abstract
This study examines the turning of cadmium as part of a broader investigation into machinability and the cutting behavior of soft metals. Experiments on a lathe using high speed steel tools varied rake angle, feed, and cutting speed while measuring chip shrinkage, cutting forces, cutting temperature, and chip contact length, including observations from instantaneous cutting stoppage. The results indicate that cutting speed and feed have little effect on chip shrinkage, that rake angle strongly reduces relative shear while leaving chip shrinkage nearly unchanged over a wide range, and that the apparent friction coefficient increases mainly because normal specific force decreases. The findings question the use of chip shrinkage as a deformation measure and suggest that several relationships observed previously in steel cutting may reflect more general features of metal cutting.
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STUDY OF THE CADMIUM CUTTING PROCESS
M. F. POLETIKA, M. Kh. UTESHEV
(Presented by Professor Doctor A. M. Rozenberg)
The substantiation of a theoretical solution to one of the fundamental questions of metal cutting—the problem of machinability—requires the study of the features of the cutting process for a great variety of materials, including those not used directly for the manufacture of parts. This prompted us to carry out experiments on the cutting of a number of soft metals: cadmium, lead, tin, and others. The present work sets forth the results of a study of the cadmium turning process.
The experiments were conducted on a 1K62 lathe with cutters made of high-speed steel grade R9. The tool-grinding angles were: clearance angle \(\alpha = 10^\circ\), plan approach angle \(\varphi = 70^\circ\), auxiliary plan angle \(\varphi_1 = 10^\circ\), inclination angle of the main cutting edge \(\lambda = 0^\circ\). The rake angle was varied within the range from \(-30^\circ\) to \(60^\circ\).
The depth of cut in all experiments was taken as equal to \(2.5\) mm. The feed was varied from \(0.07\) to \(0.47\) mm/rev, and the cutting speed from 5 to 60 m/min.
In each experiment the chip shrinkage, cutting force, cutting temperature, and length of chip contact with the cutter were measured. To measure the cutting force, a three-component elastic-electric dynamometer with inductive sensors was used. The temperature was determined by the natural-thermocouple method.
In addition, special experiments were carried out with instantaneous stoppage of the cutting process, for which we used a device of the “falling tool” type.
Measurement of the imprints on the contact side of the chip made it possible for us to determine, by the method of N. N. Zorev, the length of the plastic contact between the chip and the cutter.
Figure 1 presents the obtained dependence of chip shrinkage on cutting speed. As the speed increases, the shrinkage decreases, although only very slightly. The influence of speed on shrinkage becomes somewhat more noticeable at a negative rake angle, and even then only in the region of low speeds. The absolute values of shrinkage in cutting cadmium are small and much lower than in machining lead, aluminum, and other nonferrous metals.
No noticeable influence of feed on chip shrinkage was found.
Completely analogous regularities were obtained for all components of the cutting force. With an increase in cutting speed they at first decrease slightly, and beginning at a speed of \(20 \div 25\) m/min;
practically remain constant. Under these conditions the dependence of the cutting force on feed will be linear at constant speed. This made it possible, when determining the forces on the back face by the method of extrapolating the function \(P = f(a)\) to zero chip thickness, not to require conditions of constant temperature, which simplified the calculation of the forces on the front face of the tool and of the coefficient of friction of the chip on the tool.
Fig. 1. Dependence of chip shrinkage on cutting speed. Tool made of high-speed steel R9; front angle \(\gamma = 10^\circ\); plan angle \(\varphi = 70^\circ\); cutting depth \(t = 2.5\) mm; feed \(S = 0.26\) mm/rev.
Measurement of the length of the visible contact and of the length of the plastic contact of the chip with the tool showed that these quantities are practically independent of cutting speed. As the front angle increases they decrease slightly. The plastic portion of the contact is approximately 0.7 of the length of the visible contact.
Finally, Fig. 2 shows the values of some characteristics of the cadmium cutting process as a function of the front angle. The most noteworthy point is that the front angle, over wide limits of its variation, has practically no effect on chip shrinkage. Only at very large front angles is some decrease in shrinkage observed.
We have not yet succeeded in explaining the absence of an influence of the front angle on chip shrinkage. It should be noted, however, that this experimental fact once again indicates the unsuitability of shrinkage for evaluating the degree of deformation of the metal converted into chip. The relative shear, which accurately characterizes the degree of deformation, decreases almost fivefold as the front angle increases.
The coefficient of friction of the chip on the tool when cutting cadmium remains approximately the same in magnitude as in machining other metals by cutting. With an increase in the front angle it also increases, reaching values of 1.0 and more.
In addition to the coefficient of friction, Fig. 2 gives the magnitudes of the specific forces on the front face of the tool, calculated from the visible contact area. Comparing the regularities in the variation of the specific forces \(q_F\) and \(q_N\)
with the curve of the coefficient of friction, we become convinced that the specific friction force \(q\) depends only weakly on the rake angle, and that the increase in the coefficient of friction with an increase in the latter is due chiefly to the decrease in the normal specific force.
Fig. 2. Characteristics of the cutting process as a function of the rake angle of the tool. Tool R9, plan angle \(\varphi = 70^\circ\), depth of cut \(t = 2.5\) mm, feed \(S = 0.26\) mm/rev.
Something similar was obtained by N. N. Zorev in cutting various steels. Cadmium, in its mechanical and other characteristics, differs substantially from steel. Consequently, the relation between the coefficient of friction and the specific normal force on the rake face is a general regularity of the cutting process. This, incidentally, convincingly emphasizes the conventional nature of the concept of the “coefficient of friction” as applied to metal cutting.
In cutting cadmium there is also observed the paradox discovered by N. N. Zorev in machining certain high-alloy steels. There is no logical interrelation between the relative shear and the tangential stress in the shear zone. As follows from a comparison of the curves in Fig. 2, the continuous and significant (up to fivefold) decrease in relative shear is accompanied first by an increase and then by practical constancy of the tangential stress.
Apparently, this too is a manifestation of a certain general dependence characteristic of the cutting process. In any case, N. N. Zorev’s hypothesis, which explains the decrease in stress at high degrees of deformation by the dislocation of shears along grain boundaries, is not applicable here, since cadmium has a homogeneous structure.