PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Volume 157 1970

ON THE DEFORMED STATE IN THE CUTTING ZONE

G. L. KUFAREV, M. G. GOLDSHMIDT

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

Experimental investigation of deformations in metal cutting is a necessary and important step toward understanding the essence and regularities of the process. During chip separation in the cutting zone, shear deformations occur that are very considerable in magnitude and gradient. Since the only reliable method for the experimental study of the deformed state in comparatively small volumes is the study of the distortion of a dividing grid, special requirements are imposed on the latter because of the specific nature of the cutting process. The most preferable dividing grid is a square system of circular marks or circular impressions.

Experiments on the study of the deformed state in the cutting zone by means of screen clichés were carried out in the metal-cutting laboratory.

The surface of a screen cliché, corresponding to the dark background of the original, is a system of etched points close to circles. Depending on the screen and the background, the grid may have different pitches (0.143—0.255 mm) and circle diameters [1]. Screen clichés are usually made of zinc, less often of copper and brass.

The etched points are close in form to conical depressions, and the study of large deformations with ordinary clichés is associated with substantial errors. As experiments have shown, when zinc clichés are cut, the circles are not deformed into geometrically regular ellipses; along the major axis the edges of the ellipses merge. In addition, the use of ordinary clichés greatly limits the range of experimental materials. These shortcomings can be eliminated if the dividing grid is applied to a specimen coated with zinc or copper. In this case the etched points will have a depth equal to the thickness of the electrodeposited coating. It is also significant that such a grid is relief-like and, even under large plastic deformations, is convenient for analysis and photography. The choice of metals for coatings is predetermined by their plastic properties, the strength of their bond with the base material during electroplating, and also by the level of refinement of the technology for making the cliché.

The experiments were carried out on a dividing grid with a pitch of 0.3 mm and an etching diameter of 0.2 mm. The thickness of the zinc coating was 20 microns; the specimen was assembled from four steel (St. 15) plates.

1.5 mm thick, in such a way that the grid was located in the midsection. Cutting was carried out at a speed \(V = 1.84\) m/min with a chip thickness \(a = 1.85\) mm, using a tool with a rake angle \(\gamma = +30^\circ\). The fixed chip root was photographed (Fig. 1) and processed on a projector at a total magnification of 400 times. No broadening was observed on the plate with the division grid, which indicates plane deformation.

Fig. 1. Microphotograph of the chip root. Parameters of the division grid: pitch 0.3 mm, circle diameter 0.2 mm.

Analysis shows that the circles were deformed mainly into regular ellipses. In the narrow near-tool layer, where significant secondary deformation takes place, the pattern is not so definite: apparently, the pitch of the division grid and the diameters of the etchings proved insufficiently small to obtain information on the deformed state in such small volumes. In general, however, the experiments show that the method described above for obtaining division grids can be used to study large plastic deformations and, in particular, the cutting process. Investigation of the deformed state in the cutting zone by means of division grids in the form of raster plates is advisable for chip thicknesses of not less than 0.8–1 mm.

On the basis of the data obtained, it appeared possible to resolve experimentally the question of whether the directions of the principal strains and the principal stresses coincide.

It is known [2] that the axes of ellipses in a plastically deformed zone coincide with the directions of the principal strains. In the cutting zone, the angles of inclination of the major axis of the ellipse to the cutting plane were determined, and in this way a picture of the directions of the principal strains was obtained (Fig. 2).

The distribution of the directions of the principal stresses at various points of the cutting zone was obtained from the results of investigating the stressed state by the hardness-measurement method [3].

It should be noted that the insufficiency of the boundary conditions did not make it possible to determine the stressed state throughout the entire cutting zone. In Fig. 2, numerical values are plotted for the angles of inclination, to the cutting plane, of the directions of the principal stresses determined at the nodes of the slip-line network.

— directions of principal stresses
— — directions of principal strains

Fig. 2. Combined pattern of the directions of the principal strains and principal stresses.

Analysis of the combined pattern shows that, in the main, the directions of the principal stresses and the principal strains do not coincide, and this discrepancy increases sharply as one moves from the free surface of the chip toward the tool tip.

REFERENCES

1. A. Geodokov. Zincography, Moscow, 1962.
2. G. A. Smirnov-Alyaev. Resistance of Materials to Plastic Deformations. Mashgiz, 1949.
3. G. L. Kufarev, G. D. Del, M. G. Goldschmidt. On a method for investigating plastic deformation by measuring hardness. Factory Laboratory, No. 8, 1965.