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Tribologiczne aspekty procesu skrawania narzędziami z niskostopowej stali szybkotnącej SW3S2

Jaworski J.

Tribologia

2008

nr 2

215-222

The way of wear or destruction of the tool determines the material selection for a proper tool. This selection is facilitated by the characteristics, which determine relations between the structure and the properties of the tool material, on the one hand, and the tool working conditions and the way of wear, on the other. The permissible cutting speed level is determined primarily by the temperature at the contact surface of the tool and the stock material. There is a relationship between the temperature and wear of the cutting edge surface of the tool caused by a progressive loss of its hardness. When the temperature in the cutting process exceeds the value at which the cutting edge of the tool loses its hardness, then dynamic recrystallization causes its immediate catastrophic wear, even at a small value of dullness. The nature of temperature distribution qualifies the place of the wear of the cutting edge. At a small cut layer thickness the maximum temperature occurs near the cutting edge which is the reason of wear of the cutting edge on the flank. Together with the raising thickness of the cut layer, the highest temperature moves away from the cutting edge and its centre lies in the place of contact of the chip and the rake surface. For such cutting conditions, wear in the form of a groove on the rake surface is characteristic. For the same tool material and stock material the thickness of the cut layer has an influence on the temperature distribution, and the cutting speed has an influence - on the value of the temperature. It is known that the growth of cut layer thickness is followed by high increase of build-up and, simultaneously, the range of cutting speeds restricts, where it occurs. Figure 2 shows the cutting forces, the length of contact line bu and chip upset coefficient, for the edges made of SW3S2, which are less than for those made of SW7M. A decrease of the parameters becomes more intensive at the higher cutting speeds, out of the range of build-up creating, vc>60 m/min for both kind of steel. The mean friction coefficient on the friction surface is the basic factor influencing on build-up creating. As it is shown in Fig. 3, the build-ups, occurring in the conditions, are different from one another. On the edge of cuter, made of SW7M, a massive and high buildup occurs which enlarges the tool rake. A build-up with the shape prevents the tool flank against direct contact with the cutting surface and facilitates the chip rising process. The height of the buildup, on the cutting edge made of SW3S2, is smaller so it can be supposed that its influence on decrease of friction on the tool flank is inconsiderable which prevents the cutter against the heat created in the area of rising chip. Consequently the maximum temperature in the cutter rises in the vicinity of the side cutting edge under the base of the build-up. It was shown experimentally that for the thick cut layers the build-up mainly determines the position of the point of the highest temperature on the cutting edge.