INDEXABLE INSERT WITH CORNER PORTIONS HAVING FIVE DIFFERENT RADIUS SEGMENTS

Abstract: An indexable cutting insert for turning has in each operative corner portion a sequence of at least five different radius segments (5A, 5A', 5B, 5B', 5C). The middle segment (5C) has a first radius (r epsilon ), the two adjacent segments (5B, 5B') both have a second radius (r2), and the two segments (5A, 5A') located outside those segments both have a third radius (r3), the relation between the radii being r3 > R epsilon > R2. Possibly, there may also be two fourth segments with a fourth radius (r4) adjacent and outside the third segments (5A, 5A'). Cutting inserts with nose radii modified in this way have made possible considerably better surface smoothnesses than cutting inserts with a constant radius. Furthermore, the sensitivity to the positioning of a correct entering angle has been considerably reduced.

Description: INDEXABLE INSERT WITH CORNER PORTIONS HAVING FIVE DIFFERENT RADIUS SEGMENTS The present invention relates to a metal cutting insert which is primarily intended for turning, with a modified corner or nose radius according to the characterizing portion of claim 1. The insert is normally indexable, i.e., it has more than one operative corner cutting edge and when one corner has been worn out, the insert can be turned or indexed so that a new corner becomes active. For all sorts of turning, there is a certain interplay between the feed rate and the corner radius, the latter being the connection between the main cutting edge and the secondary cutting edge. Thus, for rough machining, a largest possible corner radius should be chosen in order to obtain a strong cutting edge. On the other hand, a corner radius too large may cause vibrations. Therefore, for fine turning usually a smaller corner radius is selected, normally always smaller than 2 mm. The disadvantage with a small corner radius is that it increases the wear-down speed of the insert, thereby deteriorating the life and the performance. The surface smoothness attined on the workpiece is specifically influenced by the interplay between the corner radius and the feed rate. The entering angle is defined as the angle between the main cutting edge and the direction of feed. This angle has a considerable influence on the interrelation between the different cutting force components, and thereby also on the surface smoothness and dimension accuracy. The angle of tool back clearance is the angle between the secondary cutting edge and the direction of feed and it affects the smoothness of the turned surface. It is evident that for a specific insert it is not possible to alter the entering angle without simultaneously changing the angle of tool back clearance. Thus, surface smoothness and dimension accuracy are very sensitive to changes of the entering angle. A fundamental problem for all turning operations is that the desired smoothness is not achieved. Sometimes the surface smoothness can be improved by using higher cutting speeds and neutral or positive rake angles. However, the CONFIRMATION sensitivity to the adjustment of the entering angle remains. Thus, a first object of the present invention is to improve the smoothness of the machined surface. A second object of the present invention is to reduce the sensitivity of a turning cutting insert relative to the positioned entering angle. These and further objects have been attained in a surprising way by forming a turning cutting insert with a specially composed sequence of corner radii in accordance with the characterizing portion of claim 1. For illustrative but non-limiting purposes, some preferred embodiments of the invention will now be further described with reference to the appended drawings, which are herewith presented: Figure 1 shows a side view obliquely from above of a corner of a turning cutting insert according to the invention. Figure 2 shows a corner portion of a turning cutting insert according to the invention straight from above, with the different corner radii illustrated. Figure 3 shows a corner portion of another turning cutting insert according to the invention, straight from above, with the different radii illustrated. Figure 4 shows schematically the different radii which may be comprised in a corner portion according to the present invention. Figure 5 shows a diagram relative to how the surface smoothness of the workpiece is improved thanks to the present invention, at different feeds. Figure 1 shows a corner or nose portion of a turning cutting insert according to the present invention, which may be either single- or double-sided. It is usually made of cemented carbide but it may also be made of different ceramic materials. Because of the cost of production, it is suitably directly pressed. According to this embodiment (Fig. 1) , each operative cutting corner comprises on the upper side of the insert (and at double-sided inserts, also on the opposed side) , a curved protrusion 1 according to our Swedish patent application No. 9301132-8 (& WO-A-94/22622) , which application is hereby incorporated by this reference. In immediate connection to cutting edges 2 and 2', and also to the corner cutting edge, a primary land 3 adjoins on the upper side of the insert, which primary land is substantially perpendicular to the side surfaces 4,4' of the insert and to the rounded corner radius surface 5 of the insert . A rake surface 6 slopes downwardly and inwardly from the primary land 3. The curved protrusion 1 merges into and overlaps with the rake surface 6 and delimits thereby a corner surface portion 7. The cutting insert is rhombic, the corner portion shown in Figure 1 having a total corner radius of substantially 80°. However, it may also be square, rectangular, triangular or hexagonal. When it is hexagonal, it may also be in the form of a so called trigonal insert. The essential feature of the present invention is that the corner radius is differentiated, which is illustrated in Figures 2, 3 and 4. According to a number of tests, this differentiation of the corner radius has brought about a surprisingly better surface smoothness of the machined surface in comparison with the same insert with a constant radius, and has also made the insert considerably less sensitive to the positioning of the entering angle. The differentiation may be asymmetric in relation to the corner bisector; however it is preferred to make the corner radii sequences symmetrically on the two sides of the corner bisector. This results in the essential advantage that the cutting insert becomes symmetrical when turning it over, thereby enabling both left-hand and right- hand turning with the same insert, i.e., both left-hand and right-hand holders may be used. Further, a symmetrical corner geometry makes possible a symmetrical turning axially as well as radially. In order to illustrate the different corner radii, the imaginary circle sectors are reproduced in Figures 2 and 3. The cutting corner in Figure 2 has an angle of substantially 80° and is divided into five different circle or cylinder-surface segments 5A, 5B, 5C, 5A' and 5B' . The middle segment 5C has a radius r £ . The two segments 5B and 5B' have substantially the same radius, which is designated r 2 . Also the segments 5A and 5A' have substantially the same radius, which is designated r 3 . For all cutting inserts according to the invention, it is valid that The smallest difference between two radii should be 0,1 mm, preferably 0,2 mm, and in particular 0,3 mm. As long as this minimum difference is upheld, the three radii may vary within rather wide ranges. Thus, r £ may be between 0,2 and 5 mm, preferrably between 0,4 and 3, 2 mm and in particular between 0,4 and 1,6 mm. Radius r 2 may be between 0,1 and 4 mm, preferrably between 0,2 and 3 , 2 mm and in particular between 0,4 and 1,5 mm. Radius r 3 may be between 1 and 10 mm, preferrably between 2,4 and 10 mm and in particular between 4 and 8 mm. Depending upon the corner radius of the cutting insert, the center angles a , & and δ of the imaginary circle sectors may also vary within wide ranges. However, considering the desired turning-over symmetry of the insert, the center angles δ of segments 5A and 5A' should be substantially equally large. The same is of course true for center angles β. It may be generally said that the center angle should be between 1 and 145°, preferably between 5 and 90° and in particular between 10 and 40°. Each of the angles β should be between 1 and 45°, preferrably between 10 and 40° and in particular between 15 and 35°. Each of angles δ should be between 1 and 90°, preferrably between 1 and 15°, and in particular between 2 and 10°. According to a preferred embodiment of the present invention, and particularly at smaller point angles (for instance less than 80°) , also a fourth radius r 4 should be provided symmetrically on both sides of the bisector of the cutting corner. As may be seen in Figure 3, this radius is then positioned between segment 5A and side surface 4, and between segment 5A' and side surface 4', respectively. Radius r 4 should be at least 1 mm smaller than radius r 3 and is suitably between 0,1 and 5,0 mm, preferrably between 0,1 and 2,4 mm and in particular between 0,2 and 1,0 mm. The corresponding center angle σ of the sector should be between 1 and 45°, preferably between 5 and 40° and in particular between 15 and 35°. The connections between the different circle segments should be tangential. Thus, this for example means that the tangent of radius r 2 at the transition point to radius r 3 coincides with the tangent of radius r 3 in the same point . This results in smooth radius transitions and increases the insensitivity to a correct adjustment of the entering angle. Figure 4 illustrates the different radii which together constitute a modified corner radius according to the present invention. The straight lines 2 and 2' designate the two adjacent cutting edges. In the area around the intersectional point of the cutting edge and the bisector B, r e is located radially inside the circle of radius r 2 , thereby forming two intersectional points or transitions between r 2 and r £ , one on each side of the bisector. Between such an intersectional point and cutting edge 2 (or 2') , there is a further intersectional point or transition, namely the one between radius r 3 and radius r 2 . The tangent in the latter point forms a clearance angle θ with cutting edge 2 (or 2 ' ) . This clearance angle θ is suitably between 4 and 29° and preferably between 3 and 7° for rhombic inserts or so called trigonal inserts. For the sake of comparison, a cutting insert according to the present invention was tested together with and compared to a usual turning cutting insert with constant nose radius. Both inserts had a nose angle of 80°. The insert according to the invention had the following data: r e = 0,79 mm α = 24° r 2 = 0,65 mm β = 31° r 3 = 4,7 mm δ = 7 The known insert had a continuous radius r e of 0,79 mm. Both cutting inserts had a chip breaking geometry according to Figure 1 and a clearance angle θ of 5°. The results at machining of low alloy wrought steel (SS 2511) are shown in the diagram in Figure 5. On the y-axis is given the obtained surface smoothness R a in μm and on the x-axis is given the feed in mm/revolution. The cutting speed was 180 m/min and the cutting depth was 2 mm. From this diagram it may for instance be seen that the R a -value at a feed of 0,8 mm/rev. was substantially halved. From this, the superiority of cutting inserts with a nose radius sequence according to the present invention, is clearly shown in comparison with otherwise equal inserts but with a constant nose radius. Moreover, contrary to the known cutting insert, an alteration of the entering angle from 91 to 93° had no prejudicial influence on the surface smoothness attained by turning with the insert according to the invention. According to the above preferred embodiments, double- sided cutting inserts have been described and, therefore, the side surfaces 4 and 4' are perpendicular to the upper and lower surfaces. However, it is evident that if the insert is single- sided, the flank or side surfaces 4,4', and the nose surfaces 5A-5C, preferrably form an acute angle with the upper surface, the preferred insert-inherent angle of clearance being between 5 and 11°.

Claims: C A I M S 1. A cutting insert primarily intended for turning comprising an upper surface, a bottom surface which is substantially plane- parallel with said upper surface, and at least three side surfaces (4,4') extending between said upper and bottom surfaces, the transition between two adjacent side surfaces forming a rounded nose radius surface (5) and the intersection line between said upper surface and side surface or nose radius surface, respectively, forming cutting edges, c h a r a c t e r i z e d in that the nose cutting edge is divided into at least five circle segments. 2. Cutting insert according to claim 1, c h a r a c t e r i z e d in that at least the part of the nose radius surface (5) adjacent to the nose cutting edge is correspondingly divided into at least 'five cylinder-surface segments (5A-5C) . 3. Cutting insert according to claim 2, c h a r a c t e r i z e d in that the whole height of the nose radius surface (5) , from the upper surface to the lower surface, is divided into said cylinder-surface segments. 4. Cutting insert according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the five segments have at least three different radii, adjacent segments always having different radii. 5. Cutting insert according to any of claims 1 to 4, c h a r a c t e r i z e d in that the bisector (B) of the nose radius intersects a first segment (5C) ' with a first radius (r ε ) , in that on both sides of this segment adjoin second segments (5B, 5B') with a substantially equally large second radius (r 2 ) , and in that to these second segments adjoin two third segments (5A, 5A' ) with a substantially equally large third radius (r 3 ) , the relation being r 3 > r 6 > r 2 . 6. Cutting insert according to any of ,the preceding claims, c h a r a c t e r i z e d in that two further segments with a substantially equally large fourth radius (r 4 ) are located between the third segment (5A, 5A' ) and the respective side surface (4, 4' ) . 7. Cutting insert according to any of the preceding claims, c h a r a c t e r i z e d in that the smallest radius difference between two radii is 0,2 mm. 8. Cutting insert according to any of the preceding claims, c h a r a c t e r i z e d in that the first radius (r £ ) is between 0,4 and 1,6 mm, that the second radius (r 2 ) is between 0,4 and 1,5 mm and in that the third radius (r 3 ) is between 4 and 8 mm. 9. Cutting insert according to any of the preceding claims, c h a r a c t e r i z e d in that the first segment (5C) corresponds to a circle sector with a center angle (α) of between 5 and 90°, that the second segments (5B, 5B') correspond to two substantially equally large circle sectors, both with a center angle (δ) of between 10 and 40°, and in that the third segments (5A, 5A' ) correspond to two further, substantially equally large circle sectors, both with a center angle (δ) of between 1 and 15°. 10. Cutting insert according to claim 6, c h a r a c t e r i z e d in that the fourth radius (r 4 ) is between 0,2 and 1,0 mm and corresponds to a circle sector with a center angle (σ) of between 15 and 35°.

Inventors: LUNDSTROEM JAN, WIMAN JOERGEN, LUNDSTROEM, JAN, WIMAN, JOERGEN
Applicants: LUNDSTROEM JAN, SANDVIK AB, WIMAN JOERGEN, LUNDSTROEM, JAN, WIMAN, JOERGEN
Application year: 1995
Main classification: B23B27
IPC codes: B23B27, B23C5