INDEXABLE CUTTING INSERT

Abstract: An indexable cutting insert (3) for cutting metal machining has substantially the shape of a star. This makes possible a large number of cutting edges per insert and, thereby, a very good cutting economy. E.g., for an eight-pointed, double-sided cutting insert there are sixteen cutting edges.

Description: INDEXABLE CUTTING INSERT The present invention relates to an indexable cutting insert for cutting metal machining according to the preamble of claim 1. More precisely, in first hand the invention relates to an indexable cutting insert for turning, that includes a large number of cutting edges. Both external turning and internal turning may be involved. For internal turning either the workpiece may rotate, in the same way as for external turning, or the workpiece and the boring bar may rotate. The use of detachable, indexable cutting inserts is nowadays common practice for almost all types of cutting metal machining, i.e., for turning, milling, drilling, etc. They are normally made of coated or uncoated cemented carbide, although certain ceramic materials occur. Further, cemented carbide inserts with an inlay or a so called "flake" of an extremely hard material in the operative parts of the cutting edge also occur, which inlay may be produced of diamond or cubic boron nitride. These different indexable cutting inserts may have a very varying geometrical basic shape, depending on cutting economy and application. For instance, for turning they may be triangular, square or rhombic. Furthermore, they may be double- or single-sided. In principle, a double- sided cutting insert has twice as many operative cutting edges as a corresponding single- sided insert. The highest number of operative cutting edges which is possible to attain for the time being is on a double-sided, square cutting insert, where maximally eight cutting edges are obtained. It is true that it would be feasible to use, e.g., hexagonal cutting inserts for turning, but in practice they do not occur, since the entering angle by necessity becomes small and also the cutting depth becomes restricted. Therefore, the endeavour continues to achieve a higher number of operative cutting edges per turning cutting insert and the improved cutting economy associated therewith. Thus, a primary object of the present invention is to increase the number of cutting edges per indexable cutting insert, in particular for indexable cutting inserts for turning. A second object of the present invention is to provide an insert with a large number of cutting edges, which is strong, has a good capability of leading away heat from the cutting location and is easy to press. Still another object of the present invention is to make possible a simple insert seat, in spite of the large number of cutting edges, with only a small number of point or straight line contacts. Yet another object of the present invention is to provide a cutting insert with which you may perform a number of turning operations, without having to change cutting inserts and without having to turn the insert in the holder. These and further objects have been achieved in a surprising way by shaping an indexable cutting insert that comprises the features as defined in the characterizing clause of claim 1. For illustrative but non-limiting purposes, the invention will now be further described under reference to the appended drawings. These are herewith briefly presented: Fig 1 shows an exploded view in a perspective obliquely from above of a mounted insert seat according to the invention. Fig 2 shows an indexable cutting insert according to the invention in a perspective obliquely from above. Fig 3 shows a top view of an indexable cutting insert according to fig 2. Fig 4 shows a boring bar with a mounted insert seat according to the invention and a workpiece. Fig 5 shows a boring bar with a mounted insert seat according to another embodiment of the invention. Fig 1 shows generally a shaft holder 1 with an insert seat 2 for the accomodation of an indexable cutting insert 3 according to the present invention. The indexable cutting insert is suitably, but not necessarily, mounted on a shim 4, which protects the shaft holder at possible insert break-downs. The shim 4 rests against the bottom support surface 5, whilst the indexable cutting insert 3 bears against the shim. Further, the indexable cutting insert is clamped in the insert seat to a stable abutment, on one hand against the shim and on the other hand against the side support surfaces 6, 7, 8 and 9. Alternatively, the cutting insert abuts only against three bearing points, e.g. the surfaces 6, 8 and 9, in order to give a statically well-defined position. The clamping mechanism as shown in the figure consists of a lever clamp 10 of an L-shaped basic form that comprises a lower support shoulder 1 1. Said shoulder is introduced into the hole 12, which has an elongated cross-section in order to make possible the introduction of the clamp. The end of the substantially horizontal leg of the clamp engages into a circle-shaped narrower portion 13 in a locking screw 14. As is easily understood, by screwing the clamping screw 14 downwards, the other, substantially vertical leg of the clamp is brought to tip towards the insert seat and clamp the cutting insert 3, by pressing against the hole wall in the center hole 15 of the latter. In order to position the shim during the clamping of the indexable cutting insert, the former is stabilized by a slitted sleeve 16, which is introduced into the center hole of the shim, over the upper leg 10 of the clamp, before the cutting insert 3 is placed on the shim. However, it should be pointed out that the method of clamping per se does not constitute any essential feature of the present invention. Thus, the indexable cutting insert according to the invention may also be clamped by a threaded locking screw through a corresponding through-hole in the insert, or by a clamp that presses against the top surface of the insert, suitably in a recess on the top side of the insert. An eccentric screw according to US-A-4 430 031 may also be used. Nor is the presence of a shim essential for the invention. Flowever, if a shim is present, it should comprise protruding portions corresponding to the protruding portions of the insert (although they do not protrude as far as the portions of the insert), to avoid too large free overhangs for the insert. Correspondingly, also the bottom support surface 5 is shaped along its free, open edge 40 with protruding portions 5a to give sufficient support to the cutting insert and the shim, if a shim is present. Further, the existence of protruding portions 5a in the two positions adjacent the operative cutting corner gives the essential advantage that the insert becomes considerably more difficult to tilt in an undesirable way. The cutting insert according to the invention has a star-like basic shape, as may be seen in all five figures. Thus, in figures 2 and 3 an eight-pointed star-shaped indexable cutting insert is shown in more detail. The star-like basic shape is conferred by the substantially planar clearance or side surfaces 18, which pair-wise form inwardly directed corners 34, at the same time as they form outwardly directed corners or points 17 with the two side surfaces 18 adjacent to either side of said pair. If the insert is single- sided, the side surfaces may form an acute angle to the chip surface and an obtuse angle to the bottom surface. The acute angle may for instance be between 75 and 88°. If the insert is double-sided, then the clearance surfaces normally form right angles to the two chip surfaces. However, it may also be double-positive in accordance with US-A- 5 275 633, whose description is hereby incorporated by this reference. According to the shown example, the two clearance surfaces 18 of each point 17 form a right angle with each other, as is best seen in fig 3. Typically, each point 17 has a nose radius 19, which is chosen having regard to feed and cutting speed. The transition between the substantially straight cutting edge and the nose radius edge may also be provided with a section of modified nose radius, a so called "wiper", to give a better surface finish at high feed rates. The shown insert is formed as two equal squares that have been placed on top of each other, whereafter one of them has been turned by 45° around the common central axis 20. Thereby an indexable cutting insert is obtained with strong points, which enable the normally occurring cutting depths. According to fig 2, the transition between two adjacent points 17 takes place over a break line 28, but it may also take place over a fillet or a radius. The indexable cutting insert may have a number of different sintered-in chip breakers, such as the C-like ridges 29. However, the geometry of the chip surface is not an essential feature of the invention, the geometry as shown in fig 2 and 3 only being an illustrative example that is the object of a previous patent application (see WO 94/22622), which is hereby incorporated into the present application by this reference. Moreover, the shown cutting insert is double-sided, wherefore it on the chip surface has bearing surfaces 30. Another example of a possible embodiment is a completely smooth top side, as shown in fig 1 and 4. With this cutting insert, it is possible to make a sequence of different turning applications using different cutting edges of the same insert, without having to replace or indexate it and without altering the axial direction of the boring bar, as may be seen in fig 4. Thus, on a rotating workpiece 21 one may first make a facing machining 22 by means of the point 23, thereafter a chamfering 24 by means of the point 25, and thereafter a longitudinal turning 26 by means of the point 27. This shows clearly the high poly valence of the indexable cutting insert according to the invention, with a higher productivity as a consequence thereof. Moreover, the eight-pointed, double-sided insert has the hitherto unrivalled number of sixteen operative cutting edges, which doubles the cutting economy in comparison with the usual double-sided inserts of a square basic shape. In relation to each operative cutting edge, also the material consumption for the production of inserts decreases, and so does the insert surface to coat, the weight, the storage volume, etc. For practically all steps during the insert manufacturing, the labour per cutting edge may be halved, such as for instance pressing, sintering, grinding, marking, etc. Moreover, by the fact that the cross-section of the points augments rapidly, strong cutting edges are obtained and the tranport of heat is increased. As indicated by the black point corners 31 in fig 1 and 4, the indexable cutting inserts may be provided with an inlay of, e.g., diamond or cubic boron nitride (CBN) in order to further increase the hardness and useful life of the insert. For double- sided inserts, these inlays may be through, i.e., from the top side to the bottom side of the insert. Although the points in fig 1-4 are right-angled, the point angles may be both larger and smaller than 90°. However, too large point angles cause small entering angles and small cutting depths. Too small point angles result in weak inserts and a deteriorated heat transport. Suitable point angles α may be between 50 and 120°, preferably between 70 and 100°. In order to give place to the non-operative, inwardly directed points, the insert seat may suitably be provided with borings 32 and 33, respectively. Fig 5 shows another embodiment of the present invention. Equally to fig 4, the cutting insert is mounted by means of a lever clamp in a boring bar 1 '. The difference to fig 4 is that the cutting insert 35 is six-pointed instead of eight-pointed. It is shaped as two regular triangles laid over each other, from which follows that the point angles are 60°. However, in the same way as the eight-pointed cutting insert, also other point-angles may be considered, suitably within the ranges given for the eight-pointed cutting inserts. Also this six-pointed cutting insert gives a possibility to make longitudinal turning as well as facing machining with the same insert positioning, as indicated by the two arrows in fig 5, and to some extent also make chamferings. The shown cutting insert has a simpler sintered-in chip breaker 36, which however is unessential for the inventive concept. The insert bears against three side support surfaces, while the remaining side surfaces are free surfaces. Without departing from the basic star-like shape of the cutting insert, a cutting insert according to the invention may suitably comprise between 5 and 12 points, preferably between 6 and 8. The limiting factor upwards is that the possible cutting depth becomes smaller and smaller. The limiting factor downwards is that the advantage with a large number of cutting edges gets lost. Also the relation between the radii for the inner corners and the point tips (i.e., the relation b/a in fig 3) may vary within wide limits. Suitably, this relation is between 0,50 and 0,90, preferably between 0,55 and 0,85.

Claims: CLAIMS 1. Indexable cutting insert for cutting metal machining, characterized in that it has a star-like basic shape. 2. Indexable cutting insert according to claim 1, characterized in that the star shape has between 5 and 12 points. 3. Indexable cutting insert according to claim 1 or 2, characterized in that the tips of the points ( 17) arc substantially equidistantly placed around a common circle. 4. Indexable cutting insert according to any of the preceding claims, characterized in that the intermediate distance between two point tips or nose radii essentially consists of two straight lines, the intersection (28) between these being closer to the center (20) of the insert than the point tips. 5. Indexable cutting insert according to any of the preceding claims, characterized in that the bisector for each point extends substantially through the center (20) of the cutting insert. 6. Indexable cutting insert according to any of the preceding claims. characterized in that the point angles (α) are between 50 and 120°. 7. Holder for cutting metal machining comprising an insert seat intended to accomodate an indexable cutting insert according to any of claims 1-6, characterized in that the insert seal comprises a bottom support surface (5) for the indexable cutting insert or for a shim (4), which bottom support surface along its open ede (40) has at least two protruding portions (5a) which are intended to support corresponding points of the cutting insert or the shim. 8. Holder according to claim 6, characterized in that the protruding portions (5a) have a substantially triangular basic shape.

Inventors: GYLLENGAHM STEFAN, GYLLENGAHM, STEFAN
Applicants: GYLLENGAHM STEFAN, SANDVIK AB, GYLLENGAHM, STEFAN, SANDVIK AB (PUBL)
Application year: 1997
Main classification: B23B27
IPC codes: B23B27