CUTTING INSERT FOR HEAVY TURNING PURPOSES

Abstract: The invention relates to a cutting insert (1) intended for heavy turning having a top side (2), a bottom side (3) and an edge (6) in the area between the top side (2) and a front surface (4). Characteristic for the cutting insert is that the same, in addition to a comparatively thick front part (20) having a chip breaker (9) positioned behind the edge (6), comprises a rear part (21), which is thinner than the front part (20) and tapers in the rearwards direction therefrom, in the top side of the rear part (21) a recess (22) ports for a clamping unit for fixing the cutting insert in a co-operating seat.

Description: CUTTING INSERT FOR HEAVY TURNING PURPOSES Technical field of the invention This invention relates to a cutting insert intended for heavy turning, which has a top side, a bottom side and an edge in the area between the top side and a front surface, the cutting insert comprising, in addition to a comparatively thick front part having a chip breaker positioned behind the edge, a rear part, which is thinner than the front part. Prior art In comparison with conventional turning, heavy turning is generally distinguished by the fact that in the main all parameters involved are considerably enlarged, e.g. greater cutting forces, larger chip thickness, larger and heavier workpieces, greater heat, etc. The feed during conventional turning usually is within the range of 0,05-0,8 mm/revolutions, while the feed during heavy turning is within the range of 1-40 mm/revolutions at the same time as the cutting depth may vary within the range of 0,2-6 mm. It is not unusual for the workpieces in question, e.g. rough shafts, to have a weight of one or more tons. Under these circumstances, it is not only necessary that the turning inserts but also cassettes and carriers for the cutting inserts have to be generously dimensioned. Cutting inserts for heavy turning purposes are previously known in a plurality of different embodiments. A usual embodiment is characterized by a large thickness in relation to the depth or length of the cutting insert (the height of the front surface which extends between the edge and the bottom side of the cutting insert is only slightly smaller than the length of the cutting insert, counted as the distance between the front surface and a rear end surface). Rearwards from the edge, a surface sloping obliquely downwards/rearwards extends in which a straight chip breaking shoulder is formed. Said inclined top surface ends in a delimitation surface substantially parallel to the front surface, which delimitation surface in turn at the bottom transforms into a surface inclined rearwards/upwards on a short rear end portion in which a central, rearwardly porting opening is formed for a pin which has the purpose of fixing the cutting insert with the aim of counteracting lateral movements of the cutting insert. A clamping unit may be pressed against the surface inclined rearwards/upwards on the rear end portion of the cutting insert, by means of which the cutting insert is kept clamped with the rear end surface thereof in contact with an abutment surface in the cassette in which the cutting insert can be assembled, at the same time as the bottom side of the cutting insert is kept pressed against a bottom surface in the cassette. However, a disadvantage of this known cutting insert for heavy turning is that the same in practice is only suitable for grooving operations, but not for longitudinal turning, inasmuch as the lateral stability of the cutting insert is mediocre. Thus, disturbances of the cutting insert longitudinally are only counteracted by the above-mentioned pin, which for technical, dimensioning reasons is comparatively weak. Another disadvantage of the cutting insert is that the same requires extensive and expensive grinding. Thus, not only the top surface sloping obliquely downwards and from the edge rearwards but also the front surface and the bottom side must be precision-ground in a plurality of steps. An additional disadvantage is that the clamping unit for the cutting insert is exposed to considerable stresses caused by the arrival of hot chips since the top surface of the cutting insert slopes obliquely downwards/rearwards against the clamping unit. Furthermore, the known cutting insert has a limited ability to reduce the width of the chip after separation of the same from the workpiece by the cutting edge. This is something that makes chip release from the groove which is cut more difficult. Aims and features of the invention The present invention aims at obviating the above-mentioned inconveniences of the previously known cutting insert for heavy turning and at providing an improved cutting insert for heavy turning purposes. Therefore, a primary aim of the invention is to provide a cutting insert for heavy turning, which may be secured in a stable and reliable way, and thereby not only be used for plunge-cut operations, but also for longitudinal turning during which the cutting insert is exposed to considerable lateral forces. An additional aim is to create a cutting insert for heavy turning, which is suitable for manufacture by direct pressing, e.g. of cemented carbide powder, while minimizing the number of grinding operations. Yet another aim of the invention is to create a cutting insert for heavy turning having good chip breaking properties with the purpose of improving the protection of the clamping unit against the destructive effect of the hot chip and thereby increase the service life of the clamping unit. In a particular aspect, the invention also aims at providing a cutting insert for heavy turning which quickly and effectively reduces the width of the separated chip with the purpose of facilitating chip transport. According to the invention, at least the primary aim is attained by the features defined in the characterizing clause of claim 1. Preferred embodiments of the invention are furthermore defined in the dependent claims. Brief description of the appended drawings In the drawings: Fig 1 is a perspective view of a cutting insert according to the invention as seen obliquely from above and from the front, Fig 2 is a planar view from above of the cutting insert according to fig 1, Fig 3 is a longitudinal section through the same cutting insert, Fig 4 is a section showing the cutting insert mounted in a cassette by means of a clamp, Fig 5 is a planar view from above of the cutting insert, the cassette and the clamp according to fig 4, and Fig 6 is a perspective view illustrating an alternative embodiment of the cutting insert according to the invention. Detailed description of preferred embodiments of the invention In figs 1-3, a first embodiment of a cutting insert according to the invention designated 1 is illustrated. At the top, said cutting insert is defined by a topside generally designated 2, which is opposite a bottom side designated 3. Between the topside 2 and the bottom side 3, a front surface 4 extends which in turn is opposite a rear end surface 5. hi the area between the front surface and the topside 2, a cutting edge 6 is formed, which in a conventional way consists of a sharp edge between the front surface 4 and a plane reinforcement surface 6'. At opposite ends thereof, the front surface 4 transforms into two lateral flank surfaces 7, more precisely via rounded transition surfaces 8. The width B of the cutting insert is defined by the distance between said lateral flank surfaces 7, while the depth or length L of the cutting insert is defined as the distance between the front surface 4 and the rear end surface 5. In the area behind the cutting edge 6, a chip breaker in its entirety designated 9 is formed in the topside of the cutting insert. In practice, the cutting insert according to the invention may be made by direct pressing of cemented carbide powder, the only requisite grinding consisting of face grinding of the bottom side 3. Before the features of the cutting insert characteristic for the invention are described in more detail, reference is made to figs 4 and 5, which illustrate the cutting insert 1 mounted in a cassette or holder generally designated 10, which in turn is, in a suitable way, detachably connected to a larger carrier (not shown), (e.g. a robust plate). In the cassette, a seat 11 intended for receipt of the cutting insert 1 is formed (by those skilled in the art also denominated insert seat), which is delimited by a plane bottom surface 12, as well as two inclined side surfaces 13. The cutting insert can be secured in said seat by means of a clamp 14, which at a front end has a nose 15. The clamp 14 is tightenable by means of a screw 16 at the rear end of the clamp. In the usual way, the geometry of the clamp and of the screw is so formed that the nose 15 on the tightening of the screw moves rearwards/downwards in relation to the seat. A shim 17 is advantageously arranged between the cutting insert 1 and the bottom surface 12 of the seat 11, which shim like the cutting insert consists of cemented carbide. The shim 17 is kept in place by means of a countersunk screw 18, which is fastened in a threaded hole 19 porting in the bottom surface 12. Now reference is made again to figs 1 -3. Characteristic for the cutting insert according to the invention is that the same, in addition to a comparatively thick front part 20, on which the chip breaker 9 is formed, comprises a rear part 21, which is thinner than the front part 20 and tapers in the rearwards direction therefrom, a recess 22 for the nose 15 of the clamp 14 being formed in the rear part 21 and porting in the top surface 23 thereof. In the example, the tapering shape of the rear part 21 is determined by two plane side surfaces 24, which extend at an acute angle ° to each other. In the example shown, the nose angle α° amounts to 60°, although variations upwards as well as downwards from this angular value are feasible. However, the angle α° should be within the range of 30-80°, preferably 45-70°. At the rear ends thereof, the side surfaces 24 transform into the rear end surface 5, which in the example consists of a plane surface, which is parallel to the edge 6 and extends perpendicularly to the bottom side 3. At the front ends thereof, the side surfaces 24 transform into the aforementioned lateral flank surfaces 7, more precisely via comer edges 25. The thick front part 20 transforms into the thinner rear part 21 via a surface 26 sloping rearwards/downwards, a softly rounded surface portion 27 being formed between the sloping surface 26 and the plane top side 23 of the rear part. The length Li of the front part 20, counted as the distance from the edge 6 to the sloping surface 26 is in the example somewhat smaller than the length L of the rear part 21 counted as the distance between the surface 26 and the end surface 5. As may be clearly seen in fig 2, the comer edges 25 are situated in an area in front of the surface 26. In other words, also the rear portion of the front part 20 tapers in the same way as the rear part 21. The thickness Ti of the front part 20, counted as the distance between the cutting edge 6 and the bottom side 3 of the cutting insert, is considerably larger than the thickness T 2 of the rear part 21. In practice, the thickness T t should be 20-40 % larger than the thickness T 2 . The recess 22 for the clamp nose 15 only opens upwards in the cutting insert by ending in a bottom 28 situated on a level above the bottom side 3 of the cutting insert. In this way, a considerable quantity of material is retained in the area below the recess. This is something that vouches for maximal strength and minimal susceptibility to breakage of the cutting insert. Although it is feasible to form the recess with a depth all the way down towards 70 % of the thickness T 2 , a depth not exceeding 50 % of the thickness T 2 is preferred. In other respects, it should be noted that the recess 22 is located at a large distance from the rear end surface 5 of the cutting insert. More precisely, the recess is located in the immediate vicinity of the rounded surface 27 adjacent to the sloping transition surface 26. The recess 22, which advantageously has a rotationally symmetrical basic shape and which may widen upwards by having a certain conicity, is centered between the two inclined side surfaces 24. It should also be mentioned that the top surface 23 of the rear part 21 advantageously - though not necessarily - is plane. In the embodiment exemplified in the drawings, which constitutes a prototype embodiment, the cutting insert has a width B of 40 mm and a length L of 31,5 mm, the length Lj; of the front part amounting to about 12,5 mm, i.e. approximately 40 % of the total length L. Furthermore, the thickness Ti of the front part amounts to 12,7 mm, while the rear part 21 has a thickness T of 9,5 mm. Thus, although the front part is shorter and thicker than the rear part 21 , the front part has in spite of this a marked length extension L t (in practice 10 to 15 mm). This means that the draftsman is given a large degree of freedom to form the cutting insert with efficient chip breaking geometries for many different purposes. In the illustrated, preferred embodiment, the chip breaker in its entirety designated 9 is formed with a highest ridge 29, which is maximally distanced from the cutting edge 6, more precisely by being formed in the immediate proximity to the sloping transition surface 26. The level difference between this ridge 29 and the front edge 6, which in fig 3 is designated N, may vary most considerably depending on other dimensions of the cutting insert (which are dependent on the area of application of the cutting insert), but should be within the range of 0,3-3,0 mm. Forwards from the ridge 29, a sliding surface 30 sloping forwards/downwards extends, which in turn in the front transforms into a cross-section-wise arched flute 31 immediately behind the cutting edge 6. Each chip released from a workpiece by the cutting edge 6 passes cross-section- wise first the arched flute 31 and then the sloping sliding surface 30 in order to finally leave the cutting insert in the area of the ridge 29 which is situated highest. In the illustrated, preferred embodiment a central countersink 32 is formed in the ridge 29 said countersink having a concavely arched basic shape, which in turn, in the forward direction, transforms into arched transition surfaces 33, 34 of which the rear surface 33 slopes forwards/downwards at a comparatively flat angle, while the second surface 34 slopes forwards/downwards at a steeper angle and transforms into a plane surface 35, which constitutes a central interruption in the flute 31 and is countersunk in relation to the bottom of the flute 31. Thanks to the fact that these centrally situated surfaces 32, 33, 34 and 35 form centrally located interruptions in the ridge 29 and the surfaces 30 and 31, respectively, the centrally situated portion of the chip will be given a tendency to move at a higher speed than the two portions of the chip which are situated closest to the edges the chip. This means that the chip will be given a cross- section- wise curved shape. In this way, the width of the chip is reduced in relation to the nominal width, which is determined by the width B of the cutting insert, whereby release of the chip from a groove cut by the cutting insert is facilitated to a high degree. In this connection, it should be noted that the edge 6 in the embodiment according to figs 1-3 is straight, i.e. extends unbroken between opposite ends. The two opposite side surfaces 7 of the cutting insert have clearance not only in the rearwards direction, as is marked by means of the clearance angle β° in fig 2, but also in the direction downwards towards the bottom side of the cutting insert. Although said clearance angles may vary depending on the field of use of the cutting insert, the same should amount to at least about 3°. In the concrete embodiment example according to figs 1-3, the cutting insert is formed with a neutral cutting geometry inasmuch as the front surface 4 extends perpendicularly to the bottom side 3. As is outlined at the angle ε°, the cutting insert may, however, also be formed with an at least slightly positive cutting geometry. Concerning the bottom side 3 of the cutting insert, it should be pointed out that this advantageously may be in the form of a face-ground surface, which forms a reference plane for the cutting insert in other respects. However, it is also possible to instead of a whole, plane surface use either three triangularly located supports or a triangular, frame-like and face-ground surface, which surrounds a central recess. What is important however is that the bottom side of the cutting insert may be pressed in close contact against a plane support surface, such as the topside of the shim 17 according to fig 4. In fig 6, an alternative embodiment of the cutting insert according to the invention is shown. In this embodiment, not only the actual chip breaker 9, but also the edge 6, is interrupted by a central countersink 36 having an arched basic shape. In this way, the cross-section- wise bending of the chip is initiated already by the arched portion 36 of the edge. This is something that additionally renders the reduction of the width of the chip more efficient. Advantages of the cutting insert according to the invention A substantial advantage of the cutting insert for heavy turning according to the invention is that the rather long rear part of the cutting insert resulting from the tapered shape thereof may be secured in a distinct and reliable way in an appurtenant seat having the same shape. Thanks to the fact that the clamp, on tightening of the appurtenant screw, pulls the cutting insert in the rearwards direction, the inclined side surfaces of the cutting insert will thus be pressed against and wedged up between the wedge-shaped inclined side surfaces of the seat with a powerful force at the same time as the bottom side of the cutting insert is kept pressed in close contact against the bottom surface of the seat or against a shim when one is used. Furthermore, thanks to the fact that the rear part of the cutting insert is considerably thinner than the thicker front part which carries the chip breaker, the clamp will be located in a comparatively protected position behind and below the chip breaker because the chip is encouraged by the chip breaker to distanced itself from the clamp. Thanks to the stable and reliable fixation of the cutting insert in the appurtenant seat, the cutting insert will be flexibly usable not only for grooving operations but also for longitudinal turning in that the cutting insert can withstand large lateral forces. Furthermore, in the described, preferred embodiments, the cutting insert offers the advantage that the width of the chip can be reduced quicldy and effectively, whereby the removal of chips from deep grooves is also facilitated to a large extent. The fact that the cutting insert may be secured by means of a clamp nose engaging in a comparatively shallow recess, allows the manufacture of a cutting insert with great strength and long service life in comparison with cutting inserts formed with through holes or openings, because a robust material portion can be retained in the area below the recess. Furthermore, by forming the recess so that it widens conically upwards at the same time as all surfaces in the chip breaker are exposed upwards, the cutting insert may be manufactured by direct pressing of cemented carbide powder, in an advantageous manner. Therefore, the only grinding operation that is required is a simple face grinding of the bottom surface of the cutting insert. Feasible modifications of the invention The invention is not solely restricted to the embodiments described above and exemplified in the drawings. Thus, it is possible to vary the chip breaking geometry within rather wide limits. Furthermore, the tapering shape of the rear part of the cutting insert may be determined in another way than by means of two plane, inclined side surfaces, although plane surfaces are preferred in practice. Also, the shape of the recess for the clamp nose porting in the topside of the rear part of the cutting insert may deviate from the illustrated, rotationally symmetrical shape. Nor does the rear part of the cutting insert need to be terminated by a plane surface. Thus, the rear end of the cutting insert may be delimited by, for instance, a curved surface.

Claims: Claims 1. Cutting insert for heavy turning purposes, having a top side (2), a bottom side (3) and an edge (6) in the area between the top side and a front surface (4), the cutting insert in addition to a comparatively thick front part (20) having a chip breaker (9) positioned behind the edge (6), comprising a rear part (21), which is thinner than the front part, characterized in, that the rear part (21) of the cutting insert tapers rearwards by being delimited by two side surfaces (24) which extend at an acute angle ( °) in relation to each other, and that in the top side (23) of the rear part (21) a recess (22) ports for a clamping unit (14) for fixing the cutting insert in a co-operating seat. 2. Cutting insert according to claim 1, characterized in, that the recess (22) has a rotationally symmetrical basic shape and ends in a bottom (28), the same having a depth which is less than 70 %, suitably 50 %, of the thickness (T 2 ) of the rear part (21). 3. Cutting insert according to claim lor2, characterized in, that the angle (α°) between the side surfaces (24) of the rear part (21) is within the range of 30-80°, preferably 45-70°, and suitably amounts to about 60°. 4. Cutting insert according to any one of the preceding claims, characterized in, that the tapering rear part (21) has a length - measured as the distance between a rear end surface (5) and a transition surface (26) against the front part (20) - which is larger than the length of the front part - measured as the distance between said transition surface (26) and the front surface (4) of the cutting insert. 5. Cutting insert according to any one of the preceding claims, characterized in, that the recess (22) for the clamping unit is located in a front portion of the rear part (21). 6. Cutting insert according to any one of the preceding claims, characterized in, that the chip breaker (9) comprises a ridge (29) distanced from the edge (6), which ridge is situated at a higher level in relation to the bottom side (3) of the cutting insert than the edge (6), and behind which a surface (26) sloping rearwards and downwards forms a transition surface between the front part (20) and the rear part (21). 7. Cutting insert according to claim 6, characterized in, that a central countersink (32) with an arched basic shape is formed in the ridge (29) of the chip breaker, which countersink has the purpose of giving a chip arriving from the edge (6) a cross-section- wise arched shape with the aim of reducing the width of the chip. 8. Cutting insert according to claim 7, characterized in, that a sliding surface (30), sloping obliquely forwards/downwards in the direction of the edge (6) extends forwards from the ridge (29), and that the arched countersink (32) in the ridge (29) also extends forwards through this sliding surface. 9. Cutting insert according to claim 7 or 8, characterized in, that a central countersink (36) with an arched basic shape is also formed in the edge (6).

Inventors: ISAKSSON ROBERT, ISAKSSON, ROBERT
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 2001
Main classification: B23B27
IPC codes: B23B27