Indexable insert for rotary milling tools

Abstract: A cutting insert comprises an upper chip surface (16), a bottom surface (17) and side faces (18, 19) therebetween. At each of the end faces the cutting insert has axially protruding portions (22, 23), each of which appears with a bevel face (26, 27). Main cutting edges (24, 25) are provided in the intersection between the chip surface (16) and the side surfaces (18, 19). On each side surface is provided a clearance surface (30) formed on a protruding portion (32) which via a step clearance extends into a secondary helically twisted clearance surface, the chip angle of which increases with increasing cutting depth. The upper face (16) includes along the main cutting edges (24, 25) a planar inclined surface (34, 35) with constant width that extends inwards and intersects with an upwardly inclined secondary chip breaking surface (36, 37). Thanks to this embodiment is achieved constant functional edge angle along a wave-shaped edge line, which combined with more optimized chip forming at large cutting speeds contributes to improved machining in long chip producing workpiece materials.

Description: The present invention relates to a new type of polygonal cutting inserts for application in rotary milling tools for machining of metallic workpiece. In particular the invention relates to a new type of cutting insert with optimized insert geometry so as to be useful in both end mills and milling cutters for flat milling. A basically similar type of insert is known from EP-A-0 707 911 representing the closest prior art. Milling inserts are generally manufactured through form pressing and sintering of a cutting material forming powder. The development is concentrated today on positive cutting edges with positive rake angles since these showed the ability to reduce the cutting forces and the power supply needed in a tooling machine at the same time as undesirable vibrations can be diminished thereby. It is often aimed to provide the tool with a geometry implying positive axial angle and simultaneously negative radial angle. In certain cases also a combination of positive axial angle and zero degree radial angle can be used in order to obtain a helical chip that provides best chip transport. The cutting insert is usually clamped with a centre locking screw in order to provide best possibilities for chip transport without problems. One of drawbacks with existing usual indexable inserts for end mills has been that they do not enable avoiding entirely problems with chip entanglement. This problem occurs specifically at the machining of materials that result in long chips. As a solution on this problem has been proposed to use a cutting insert, the cutting edges of which are spirally curved. With such spiral cutting edges the cutting edge can easier come into engagement with the workpiece with an inclined curve of the cutting force up to a maximum value which lies under that one achieved with a straight cutting edge. The tool has in this way obtained a somewhat better stability but the problem with chip entanglement has remained unresolved when using inserts with a central hole, especially at the machining of long chip producing materials. In view hereof it is a purpose with the invention to achieve a modified cutting insert of the type which described in EP-A-707 911 with improved geometrical optimization, implying constant functional edge angle in combination with step clearance along a wave shaped edge line intended to be mounted with large axial inclination in the tool at the same time as the rake face has obtained a more optimized contour. The specific features of the invention are more precisely defined in the following claims. The invention will now be described in more detail in connection with a preferred embodiment as shown in the enclosed drawings where Fig 1 shows a side view of a milling body with a clamped cutting insert according to the invention. Fig 2 shows another partial side view of the tool according to Fig 1. Fig 3 shows an end view of the cutting insert according to Fig 1-2. Fig 4 shows another partial side view of an alternative embodiment of the insert of the invention. Fig 5 shows a sectional view along line V-V in Fig 4. Fig 6 shows a sectional view along the line A-A in Fig 4. Fig 7 shows a sectional view of the cutting insert in Fig 1. Fig 8 shows a sectional view along the line C-C in Fig 4. Fig 9 shows an end view of the insert in Fig 2. Fig 10 shows a sectional view along line E-E in Fig 9. Fig 11 shows a sectional view along line F-F in Fig 9, and Fig 12 shows a sectional view along the line G-G in Fig 9. In figure 1-2 is illustrated an embodiment of an end mill, which is provided with a prismatic elongated indexable insert 13 according to the invention. Such a cutting insert is produced by means of a direct pressing method at which a cemented carbide forming powder first is given desired shape in a press and then is sintered in an oven at a temperature above 1000°C. The shrinking obtained at the sintering is something that must be considered at the dimensioning of the press device. The end mill comprises an essentially cylindrically shaped body 10, the rear upper part (not shown) of which is conical and intended to be clamped in a chuck or spindle of a tooling machine by means of which the body can be brought to rotate around a center axis CL. The forward part of the end mill has a number of spaced recesses 11 round the periphery, each of which has a pocket 12 confined by a bottom support surface 14 and one or more side supporting surfaces upstanding therefrom to receive a releasable clamped indexable insert 13 in a work position therein. Alternatively a number of cutting inserts can furthermore be piled axially in matching pockets in said milling body. The bottom surface 14 of said pocket 12 is provided at a longitudinal axis that forms a positive axial angle α simultaneously as it is provided at negative radial angle from a radial plane. The indexable insert 13 is preferably intended to be fastened in said pocket 12 by means of a clamp screw (not shown) which is received through a central aperture 15 of the insert and threadably engaged in the milling body 10. The axis of said aperture 15 is preferably perpendicular to the bottom face of said insert. The size of this positive axial angle α should be chosen in the area of 0-20°, suitably 5-15°. The positive axial angle α will enable the chips to be easily lifted and thrown away from the workpiece. The indexable insert 13 is essentially formed as a parallelogram, including two essentially parallel rectangular side faces, the top face 16 and the bottom surface 17 which surfaces may be mutually angled away from each other in the manner described in Swedish patent application 9801576-1. Between said top and bottom faces extend two longitudinal side surfaces 18, 19 and two end faces 20, 21 essentially perpendicularly oriented from said top and bottom faces wherein said side surfaces 18, 19 have considerably larger longitudinal extensions than the laterally extending end faces 20, 21. These side surfaces 18, 19 have increasing width towards the active cutting corner whilst being generally inclined in acute angle from the top face 16 and in obtuse angle from the bottom surface 17. The end faces located in diametrically opposite corners are provided on portions 22, 23, which protrude axially from the remainder of said insert. Each of the intersection lines between the side surface 18, 19 and the top surface 16 of the cutting insert forms a main cutting edge 24 and 25 respectively whereas the intersections between said portions 22, 23 and the top surface 16 constitutes bevel faces 26 and 27 formed essentially right-angled in relation to said main cutting edges on said protruding parties 22 and 23 respectively intended to plane out the machined surface and achieve an improved surface finish. The main cutting edges and said bevel faces are separated in each corner of smoothly rounded corner edges 28 and 29 respectively, the curvature of which is chosen such that during machining of the workpiece a curved portion with radius 0.4-6.4 mm is obtained. In order to obtain strengthening of the edges a narrow chamfer 30 has been provided along the main cutting edges 24, 25 with a suitable width, whilst providing a somewhat wider bevel face 31 along said first bevel faces 26, 27 whereby said bevel face 31 extends with substantially constant width along the entire end face 20, 21. Additionally, there is a smoothly curved edge portion 26a, 27a, adjacent to the bevel face 26, 27. This edge portion 26a, 27a is intended to be an active edge at axial ramping and drilling. This curved radius becomes successively bigger at positions more remote from the bevel face. The longitudinal side surfaces 18, 19 are designed with increasing width towards the active cutting corner so that the cutting corner and the curved corner edge 28, 29 becomes situated on an elevated portion of the cutting insert. The cutting insert is simultaneously formed such that both the side surfaces 18, 19 and the end faces 20, 21 appear with positive clearance angle along the entire cutting edge in relation to the top face 16 of the cutting insert. It is furthermore distinguishing that the longitudinal side surfaces 18, 19 of the cutting insert appear with wave shaped primary clearance surfaces 32 along the entire main cutting edge 24 (similarly along the main cutting edge 25), with large axial inclination from one edge face 21 and downwards towards the other end face 20. It is to be noticed that this primary clearance surface 32 is provided on a portion that is protruding laterally from the remainder of the insert which via a step clearance with certain radius extends into a secondary helically twisted clearance surface 18, formed such that its clearance angle increases with increasing cutting depth. The primary clearance surface 32 and the adjacent main cutting edge 24, 25 are smoothly S-curved in the longitudinal direction. The primary clearance surface 32 is formed with essentially constant width along its entire length. The detailed form of the top face 16 of the insert appears best in Fig 2, Fig 4, Fig 9 and from the corresponding sectional views in Fig 5-9 and Fig 10-12. Hence, the top face has a recessed flat central surface 33 round the center hole 15. The surrounding surfaces are elevated therefrom. Adjacent every main cutting edge next to the chamfer 30 there are obliquely downwards inclined surfaces 34 and 35 which are helically shaped in longitudinal direction in order to act as primary rake surfaces. This rake face 34, 35 has constant width along the entire main cutting edge 24, 25. In the embodiment shown in Fig 4 a portion of the rake faces 34, 35 is located adjacent an upwardly inclined surface 36, 37 intended to act as a secondary chip breaking surface having an increasing width towards the axially protruding portions 22, 23, said width then diminishing towards said portions 22, 23. At the cross section taken at the transition area between the chamfer and the curved radius 26a, 27a, said inclined surface 36, 37 meets a ridge 36a, 37a adjacent a downwards inclined surface 38, 39 which is having a convexly curved shape and extends all the way down to the intersection with the opposed rake face 34. In a direction toward the insert centre said secondary chip face 36, 37 extends to meet a recessed surface 40, 41, the level of which is somewhat elevated in comparison with the surface 33 that extends as an annular surface all around the aperture 15. This recessed surface 40, 41 is also the transition surface located between the central portion of the convexly curved surface 38, 39 and the centre of the insert. There should be such inclination of surfaces 35 and 37 so that the obtuse angle β therebetween amounts to 130-160°, preferably 140-150°. The more narrow angular range should be selected at the less heavy machining of a workpiece. The chip angle γ provided by the rake face 38 at section line D-D in Fig 4 should amount to 0-10°. As appears from Fig 2, Fig 4 and Fig 9 the portion of inclined surface 34, 35 located adjacent the curved edge 28, 29 ought to be smoothly curved whilst having a width larger than the width than the width of said surfaces 34, 35 extending along the main cutting edge. As a consequence thereof the width of said secondary chip face 36, 37 will appear with diminishing width when approaching the bevel face 26, 27. In Fig 2 and Fig 9 is shown an alternative embodiment where the insert has been provided with an elongated protrusion 42, 43 that extends parallel with the main cutting edge 24, 25, said protrusion being located on the upward slope of the surface 36, 37 whilst having a convex cross-sectional contour. Hence, the protrusion 43 is located at a certain distance from the intersection line between surfaces 35 and 37 whilst being parallel with main cutting edge 25 and, in same manner, protrusion 42 is located at a certain distance from the intersection between the surfaces 34 and 36 whilst extending parallel with the main cutting edge 24. The longitudinal extension of the protrusion 42, 43 should amount to 0.8-1.2 times preferably 0.9-1.1 times the size of the diameter of the central aperture 15. Thanks to the arrangement above described a new insert has been provided for a cutting tool that becomes useful at large cutting speeds in difficult machining operations such as in slotting cutters machining long chip producing materials, as well as ramping or end milling operations in such difficult materials. At the same time very high precision and good finish of the machined surface is possible to achieve. Although the insert has been described as a prismatically elongated insert for end mills it is to be understood that a corresponding optimization of the primary and secondary rake faces, as well as those protrusions provided could similarly be applied to other polygonal shapes of the insert, such as square-shaped rhombic or diamond shaped inserts.

Claims: A cutting insert for chip forming machining, which comprises a basically polygonal body including an upper chip surface (16) and a planar bottom surface (17), and a plurality of peripheral side surfaces (18, 19, 20, 21) therebetween which are forming main and secondary cutting edges (24, 25, 26, 27) at the intersection of said upper face wherein each main cutting edge appears with a chamfer (30,31) and a primary chip forming surface (34, 35) extending from said chamfer (30,31) wherein the longitudinally extending surfaces (18, 19) having a width increasing towards the active cutting corner such that the cutting corner becomes situated on a raised portion, and there being sloping surfaces (37, 39) extending inwards from said primary chip forming surface and one or several upper chip surface arranged inside said sloping surfaces whilst extending towards the centre of the insert so as to provide chip curling zones therebetween, wherein a) the peripheral surfaces are inclined at an acute angle from the upper surface (16) whereby the upper portion of two opposed peripheral surfaces (18, 19) are located on a laterally extending portion (32) whereby both said portion and the lower of the adjacent peripheral surface are provided with a helical contour along their entire longitudinal extension b) the primary chip forming surface (34, 35) intended to act as primary chip breaking surface next to the main cutting edges is helical along its entire extension and has a constant width along the entire main cutting edge c) the primary chip forming surface (34, 35) extends inwards to an upwardly inclined surface intended to act as a secondary chip breaker such that a V-shaped chip-curling cavity is formed therebetween characterized in, that said secondary cutting edges being provided on portions (22, 23) which protrude axially from the remainder of the insert body and wherein the secondary chip forming surface (36, 37) does, at a section in level with the area where the secondary cutting edge (26, 27) intersects with the curved edges portion (26a, 27a) extend into a ridge (36a, 37a) located at a downwards inclined surface (38, 39) with substantially convexly curved contour Cutting insert as defined in claim 1, characterized in, that the size of the angle (β) that includes chip forming surfaces (35, 37) and (34, 36) amounts to 130-160°, preferably 140-150°. Cutting insert as defined in any of the claims 1-2, characterized in, that an elongated protrusion (42, 43) has been provided on the upwardly inclined secondary chip forming surface (36, 37) at a certain distance from the intersection between the primary and secondary chip forming surfaces (34, 35) and (36, 37). Cutting insert as defined in claim 3, characterized in, that the longitudinal extension of said protrusions (42, 43) amounts to 0.8-1.2, preferably 0.9-1.1 times the size of the diameter of the central aperture (15) of the insert. Cutting insert as defined in any of the claims 3-4, characterized in, that the protrusions (42, 43) are oriented parallel with the main cutting edge (24, 25) and that the cross-section of said protrusion has a convexly curved contour. Cutting insert as defined in any of the claims 1-5, characterized in, that it is in the shape of a prismatically elongated body where the main cutting edges (24, 25) extend in the longitudinal direction of said insert. Cutting insert as defined in any of the claims 1-6, characterized in, that the insert has a central hole (15) that extends entirely through the insert preferably in a direction that is perpendicular towards the bottom surface (17) of said insert.

Inventors: ASTROEM MAGNUS, HANSSON LARS-OLA, ASTROEM, MAGNUS, HANSSON, LARS-OLA, Aström, Magnus
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1999
Main classification: B23C5
IPC codes: B23C5, B23B27