Indexable insert for end mill.

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 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 (31) which via a step clearance extends into a secondary helically twisted clearance surface, the chip angle of which increases with increasing cutting depth. With the cutting insert regarded in plan view the bevel face is oriented in straight angle towards the main cutting edge (24, 25). Thanks to this embodiment is achieved constant functional edge angle along a wave-shaped edge line, which combined with step clearance and large positive axial angle (α) enables improved precision when machining a 90-degree shoulder in a work piece, which contributes to a more even wear along the main cutting edge and increased life-time of the cutting insert.

Description: The present invention relates to a new type of prismatic elongated cutting insert for application in rotary end mills for machining of metallic workpiece according to the preamble of claim 1. In particular the invention relates to a new type of cutting insert for end mills in order to with bigger precision be able to machine a 90-degree shoulder. The development of milling inserts for end mills generally manufactured through form pressing and sintering of a cutting material forming powder 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 with desired exactitude the machining of a 90-degree shoulder of a work piece. The problem with this type of machining is that of the workpiece wall does not become wholly straight. As a solution on this problem has been proposed to use a cutting insert, the cutting edges of which are spirally curved. EP-A-416 901 shows and describes such a tool. 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 at the same time as the tendency of tool vibrations has been somewhat diminished. In view hereof it is a purpose with the invention to achieve a modified cutting insert of the type which described in EP-A-416 901, 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. The specific features of the cutting insert according to the invention have been defined in the subsequent claims. It is another purpose to achieve a milling tool, which become useful both for machining of a 90-degree shoulder that furthermore is useful for axial machining so called ramping, i.e. a tool, which machines the workpiece in both axial- and radial directions. The invention will now be described in more detail in connection with 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 a plan view of the cutting insert according to the invention. Fig 4 shows a side view of the cutting insert in Fig 3. Fig 5 shows a sectional view along line V-V in Fig 3. Fig 6 shows an end view of the cutting insert in Fig 1. Fig 7 shows a perspective view of the cutting insert in Fig 1. Fig 8 shows a plan view of the cutting insert. Fig 9 shows a sectional view along line H-H in Fig 8. Fig 10 shows a sectional view along line L-L in Fig 8, and Fig 11 shows a side view of a milling body at machining in both axial- and radial direction. 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 releaseably 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 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 such as is shown in Fig 2-3. 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 are 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 attain this the corner edges 28, 29 have such a shape that the radius closest to the straight main cutting edge and the bevel face 26, 27 is somewhat larger than the curved portion therebetween. As is best shown for example in Fig 3 there is a smoothly curved edge portion 27a, adjacent to the bevel face 27. This edge portion 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 27. A separate ramping edge X1, X2 is provided in connection to the curved edge R1 which forms a transition to the portion 26a. The size of the cutting edge R1 and R2 should be 0.4-0.6 mm. It is distinguishing that 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 30 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 30 is provided on a portion 31 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 30 is formed with essentially constant width along its entire length. The detailed form of the top face 16 of the insert appears closer in Fig 6-7. Hence, the top face has a recessed central region round the center hole 15. From this region and in direction towards both end faces curved rake surfaces 32 and 33 are extending. Such as is best shown in Fig 7 the top face is confined by a circumferentially extending planar reinforcing chamfer 34, the width of which has been enlarged for clarification purpose. This chamfer 34 forms along the main cutting edge an acute angle with the primary clearance surface 30 such as shown in Fig 8-10, suitably in the magnitude of 75-90°, preferably 84-89°. The reinforcing chamfer 34 along the bevel face 26, 27 shall, however, form a sharper angle with the clearance surface 21a located next to the bevel face 26, 27. This sharper angle should lie in the area 70-80°. Those rake surfaces of the top face which connect to and form an intersection with the longitudinal side surfaces 18, 19, have the form of obliquely upwards inclined essentially planar surfaces 35 and 36. Between the rake surfaces 33 and 35 there is provided a substantially planar surface 37 that is recessed in relation to its surrounding surfaces, said surface being inclined downwards whilst having an increasing width up to the corners and in corresponding manner there is a similar narrow surface 38 inclined downwards towards the insert centre in diametrically opposite position. Both these narrow surfaces 37, 38 connect to a recessed, round the center hole 15 located planar surface which is planar and parallel with the bottom surface 17. Thanks to this arrangement with alternating raised and recessed surface parts in near connection to the active cutting corner it has become possible to achieve a favorable optimized form of the chip shape such that it is easier broken up and can be removed more easily from the workpiece. Such as also shown in Fig 6-7 the upper part of the end face 20 is formed as a clearance surface 20', which under acute angle forms an intersection with the upper reinforcing chamfer 34. The surface portion 40, of the end face 20, which connects to the secondary cutting edge 26, has however, the shape of a planar surface, which extends as an unbroken surface from the top face 16 to the bottom surface 17. In the figures 8-10 is illustrated the cutting insert by a number of sectional views, wherefrom the contour of the top face 16 should appear more closely. The helically twisted contour of the upper reinforcing chamfer 34 and its connecting chip surface 35 along the main cutting edge 24 should appear therefrom as well as a corresponding helically twisted form of the primary clearance surface 30. In this manner it will be apparent that the angle β constitutes the angle between the primary clearance surface 30 and a normal to the bottom surface 17. It varies such that it becomes smaller in the section H-H and then appears with a larger value in the section L-L. The size of the angle β should be 0-25°. This means that after being functionally secured in the milling body there is obtained a constant clearance angle which makes the chamfer wear to become even. The rake angle γ, however, remains constant. With  is designated the angle between the surfaces 30 and 35 and with σ is designated the angle between the surfaces 34 and 30. It follows herefrom that the angles γ,  and σ remain constant in all cross sections along the main cutting edge. Thereby is always achieved a uniform chip formation along each cross section along the main cutting edge which gives increased edge strength which contributes to extended tool life of the cutting insert. In Fig 11 is shown how the milling tool with cutting insert according to the invention becomes useful for ramping such that a ramped surface with certain inclination angle, obliquely downwards to the right in the figure, becomes possible. The cutting edges that are operative are designated more darkly in Fig 3 and designated X1, X2 as well as 27a, 27b. Thanks to the arrangement above with constant functional edge angle along a wave shaped edge line combined with step clearance and large positive axial angle α it has become possible to establish desired precision when providing a 90-degree shoulder of the workpiece. One attains even chamfer wear along the main cutting edge, which adds to extended tool life. Simultaneously the chip formation becomes favorable and machining can occur with moderate cutting forces. Furthermore, such a tool has turned out to manage appropriate machining by circular interpolation such that a ramped surface with inclination angle up to about 10° becomes possible. Another effect is that one limits increments of appearing notch or spark wear on the cutting edge. Thereby one limits also the drawback with otherwise increasing cutting forces during machining, and contributes additionally to increased tool life.

Claims: Cutting insert for chip forming machining, in particular for end mills, which comprises a basically parallelogram formed body including an upper chip surface (16) and a planar bottom surface (17), wherein the two longitudinal side surfaces (18, 19) therebetween form main cutting edges (24, 25) and the two end faces (20, 21) each has a bevel face (26, 27), said bevel faces are provided on portions (22, 23) which protrude from the remainder of the insert body, which said side surfaces having a width increasing towards the active cutting corner such that the cutting comer becomes situated on a raised portion, characterized in that in combination, the longitudinal side surfaces are generally inclined at acute angle towards the upper chip surface (16) and in obtuse angle towards the bottom surface (17), and the upper portions of said side surfaces (18, 19) are provided as wave shaped primary clearance surfaces (30) along the entire main cutting edge with an overall inclination from one end face (21) downwards towards the second end face (20), wherein said clearance surface is provided on a portion (31) extending laterally from the remainder of the cutting insert, and that these primary clearance surfaces (30) via a step clearance extend to a lower portion of adjacent side surface (18, 19), which constitutes a secondary helically twisted clearance surface, the upper portion of each side surface defining a clearance angle with a normal to the bottom surface, the clearance angle increasing in size as a distance away from the active cutting comer increases, and with the cutting insert regarded in plan view said bevel face (26, 27) is provided in essentially straight angle from said main cutting edge (24, 25) whilst separated therefrom by a smoothly curved edge portion (28, 29). Cutting insert according to claim 1, characterized in that the primary clearance surface (30) appears with substantially constant width along the entire main cutting edge, and that the clearance surfaces (30) extend along the entire side surface between the two opposed end surfaces (20, 21). Cutting insert according to claims 1-2, characterized in that the top surface is confined by a recessed central region and chip surfaces around said region of which the chip surfaces facing said end surfaces (20, 21) are forming convex surfaces (32, 33) whereas the directed chip surfaces adjacent said side surfaces consist of obliquely upwards inclined essentially plane surfaces (35, 36), which under acute angle connect to the primary clearance surfaces (30) located adjacent the main cutting edges. Indexable insert according to anyone of the claims 1-3, characterized in that the top face (16) and the bottom surface (17) are non-parallel. Indexable insert according to anyone of the claims 1-4, characterized in that a planar reinforcing chamfer (34) is provided along the main cutting edges (24, 25) and along the bevel face (26, 27). Indexable insert according to claim 5, characterized in that the planar reinforcing chamfer (34) and the primary the clearance surface (30) form mutually an angle of 84-89E along the entire main cutting edge (24, 25). Indexable insert according to claim 3, characterized in that between the convex the convex surface (32) and the planar surfaces (36) and between the convex surface (33) and the planar surfaces (35) is provided a recessed, essentially planar surface (38) and (37) respectively with a width that increases towards the corner. Indexable insert according to claim 3, characterized in that the upper part of the end face is formed as an inclined clearance surface (20') at the portion adjacent the convex chip surface (32, 33) whereas the portion of the end face adjacent the bevel face (26, 27) has the shape of a planar surface (40) which extends as an unbroken surface between the top face (16) and the bottom surface (17). Indexable insert according to claim 5, characterized in that the angle between the reinforcing chamfer (34) and the clearance surface (21a) of the bevel face is smaller than angle between the reinforcing chamfer (34) and the clearance surface (30) along main cutting edge (24). Indexable insert according to anyone of the claims 1-9, characterized in that the comer edges (28, 29) each has a first curved portion joined to the nearest main cutting edge (24, 25), a second curved portion (26a, 27a) joined to a respective bevel face (26, 27), and a third curved portion interconnecting the first and second curved portions. Milling tool for rotary machining of metallic workpieces which has a cylindric basic form and is rotatable around a center axis (CL) and the rear part of which is equipped with a shank portion (10) intended to be rotatably clamped in an associated chuck or machine spindle, and which round its periphery is equipped with a number of mutually spaced recesses, each of which is confined by a bottom support surface (14) and one or several side support surfaces upstanding therefrom that extend in the axial and radial directions, characterized in that indexable inserts according to any of the preceding claims 1-10 are releaseably clamped in the milling body such that they define clearance angles along main cutting edges (24) and along bevel faces (26) thereof, and each cutting insert in the milling body has a positive axial angle orientation and a negative radial angle orientation. Milling tool according to claim 11, characterized in, that the cutting inserts are axially secured in the milling body (10) such that they extend axially beyond the holder body to such extent that an axially oriented ramping edge (X) can come in engagement with a workpiece whilst a curved radial edge (26a, 27a) located radially inside the bevel faces (26, 27) can come in engagement with the workpiece during combined ramping and drilling.

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