Face-milling cutter

Abstract: The invention relates to an indexable cutting insert for face milling of engine blocks of cast iron. The insert (10) has a free surface (16) and a clearance surface (13A) and a cutting edge extending peripherally about the insert. The corners of the insert have a relatively large radius and therefore the setting angle gets close to zero degrees during machining of the engine block when the insert gets close to the border line of the work piece. The free surface (16) and the clearance surface (13A) enclose an acute angle and therefore the insert has a positive cutting geometry. A land (18) is provided between the the free surface (16) and the cutting edge (19). The land (18) has a width increasing towards each insert corner.

Description: The present invention relates to a face-milling cutter comprising a cutter body and an indexable cutting insert in accordance with claim 1, see EP-A-207914. Cutting inserts for face milling which are provided with corner chamfers are previously known. A drawback of these known inserts is that the active corner chamfer has a constant setting angle during the machining of the entire work piece. At machining of engine blocks of cast iron the constant setting angle results in that the cutting insert tears material away from the work piece in stead of cutting it away and therefore the surface of the work piece becomes impaired, especially at its border edges and its cylinder borings. A drawback of conventional inserts which have a strengthening land of constant width is that in the active corner of the insert, where the largest cutting forces arise, the land becomes more worn than at a distance from the corner. One object of the present invention is to provide a face-milling cutter which makes a clean cut also at the periphery of the work piece. Another object of the present invention is to provide a cutting insert having maximum length of life. Still another object of the present invention is to provide a cutting insert which demands little power to cut. Still another object of the present invention is to provide a cutting insert which in spite of a negative basic shape achieves a positive cutting geometry. The invention will be more closely described hereinafter in connection with the appended drawings. Fig. 1 shows an embodiment of the indexable cutting insert according to the present invention in a top view. Fig. 1A shows a cross section according to the line IA-IA in Fig. 1. Fig. 2 and 2A show the cutting insert in two side views. Fig. 3 shows the cutting insert in a perspective view. Fig. 4 shows a corner of the cutting insert according to the present invention and a corner of a conventional insert in engagement with a work piece. Figs. 5 to 7 show cross sections according to the lines V-V, VI-VI and VII-VII, respectively, in Fig. 1. Fig. 8 shows a corner of a cutting insert according to the invention in engagement with a work piece. Figs. 1 to 8 show a cutting insert 10 according to the invention for chip removal machining, preferably for face milling of work pieces such as engine blocks of cast iron which are partly thin and deformation sensitive. The insert 10 is to be inserted into a cutting insert pocket positioned at a peripheral part of a milling cutter body. The milling cutter body, not shown, is adapted for axial rotation with respect to the work piece. The insert comprises two plane-parallel substantially identical upper and lower surfaces 11 and 12, respectively, which are perpendicularly connected by four side surfaces 13A, 13B, 13C and 13D, i.e. the insert has a strong, negative and square shape. Two adjacent side surfaces connect to each other via a smoothly rounded corner 14. The radius R of the corner 14 is 1/4 to 1/8 of of the length X of the insert 10, i.e. the radius R is 2 to 5 mm or at least 4 times the tooth feed of the milling operation. A peripheral land 18 is connected to the side surfaces 13A-13D, the lines of intersection of the land and the side surfaces form the peripheral cutting edge 19. The relatively straight part of the land 18 is arranged generally beyond a straight imaginary line Z , relative to the center of the insert, between points of intersection of the bisectors B and the cutting edge at two adjacent corners 14. A groove 15 is formed by an outer portion 16 or free surface, since the chips will never be formed against the portion 16, sloping downwardly and inwardly from the connected land and a planar inner portion 17 rising from the inner end of the outer portion 16. The land has a substantially constant width w along the major part of the substantially straight cutting edge 19. The land becomes wider at each cutting corner 14 up to the bisector B of the cutting corner. The land widens from about 0.5 mm to about 0.7 mm, preferably from about 0.45 mm to about 0.58 mm. The width w of the land is determined by the formula w = K x L . cos β where K is a factor between 1 and 3 dependent of the material to be cut, where L is a constant length of contact of the land and the chip and where β is the angle between a normal N of the cutting edge in the corner and the flow direction A of the chip, said formula being true for a circular sector of about 45° where the bisector B forms an imaginary border line. The length of contact L is always measured parallel with the flow direction A . The land defines a rake angle σ, which can be more negative at the cutting corners than at a distance from the corner. The rake angle σ₁ (Fig. 5) along the bisector B lies within the range ± 15° and the rake angle σ₂ according to the section VII-VII (Fig. 7) lies within the range ± 15°. The cutting edge 19 follows a path which at the corner is parallel with the upper surface 11A and which succesively slopes thereafter with increasing distance from the corner. The sloping part of the cutting edge 19 forms an acute angle  between 5 and 20° with the plane of the upper surface 11A alternatively with the plane which defines the highest points of the insert in case the upper surface has been arranged below said plane or in case the upper surface is non-planar. The land at the corner portion has said shape in order to provide for maximum strength due to the increasing constant contact length between the chip and the land. The more positive and sloping part of the cutting edge 19 provides for smaller power requirements, i.e. the insert is subjected to smaller tangential forces during milling. The free surface 16 connects to the land 18 via a wall 16A. The wall which can be straight or curved slopes downwardly and inwardly just like the free surface. The slope of the free surface at the corner portion relative to the plane of the upper surface is defined by the angle ε₁, which is 0 to 15°, while corresponding angles ε₂ at a distance from the corner is 5 to 15°. The free surface 16 preferably has a constant width, inplane projection, along the straight portions of the cutting edges 19. The free surface is 20 to 50 % wider in the corner portions. The width of the groove 15 succesively increases from the corner 14 towards an adjacent corner, said width being largest at the area between the two corners. The part of the cutting edge 19 which slopes provides for a positive axial angle for the insert in spite of the insert being neutrally and radially mounted or negatively mounted in a milling body. The groove connects to a planar mid plane 11A, 12A. Said plane has the shape of a four-armed star, each said arm extending symmetrically about the bisector B of each corner. The end of each arm is truncated. The outer portion 16 or the free surface of the groove 15 can be planar, concave or broken, while the mid-portion of the groove is concave in cross section. The side surfaces 13A-13D can be planar but preferably preferably they have a somewhat concave shape and therefore the insert will achieve carefully defined line-shaped support surfaces abutting the site of the milling body. Said support surfaces are separate from each other and are arranged in the vicinity of the insert corners 14. The side surfaces can be provided with planar or convex chamfers which serve as support surfaces at the same time as the cutting edge of each chamfer is provided to generate the surface of the work piece. The above-mentioned corner 14 has a relatively large radius R which results in maximum spread of the cutting forces acting upon the insert from the work piece and therefore the ultimate breaking strength of the material will not be reached during the last passes in the work piece, e.g. at an external corner as shown in Fig. 4. The surface of the work piece will become planar also at the last pass. During such machining with conventional inserts having non-round corners, the last parts to be cut from the work piece will break according to the dotted line due to the large power concentration arising at sharp corners. The setting angle K of the corner chamfer of the conventional insert is constant and the fracturing of the edge 30 of the work piece is defined by approximately the same angle. The setting angle and thus the cutting forces for the corner at the present insert decreases the more close the work piece's edge the insert machines. Spreading of the cutting forces in the area of the cutting corner is achieved with the present insert and a lenient cutting engagement is achieved due to the positive rake angles and axial angles. The circle C which is defined by the corner radius R is inscribed in the schematically drawn corner portion 14. The cutting edges 19 touch the circle at two points. A chord E between these points forms the angle α, which is between 30 to 50 degrees, relative to an extension line of the cutting edge 19. In the cross-section IA-IA is shown how the chip surface 16 varies in the cutting corner. The chip surface 16 has a local projection which serves to strengthen the corner. In Fig. 8 is shown a corner 14 of the insert in engagement with a work piece 30. The chip flow direction is identified as A . With a setting angle K 90 degrees the chips will flow perpendicularly over the land 18 along the area where the land has a constant width. Thus the flow direction A coincides with the normal N to the cutting edge 19. The land deflects towards the bisector B of the corner during about 45 degrees, and therefore the difference in angle between the normal N and the flow direction A successively increases. In the insert according to the present invention the land has been widened in order to achive a distinct increase of length of contact L between the chip and the land in order to strengthen the insert. In the curved part of the land the normal N coincides with the radius R . The width of the land is defined by the formula w = K x L x cos β as mentioned earlier. The cutting insert corner is symmetrically shaped about the bisector B and the curved part of the land encloses about 90 degrees, i.e. the angle between the two dotted lines in Fig. 8 is about 90 degrees where the lines meet in the center C of the radius. In an alternative embodiment of the present invention the land in the corner area is connected to the upper surface of the insert such that the land does not have a radially inner border line in the corner area. The land and the upper surface are arranged in the same plane. The alternative embodiment is equal to the first-described insert in all other respects. Thus the invention relates to an indexable cutting insert for chip removing machining of deformation sensitive work pieces such as face milling of engine blocks of cast iron, through which shape fracturing of the edge of the work piece is avoided and through which shape a long life for the insert is obtained.

Claims: Face-milling cutter including a cutter body having at least one cutting insert pocket positioned at a peripheral part thereof, and being adapted for axial rotation with respect to a work piece, and an indexable cutting insert (10) comprising a substantially square-shaped body having an upper surface (11), a lower surface (12) and them generally perpendicularly connecting side surfaces (13A-13D) of which two adjacent side surfaces meet in a corner, a peripheral land (18) connecting to said surfaces and forming a peripheral cutting edge (40), said land (18) further at least partly connecting to a downwardly and inwardly sloping free surface (16A), said corner having a bisector (B), whereby the width of the land is larger at the corner than at a distance from the corner, and the land (18) has a rake angle (σ₁, σ₂) which is constant or varies around the insert, characterized in that the corner is rounded and defined by a radius (R) which is 2 to 5 mm and amounts to 1/4 to 1/8 of the length (X) of the insert, and that the land is spaced outwardly of a straight imaginary line between points of intersection of bisectors and the cutting edge at two adjacent corners. Cutting insert according to claim 1, characterized in that the rake angle (σ₁, σ₂) is between +15 and -15 degrees. Cutting insert according to claim 1, characterized in that the land (18) widens from about 0,5 to about 0,7 mm, preferrably from about 0,45 to about 0,58 mm. Cutting insert according to claim 1, characterized in that the cutting edge (19) follows a path, symmetrical about the bisector (B), which at the corner (14) is parallel with the upper surface (11A) of the insert or the plane which is defined by the outermost points of the corner and which thereafter successively slopes with increasing distance from the corner. Cutting insert according to claim 4, characterized in that the cutting edge (19) slopes at an angle (Θ) relative to anyone of said planes, said angle (Θ) being about 5 to 20 degrees. Cutting insert according to claim 1, characterized in that the free surface (16) has a constant width adjacent to the straight parts of the cutting edge (19) and increasing width at the corners (14).

Inventors: HESSMAN INGEMAR, NYSTROM LEIF, HESSMAN, INGEMAR, NYSTROM, LEIF
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1988
Main classification: B23B27
IPC codes: B23B27, B23C5