TECHN ICAL FI ELD OF TH E I NVENTION

The present invention belongs to the technical field of metal cutting.

More specifically the present invention belongs to the field of turning inserts and turning tools used for metal cutting in machines such as computer numerical control, i.e. CNC, machines. BACKG ROUN D OF TH E I NVENTION AN D P RIOR ART

The present invention refers to a turning i nsert and a turning tool according to the preamble of claim 1 . A similar turning insert is known from US6241 430 or US4681 487.

A turning insert is commonly used in metal cutting, for machining a metal work piece, which after machining is rotationally symmetrical, such as e.g. cylindrical. I n turning, commonly the work piece rotates around an axis of rotation, i.e. a rotational axis. A turning insert is clamped in a turning tool, which moves in relation to the metal work piece. The moving of the turning tool is commonly in a linear motion, and is designated feed. When turning a cylindrical surface, the movement of the turning tool is in a direction which is parallel to the axis of rotation of the work piece. I n such longitudinal turning, the machined surface is commonly formed by an active corner cutting edge, also known as nose cutting edge.

The chips cut from the metal work piece during a tu rning operation are preferably short or have a shape such that they do not affect the metal work piece, the turning tool or the turning i nsert in a negative way. Further, chips should be shaped such they can be handled and removed from the machine in a convenient manner.

A common problem in turning of metal is that chips are long or otherwise have a shape which is not desi rable, especially when turning e.g. low carbon steels at low depth of cut, i.e. a depth of cut which is smaller than the active nose radius of the turning insert.

Commonly, attempts to solve such problems have included : a) choosing smaller nose radius, which decrease the life of the turning insert; b) choosing a higher feed, which reduces the machined surface finish ; c) using high pressure coolant to break chips, which may requi re expensive investments; and d) modifying the top surface such that a chip breaker, spaced apart from the active nose cutting edge, improves the shape of the chip after it is generated.

There are often substantial cutting forces during turning operations. Especially during turning of ISO-M and ISO-S materials temperature increases at the cutting zone, and hence reduces the tool-life.

Although improvements of turning insert design have been made the current inventor has seen a need for further improvements. SUMMARY OF TH E I NVENTION

According to an aspect of the present invention, a turning insert for longitudinal turning of metal work pieces comprises a top surface, a bottom surface and a side surface connecting the top and bottom surfaces, wherein a midplane extends midway between and parallel to the top and bottom surfaces, intersections of the top surface and the side surface comprising a first and second or leading or major cutting edges connected by a nose cutting edge at an acute angled cutting corner, the nose cutting edge being defined by a nose radius, wherein the nose cutting edge slopes from the first or trailing or minor cutting edge in direction towards the second or leading or major cutting edge so as to reduce radial cutting forces during turning of a work piece.

According to another aspect of the present invention, the nose cutting edge extends from a first transition point to a second transition point, wherein the first or trailing or minor cutting edge is connected to the nose cutting edge at the first transition point, wherein the second or leading or major cutting edge is connected to the nose cutting edge at the second transition point, wherein a bisector extends between the first and second or leading or major cutting edges wherein the bisector in a top view intersects a mid-point of the nose cutting edge, wherein the trailing and leading cutting edges in a top view forms a nose angle of from 35 to less than 90° for favorable accessibilit y during turning.

According to another aspect of the present invention, the top surface comprises a chip forming space, wherein the chip forming space borders on the first, second and nose cutting edges, wherein a distance from the midplane to the first transition point is greater than a distance from the midplane to the second transition point for favorable chip forming and for further reducing radial cutting forces.

According to another aspect of the present invention, the second or leading or major cutting edge is disposed at a right-hand side of the nose cutting edge in a front view where the top surface is located above the bottom surface for reducing radial cutting forces during right-hand turning.

According to another aspect of the present invention, the chip forming space comprises a first rake surface adjacent to the first or trailing or minor cutting edge, a second rake surface adjacent to the second or leading or major cutting edge, and a third rake surface adjacent to the nose cutting edge, the chip forming space further comprising a first upstanding wall facing the first or trailing or minor cutting edge, a second upstanding wall facing the second or leading or major cutting edge, and a third upstanding wall facing the nose cutting edge, wherein the third upstanding wall is located between and

connecting the first and second upstanding walls, wherein the third upstanding wall is convex in a top view for facilitating favorable chip forming.

According to another aspect of the present invention, each rake surface connects to an upstanding wall via a transition portion, wherein the transition portion associated with the second or leading or major cutting edge is located closer to the midplane as compared to a distance between the transition portion associated with the first or trailing or minor cutting edge and the midplane for balancing favorable cutting forces.

According to another aspect of the present invention, the nose cutting edge slopes at an angle relative to a plane parallel to the midplane wherein the angle is in the range of 1 0 to 30°, preferably 1 5 to 25° for further reducing radial cutting forces.

According to another aspect of the present invention, the nose cutting edge extends from a first transition point to a second transition point, and wherein the second transition point defines the point being located closest to the midplane.

According to another aspect of the present invention, all rake surfaces form acute angles with the associated side surface for generally reducing cutting forces.

According to another aspect of the present invention, the nose cutting edge is asym metrically arranged relative to a plane containing the bisector to be used in one of right-hand and left-hand turning directions, and wherein the first or trailing or minor cutting edge is convexly curved when seen in a top view for providing a wiper effect on the workpiece.

According to another aspect of the present invention, a midplane extends equidistantly between the top and bottom surfaces, wherein the top surface comprises a first flat surface extending parallel to the midplane, and wherein the turning insert has at least one set of cutting edges - in an each acute angled corner. An acute angled corner may comprise one or two sets of cutting edges on opposite sides of the midplane. Often the turning insert according to the present invention comprises four sets of cutting edges, i.e. two acute angled corners, for being an indexable turning insert. The number of acute angled corners is of course optional in a turning insert having a geometry such as a star shape to allow for more acute angled corners.

According to another aspect of the present invention, a midplane extends equidistantly between the top and bottom surfaces, wherein a shortest distance from a second transition point of the nose cutting edge to the midplane is less than a shortest distance from the first transition point to the midplane.

According to another aspect of the present invention, the turning insert comprises at least 99% cemented carbide or at least 99% cermet, wherein the first surface is formed by pressing and sintering for favorable tool life.

According to another aspect of the present invention, the turning insert in a top view is shaped as a parallelogram or a rhomboid, wherein the turning insert, including the first surface, is made from cemented carbide or cermet.

According to a further aspect of the present invention, a turning tool comprises a tool body and the turning insert, wherein the tool body comprises a seat in which the turning insert is mountable, wherein the tool body comprises a front end, a rear end, and a longitudinal axis intersecting the front and the rear ends, and wherein the bisector intersecting a mid-point of the active nose cutting edge in a top view forms an angle of 40-50° relative to the longitudin al axis of the tool body.

DESCRI PTION OF TH E DRAWI NGS

The present invention will now be explained in more detail by a description of an embodiment of the invention and by reference to the accompanying drawi ngs, wherein FIG. 1 A shows a perspective view of a turning insert according to the present invention ;

FIG. 1 B shows a side view of the turning insert in FIG. 1 A;

FIG. 1 C shows a top view of the turning insert in FIG. 1 A;

FIG. 1 D shows a cross-sectional view along line D - D in FIG. 1 C; FIG. 1 E shows a cross-sectional view along line E - E in FIG. 1 C; FIG. 1 F shows a magnified perspective view of an alternative section corresponding to FIG . 1 A, schematically;

FIG. 1 G shows a cross-sectional view along line G - G in FIG. 1 C; FIG. 1 H shows a portion of the turning insert in top view;

FIG. 1 1 shows a portion of the turning insert in side view;

FIG. 1 J shows a portion of the turning insert in perspective front view;

FIG. 1 K shows the turning insert in a frontal side view in the direction shown by arrow K in FIG. 1 C;

FIG. 2A shows the turning insert and a work piece at a small cutting depth ;

FIG. 2B shows a magnified view of a section 2B in FIG. 2A;

FIG. 3A shows the turning insert and a work piece at a greater

cutting depth than in FIG. 2A;

FIG. 3B shows a magnified view of a section 3B in FIG. 3A;

FIG. 4 shows a graph of radial cutting forces in relation to cutting insert geometry and cutting depth ; and

FIG. 5 shows an example of measured radial forces at feed 0.1 2 and 0.3 mm/rev and cutting depth 1 .5 mm for two turning inserts.

All turning insert drawings or figures have been drawn to scale.

DETAI LED DESCRI PTION OF EMBODI M ENTS OF TH E I NVENTION

Reference is made to FIGS. 1 to 3B, which show a turning insert 1 according to an embodiment according to the present invention. The turning insert 1 has the dimension or general shape of the type commonly known as CNMA 1 20408 or CN MG 1 20408, i.e. a single-sided or a double-sided turning insert, respectively.

The turning insert 1 comprises a top surface 2, an opposite bottom surface 3, a circumferential side surface 4 connecting the top and bottom surfaces 2, 3 and a circumferential edge 1 5 formed at an intersection of, or between, the top surface 2 and the side surface 4. The side surface 4 is a clearance surface during turning. As seen in FIG. 1 B, a midplane P2 extends equidistantly between the top and bottom surfaces 2 and 3. The top surface 2 comprises a first flat surface 1 7 that may extend parallel to the midplane P2. The bottom surface 3 comprises a second flat surface 1 8 that may extend parallel to the midplane P2. When a circumferential edge 1 5 adjacent to the top surface 2 faces towards a work piece, the second flat surface 1 8 functions as a seating surface when the turning insert is mounted in a seat formed in a tool body 25 (FIG. 2A).

The top surface 2 and the bottom surface 3 may be identical when rotated 1 80° relative to an imaginary axis extendin g in the midplane P2 at a longest diagonal of the turning insert (e.g. from the position of the turning insert shown in FIG. 1 A). A central through hole 1 6 suitable for a clamping means such as a screw or a clamp, not shown, may extend along a center axis A1 of the turning insert 1 and may intersect the top surface 2 and the bottom surface 3. The center axis A1 of the turning insert 1 can be perpendicular or substantially perpendicular to the midplane P2.

The circumferential edge 1 5 comprises a nose cutting edge 5, a first or trailing or minor cutting edge 6 and a second or leading or major cutting edge 7. A first end of the nose cutting edge 5 and the first or trailing or minor cutting edge 6 are connected at a first transition area or point 8, and an opposite second end of the nose cutting edge 5 and the second or leading or major cutting edge 7 are connected at a second transition area or point 9. The circumferential extension of each said area or point is 1 micron to 0.1 mm, e.g. much less than a usual radius R of curvature of the nose cutting edge 5.

A clearance or relief surface below the or each nose cutting edge 5 is part cylindrical or part conical in a conventional manner at least about the level of the midplane P2.

A shortest distance from the midplane P2 to a point on the first flat surface 1 7 may be greater than a shortest distance from the midplane P2 to a point on the major cutting edge 7. The first flat surface 1 7 may be spaced apart from the circumferential edge 1 5 at cutting corners of the turning insert by means of a recess or chip breaker groove 1 2.

As can be seen in e.g. FIG. 1 C, the first and second or leading or major cutting edges 6, 7 are straight or substantially straight i n a top view. The first and second or leading or major cutting edges 6, 7 may extend to about the half of a length L (FIG. 1 C) of the turning insert.

The first or trailing or minor cutting edge 6 may be flat or convexly curved edge to form a wiper edge. The wiper edge for turning is based on one or several radii that make up the cutting edge. The wiper edge may be made up of a large, main radius complemented by several smaller radii. A long wiper edge may add to the radial cutting forces. The wiper edge will smooth the scalloped tops that would otherwise have been created in the generated surface during turning.

The nose cutting edge 5 is convex in a top view. As seen in FIG. 1 C, a bisector B extends equidistantly between the first and second cutting edges 6, 7 and intersects a midpoint 1 1 of the nose cutting edge 5 in a top view, i.e. a point halfway between the two ends 8 and 9. A plane P3 containing the bisector B and being perpendicular to the midplane P2 may intersect the midpoint 1 1 .

The nose cutting edge 5 has a constant or substantially constant radius R of curvature in a top view. The radius of curvature is i n the range of 0.1 to 2.0 mm. The nose cutting edge 5 in the top view of FIG. 1 C is shaped as an arc of a circle, for example with a radius of 0.8 mm. The nose cutting edge 5 at the first transition point 8 may be tangent to the first cutting edge 6 in a top view, and the nose cutting edge 5 at the second transition point 9 may be tangent to the second cutting edge 7 in a top view. As best seen in FIGS. 1 1 , 1 J and 1 K, at least 90 % of the nose cutting edge 5 may be concave in a front view.

The transition point 9 at the end of the nose cutting edge 5 being located closest to the major cutting edge 7 may be the most depressed portion of the nose cutting edge 5.

The major cutting edge 7 may connect to the nose cutting edge 5 via a local peak, see e.g. FIG. 1 K.

As can be seen in FIG. 1 C, the midpoint 1 1 of the nose cutting edge 5 of the acute angled cutting corner 1 0 of the turning insert 1 i n a top view is the part of the circumferential edge 1 5 which is positioned at the greatest distance from the center axis A1 of the turning insert 1 . By the term "acute angle" is here meant forming an angle that is less than 90°.

As evident from FIG. 1 B a distance from the midpoint 1 1 of the nose cutting edge 5 to the midplane P2 is shorter than a distance from the first transition point 8 to the midplane P2. The distance from the midpoint 1 1 of the nose cutting edge 5 to the midplane P2 is greater than a distance from the second transition point 9 to the midplane P2.

Stated another way, the distance or drop d between the first transition point 8 and the second transition point 9 may be in the range of 0.05 to 0.5 mm, preferably in the range of 0.1 to 0.3 mm.

All portions of the leading cutting edge 7, 7' may be located below all portions of the trailing cutting edge 6, 6' and the drop or difference in height direction is bridged by the nose cutting edge 5, 5'.

The nose cutting edge 5 slopes linearly or non- linearly towards the midplane P2. More than 50% of the arc length of the nose cutting edge 5 slopes as measured in direction away from the first cutting edge or minor cutting edge 6 towards the second cutting edge or major cutting edge 7. More than 50% of the arc length of the nose cutting edge 5 can be located above the second cutting edge or major cutting edge 7, preferably 80 to 1 00% is located above the second cutting edge or major cutting edge 7.

A line as seen in the frontal side view of FIG. 1 K intersecting the transition points 8 and 9 may form an acute angle β with the midplane P2 or top surface 1 7 or bottom surface 1 8 that is 1 0 to 30°, preferably 1 5 to 25°. Of course, the acute angle β value depends on what drop d value is chosen. I n a double-sided insert each such line would be parallel with the other line at the same cutting corner as seen in FIG. 1 K.

As can be seen in e.g. FIG. 1 F, the major cutting edge 7' may smoothly or without local peaks (or valleys) connect to the nose cutting edge 5' and the latter follows a path that rises along the side surface 4' towards the minor cutting edge 6', that is located below the plane of the top surface 1 7'. The major cutting edge 7' and the nose cutting edge 5' are located under dashed lines in FIG. 1 F. The dashed line and curve immediately above edges 5' and 7' represent conventional turning edges.

At least one acute angled cutting corner 1 0 of the top surface 2 comprises a recess or chip breaker groove 1 2. The chip breaker groove 1 2 comprises a first or rake surface 1 3 in the form of an inclined surface, which borders to all cutting edges 5, 6 and 7 or at least a major portion, or to at least 75 %, of the cutting edges in the acute angled cutting corner 1 0. The inclined surface or rake surface 1 3 forms an acute edge angle a with the clearance surface. The acute edge angle is chosen in the range of 60 to 85°, preferably 70 to 80°.

The width of the rake surface 1 3 perpendicular to the associated cutting edge 6 or 7 may be constant or substantially constant. The width may alternatively vary, e.g. such that the width increases as it is measured farther away from the nose cutting edge 5.

The width of the rake surface 1 3 perpendicular to the associated nose cutting edge 5 may vary such that the width measured along a normal to the midpoint 1 1 is greater than at the end points 8 and 9.

Alternatively the width may be constant or substantially constant. The rake surface 1 3 of the chip forming space 1 2 may comprise a first rake surface 1 3A adjacent to the first cutting edge 6, a second rake surface 1 3B adjacent to the second cutting edge 7, and a third rake surface 1 3C adjacent to the nose cutting edge 5.

The trailing and leading cutting edges 6,7; 6', 7' form a nose angle μ in a top view in the range of from 35 to less than 90°, (FIG 1 H). I n a top view, as seen in FIG. 1 C, the first and the second cutting edges 6, 7 forms a nose angle which is 80°.

The rake surface 1 3 of the chip forming space 1 2 connects to an upstanding wall 1 4 via a transition portion 1 9. A first upstanding wall 1 4A faces the first cutting edge 6, a second upstanding wall 1 4B faces the second cutting edge 7, and a third upstanding wall 1 4C faces the nose cutting edge 5. The third upstanding wall 1 4C is located between and connecting the first and second upstanding walls 1 4A, 1 4B. The third upstanding wall 1 4C may be convex in a top view.

A depth h of the chip forming space 1 2 is measured between the transition portion 1 9 and a plane P1 at least partially containi ng the top surface 2. The plane P1 is usually parallel with the midplane P2. The depth h may vary such that it may be different on each side of the bisector B. The depth h may be deeper on the side of the bisector which is associated with the main or second cutting edge 7 where greater chips usually are being formed during turning.

At least the majority of the first surface 1 3C is concave when seen in cross sections in planes perpendicular to the bisector B, from the midpoint 1 1 of the nose cutting edge 5 towards the third upstanding wall 14C.

A shortest distance in a top view between the midpoint 1 1 of the nose cutting edge 5 and the third upstanding wall 1 4C is at least 1 25% and less than or equal to 300% of the radius of curvature of the nose cutting edge 5 in a top view.

As can be seen in e.g. FIG. 1 K, the top surface 2 of the turning insert 1 , including the nose cutting edge 5 and the first surface 1 3, is asymmetrically arranged relative to the plane P3 containing the bisector B.

I n the embodiment, as best seen in FIG. 1 C the top surface 2 comprises two diametrically opposite 80° cutting co rners, in a top view, where each 80° corner comprises cutting edges 5-7 a nd an adjacent first surface 1 3 in the form of a depression. The turning insert 1 comprises two opposite obtuse or 1 00° corners, in a top view, that may or may not have adjacent inclined rake surfaces and can be called non-cutting corners. Alternatively the obtuse corners may comprise a nose cutting edge and an adjacent inclined rake surface.

As is shown in FIG. 1 D, an edge angle a formed between the first rake surface 1 3 and the side surface 4 along at least the nose cutting edge 5, is less than 90°. I n FIG. 1 D the edge angle a is 65° - 75°. The edge angle a is measured in a plane being normal to the relevant cutting edge.

As can be seen in e.g. FIG. 1 B, the first and second cutting edges 6, 7 are spaced apart in relation to the midplane P2. The second cutting edge 7 is located closer to the midplane P2 as compared to the distance between the first cutting edge 6 and the midplane P2.

Reference is now made to FIG. 4 which shows a graph of radial cutting forces in relation to cutting insert geometry and cutting depth Ap. The Prior art insert used as comparison is of the type commonly known as CNMG 1 20408 of a rhombic basic shape, with an 80° active nose angle. The insert has a nose radius of 0.80 mm and thus a drop d=0.0 mm. Tests have been performed by longitudinal turning, i.e. turning with a feed direction parallel to the rotational axis of the metal work piece to be machined. The entering angle during machining was 95°. The active cutting corner of the insert has an 80° nose angle in a top view. Coolant in the form of emulsion at approximately 1 0 bars has been used. The cutting speed has been 1 20 m/min. The metal work piece is a material according to Swedish standard SS 2348 (corresponding to EN 1 .4404), i.e. a low carbon stainless steel containing an addition of molybdenum for improved corrosion resistance.

Tests have been performed at different cutting depths Ap at feed rates of 0.1 2 and 0.3 mm/rev with different turning inserts. The feed was changed from 0.1 2 mm/rev after 4 seconds of turning to 0.3 mm/rev for an additional 4 second period of turning during each turning operation. Twelve turning operations were completed while radial cutting forces were measured.

The rows in FIG. 4 show radial cutting force averages for different cutting depths and nose cutting edge 5 drop d for the insert according to the present invention (named "New I nsert" in FIG. 4) and for a

conventional turning insert (named "Prior art" in FIG. 4). From the top row to the bottom row, the cutting depths Ap was 1 .5; 0.5 and 0.1 mm for each nose cutting edge 5 drop d of 0.3 ; 0.2 and 0.1 mm. The prior art insert was tested with identical cutting data as the above-captioned inserts but without having any built-in drop. The substrate and the coating were the same for all tested inserts, i.e. only the geometry differed.

FIGS. 2A and 2B schematically show the test set-up at cutting depth less than the nose cutting edge radius, e.g. where the cutting depth is about 0.5 mm. FIGS. 3A and 3B schematically show longitudinal turning at a cutting depth much greater than the nose cutti ng edge radius, e.g. about 2.0 mm.

A turning tool is partially shown in FIG. 2A and it comprises a tool body 25 and the turning insert 1 . The tool body 25 has a seat in which the turning insert 1 is mountable. The tool body 25 has a front end 26, a rear end 27, and a longitudinal axis A3 intersecting the front and the rear ends 26, 27. The bisector B intersects the midpoint 1 1 of the active nose cutting edge 5 in the top view and forms an angle of 40 - 50° relative to a longitudinal axis A3 of the tool body 25.

The test results show that the insert according to the present invention makes it possible to reduce the radial cutting force component and is thus able to implement the invention in products, where radial cutting forces are crucial to be kept as low as possible, e.g. at turning of vibration prone components, or to balance forces when using wiper geometries. The turning insert according to the present invention may reduce the overall radial cutting force components with up to -39% compared to the conventional insert having no drop.

Referring now to FIG. 5 as an example, when looking at measured forces at feed 0.3 mm/rev and Ap=1 .5 mm for the prior art insert (see d=0.0), the radial force value was about 0.2 kN to be compared with the turning insert according to the present invention (see d=0.3) having 0.3 mm drop where the value was about 0.1 kN. The drops 0.1 and 0.2 mm have been substantially masked from the graph for illustrative purposes, but they lie in between the two shown force curves, where an insert with drop=0.1 mm had a higher radial force value than an insert with drop=0.2 mm.

It was further noted at the test that tangential force values did not differ in a significant manner for all inserts but the axial force values was highest for the insert with drop=0.3 mm since it has the nose cutting edge 5 with the greatest length of engagement with the work piece for all cutting depths Ap.

The conclusion is that from a tool life perspective, an increase of tool life in turning steel with a turning insert according the invention can be expected at any depth of cut.

Although the embodiments described above have been of the general shape or dimension commonly known as CNMA or CN MG, other shapes of inserts are feasible. For example, WNMG-type inserts are possible, where the trigon top surface comprises th ree 80° corners in a top view. Also other types of turning inserts are possible. For example, single sided, or positive, turning inserts are possible, such as CCMT-type turning inserts.

The turning insert may comprise at least 99 % cemented carbide or at least 99 % cermet. The first surface 1 3 may be formed by pressing and sintering. The turning insert when seen in a top view may be shaped as a parallelogram or a rhomboid. The turning insert, including the first surface 1 3, may be made from cemented carbide or cermet.

The turning insert and turning tool accordi ng to the present invention may reduce the cutting forces at any depth of cut, i.e. even at depths of cut that is less than the nose radius size. I n the physical experiments has been shown that the turning insert and turning tool according to the present invention may reduce the radial cutting force components with up to about 39% compared to a conventional insert. This reduction will decrease the heat generated in the cutting zone and hence, increase the tool-life. The turning insert geometry according to the present invention can also be used in combination with other types of products/applications - especially in combination with wiper inserts.

Wiper inserts are designed with a large flat or convexly curved edge to be able to generate a smoother surface finish. The drawback of the latter is of course the increased radial cutting force components. Combining the turning insert geometry according to the present invention with a wiper design, the additional radial cutting force component can be reduced or completely removed.

The nose cutting edge is sloping downward from a contact point with the work piece towards the main cutting edge. By tilting or sloping the nose cutting edge the design will be either of left- or right-hand style.

I n the present application, the use of terms such as "including" is open-ended and is intended to have the same meaning as terms such as "comprising" and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as "can" or "may" is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. Terms such as "upper", "lower", "top", "bottom", "forward" and "rear" refer to features as shown in the current drawings and as perceived by the skilled person.

The term "slopes" or "sloping" shall here be interpreted as moving in direction towards the midplane P2. The terms " trailing" and "leading" refer to the normal cutting direction of a turning insert such that the indexable turning insert has a leading cutting edge for coarse cutting operations and a trailing cutting edge for fine cutting operations, as best illustrated by FIG. 3B.