JP2001025907A - Throw-away tip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the chipping resistance of a sub cutting blade to chipping at the time of cutting machining. SOLUTION: This throw-away tip 50 is formed with a protruding part 80 on a third upper side sloped part 73 down-sloped from a third honing 63, whose ridge part intersecting the side surface thereof serves as a sub cutting edge 14, toward the central part of the throw-away tip 50. The protruding part 80 is formed by a protruding part upper side 81 of roughly triangular flat surface and a protruding part sloped side 82 intersecting the protruding part upper side 81 and the third upper side sloped part 73. A distance (h) from the base 81A of the protruding part upper surface 81 to the vertex 81B is set, relative to a distance H between the two sub cutting edges 14, 14 formed respectively at the opposing ridge parts of the throw-away tip 50, so as to be 0<h<=(1/2)H. The two basic angles α, β of the protruding part upper side 81 is set so as to be in the range of 0 deg.<α<60 deg., 0 deg.<β<60 deg. respectively. The protruding part 80 is disposed in an area between the two bisectors T, T which bisect the angles formed by adjacent main cutting edge 13 and each of the sub cutting edges 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a throw-away insert which is mounted on various milling tools such as a face mill and an end mill and is used particularly for interrupted cutting.

[0002]

2. Description of the Related Art FIG. 6 is a front view of a throw-away tip according to an example of the prior art, FIG. 7 is a cross-sectional view taken along the line XX of the throw-away tip shown in FIG. 6, and FIG. It is a center sectional view of the face milling machine in which the away tip was attached. As shown in FIG. 6, the throw-away tip 10 is formed in a substantially square (approximately square in FIG. 6) plate-like appearance, and has a lower surface 11 serving as a seating surface and a lower surface 1.
1 and an upper surface 12 facing the same. A main cutting edge 13 is formed on each of the ridges forming approximately four sides of the upper surface 12, and a substantially straight sub-cutting edge 14 is formed between two main cutting edges 13, 13 which are substantially orthogonal to each other. And the main cutting edge 13,
A substantially arc-shaped corner blade 15 for smoothly connecting the two cutting blades 13 and 14 is formed between the sub-cutting blade 14. As shown in FIG. 7, the side surface 16 between the lower surface 11 and the upper surface 12 is inclined outward from the lower surface 11 toward the upper surface 12 to form a positive chip. The upper face 12 is a rake face, and the side face 16 is a flank face. Also,
An insertion hole 17 for screw insertion is provided through the center of the upper surface 12 and the lower surface 11.

[0006] As shown in FIGS. 6 and 7, a honing (honing surface) 18 is provided over the entire periphery of a ridge portion that forms substantially four rounds of the upper surface 12. That is, honing 18
The intersection ridge line between the main cutting edge 13 and the side cutting edge 16 is
4 and a corner blade 15. This honing 1
Reference numeral 8 denotes a cross section orthogonal to the ridge line portion, for example, a straight line, and the inclination angle (the honing angle) θ inclined toward the outside of the throw-away tip 10 so as to have a downward slope with respect to the horizontal plane is formed over the entire circumference. For example, it is fixed. The upper surface 12 of the throw-away tip 10 intersects the honing 18 and faces the center of the throw-away tip 10 and is inclined so as to have a downward slope with respect to a horizontal plane, and is connected to the upper surface inclined portion 12A. And a top horizontal portion 12B that is horizontal near the center of the throw-away tip 10. The upper inclined portion 12
The connection between A and the upper horizontal portion 12B is smooth.

As shown in FIG. 8, the indexable insert 10 is mounted, for example, on the outer periphery of the front end of a cutter body 21 of a face mill 20, and is intermittently cut by rotating the cutter body 21 against a work material. A part of the cutting material is cut off. The indexable insert 10 is mounted so that the main cutting edge 13 protrudes toward the outer peripheral side of the cutter body 21 and the sub-cutting edge 14 serves as the bottom blade of the cutter body 21. When the work material is rotationally cut in the main cutting edge 13, the main cutting is performed, and the sub cutting edge 1
At 4 a finished surface is generated.

[0005]

When cutting is performed by the face mill 20 equipped with the above-described indexable insert 10, for example, when each of the cutting edges 13, 14, 15 starts to hit the work material, an impact such as a bite occurs. Therefore, there is a problem in that the cutting edges 13, 14, 15 are liable to be damaged. In particular, since the sub-cutting edge 14 is for performing the finish cutting of the cut surface of the work material, there is a problem that if the sub-cutting edge 14 is lost, the surface roughness of the cut surface of the work material is deteriorated. . The present invention has been made in view of the above circumstances,
It is an object of the present invention to provide a throw-away insert capable of improving the fracture resistance of a sub cutting edge during cutting.

[0006]

In order to solve the above-mentioned problems and to achieve the above object, the indexable insert of the present invention according to the first aspect of the present invention is provided with a rake face having a ridge line at a corner portion of an upper surface. In a throwaway tip including a sub-cutting edge, the upper surface is provided with an upper surface inclined portion inclined so as to have a downward slope from the ridge line portion toward the central portion, and the upper surface has a central portion following the sub-cutting edge. Side, a convex portion protruding from the upper surface inclined portion is formed, and the upper surface of the convex portion is formed of a substantially triangular flat surface, and a bottom disposed on the sub-cutting edge side, and the upper surface is formed from the bottom. And a vertex arranged on the central portion side of.

[0007] In the throw-away insert having the above-described structure, a projection protruding from the inclined upper surface is formed between the central portion of the throw-away tip and the sub-cutting edge. The thickness of the away tip is increased, and the fracture resistance of the minor cutting edge can be improved. For example, it is possible to suppress the occurrence of chipping or the like of the minor cutting edge due to an impact acting on the cutting edge during cutting. . Moreover, the upper surface of the convex portion is substantially triangular, and the base is disposed on the side of the sub-cutting edge. For example, by appropriately setting the position of the base angle or vertex of the triangle, during cutting, In particular, it is possible to prevent chips generated by the main cutting by the main cutting edge from colliding with the convex portion. Further, since the upper surface is flat, the collision of the chip with the upper surface of the convex portion is suppressed even when the chip extends toward the convex portion, and the chip is smoothly discharged. Thus, it is possible to prevent the chip discharge property from being deteriorated.

Further, in the throw-away tip according to the present invention, the throw-away tip has a polygonal plate shape, and at least a pair of opposed sub-cutting blades on the upper surface are formed, and the convex is provided. The distance h from the bottom to the apex of the upper surface of the portion is set in a range of 0 <h ≦ (）) H with respect to the distance H between the opposed sub-cutting edges, and the upper surface of the convex portion Are 0 ° <α <60.
° and 0 ° <β <60 °.

In the indexable insert having the above structure, if the distance h from the bottom to the apex of the upper surface of the convex portion is zero, the effect of providing the convex portion, that is, the effect of improving the fracture resistance of the sub cutting edge cannot be obtained. , Conversely, if it is larger than (1/2) H,
Chips generated at the time of cutting work collide with the projections, resulting in poor chip dischargeability. Furthermore, two base angles α and β of the upper surface of the convex portion
Are 0 ° <α <60 ° and 0 ° <β <60 °, respectively.
When one of the base angles α is zero, the effect of providing the convex portion, that is, the effect of improving the fracture resistance of the sub-cutting edge cannot be obtained, and conversely, when it is larger than 60 °,
Chips generated at the time of cutting work collide with the projections, resulting in poor chip dischargeability. In addition, the same operation and effect as the above-described one base angle α occur for the other base angle β.

A third aspect of the present invention is directed to a throw-away insert, comprising: a sub-cutting edge at a ridge of a corner portion of an upper surface which is a rake face;
The upper surface is provided with an upper surface inclined portion inclined so as to have a downward slope from the ridge line portion toward the central portion, and the upper surface protrudes from the upper surface inclined portion to a central portion side following the sub cutting edge. A convex portion is formed, and the convex portion is configured such that the sub cutting edge is formed with respect to a bisector that bisects an angle between the sub cutting edge and a main cutting edge adjacent to the sub cutting edge. Are arranged in a region sandwiched between the two bisectors located at both ends of the line.

In the indexable insert having the above-mentioned structure, the thickness of the indexable insert in the vicinity of the sub-cutting edge is increased by the formation of the projection, so that the fracture resistance of the sub-cutting edge can be improved. Since the area where the convex portion is formed is set so that the chip generated during the cutting process, particularly the chip generated by the main cutting edge, is deviated from the extending direction, it is possible to further prevent the chip from colliding with the convex portion. Thus, it is possible to prevent the chip discharging property from being deteriorated.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a throw-away tip according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a perspective view showing one embodiment of the throw away tip of the present invention, FIG. 2 is a front view of the throw away tip shown in FIG. 1, and FIG. 3 is a YY of the throw away tip shown in FIG. FIG. 4 is a sectional view of FIG.
5 is an enlarged front view of a corner portion of the throw-away insert shown in FIG. 5, and FIG. 5 is a diagram showing a shape of a chip generated when a work material is cut by a face mill equipped with the throw-away tip shown in FIG. (A) shows a case where a helical chip is generated, and (b) shows a case where a spiral chip is generated. Note that the same reference numerals are assigned to the same parts as those in the above-described conventional technology, and the description will be simplified or omitted. The throw away tip 50 according to the present embodiment is
For example, the outer surface is formed in a substantially square (approximately square in FIG. 2) plate shape, and a side surface 16 serving as a flank faces from a lower surface 11 serving as a seating surface of the throw-away tip 50 toward an upper surface 12 serving as a rake surface. It is a positive tip inclined outward. As shown in FIG. 2, a main cutting edge 13 is formed on each of ridges forming substantially four sides of the upper surface 12, and a corner portion is cut off between two main cutting edges 13, 13 which are substantially orthogonal to each other. A substantially straight sub-cutting edge 14 is formed. Further, between the main cutting edge 13 and the sub-cutting edge 14, a substantially arc-shaped corner for smoothly connecting the two cutting edges 13, 14 is formed. A blade 15 is formed. And honing (honing surface) over the entire circumference of the ridge line portion that forms approximately four circumferences of the upper surface 12
60 is given.

The honing 60 has a first honing 61 whose intersection ridge line with the side surface 16 is the main cutting edge 13.
And a second honing 62 serving as the corner blade 15 and a third honing 63 serving as the sub-cutting blade 14. The cross section of each of the honings 61, 62, 63 orthogonal to the ridge is, for example, a straight line. It is in the shape. Further, the honing 60 is inclined so as to have a downward slope from the center side of the throw-away tip 50 toward the outer peripheral side. On the inner peripheral side of each of the honings 61, 62, 63, a first upper surface inclined portion 71, a second upper surface inclined portion 72, which is inclined so as to have a downward slope toward the center of the throw-away tip 50, 3 upper surface inclined portions 73 are connected to each other, and these upper surface inclined portions 71, 72, and 73 are respectively intersecting ridge lines 61 a and 62 with the honings 61, 62, and 63.
a, 63a, and is smoothly connected to the upper surface horizontal portion 12B near the center of the throw-away tip 50 located vertically below.

As shown in FIG. 3, a third honing 63 having an auxiliary cutting edge 14 at the intersection ridge line with the side surface 16 is inclined downward by a predetermined honing angle θ from the horizontal line S in the vertical direction. The third upper surface inclined portion 73 formed between the third honing 63 and the upper horizontal portion 12B is provided with a projection (convex portion) 80 that is substantially triangular in plan view.
The protruding portion 80 includes a protruding portion upper surface 81 formed of a flat surface intersecting and connected to the third honing 63, and a protruding portion inclined surface intersecting and connected to the protruding portion upper surface 81 and the third upper surface inclined portion 73. 82. Projection upper surface 81
Is arranged such that a flat surface of a substantially triangular shape (a substantially isosceles triangle in FIGS. 2 and 4) is horizontal, that is, orthogonal to the central axis M of the throw-away tip 50, and the third honing 63 and the third upper surface A predetermined length of the intersection ridgeline portion 63a with the inclined portion 73 is a bottom 81A of the projection upper surface 81, and an appropriate position from the bottom 81A toward the central axis M of the throw-away tip 50 is a vertex 81B of the projection upper surface 81. It has been. However, the vicinity of the vertex 81B is formed in, for example, an arc shape as shown in FIG.

The projecting portion inclined surface 82 is formed in a substantially curved shape that is inclined from the outer periphery of the projecting portion upper surface 81 except for the bottom side 81A toward the outside of the projecting portion upper surface 81 so as to have a downward slope. As shown in FIG.
And the valley line portion 82a formed by intersecting with the third upper surface inclined portion 73 has, for example, a substantially triangular shape in which the vicinity of the vertex is formed in an arc shape. In addition, as shown in FIG. 3, in the cutting plane including the center of the sub-cutting edge 14 and the center axis M of the throw-away tip 50, the inclination angle γ1 of the projection inclined surface 82 inclined vertically downward from the horizontal line S. Is larger than the inclination angle γ2 of the third upper surface inclined portion 73.

Here, as shown in FIG. 4, the center axis M of the throw-away tip 50 passes through the center of the sub cutting edge 14.
As shown in FIG. 2, the length h of the protrusion upper surface 81, that is, the distance h between the bottom 81A of the protrusion upper surface 81 and the vertex 81B of the protrusion upper surface 81, on a horizontal line S orthogonal to
With respect to the distance H between the two sub-cutting edges 14 facing each other across the central axis M of the throw-away tip 50, 0 <h
.Ltoreq. (1/2) H. Here, the protrusion upper surface 81
If the length h is zero, the effect of providing the protruding portion 80, that is, the effect of improving the fracture resistance of the sub-cutting edge 14 cannot be obtained. The generated chips collide with the protruding portion 80, and the chip discharging property is deteriorated.

Further, as shown in FIG. 4, the two base angles α and β of the upper surface 81 of the substantially triangular projecting portion are each 0 °.
<Α <60 °, 0 ° <β <60 °, and more preferably, 0 ° <α ≦ 45 ° and 0 ° <β ≦ 4, respectively.
5 °. Here, if the base angle α or β is zero, the effect of providing the projecting portion 80, that is, the effect of improving the fracture resistance of the sub-cutting edge 14 cannot be obtained, and conversely, the base angle α or β
Is larger than 60 °, the chips generated during the cutting work collide with the protruding portion 80, and the chip discharging property is deteriorated. In FIG. 4, the base angles α and β are made equal to each other. Further, as shown in FIG. 4, the main cutting edge 13 and the sub cutting edge 1
Assuming that a bisector that bisects an angle formed by each extension line with T is T, the protruding portion 80 is located within a region sandwiched between the two bisectors T on both sides of the sub-cutting edge 14. Are located.

The indexable insert 50 according to the present embodiment has the above-described configuration. Next, the indexable insert 50 is mounted on, for example, the face mill 20 shown in FIGS. 5A and 5B. Next, the operation when the work material 91 is cut intermittently will be described. The indexable insert 50 is mounted such that the main cutting edge 13 protrudes toward the outer peripheral side of the cutter main body 21 and the sub cutting edge 14 serves as the bottom blade of the cutter main body 21.
When the cutter body 21 is rotationally cut with respect to
The main cutting by 3 generates a helical chip as shown in FIG. 5 (a) or a spiral chip as shown in FIG. 5 (b).

These chips are throw-away chips 50
However, the projecting portion upper surface 81 of the projecting portion 80 projecting from the third upper surface inclined portion 73 is a flat surface, and furthermore, the projecting portion 80 is adjacent on both end sides of the sub-cutting edge 14. Angle between each of the main cutting edges 13
Since the chip is disposed in the area sandwiched by the two bisectors T, T that divide equally, the chips generated by the main cutting edge 13 are prevented from colliding with the protruding portion 80 and the like, and are smooth. Will be discharged. Moreover, the protrusion 8
0, the indexable insert 50 near the sub cutting edge 14
By increasing the thickness of the cutting edge, the occurrence of chipping or the like of the sub-cutting edge 14 due to an impact acting on the throw-away tip 50 when biting into the work material 91 or the like is suppressed. Further, the chips generated at the time of finish cutting with the sub cutting edge 14 are thin and run in the region of the third honing 63 and curl, so that the protrusion 80 does not rub.

As described above, according to the indexable insert 50 of the present embodiment, the third honing 63 in which the ridge line intersecting with the side surface 16 is the sub-cutting edge 14 and the upper horizontal portion 12
B, the third upper surface inclined portion 73 is provided with a substantially triangular projection 80 in a plan view.
4, the thickness of the indexable insert 50 is increased, and the fracture resistance of the sub-cutting edge 14 is improved without reducing the sharpness of the sub-cutting edge 14 and biting of the work material. be able to. Thereby, for example, when the secondary cutting edge 1 is impacted by the impact acting on the cutting edge when the cutting edge bites into the work material or the like.
4 can be prevented from being chipped. Moreover, the projecting portion upper surface 81 of the projecting portion 80 is a substantially triangular flat surface, and the base 81 of the projecting portion upper surface 81 formed of a predetermined length of the intersection ridge 63 a with the third upper surface inclined portion 73.
The distance h from A to the vertex 81B on the side of the center axis M of the throw-away tip 50 is greater than the distance H between the two sub-cutting blades 14 facing each other across the center axis M of the throw-away tip 50. 0 <h ≦ (１ ／) H, and the two base angles α and β of the upper surface 81 of the protrusion are 0 ° <α <60 ° and 0 ° <β <60 °, respectively. Because of this, it is possible to further improve the dischargeability of chips generated during cutting. That is, when the distance h between the bottom 81A and the vertex 81B of the protrusion upper surface 81 is zero, the effect of providing the protrusion 80, that is, the sub cutting edge 1
No effect of improving the fracture resistance of No. 4 cannot be obtained, and conversely, (1 /
2) If it is larger than H, the chips generated during the cutting work will collide with the protruding portion 80, and the chip discharging property will be poor. In addition, if the base angle α or β is zero, the effect of providing the protruding portion 80, that is, the effect of improving the fracture resistance of the sub cutting edge 14 cannot be obtained, and conversely, the base angle α or β is larger than 60 °. In this case, chips generated during the cutting work collide with the protruding portion 80, and the chip discharge performance is deteriorated.

The adjacent main cutting edge 13 and sub cutting edge 14
The projection 80 is disposed in a region between the two bisectors T, T at both end sides of the sub-cutting edge 14, where T is a bisector that bisects the angle formed by each extension line with As a result, the chips generated during the cutting process, particularly the chips generated by the main cutting by the main cutting edge 13, can be projected.
The collision with zero can be further suppressed, and the chip discharge performance can be improved.

In this embodiment, as shown in FIG. 4, the upper surface 81 of the protruding portion is formed in an isosceles triangular shape. However, the present invention is not limited to this, and other triangular shapes may be used. . In this case, the two bottom angles α and β of the protrusion upper surface 81 have different angles from each other, but the vertex 81B of the protrusion upper surface 81 is divided into two equal parts on both sides of the sub-cutting edge 14. What is necessary is just to arrange | position in the area | region between the lines T and T. Further, in the present embodiment, the upper surface 81 of the protruding portion is horizontal,
Although it is arranged to be orthogonal to the central axis M of 0,
The present invention is not limited to this, and may be inclined in the vertical direction by an appropriate angle from the horizontal direction. Further, in the present embodiment, the throw-away tip 50 has a substantially square plate shape, but is not limited to this, and may have a substantially triangular or other polygonal plate shape. In the present embodiment, the case where the indexable insert 50 is mounted on the face mill for shoulder milling has been described. However, for example, other cutting tools such as an end mill, and various cutting tools such as a turning tool such as a cutting tool. It may be mounted on a tool.

[0023]

As described above, according to the indexable insert according to the first aspect of the present invention, between the central portion of the indexable insert and the sub-cutting edge, the convex portion projecting from the upper surface inclined portion is provided. Since it is formed, the chipping resistance of the sub-cutting edge can be improved without lowering the sharpness of the sub-cutting edge and biting of the work material. In addition, since the upper surface of the convex portion is a substantially triangular flat surface and the bottom side is disposed on the side of the sub-cutting edge, chips generated during cutting can be prevented from colliding with the convex portion. In addition, it is possible to prevent the chip discharging property from being deteriorated. Furthermore, according to the indexable insert of the present invention, when the distance h from the bottom to the top of the upper surface of the projection is zero, the effect of providing the projection, that is, the fracture resistance of the sub-cutting edge is improved. On the other hand, if it is larger than (1/2) H, chips generated at the time of cutting work will collide with the convex portion, and the chip discharging property will be deteriorated. The two base angles α and β of the upper surface of the convex portion are set in the ranges of 0 ° <α <60 ° and 0 ° <β <60 °, respectively, and one base angle α is zero. And the effect of providing the convex portion, that is, the effect of improving the fracture resistance of the sub-cutting edge cannot be exhibited. become worse. In addition, the same operation and effect as the above-described one base angle α occur for the other base angle β. According to the indexable insert of the present invention described in claim 3, the area where the convex portion is formed is set so as to be out of the direction in which chips generated during cutting, particularly chips generated by the main cutting edge, extend. Therefore, it is possible to further prevent the chips from colliding with the convex portions, and to improve the chip discharge performance.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a throw-away tip of the present invention.

FIG. 2 is a front view of the throw-away tip shown in FIG.

FIG. 3 is a sectional view taken along line YY of the throw-away tip shown in FIG. 2;

FIG. 4 is an enlarged front view of a corner portion of the throw-away tip shown in FIG. 2;

FIG. 5 is a view showing a chip shape when a work material is cut by a face mill equipped with the throw-away tip shown in FIG. 1, and FIG. 5 (a) is a view showing a helical chip. (B) is a diagram showing a spiral-shaped chip.

FIG. 6 is a front view of a throw-away tip according to an example of the related art.

FIG. 7 is a sectional view taken along line XX of the throw-away tip shown in FIG. 6;

FIG. 8 is a central sectional view of the front milling cutter to which the throw away tip shown in FIG. 6 is attached.

[Explanation of symbols]

 10,50 indexable insert 13 main cutting edge 14 sub cutting edge 15 corner blade 60 honing (honing surface) 61 first honing 62 second honing 63 third honing 71 first upper surface inclined portion 72 second upper surface inclined portion 73 third Upper surface inclined portion 80 Projected portion 81 Projected portion upper surface 81A Bottom 81B Vertex 82 Projected portion inclined surface 82a Valley line portion α, β Base angle T bisector

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiro Kitagawa 1511 Furimagi, Ishishita-cho, Yuki-gun, Ibaraki Prefecture Inside Tsukuba Works, Mitsubishi Materials Corporation (72) Inventor Yuichi Suzuki 1511 Furamaki, Ishishita-cho, Yuki-gun, Ibaraki Prefecture Mitsubishi Materials Corporation Tsukuba Works F-term (reference) 3C022 HH01 KK03 KK06 KK12 KK25 LL02 LL05 3C046 CC06 JJ02 JJ03

Claims (3)

[Claims] 1. A throw-away insert having a sub cutting edge at a ridge at a corner of an upper surface to be a rake face, wherein the upper surface is inclined so as to have a downward slope from the ridge toward a center. On the upper surface, a convex portion protruding from the upper surface inclined portion is formed at a central portion side following the sub cutting edge, and the upper surface of the convex portion is formed of a substantially triangular flat surface. A throw-away tip, comprising: a base arranged on the side of the sub-cutting edge; and a vertex arranged on the center side of the upper surface from the base. 2. The throw-away tip has a polygonal plate shape, and at least a pair of opposed sub-cutting edges on the upper surface are formed. A distance h from a bottom to a vertex of the upper surface of the convex portion is: With respect to the distance H between the opposing sub-cutting edges, 0 <h ≦ (1/2)
H, and the two base angles α and β of the upper surface of the projection are 0 ° <α <60 ° and 0 ° <β <60 °
The throw-away tip according to claim 1, wherein the throw-away tip is set in a range of: 3. A throw-away insert having a sub-cutting edge at a ridge at a corner of an upper surface to be a rake face, wherein the upper surface is inclined so as to have a downward slope from the ridge toward a center. The upper surface is provided with a convex portion protruding from the upper surface inclined portion on a central portion side following the sub cutting edge, and the convex portion includes the sub cutting edge and the sub cutting edge. With respect to a bisector that bisects the angle between the main cutting edge adjacent to the blade and the bisector, it is disposed in a region sandwiched by the two bisectors located at both ends of the sub cutting edge. A throwaway tip that is characterized by