Milling insert and milling tip-replacement-type rotary cutting tool

1. A milling insert, comprising:

an insert body having a polygonal tabular shape, the insert body comprising:

rake faces provided on one polygonal surface of the insert body;

flanks composed of circumferential surfaces adjacent to the polygonal surface; and

cutting edges defining a ridge line where the rake faces and the flanks intersect each other,

wherein, the rake faces include serrations having pluralities of mount portions and valley portions that are arrayed alternately to intersect the cutting edges,

wherein, when the flanks are viewed along an axis transverse to a longitudinal axis of the insert body, the ridge line has a waveform shape in which bottom portions and top portions are formed into concave circular arc portions and convex circular arc portions, respectively, and are alternately repeated,

wherein, both of the concave circular arc portions and the convex circular arc portions are a ¼ circular arc or more to a ⅓ circular arc or less,

wherein, each concave circular arc portion that is located between two adjacent convex circular arc portions is positioned so as to be closer to one of the two adjacent convex circular arc portions and farther from the other of the two adjacent convex circular arc portions, and

wherein the ridge line is formed with a pitch that narrows as the ridge line extends in a direction corresponding with an arc sequence of: farther-spaced convex circular arc portion, concave circular arc portion, closer-spaced convex circular arc portion, and the pitch gradually narrows over at least three iterances of the arc portion sequence.

2. The milling insert according to claim 1 , wherein the ridge line is shaped such that a concave circular arc portion is connected by a straight line to the farther-spaced of two adjacent convex circular arc portions on opposite sides of the concave circular arc portion.

3. The milling insert according to claim 2 , wherein the ridge line is shaped such that the concave circular arc portion is connected by a straight line to the closer-spaced of the two adjacent convex circular arc portions on opposite sides of the concave circular arc portion.

4. The milling insert according to claim 2 , wherein the ridge line is shaped such that the concave circular arc portion is directly connected to the closer-spaced of the two adjacent convex circular arc portions on opposite sides of the concave circular arc portion.

5. The milling insert according to claim 1 , wherein a virtual chord that connects both end points of a convex circular arc portion to each other is inclined relative to a closer-spaced of two adjacent concave circular arc portions on opposite sides of the convex circular arc portion.

6. The milling insert according to claim 5 , wherein the milling insert is configured so as to be attachable to a body of a rotational cutting tool while setting, within ±5 degrees, a deviation angle between a tangential line of a convex circular arc portion, the tangential line passing through a midpoint of the convex circular arc portion, and a straight line that passes through the midpoint and is perpendicular to a tool revolution axis.

7. The milling insert according to claim 1 , wherein the milling insert is coated with a polycrystalline diamond film.

8. The milling insert according to claim 1 , wherein

a base body of the milling insert is composed of a cemented carbide-made member, and

the cutting edges are composed of a member made of:

cubic boron nitride sintered body bonded to the base body, or

polycrystalline diamond bonded to the base body.

9. The milling insert according to claim 1 , wherein the cutting edges include wiper edges between a range of the waveform shape, in which the concave circular arc portions and the convex circular arc portions are alternately repeated, and corners of the polygonal surface.

10. A cutting tool, comprising

an indexable face milling cutter having a milling cutter body, and

the milling insert according to claim 1 detachably attachable to a tip end portion of the milling cutter body and configured to be rotationally driven when attached to the tip end portion of the milling cutter body.

11. The cutting tool according to claim 10 , wherein

in a state where the milling insert is mounted on the tip end portion of the milling cutter body, there is an angle (δ) measuring 5°≦δ≦30°, the angle (δ) being defined by:

a line segment that connects a lowest point of the cutting edges and an equivalent point on a side of the polygon shape that is adjacent to and radially outward, relative to the tool revolution axis, from the side including the lowest point, and

a plane that includes the lowest point and is perpendicular to the tool revolution axis.

12. The cutting tool according to claim 10 ,

wherein the insert is attached to the milling cutter body with a deviation angle measuring ±5 degrees, the deviation angle being an angle between a tangential line of a convex circular arc portion, the tangential line passing through a midpoint of the convex circular arc portion, and a straight line that passes through the midpoint and is perpendicular to a tool revolution axis.

13. A cutting tool, comprising

an indexable face milling cutter having a milling cutter body, and

the milling insert according to claim 9 detachably attachable to a tip end portion of the milling cutter body to be rotationally driven when attached to the tip end portion of the milling cutter body,

wherein, in a state where the milling insert is mounted on the tip end portion of the milling cutter body, under conditions where a plane that includes a lowest point of the cutting edges and is perpendicular to a tool revolution axis is taken as an angle reference, and where the lowest point is taken as an observation point, each of the wiper edges is formed over a predetermined length within a range where an angle of elevation is 1 degree or less.