A tunable or tuned boring bar having increased stiffness is provided. Increasing the stiffness of the bar increases the natural frequency, thereby reducing directional deformation of the bar during use. The tunable boring bar includes a distal portion configured to support a tool, a proximal portion configured for attachment to a support structure of a metalworking machine, and a body, which is at least partially tubular, extending between the proximal portion and the distal portion. The tubular portion of the body has an elongated cylindrical cavity. The body of the boring bar includes a core layer formed from a first material and a coating layer formed from a second material. The second material has a higher modulus of elasticity than the first material. In certain configurations, the coating layer is bonded to the core layer by cladding, welding, chemical adhesives, chemical vapor deposition, pulsated plasma diffusion, or combinations thereof.

A cutting insert is used in a cutting edge replaceable rotary cutting tool. The cutting insert includes a first cutting edge formed on an intersecting ridge portion between a first end surface of two opposite end surfaces and a circumferential side surface, and a first depressed portion formed along a part of the first cutting edge. The first depressed portion is provided to form a larger rake angle at the first cutting edge than that at a part not having the first depressed portion of the first cutting edge.

A straight line along the second line segment and a straight line along the fourth line segment form an acute angle. The straight line along the second line segment and the straight line along the third line segment form an obtuse angle. The straight line along the first line segment and the straight line along the fourth line segment form an obtuse angle. Each of the third curved portion and the fourth curved portion is larger in radius of curvature than the first curved portion and larger in radius of curvature than the second curved portion. In a direction perpendicular to the bottom surface, a distance between the top surface and the bottom surface is equal to a distance between the cutting edge and the bottom surface, or is shorter than the distance between the cutting edge and the bottom surface.

A drill structure includes a shank part and a flute part. A chisel edge is formed on the front end of the flute part and two primary relief faces with tile directions toward the shank part are symmetrically formed on the two sides of the chisel edge. Each primary relief face has a cutting edge, a knife-back edge, and an outer edge. The outer edges are respectively helically extended around the periphery of the flute part to form two helical cutting edges and two helical grooves. Two assist relief faces are respectively formed on the inner walls of the two helical grooves, and every assist relief face is connected with the cutting edge of one primary relief face and portion of the knife-back edge of another primary relief face. The drill structure can decrease the thickness of the chisel edge, reduces the resistance during drilling, and increases the life.

The present invention provides a tool for drilling, the tool comprising two or more cutting edges and a web thinning, in which a thinning heel surface facing backward in a rotating direction of the tool is formed by a convex surface, and in which a thinning angle gradually becomes smaller from a leading end of the tool toward a base end thereof. A thinning rake surface of the web thinning may be shaped straight so as to extend from an area around a center of the tool and in a radial direction or a parallel direction thereto in a tool front view. A thinning rake surface may be shaped in a helix in an opposite direction to the rotating direction of the tool so as to extend from an area around a center of the tool and in a radial direction in a tool front view.