In an exemplary embodiment, a blade portion 41 is formed by a plurality of blades 42 being arrayed on a front surface of a main body at an even interval while forming blade striation portions 43 between adjacent blades 42. Each blade 42 is cut into a circular-arc shape. A pitch between the blades 42 is in a range of 1 to 3 mm. A recess-portion apex composed of a cut surface of each blade 42 has a rounded shape with a radius of 1.0 to 3.0 mm. A protrusion-portion apex of the blade striation portion 43 has a rounded shape with a radius of 0.1 to 0.5 mm. A tip of the blade 42 formed by the blade striation portion 43 has a rounded shape with a radius of 0.1 to 0.5 mm.