Abstract: In the operation of cutting a metallic work material, if the rotation of an electric motor 21 is reduced by a reduction gear mechanism including a gear mechanism and transmitted to a main shaft 23 and the metallic work material is cut by a circular saw 28 attached to one end of the main shaft, the moment of inertia I (kgf·m·sec2) of the main shaft is set to be more than F×(D/2)/[1000×250×(N/60)2] while an outside diameter of said circular saw is D (mm), the number of revolution is N (rpm) and a cutting force per tooth as a component of force in tangential direction is F (kgf). By so setting, at the initial cut positions of the work material and in a region in which the number of cutting related teeth Zi is 1.0 or less at the final cut positions, the moment of inertia is high compared with a cutting torque applied to the main shaft by intermittent cutting resistance, so that the rotational variation of the main shaft is very small.

Abstract: A rotating boring tool (10) comprises a cylindrical shank (11) and a body (12), both of which are coaxial with each other, said body being provided at its tip end with cutting blades (13a, 13b) which are in point symmetry with respect to an axis of the shank. Provided on the body are two flutes (14a, 14b) continuous from the cutting blades. The both flutes are made to approach each other at the lower half lead portions (designated hereinbelow as f1 portions) of terminal end portions f0 of the flutes and positioned about 35 degrees or more offset from the position of point symmetry. Alternatively, the both flutes are merged at the f1 portions. Accordingly, thickened portions y0 are formed on the f1 portions to increase rigidity of the f1 portions. As a result, it is possible to suppress generation of fatigue failure at the f1 portions whereby a rotating boring tool of long service life which is hard to break is obtained and a product cost and a cost for tool replacement are considerably reduced.