Patent ID: 11865627
Assignee: nan
Field: Machine tools (Mechanical engineering)
Classification: CPC B | IPC B

Claim 17:
18. A method for improving centering accuracy of a twist drill when drilling a drill hole in a workpiece using multistage centering and using both a cone portion and a cylinder portion, the method comprising:
providing the twist drill comprising:
a shank portion;
an operating portion axially fixed to the shank portion, the operating portion comprising the cylinder portion and the cone portion axially fixed to the cylinder portion and a spiral flute having a sidewall and being formed into an exterior surface of the operating portion extending from a front end of the cone portion and at least partway up the cylinder portion, the shank portion axially fixed to the cylinder portion opposite the cone portion;
a plurality of composite cutting blade groups formed sequentially on a cone portion exterior from the front end of the cone portion to a rear end of the cone portion, and diameters of the composite cutting blade groups sequentially increase from the front end to a rear end of the cone portion, each of the plurality of composite cutting blade groups comprising:
a first step surface;
a second step surface adjacent to the first step surface,
a major cutting edge defined by an intersection of the first step surface and the sidewall of the spiral flute;
a minor cutting edge defined by a intersection of the second step surface and the sidewall of the spiral flute; and
a cutting tip defined by an intersection of the major cutting edge, the minor cutting edge, and the sidewall of the spiral flute; and

a top blade provided on the front end of the cone portion;
wherein the second step surface of at least one composite cutting blade group is immediately adjacent to the first step surface of a next composite cutting blade group; and the diameter of the last composite cutting blade group located at the rear end of the cone portion immediately adjacent to the cylinder portion is less than or equal to the cylinder portion diameter;
positioning the twist drill atop the workpiece;
applying a torque and an axial drilling force to the twist drill;
cutting the workpiece with the top blade to create a drill hole in the workpiece;
continuing cutting the workpiece using one or more of the plurality of composite cutting blade groups situated closest to the front end; and
continuing cutting the workpiece using all of the plurality of composite cutting blade groups, when a portion of the cylinder portion is also within the workpiece.