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

Claim 24:
25. 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:
an operation portion comprising
the cone portion,
the cylinder portion axially fixed to the cone portion,
a shank portion axially fixed to the cylinder portion opposite the cone portion,
a spiral flute formed on 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 spiral flute having a sidewall, and
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; 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 an intersection of the second step 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 a diameter of each of the plurality of composite cutting blade groups increases sequentially from the front end of the cone portion to the rear end of the cone portion, and each of the plurality of composite cutting blade groups is configured to cut fine chips and the spiral flute is configured to shunt the fine chips, and
wherein a diameter of a 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 a diameter of the cylinder portion;
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.