To substantially prevent the remaining of fine chips by chipping in an opening of an end face of a glass plate in a step of boring the glass plate and to accurately form a through-hole.
A preceding drill 1 is allowed to penetrate under cutting into a glass plate 3 in the thickness direction from the lower end face side to form a bottomed hole 31, the preceding drill 1 is retreated, and a following drill 2 is allowed to penetrate coaxially with the preceding drill 1 under cutting from the upper end face side of the glass plate 3 to form a through-hole 34 in the glass plate 3. The at least one drill 1 is composed of a body part 11 having a uniform outer diameter along the axial direction, a small diameter part 12 positioned on the drill base end side of the body part 11 and having a smaller outer diameter than the body part 11, and a connection part 13 connecting the body part 11 and the small diameter part 12, and cutting ability is provided to the part from the tip of the drill 1 to at least the body part 11 side of the connection part 13. The drill 1 used in the boring step is configured such that in a state in which the drill 1 is allowed to penetrate to the deepest part, the connection part 13 reaches a thickness-direction inner side of the glass plate 3.
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TONO MASAYUKI
Kunihiko Shiromura
Tsuyoshi Kumano