| 摘要 |
PROBLEM TO BE SOLVED: To allow a drilling work without causing any crack or crazing, by allowing a first hole to communicate with a second hole when an expanded diameter portion of a second drill is plate-faced. SOLUTION: A lower-hole machining device machines a lower hole 42 by pressing a first diamond drill 40 onto the lower surface of a glass plate 20 to provide rotation and feeding. At this time, the first diamond drill 40 defines a tapered flange 40A at a position a given length L1 spaced from the tip, and a tapered surface of this flange 40A is pressed onto a peripheral edge of the previously machined lower hole 42 to plate-face the lower hole 42. A second diamond drill 50 is provided facing the first diamond drill 40. When a flange 50A of the second diamond drill 50 is plate-facing an upper hole 48, the lower hole 42 is allowed communicate to with the upper hole 48.
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