| 摘要 |
<p>A front end portion of a core drill is formed in an arc shape in which longitudinal cross-sectional portions have the same radius. A contact width of a cutting edge of the core drill which first comes in contact with a surface of the glass sheet is small for the surface of the glass sheet, and the contact width of the cutting edge which comes in contact with the surface of the glass sheet is gradually increased as the core drill is lowered. An annular cavity into which the front end portion of the core drill is inserted is formed on an upper surface of the stage on which the glass sheet is placed. The annular cavity is formed so that its cavity width in the radial direction is wider than a drill width of the core drill.</p> |
