| 摘要 |
A screw device with an elongated shank, a self-tapping cutter located thereon on and with a cutting head within which there are two equivalent and opposing grooves the angularity of which with reference to horizontal is the same and equivalent to the angular pitch of the threading about the shank and with serrated edging about the bottom side of the cutting head and in the top side of the cutting head there being found a closed hole.
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