| 摘要 |
The dowel (1) is tubular in shape and is cut to form a spiral for at least part of its length. It has lengthwise internal (7) and external (8) projections on its inner (6a) and outer (6b) surfaces. Both sets of projections are rectangular, semicircular or triangular in cross-section, and both can be in the form of lengthwise ribs or annular collars. The dowel can be made by winding it in a spiral round a roller or cutting a tube with spiral slits.
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