| 摘要 |
The central hub of the cutter is formed in three parts including two conical portions (3,4) of different pitch and a series of twelve blades (2b) spaced angularly around its circumference. The third portion of the hub is an elongate conical tip (1) formed with a circular groove in its lower surface. The central conical portion has twelve longitudinal grooves in its exterior surface, and the lower conical portion has a star-shaped array of twelve grooves radiating from a central circular groove in its upper face. The blades engage in the grooves and are locked by engagement of inner end lugs in the circular grooves.
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