| 摘要 |
In a W-shaped twist drill which is to be suitable primarily for the machining of materials difficult to cut, provision is made for guiding one (3a) of the centre bits (3) about the drilling-tool centre (1') in the region of the tapering out (1a) of the bore, in such a way that, depending on the cutting diameter, a more or less thick workpiece material core (K) is initially left uncut and, during the further advance of the drilling tool, is removed, together with the chips, by means of the cutting groove (5a') adjacent there. Moreover, it is important, at the same time, that each of the cutting grooves (5) be subdivided (5a and 5b) at least once in the radial direction and that, as seen in the radial plane (Fig. 2), the two portions (5a and 5b) run out of a region in front of the respective tool mid-plane (E) preferably as far as approximately 6 DEG -10 DEG behind the latter. <IMAGE>
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