| 摘要 |
PURPOSE:To reduce production of burr to the outlet of a machining hole, to improve finish surface precision and roughness, and to decrease an enlargement allowance, by a method wherein the outer peripheral end of the main cutting edge of a twist drill is chamfered along a chip groove, and a finish cutting edge is formed as a cross line with a flank. CONSTITUTION:A cross line between a plane extending through an outer peripheral end S of a finish cutting edge and at right angles with tool axes 0 and 0' and a plane containing a finish cutting blade MS and extending in parallel to tool axes 0 and 0' is SM'. In this case, provided LS M'M is 90 deg., the vector of a cutting speed (V) at the outer peripheral end S is V the positive projection of the vector V on the cross line SM' is V', and the positive projection of V' on the cutting edge SM is V'', V is the positive projection of the vector V on the cutting edge SM and forms a cutting speed component extending along a finish cutting edge 7 at the outer peripheral end S, and cutting quality is improved and production of burr is reduced. Since the blade right-angle cutting speed of a main cutting edge 1 is different from that of the finish cutting edge 7, and a cutting speed component along the cutting edge is also different, if LAMS is below some limit, the chips of the main cutting edge 1 and that of the finish cutting edge 7 are discharged in a separated state. Since a tip angle with the finish cutting edge 7 is lower than that with the main cutting edge 1, centripetalism is improved and an enlargement allowance is reduced.
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