This paper presents a geometric analysis of an undeformed chip thickness at the top of tool in ball-nosed end milling. It has been firstly pointed out backward cutting near the center of tool that is irregular interference between the flank and the workpiece. So backward cutting occurs a non contact part on the cutting edge. Secondly a condition of backward cutting has been clarified. An area of backward cutting in a ball-nosed end mill is shaped circle that exists beside the center of tool axis. Thirdly a method of avoidance from backward cutting has been brought up. Then its influence for surface roughness is verified by a few experiments.
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