Abstract
It was previously found that material removal in orthogonal cutting of ceramics was characterized by extension of a macro crack: it initiates in a region near the tool tip, then unstably extends slightly downwards following the minimum tensile stress trajectory. Its further extension is affected by material crashing ahead of the tool rake face so that it turns upwards to emarge out at the upper free surface (chip formation type) or stops in the material (crack arrest type) or successively proceeds downwards (crack proceed type).
This paper reports the experimental investigation of the effects of depth of cut, cutting velocity and tool rake angle on the macro crack extension. The specimens of glass-ceramics (Macor, Corning Glass Works), mullite and soda-lime glass were cut in some length just at the incipient stage of orthogonal cutting when the depth of cut was varied, and they were cut in full length when the cutting velocities and tool rake angles were varied. The main results obtained are as follows.
(1) Increase in depth of cut changes the crack type from chip formation type to crack proceed type through crack arrest type. There appeare distinct differences among these types when interpreted in terms of l/Dt ratio (crack length/depth of cut ratio). The ratio is smallest for the chip formation type and largest for the crack proceed type. Macor and mullite have a wider range of depths of cut for attaining chip formation type and smaller values of l/Dt than soda-lime glass.
(2) Decrease in cutting velocity makes the crater of finished surface smaller and shallower. This phenomenon is caused by a decrease in fracture strength of the work which is usually observed at such relatively low strain rate as is attained in the present cuttings.
(3) The crater of finished surface becomes smallest when the tool rake angle is 15°, where the most frequent contact between the tool rake face and the work front face occurs to stimulates material crashing ahead of the tool rake face.
