A new low-feed chip breaking tool and its effect on chip morphology

This paper investigates the influence of cutting conditions on the formation mechanism of chips using a Tungsten Carbide in–house lasered cutter (grooved chip breaker) and a benchmark commercial cutter during turning of AISI1040 medium carbon steel. Microstructure of the free surface and segment underside the chips are experimentally characterised via scanning electron microscopy (SEM) and white light interferometry. The mechanism of chip formation is classified into continuous, partially segmented, segmented and discontinuous. Chip breaking ability are achieved for all tested feed speeds at depth of cut above 1.2 mm, marking the transition from continuous to segmented chips. The chip breaker manufactured via a nanosecond laser, proves to enable for the first time breaking of the chip below a feed rate of 0.1 mm/rev outperforming the commercial cutter and showing viability for the production capabilities of lasers for mass manufacture. Lamellae-type chips are revealed from machining using the lasered tool; while brush-stroke chips are discovered and introduced for the first time from machining using the benchmark cutter. While the lamellae form from cleavage cracks due to strain incompatibility at inclusions caused by an excess in critical shear strain. The brush-stroke chips are caused by a localized increase of temperature at the tool/material interface which lead to thermal softening of the workpiece: the resulting surface experiences large areas of plastic deformation. For the in-house lasered tool, at higher cutting speed the shear strain hardening reduces the flow stress of the workpiece material in the shear zone.