The Mechanisms of Chip Formation in Machining Hardened Steels

Abstract

The formation of saw-tooth chips is one of the primary characteristics in the machining of hardened steels with geometrically defined cutting tools. Catastrophic failure within the primary shear zone during saw-tooth chip formation is usually attributed to either cyclic crack initiation and propagation or to the occurrence of a thermo-plastic instability. The results presented here show that the primary instability resulting in the formation of saw-tooth chips is initiation of adiabatic shear at the tool tip and propagation partway towards the free surface. Depending on the work material hardness and cutting conditions, catastrophic failure within the upper region of the primary shear zone occurs through either ductile fracture or large strain plastic deformation. Prior to the onset of chip segmentation, which occurs with increases in work material hardness and cutting speed, there is a transition in the morphology of the free surface of continuous chips, from the familiar lamellar structure to what has been termed a “fold-type” structure. This transition is attributed to the operation of thermally softened micro-shear zones, which, it is suggested, are a precursor to adiabatic shear initiation.

Badania Wpływu Grubości Warstwy Skrawanej I Cieczy Obróbkowej Na Kształt Wiórów W Procesie Skrawania Ortogonalnego

“…Przeprowadzono wiele prac badawczych w celu określenia wpływu warunków skrawania na proces formowania wiórów [1,[3][4][5][6][7][8][9][10]. Jednym z częściej badanych zjawisk jest proces segmentacji wióra w paśmie ścinania adiabatycznego, co umożliwia diagnozowanie wpływu kształtu powstałego wióra na siłę skrawania, wahania długości powierzchni ścinania i chropowatość powierzchni obrobionej.…”

Experimental study of the brittle–ductile transition in hot cutting of SG iron specimens

“…Indeed, Barry et al (2001) showed that in the machining of titanium and its alloys, cleavage is the mechanism of catastrophic failure in the chips produced at low cutting speeds within the upper primary shear zone. Barry and Byrne (2002) showed that the machining of hardened steel involves the same phenomena. However, the underside of saw-tooth segments produced at relatively higher cutting speed exhibits evidence of ductile fracture related to a noticeable thermal softening within the primary shear zone.…”

Modelling of the Relationship of Adiabatic Shear and Cutting Conditions for Higher Cutting Speed Ranges

“…Therefore, it is extensively applied in the fields of aerospace, automotive and moulding, etc., and is widely used [1]. The main difference between high speed cutting and conventional cutting is that the serrated chip is often produced in high speed cutting [2] and [3], and the continuous chip is produced in conventional cutting. Fig.…”

“…Although some scholars ascribed the occurrence of serrated chips to a periodic crack initiated in the free surface of the workpiece ahead of the tool [4] and [5]. Other researchers, including Recht [6], Komandui [7], Davies [8], Molinari [9] and Ma [10], attributed the formation of serrated chips to a repeated thermo-plastic instability occurring in the primary shear zone, particularly for ductile materials [2] and [11]. On one side, the occurrence of adiabatic shear in high speed cutting will increase the tool wear and influence the quality of the finished surface.…”