Modeling of Cutting Forces Under Hard Turning Conditions Considering Tool Wear Effect

Modeling of Cutting Forces Under Hard Turning Conditions Considering Tool Wear Effect

Introduction

Generally, the hard turning process is recognized as the single point turning of materials with hardness from 50 to 70 HRc 1#.It differs from conventional turning in tool/workpiece material prop-erties, cutting tool geometry, chip formation mechanism, and cut-ting conditions applied. As a potential alternative to form grind-ing, the hard turning process can offer attractive benefits in terms of lower equipment costs, shorter setup time, fewer process steps, higher material removal rate, better surface integrity, and the elimination of cutting fluid @1–3#. But for hard turning to be a viable technology, there are still several issues—including tool life, part integrity, and machine stiffness requirements—that need to be addressed. A quantitative understanding of cutting forces under hard turning conditions is a critical element in addressing these issues because of its implications on thermal analysis, tool life analysis, chatter analysis, etc.

In general, significant prior research is available in the area of force modeling in turning operation. However, a comprehensive analysis of hard turning forces has not been well established in view of the unique process conditions involved. In addition to the workpiece and cutting tool material property aspects, hard turning process conditions are defined based on several key characteris- tics: negative tool rake angle, low feed rate, small depth of cut, relatively large tool nose radius, and rapid tool wear rate. These characteristics provide a set of cutting configuration, chip forma- tion mechanism, and force generation process distinctive to the commonly encountered turning conditions. Therefore, the effect of three-dimensional ~3D! engagement and the implication of tool wear must be incorporated in the modeling of hard turning forces. When the effect of flank wear is not considered, the primary scope of cutting force modeling is related to chip formation. A complication with the chip formation in hard turning is the seg- mentation of chips as a result of either catastrophic thermoplastic adiabatic shear @4# or crack/fracture generation @5#. The force variation due to chip segmentation is estimated to be on the order of 63% based on the measurement of Vyas and Shaw @6#. The variation component of cutting force is typically insignificant compared to the average forces component, and the variation frequency—on the order of 10–100 KHz—is beyond the sam- pling rate of typical dynamometers @4,6#. Therefore, the interest of cutting force modeling often focuses on the average force level, but not on the variation due to chip segmentation. One widely used method to model the average cutting force components is the mechanistic modeling approach @7#. This method has a proven prediction accuracy over a range of cutting conditions while re- quiring a minimum number of calibration test data. Suited for the orthogonal cutting configuration, the mechanistic modeling ap-proach is not readily applicable to hard turning due to the 3D cutting geometry and large tool nose radius that make the hard turning process highly three dimensional. When the effect of flank wear on cutting forces is considered, it has been observed in conventional turning that as the tool wears neither the shear angle @8# nor the chip thickness changes notice- ably @9,10#. Similar phenomena were observed in hard turning of HV760 steel @11#. Shintani et al. @12# further found that the effec- tive tool geometry did not change throughout the cubic boron nitride ~CBN! tool life when hard turning carburized steel under practical cutting conditions ~low feed rate, small depth of cut, and gentle cutting speed!. Although the underlying physical mecha- nism is not clear in this phase, these observations suggest that the chip formation process in conventional and hard turning is not significantly affected by the tool wear and also support the hy- pothesis that the total cutting forces can be treated as consisting of two uncoupled parts: forces due to chip formation regardless of the tool sharpness, and forces due to flank wear alone. But some researchers still doubt the efficacy of this decoupling property. Recently, Wang and Liu @13# argued that forces due to chip for- mation and forces due to flank wear should be coupled with each other. Based on the depth of the phase transformed white layer in hard turning, Wang and Liu @13# deduced the temperature profile.

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