This paper provides a comprehensive review of the literature, mostly of the last 10–15 years, that is enhancing our understanding of the mechanics of the rapidly growing field of micromachining. The paper focuses on the mechanics of the process, discussing both experimental and modeling studies, and includes some work that, while not directly focused on micromachining, provides important insights to the field. Experimental work includes the size effect and minimum chip thickness effect, elastic-plastic deformation, and microstructure effects in micromachining. Modeling studies include molecular dynamics methods, finite element methods, mechanistic modeling work, and the emerging field of multiscale modeling. Some comments on future needs and directions are also offered.
This paper examines the surface generation process in the micro-endmilling of both single-phase and multiphase workpiece materials. We used 508 μm dia endmills with edge radii of 2 and 5 μm to machine slots in ferrite, pearlite, and two ductile iron materials at feed rates ranging from 0.25 to 3.0 μm/flute. A surface generation model to predict the surface roughness for the slot floor centerline is then developed based on the minimum chip thickness concept. The minimum chip thickness values were found through finite element simulations for the ferrite and pearlite materials. The model is shown to accurately predict the surface roughness for single-phase materials, viz., ferrite and pearlite. Two phenomena were found to combine to generate an optimal feed rate for the surface generation of single-phase materials: (i) the geometric effect of the tool and process geometry and (ii) the minimum chip thickness effect. The surface roughness measurements for the ductile iron workpieces indicate that the micromilling surface generation process for multiphase workpiece materials is also affected by the interrupted chip-formation process as the cutting edge moves between phases resulting in burrs at the phase boundaries and the associated increases in surface roughness.
Under normal machining conditions, the cutting forces are primarily due to the bulk shearing of the workpiece material in a narrow zone called the shear zone. However, under finishing conditions, when the uncut chip thickness is of the order of the cutting edge radius, a ploughing component of the forces becomes significant as compared to the shear forces. Predicting forces under these conditions requires an estimate of ploughing. A slip-line field is developed to model the ploughing components of the cutting force. The field is based on other slip-line fields developed for a rigid wedge sliding on a half-space and for negative rake angle orthogonal cutting. It incorporates the observed phenomena of a small stable build-up of material adhered to the edge and a raised prow of material formed ahead of the edge. The model shows how ploughing forces are related to cutter edge radius -a larger edge causing larger ploughing forces. A series of experiments were run on 6061-T6 aluminum using tools with different edge radii -including some exaggerated in size -and different levels of uncut chip thickness. Resulting force measurements match well to predictions using the proposed slip-line field. The results show great promise for understanding and quantifying the effects of edge radius and worn tool on cutting forces.
In micromachining, the uncut chip thickness is comparable or even less than the tool edge radius and as a result a chip will not be generated if the uncut chip thickness is less than a critical value, viz., the minimum chip thickness. The minimum chip thickness effect significantly affects machining process performance in terms of cutting forces, tool wear, surface integrity, process stability, etc. In this paper, an analytical model has been developed to predict the minimum chip thickness values, which are critical for the process model development and process planning and optimization. The model accounts for the effects of thermal softening and strain hardening on the minimum chip thickness. The influence of cutting velocity and tool edge radius on the minimum chip thickness has been taken into account. The model has been experimentally validated with 1040 steel and Al6082-T6 over a range of cutting velocities and tool edge radii. The developed model has then been applied to investigate the effects of cutting velocity and edge radius on the normalized minimum chip thickness for various carbon steels with different carbon contents and Al6082-T6.
In the end milling process, the cutting forces during machining produce deflection of the cutter and workpiece which result in dimensional inaccuracies or surface error on the finished component. A previously developed mathematical model for the cutting force system in end milling is combined with models for cutter deflection and workpiece deflection so that the surface error profile may be predicted from the machining conditions and geometry and material properties of the cutter and workpiece. Machining experiments are performed on rigid and flexible workpieces of 7075 aluminum to verify the ability of the models to predict surface error. The model predicted surface error profiles are accurate both in magnitude and shape with the difference between measured and predicted surface errors ranging from 5 to 15 percent. This approach for the prediction of surface errors provides a useful aid for the analysis of a variety of end milling process design and optimization problems.
As more emphasis is placed on quality and productivity in manufacturing, it becomes necessary to develop models that more accurately describe the performance of machining processes. An improved model for the prediction of the cutting force system and surface error in end milling has been developed and has been implemented on the computer. This enhanced model takes into account the effect of system deflections on the chip load, and solves for the chip load that balances the cutting forces and the resulting system deflections. Such a model allows for the evaluation of cuts in which deflections significantly effect the chip load. The flexible system model predictions of forces and surface error are compared against both measured and rigid system model-predicted values associated with the machining conditions for experiments performed on the 390 casting aluminum alloy. It is shown that the enhanced chip load model gives predictions of both cutting force signatures and surface error profiles that are significantly better than the rigid system chip load model developed previously. The fact that system deflections temper the effects of runout, and reduce both peak cutting force and maximum surface error is demonstrated and discussed.
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