Investigation On Size Effect Of Specific Cutting Force And Machined Surface Quality In Micro Cutting

With the development of microelectronics industry, modern medical equipment, biological engineering, national defense industry and aerospace technology, the micro parts in the range of millimeter or micron meter are demanded more complex shape, more functions, more kinds of material, higher reliability. Micro machining technology is different from etching technology, thin-film technology and LIGA technology, and it can create a complicated three-dimensional small part of different materials quickly with higher precision at lower cost, then micro cutting has a promising application prospect. Micro cutting appears some characteristics different from macro cutting such as minimum uncut chip thickness, size effect of specific cutting force and size effect of surface integrity. Investigation on the mechanism of workpiece deform is helpful to understanding the peculiar phenomenon of micro cutting process, and improve the quality of micro parts.The extrusion effect of the cutting edge on the workpiece cannot be neglected in micro cutting and the ratio of uncut chip thickness to cutting edge radius becomes an important parameters. The deformation of workpiece is analyzed based on the slip-line field. The mechanism of size effect of specific cutting force and the effect of specific cutting force on the surface roughness and burr height are investigated. A specific milling force model of micro flat end milling is established and the factors and mechanism of top side burr in micro flat end milling experiments are analyzed.Firstly, a deformation model is established considering the round cutting edge based on the slip-line fields. The micro orthogonal cutting forces are calculated and validated with different ratios of uncut thickness to cutting edge radius. The cutting force decreases with the ratio of uncut chip thickness to cutting edge radius decreasing, the specific cutting force increases with the decrease of uncut chip thickness to cutting edge radius. Collecting and measuring the deformed chip in micro orthogonal cutting, the deformation coefficient shows the size effect due to the pile up of the workpiece in the front of the cutting edge. With the decrease of the ratio of uncut chip thickness to cutting edge radius, the shear strain, shear strain rate, and shear temperature in the primary shear zone increases and shows the size effect phenomenon. The shear flow stress in the primary shear stress decreases due to the softening effect of the shear temperature. The plasticity strain gradient strengthening coefficient increases due to the effect of the plasticity strain gradient and the shear flow stress in the primary shear zone. The deformation path increases due to the round cutting edge and the relative cutting shows size effect phenomenon.Secondly, the mechanism of size effect of surface integrity is investigated based on the variation of specific cutting force around the corner. Specific cutting force and the flow ability of workpiece around the comer increases when the ratio of uncut chip thickness to corner radius. The side flow takes place and the workpiece piles up in front of the cutting edge and behind the cutting edge. The best surface roughness will be got when the pile-up height of rake, flank face and feed rate reaches a balance. The cutting edge of cutting tool extrudes and shears the workpiece with the round cutting edge. The machined surface is first extruded by the rake face and then ploughed by the flank face. The micro hardness decreases first and then increases when the ratio of uncut chip thickness to cutting edge radius decreases, and it reaches the bottom when the stress around the rake face equals that of flank face.Thirdly, the effect of specific cutting force on the burr height is studied. Side flow takes place when the tool extrudes the workpiece and the Poisson burr forms (entrance burr and side burr). The Poisson burr height increases with the decrease of the ratio of uncut chip thickness to cutting edge radius when the uncut chip thickness is smaller than the cutting edge radius. The Poisson burr height is stable because of the stable specific cutting force around the cutting edge when the uncut chip thickness is larger than the cutting edge radius. The tool extrudes the chip on the edge and the exit burr forms. The exit burr height will increase due to the effect of size effect of specific cutting force. The exit burr is a curled burr and different from that of macro cutting. The curled radius decreases first, and then increases with the decrease of the ratio of uncut chip thickness to cutting edge radius.Finally, a specific cutting force model is established of micro flat end milling including the size effect of specific cutting force, the factors of milling parameters such as feed per tooth, axial depth of cut, tool parameters such as tool diameter, helical angle, milling method such as up milling, down milling on specific cutting force are analyzed. The milling process is different from orthogonal cutting of two dimensions due to the influence of helical angle. The specific cutting forces in the three directions increase with the decrease of the feed per tooth due the size effect of specific cutting force. The specific cutting forces in the three directions increase slowly due to the increase of cutting edge in the entrance and exit area when the axial depth of cut decreases. The specific cutting forces in the ox, oy directions increase with the decrease of the helical angle, while the specific milling force in the oz direction decrease. The specific cutting forces decrease in the three directions with the decrease of the tool diameter. The specific cutting force increases with the decrease of the radial cutting width in the ox direction, and the specific cutting force in the up milling is larger than that of down milling. The specific cutting force decreases with the decrease of the radial cutting width in the oy direction, and the specific cutting force in the up milling is smaller than that of down milling. The specific cutting force increases with the decrease of the radial cutting width in the oz direction, and the specific cutting force in the up milling is in the same level with that of down milling. The factors of milling parameters such as feed per tooth, axial depth of cut, tool parameters such as cutting edge radius, helical angle on top side burr are analyzed in micro flat end milling. The height of top side burr shows size effect with the decrease of the feed per tooth. The top side burr height decreases with the decrease of axial depth of cut due to the shortening of the cutting edge. The height of top side burr reaches the minimum when the cutting edge radius is equal to the feed per tooth. The top side burr height increases with the decrease of the helical angle due to poor clearance.This work is sponsored by the National Natural Science Foundation of China (Grant No.50905003) and Foundation of Shandong Province of China for Distinguished Young Scholars (Grant No. JQ200918) for financial supports.