JournalofManufacturingScienceandEngineering60(74).pdf

Z. Y. Yu e-mail: K. P. Rajurkar A. Tandon Department of Industrial revised September 2, 2004. Associate Editor: K. Dohda. Journal of Manufacturing Science and EngineeringNOVEMBER 2004, Vol. 126 727 tain extent from the tool 15#. Figure 1 shows the basic principle of USM process. Usually, the sinking process is used in ultrasonic machining, especially in the machining of 3D shapes. The tool has the same shape and size as that of the designed part 16#. Cur- rently, micro-USM is mainly applied to drill microholes 17,18#. The ultrasonic vibration is transmitted to the abrasive grain through an assembly of transducer, concentrator, and other related parts, resulting in a large eccentricity of tool rotation. The work- piece vibration has been proposed to solve this problem, and mi- croholes with 5mm in diameter have been drilled successfully 19#. In order to conduct the micro-USM drilling experiments, a micro-USM system has been designed and assembled as shown in Fig. 2. The workpiece fi xed on the top of the transducer is vi- brated by an ultrasonic vibrating system that includes an ultra- sonic vibration generator and a transducer at frequency of 39.5 kHz. The slurry, a mixer of abrasive grains and water, is added on the workpiece surface. An electronic balance serves as the static load sensor with a resolution of 0.1 mg (9.831027mN). The 3D tool-path movement is controlled by the X, Y, and Z mini-stages and their controller. When a tool is controlled to move along a designed tool path, the static load is monitored during machining. If the static load is larger than the set value, the tool is lifted up to a certain height to avoid the tool breakage because of overloading. Many factors have been found to infl uence the performance of macro-ultrasonic machining. The static load, amplitude of vibra- tion, type and size of abrasive grains, material and size of tool, and slurry concentration signifi cantly affect the material removal rate and surface roughness. In micro-USM, however, it is very diffi cult to maintain the slurry concentration at the same level because the water easily vaporizes during micro-USM under ul- trasonic vibrations. Additionally, the external fl ushing causes the vibration of microtool and the variation of static load during ma- chining. Therefore, for these preliminary studies, the slurry con- centration was not considered as one of the input parameters. The diamond powder is selected as the abrasive because it has high hardness and it does not fracture under the impact of ultrasonic vibrations. Experiments of microhole drilling by micro-USM were con- ducted to understand the infl uence of static load and tool size on the material removal rate MRR!, tool wear ratio, and gap. The tool tungsten! was brought down to touch the workpiece silicon wafer! surface, and this position was set as the starting point for hole drilling. The machining was completed when the set total tool feed was reached. The machining conditions used in drilling by micro-USM are listed in Table 1. The tool was moved to the same reference point at the workpiece surface before and after machining. The tool was fed down to touch the surface at the static load of 10 to 20 mg (9.831025mN to 1.9631024mN) without vibrations. The tool wear length was calculated by ob- serving the difference in the z-axis. The tool diameter was mea- sured using an optical microscope. The hole volume was calcu- lated by measuring the tool diameter and the thickness of the silicon wafer. The static load along the longitudinal axis was monitored and recorded by an electronic balance. Figures 3 and 4 show a microhole drilled by USM and the microtool after machining, respectively. The hole diameter is 66 mm. The total tool feed was 200mm. Tool wear length was 11.2 mm. Thus the gap which is half the difference between the tool diameter and the measured diameter of the drilled hole! is about 8 mm, which is almost double the largest grain diameter. The edge of the hole appears to have irregular cracks, which may be be- cause of the crystalline fracture of silicon. It may be reduced if smaller abrasive grains are used although additional experimental work is needed to ascertain it. The diameter of the tool remains constant throughout the length. However, the tool tip becomes round due to wear during machining. Figure 5 shows that the MRR increases with an increase of static load and tool diameter. The larger static load means larger pressure on the abrasive grains and eventually on the workpiece, resulting in an increase of MRR. The increase of the tool size Fig. 1Diagram of USM principle Fig. 2Structure of experimental equipment Fig. 3USM-generated microhole drilling diam 66mm Fig. 4Tool after hole drilling diam 50mm Table 1Machining conditions for micro hole drilling Amplitude of vibration5mm Workpiece materialSilicon Tool materialTungsten Tool size50, 100, 150mm Abrasive grain typePolycrystalline diamond powder Size of abrasive grains13mm Total tool feed515mm 728 Vol. 126, NOVEMBER 2004Transactions of the ASME enlarges the working area and more abrasive grains get involved in the material removal. The tool is also impacted by the abrasive grain, which causes the local deformation at the tool working area, followed by the crack generation and material removal from the tool, leading to the tool wear. When the static load increases, the force acting on the tool increases, resulting in the increase of the tool wear as shown in Figs. 6 and 7. The generated cracks at the sharp-edge portion of a tool lead to tool fracture at the edge por- tion more easily than the center part of the tool. The proportion of the wear on the edge part in a smaller size tool is higher than a large size tool. Therefore, the tool wear increases with the de- crease in tool size as shown in Figs. 6 and 7. It is observed that the gap decreases with the increase of static load Fig. 8!. The static load may also suppress the lateral vibration of the tool when the static load increases. The above-mentioned experimental results indicate the extent of tool wear during micro-USM. When a 3D microshape is ma- chined, a tool with microfeatures is needed to generate corre- sponding microfeatures. However, the preparation of a complex shaped microtool is a diffi cult task. Additionally, it is necessary to fabricate several complex shaped microtools to realize the accu- racy requirements of a design part because of the tool wear during machining 20,21#. The use of multiple tools leads to tool align- ment problems. Therefore, it is diffi cult and uneconomical to fab- ricate and use multiple microtools. To solve these problems, a simple shaped tool, such as a cylinder or square, can be used to move along a designed tool path to generate 3D microshapes. The Wire Electrical Discharge Grinding WEDG! method is used to obtain simple shaped tools with various cross sections 22#. A high tool wear occurring during machining prohibits the use of a single simple-shaped tool traveling along the CNC generated path to machine complex 3D microcavities. Therefore, an approach of integrating the Uniform Wear Method with CAD/CAM software, which compensates tool wear and generates desired tool paths, is used in the following attempt to generate 3D microcavities by USM. This approach has been successfully demonstrated for mi- cro EDM 23,24#. 3D Microcavities by USM To generate complex 3D cavities with desired accuracy, it is necessary to compensate the tool wear and generate the tool path accordingly. An introduction to the Uniform Wear Method and its integration with a commercial CAD/CAM software is presented below from 23,24#!. Uniform Wear Method.The static load used to remove ma- terial from workpiece also causes wear of the microtool, resulting in local deformation and cracks on the microtool surface. To gen- erate accurate 3D microshapes it is necessary to compensate the tool wear in micro-USM. The basic principle of Uniform Wear Method is that under certain conditions, the shape of the tool is regained due to the tool wear after machining one layer. Thus, it may become shorter in length but is able to regain the shape. To enhance this phenomenon, the tool path design must include fol- lowing rules. Layer-by-Layer Machining.The 3D microshapes are ma- chined layer-by-layer in the z-axis using simple-shaped tools. With a small tool feed to each layer, the shape of tool tip can easily be recovered after one layer machining without a deterio- rating effect on the machining surface. To-and-From Scanning.In one layer machining, the machined surface inclines from the start point of tool paths to the end point of tool paths because the tool length becomes shorter due to the wear in machining. Reversing the tool paths, i.e., to-and-from scanning, is helpful in reducing the inclination of generated sur- face. This can be verifi ed using the tool wear model. To further improve the machining accuracy, the cutting angle, i.e., the direc- tion of main paths is also changed. Tool Paths Overlapping.The edges and corners of a tool tip are worn more easily than the center part under the impact of ultrasonic vibration. The rounded edges and corners will be re- fl ected on the machined surface when the tool moves along the Fig. 5MRR versus average static load Fig. 6Tool wear length versus average static load Fig. 7Tool wear ratio versus average static load Fig. 8Gap versus average static load Journal of Manufacturing Science and EngineeringNOVEMBER 2004, Vol. 126 729 designed tool paths. Overlapping tool paths is to avoid the forma- tion of unmachined ridges at the surface due to the rounded edges and corners of the tool. Machining the Central Part and the Boundary of the Machined Surface Alternately.When the boundary is being machined, the edges of the tool become round due to wear. However, when the central part of the layer is machined, the static load at the center of the tool bottom is larger than the rounded edges, which are far from the machined surface. It causes the tool wear at the center of the tool more than at the edges of the tool. This help in recovering the original shape of the tool and a sharp corner can be obtained before entering the next boundary path. Based on the above analysis, it is expected that the tool shape can be maintained unchanged in micro USM when the tool paths are designed based on the Uniform Wear Method. Tool Wear Compensation.The tool shape similarly can be kept unchanged in micro-USM. The ultrasonic machining using a single tool can be considered as one similar to the milling process. However, it is necessary to compensate the tool wear length to machine 3D microshapes correctly with a simple-shaped tool. The compensation equation can be derived from the defi nition of tool relative volume wear ratio and the assumption that the tool feed of each layer h consists of two parts, the wear length htand the remaining length hw, which equals the average machined depth of the layer. The relationship shown in Fig. 9 can be written as h5hwS ySw St 11D(1) where the tool wear ratioycan be measured and calculated in slot machining as mentioned in the experimental section below. The cross-sectional area of the tool Stis measured after the tool is fabricated. The area of each layer Swcan be obtained by slicing the 3D microshape in layers along the z-axis. When the depth of a sliced layer hwis known, only by adjusting the depth of cut h needs to be adjusted to generate accurate 3D shapes. Integration of Uniform Wear Method With CADCAM Sys- tem.There are many CAD/CAM systems available to generate tool paths for many machining operations, such as turning, mill- ing, and wire EDM. However, these systems are not suitable to generate tool paths for micro-USM using simple-shaped tools be- cause the tool wear needs to be compensated to machine 3D mi- croshapes. Therefore, it is necessary to integrate the Uniform Wear Method with a commercial CAD/CAM software to generate tool paths. To utilize the tool path generation function of a CAD/ CAM system, the tool paths are generated using the volume mill- ing in the CAM module of this CAD/CAM software. The selec- tion of the tool path pattern is based on the Uniform Wear Method. To generate the machined surface smoothly, two sets of tool paths are generated with cutting angles 0 and 90 deg, respec- tively. A new set of tool paths is generated by selecting tool path data from these two sets of tool paths for each set of two layers and modifying the cutting depth of each layer based on the calcu- lation of the tool wear compensation equation 1!. The new set of tool paths needs to be translated into x, y, and z moving commands after interpolation calculation because of independent movement of these axes. Figure 10 shows the steps of the integration. Experimental Validation To obtain the gap and the tool wear ratio in 3D micro-ultrasonic machining, which are two key factors in the tool compensation, a basic experiment of slot machining has been conducted. The tool is fed into workpiece to the depth of 1mm, and then, it is con- trolled to move along the x-axis horizontally. After it travels 500 mm, it is fed down again at the same depth of previous layer. This process is repeated till the total tool feed 50mm! is reached. The adopted tool path is shown in Fig. 11. The slot depth was measured in the same way as the tool wear length. Several points were detected at the bottom of the slot and both sides of the slot top surface. The slot depth was estimated from the average value of difference in the z-axis. The length and width of the slot were measured using an optical microscope. The gap is half of the difference between the slot width and the diam- eter of the tool. The relative volume tool wear ratio is calculated using y5 Vt Vw (2) where Vtis the tool wear volume and Vwworkpiece removal volume. The machining conditions are given in Table 2. The re- sulting tool wear ratio and gap are also given in Table 2. The estimation of tool wear length and the depth of generated slot may not be completely accurate because of the diffi culties of complete removal of debris and abrasive grains. Fig. 9One layer machining Fig. 10Integration of uniform wear method with CADCAM Fig. 11Tool paths for slot machining 730 Vol. 126, NOVEMBER 2004Transactions of the ASME The application of the Uniform Wear Method and its integration with CAD/CAM in 3D micro-USM is verifi ed by machining dif- ferent complex 3D shapes. Figure 12 shows a tapered cavity 221.753221.75355mm3! with a 1/8 of sphere radius 50mm! in the center designed using a commercial CAD/CAM software. The tool paths as shown in Fig. 13 are generated based on the Uniform Wear Method. The part is sliced into layers with the thickness of 0.25mm. After regeneration of tool paths based on the Uniform Wear Method, the cutting depth of the initial layer is 0.823mm and the fi nal layer 0.684mm by calculation. The designed depth of the cavity in the part model is 55mm. The total tool feed after incorporating wear compensation is 212.932mm. Figure 14 shows the machined cavity. The cavity was machined with a fl at bottom surface and a 1/8 spherical ball at the center successfully. The dimensions of the generated cavity in x-y plane were measured using an optical microscope. The top size of the cavity is 231 3231mm2, which is 10mm more than design. This might be caused by tool vibration during machining. The depth of the cav- ity is 69mm, calculated by the difference of the total tool feed and the tool wear length 143.5mm!. The depth difference of 14mm between design and the measured depth was caused by the esti- mation error of wear ratio during the measurement of tool wear length in slot machining. The side walls with stairlike surfaces were caused by the layer-by-layer machining. A smoother surface can be obtained by reducing the cutting depth of each layer, thereby increasing the machining time. It took 10.2 h for generat- ing the microcavity shown in Fig. 14. It can be seen that the tool shape was kept unchanged after machining as shown in Fig. 15. Summary This paper presents the experimental results of microhole drill- ing by micro-USM and 3D microshape machining using micro- USM. Experiments of microhole drilling indicate the occurrence of extensive tool wear, which affects the accuracy of the machined part. Therefore, to generate 3D microcavities, a recently devel- oped Uniform Wear Method integrated with CAD/CAM software has been applied. The Uniform Wear Method compensates the tool wear and helps in regaining the tool shape during machining. The application of the proposed approach is illustrated by gener- ating a complex 3D microcavity. Extensive investigation into the effect of the cutting depth of each layer, amplitude of vibration, abrasive material, and grain size and frequency of vibration is needed to understand the micro-USM process performance and reduce machining time. A theoretical model based on the material removal mechanism based on the existing theory of macro-USM also needs to be developed. Acknowledgments Authors are thankful for the support from NSF CRCDEEC- 983028 and Nebraska Research Initiative Funds. Nomenclature h 5 tool feed for one layer machining ht5 tool wear length of one layer machining Fig. 12Designed parts Fig. 13Tool paths Fig. 14Machined cavity Fig. 15Tool after machining Table 2Machining conditions and results Vibration frequency39.5 kHz Vibration amplitude3mm Workpiece materialSilicon Tool materialTungsten Tool sizeAround 50mm Abrasive grain typePolycrystalline diamond powder Abrasive grain size0.51mm Tool wear ratio0.12mm Gap6mm Journal of Manufacturing Science and EngineeringNOVEMBER 2004, Vol. 126 731 hw5 machined depth of one layer St5 cross section area of the tool (x-y plane! Sw5 area of the machined layer (x-y plane! Vt5 t