The geometry of micro milling tools currently in use have been adopted from macro tools, assuming ... load acting on the entire cutting tool was modeled as a.
Geometry Optimization of Micro Milling Tools J. Fleischer, M. Deuchert, C. Kühlewein, C. Ruhs Institute of Production Science (wbk), Universität Karlsruhe (TH), Kaiserstrasse 12, 76131 Karlsruhe, Germany
Abstract The geometry of micro milling tools currently in use have been adopted from macro tools, assuming that chip formation and process kinematics are analogical in both types of tools [1]. Experience has proved that micro tools respond to influences in a very different way than macro tools [2]. Oftentimes, structural details such as the rake angle and the twist angle impede further miniaturization and are impossible to achieve with conventional manufacturing techniques. Therefore it is necessary to get a comprehensive understanding of the entire process by taking a structure mechanical and cutting technological approach to micro milling tools in order to be able to optimize them. Another objective consists in the production of these miniaturized milling tools by means of force-free procedures such as laser ablation and electrical discharge machining. The present state of research already puts the deficits of the currently available tools on display. Insufficient manufacturing tolerances of ±10 µm, constitute a substantial change of cutting conditions for the commonly used lateral infeed or feed per tooth of a few micrometers. Sometimes, only one cutting edge is engaged, which results in increased wear and, therefore, reduced durability, increased cutting forces, minor surface quality and a higher probability of milling cutter breakage. For that reason, a single-edged geometry has been proposed. It guarantees clear adjustment of the process parameters feed per edge and lateral infeed. For that purpose, stability analyses of simple stylus geometries have been conducted by means of FEM simulations. The resulting tool with a diameter down to 30 µm was machined on the EDM-machine at the wbk (Sarix SX 100). First tests have been carried out that prove the ability of these tools to cut steel. Keywords: micro milling, FEM, EDM, milling tool 1. Motivation At present the geometries of micro milling cutters are created by scaling down macro tools. Due to the increasing miniaturization of components [3], it is becoming ever more complex to produce the required tools. Furthermore researches have shown that micro tools respond to influences in a very different way than macro tools. Another problem is that the accuracy during the manufacturing process is insufficient [4]. Therefore it is necessary to get a comprehensive understanding of the entire process by taking a structure mechanical and cutting technological approach to micro milling tools in order to be able to optimize them to realize smallest diameters down to 30 µm.
subdivided in the force acting on the major flank face, on the minor flank face and on the rake face • superposition of the forces acting on the major flank face, on the minor flank face and on the rake face to the cutting force • superposition of the centrifugal load and the cutting force
2. Simulation FEM simulations using the general-purpose code ABAQUS developed by ABAQUS, Inc. have been used to analyse the stability of simple stylus geometries. In order to verify the analysis, several output values were identified: besides the stress distributions and the maximum principle on the tool also the maximum deflection of the tool was investigated. For defining an optimized geometry shape, simulations were performed for three different geometries, all with a diameter of 300 µm. Geometry 1 and 2 are trapezium-shaped geometries whereas geometry 3 has a semi-circular shape (see Fig. 1). Several simulation runs were conducted for each geometry shape to reflect the reality: • effects of the centrifugal load due to the eccentric mass and the rotation speed of the cutting tool • effects of the cutting force due to the machining,
Fig. 1. Micro cutting tool geometries.
Multi-Material Micro Manufacture S. Dimov and W. Menz (Eds.) © 2008 Cardiff University, Cardiff, UK. Published by Whittles Publishing Ltd. All rights reserved.
A fine mesh has been adopted at the major and minor flank face as well as at the rake face to allow proper representation of the stress distribution in the cutting area due to the cutting forces. Fig. 2 shows the mesh configuration of the micro cutting tool.
As a result of the simulations conducted so far FEM is a useful simulation software for designing micro milling tools. If all necessary boundary conditions are taken into account in the simulation model, a statement about the optimized geometry due to the obtained stress distribution is feasible. Furthermore, the FE simulation enables the output of some variables that cannot be obtained in the experiments or only with very high efforts, for example the deflection of the micro milling cutter due to an eccentric chucking.
Fig. 2. Meshing of the micro cutting tool (geometry 3). The forces acting on the different faces were determined experimentally by using a force measurement platform and modeled as a specified distributed surface load (*DSLOAD), the centrifugal load acting on the entire cutting tool was modeled as a specified distributed load (*DLOAD) modifying the data lines to define the centrifugal loads. In order to define the variation of the load magnitude during simulation, the *AMPLITUDE option was used in ABAQUS to represent the run-up of the cutting tool from standstill up to a maximum rotation speed of 160.000 min-1. Hereby, the acceleration is subdivided into 17 steps increasing the rotation speed by 10.000 min-1 for each step. In comparison with the trapezium-shaped geometries (geometry 1 and 2), the results showed a 30% higher stability of the semi-circular geometry (geometry 3) and therefore a smaller deflection enabling a more accurate machining of the work piece. Fig. 3 illustrates the stress distribution of the three different geometries as well as the deflection. For improving the representation of the deflection the scale factor has been increased by several orders of magnitude. Based on the obtained simulation results the analyses of trapezium-shaped structures were stopped and further investigations were performed for the semi-circular geometry. The optimized geometry was scaled down from 300 µm to different sizes, namely 150 µm, 125 µm, 100 µm, 75 µm, 50 µm and 30 µm to re-investigate the influence of the cutting force and centrifugal load for the different diameters in the next simulation runs. The simulation results allowed a comparison of the behaviour of the different tool sizes and the relocation of the front end showed, that the smaller the tool is the less the influence of the centrifugal load gets. Additionally, in order to make a statement about the concentricity tolerance, the effects of an eccentric chucking of the micro cutting tool in the machine tool are currently addressed. Therefore, the axis of rotation of the cutting tools was translated by 3 µm in 1 µmsteps in different directions (along the positive and negative x-and y-axis and along the imaginary 45° and 135° axis). First results indicate that the influence of the eccentric chucking needs to be considered. A quantitative evaluation is currently performed.
Fig. 3. Structure mechanical FE simulation of different micro cutting tool geometries. 3. Manufacturing of these single-edged micro milling tools After the optimization by simulation via the FEMsoftware tool ABAQUS the geometry was manufactured to verify the results of the researches. Therefore the manufacturing method WEDG (wire electro-discharge grinding) was chosen [5, 6]. The main advantage of this method is that there are no forces between the work piece and the electrode during the discharge process. So it is adapted for producing very small and filigree structures [7]. The micro milling tools were manufactured by an EDM-machine (Sarix SX100), which was especially developed for high precise standards. In addition to the common equipment, the facility was upgraded by a wire
unit, which were needed for the manufacturing process mentioned above. A wire diameter of 100 µm was chosen to assure a stable erosion process. Furthermore the whole facility was situated in an airconditioned area to avoid any negative effects caused by the environment. For the production of such micro milling cutters a blank made of carbide has to be chucked in the spindle. It is very important to use a clamping system with a high accuracy to ensure a defined position of the blank in relation to the wire. First of all the helical geometry of the blade has to be machined by die sinking. This procedure includes a simultaneous rotational and translational movement. The form used for this production step was machined by micro milling. After that small parts are cut of the blank by WEDG, like it is illustrated in the Fig. 4. For each cut the work piece needs to be positioned over the wire and sunk to start the erosion process. The first spiral-shaped milling cutters, which were manufactured by this routine, had a diameter of 300 µm. The clearance angle on the front side was 10°. The flanks behind the secondary cutter were reduced by 5 μm and the remaining cutter circumference is reduced by 10 μm. A total of ten flanks are processed along the circumference. There was an angle of 36° between one flank and the next.
That is why the electric current of 0 A was chosen. A voltage of 80 V was adjusted. The erosion process became instable at a voltage of less than 80 V, so this was the smallest value possible. Fig. 5 shows a micro milling cutter with a diameter of 100 µm manufactured by WEDG.
Fig. 5. Cutter Ø 100 µm, helical shape (geometry 3).
4. Results In order to verify the functionality and performance of these cutters several milling experiments were conducted. The research showed that the WEDGmachined helical geometry (geometry 3) as simulation proved was the most stable option. Milling cutters with a diameter of 300 µm (feed per tooth 5 µm, infeed 10 µm, total depth 50 µm) and 45 µm (feed per tooth 1 µm, infeed 1 µm, total depth 10 µm) were used for manufacturing the grooves in brass displayed in Fig. 6.
Fig. 4. Milling cutter production procedure. To ensure a stable erosion process the right parameters are needed. The following parameters have been optimized and set during several preliminary series of tests: Initially a frequency of 190 kHz was used, because the higher the frequency and the smaller the working pulse is, the smaller are the resulting craters, which are typical for surfaces manufactured by EDM. The working gap had to be as small as possible, because a wide spark gap needs a higher voltage to overcome the dielectric in the working gap. So the gap was about 5 µm wide to achieve a high quality of the surface. If the working gap gets too small, it cannot be sufficiently cleared of the erosion products, which causes processdisturbing short circuits. It was very important to minimize the electric current, because this leads to better surfaces of the work piece. That is why the electric current was set to 0 A, so no internal capacities will be added to energy storage, because the discharges are controlled by capacity immanent in the system only. System capacity is less than 10 pF here.
Fig. 6. Groove, machined by a 45 µm milling tool in brass. What becomes apparent is increased burr formation. The potential influencing factors include high cutting edge rounding and notchiness resulting from crater formation in the manufacturing process and the fact that spiralization is yet to be optimized. Geometry adaptation has been established as one possible solution to allow for chip removal, the minimization of
burr formation and the reduction of cutting forces. Another possibility to improve the milling process is to perfect the EDM manufacturing process and to achieve better surfaces, less erosion craters and sharper cutting edges. The comparison of the force measurement of the one edged cutter and a commercially available two edged cutter shows clearly the advantage of the one edged cutting tool (Fig. 7). The force peaks of the feeding force of the one edged milling cutter are at a similar level. The failure in the concentricity of twoedged cutters causes different equivalent chipping thicknesses and on this account different chipping forces which can lead to tool breakage. The forces were detected by a force measurement platform beneath the work piece (which is made of brass MS58).
Fig. 7. Force measurement of a two-edged cutter (top) and a one-edged cutter (bottom) 5. Summary At present the tolerance of the radial misalignments of micro milling cutters are larger in size than the feed per tooth during the manufacturing process. This leads to higher wear at the cutting edge and a lower durability of the micro milling tool. To avoid this disadvantage a single-edged cutter was created and manufactured. The optimization process was supported by FEM-simulation to reach maximum stability. The research started with a simulation of a simple stylus geometry and ended with an optimized single-edged structure. To verify the simulation results some milling tests took place and showed the potential of well defined micro milling tools.
Acknowledgements The authors wish to thank the German Research Foundation (DFG) for their support. ”Structuring guidelines and machining procedures for micro milling tools”. References [1] E. Uhlmann, M. Füting, K. Schauer: Optimierung von Mikrofräswerkzeugen in der Werkzeugplanungsphase, wt Werkstattstechnik, Jahrgang 94 (2004), H. 11/12 [2] D. Oberschmidt: 16IN 0121 – MiCuTool Innovative Herstellungsverfahren für Mikrozerspanwerkzeuge, InnoNet-Kongress, 6. November 2006 [3] J. Hesselbach, A. Raatz, J. Wrege, H. Herrmann, H. Weule, C. Buchholz, H. Tritschler, M. Knoll, J. Elsner, F. Klocke, M. Weck, J. Bodenhausen, A. Klitzing: „mikroPRO - Untersuchung zum internationalen Stand der Mikroproduktionstechnik, wt Werkstattstechnik, Jahrgang 93 (2003), H. 3 [4] E. Uhlmann, S. Piltz, K. Schauer: Dynamische Werkzeuganalysen in der Mikrozerspanung wt Werkstattstechnik, Jahrgang 93 (2003), H. 3 [5] J. Fleischer, T. Masuzawa, J. Schmidt, M. Knoll: New Applications for Micro-EDM, Euspen 2004 [6] J. Schmidt, M. Simon, H. Tritschler, R. Ebner: µFräsen und µ-Erodieren für den Formenbau, wt Werkstattstechnik, Jahrgang 91 (2001), H. 12 [7] J. Schmidt, J. Fleischer, M. Knoll: Electrodes for Micro-EDM, EUSPEN International Topical Conference on Precision Engineering, Micro Technology, Measurement Techniques and Equipment (2003), Proceedings - Volume 1, pp: 177-179, ISBN: 3-926832-30-4
