Development of electrochemical micro machining for air-lubricated hydrodynamic bearings J.-W. Park, E.-S. Lee, C.-H. Won, Y.-H. Moon Abstract A specially-built EMM (Electrochemical Micro Machining)/PECM (Pulse Electrochemical Machining) cell, a electrode tool filled with non-conducting material, a ele- ctrolyte flow control system and a small & stable gap control unit are developed to achieve accurate dimensions of spin- dle recesses. Two electrolytes, aqueous sodium nitrate and aqueous sodium chloride are investigated in this study. The former electrolyte with few pits on the surface of workpiece has better machine-ability than the latter one with many pits on the surface of workpiece. It is easier to control the machining depth precisely with pulse electrical current than direct electrical current. This paper also presents an iden- tification method for the machining depth by in-process analysis of applied electrical current and interelectrode gap size. The interelectrode gap characteristics, including pulse electrical current, effective volumetric electrochemical equivalent and electrolyte conductivity variations, are analyzed using the model and experimental results. 1 Introduction Electrochemical machining, one of the non-traditional machining techniques, can be used to achieve a desired workpiece surface by dissolving the metal workpiece with an electrochemical reaction. This machining method can be applied to the metal that is difficult to machining using other methods, for example, heat-treated steel. In elec- trochemical machining, the workpiece dissolves when it is positioned closely to the tool electrode in an electrolyte solution with an applied electrical current. Electrochemical machining has been used in industry for cutting, debur- ring, drilling and shaping the workpiece. Electrochemical micromachining (EMM) is necessary to improve the wide variety of technologies such as electro- communications, semiconductors, computer OA appli- ances and ultra precision machinery. Actually, it is unfor- tunate that theoretical developments of microshapes in a design have not been realized due to difficulties in their manufacture. For example, gas-lubricated spindle design based on its lubricating principles has a large potential to help the industries that require high speed rotation. Ex- amples include: spindle bearings for gyroscopes, hard disk drives, CDROMS, scanners, laser printers, high speed rotating machinery, dentist’s drills and rotary pumps [1, 2]. Although the theoretical dimensional analyses of gas- lubricated spindles have been developed since the early 1960’s, the machining of microshapes has been very difficult. For this reason they are not produced worldwide [3]. Hence, the aim of this work is to develop an electro- chemical micromachining (EMM) process and system for the manufacture of micro grooves by first establishing appropriate electrochemical parameters for the microma- chining [4, 5]. Application of electrical current in pulses to the cath- ode, rather than DC, offers significant improvements in dimensional accuracy as compared with conventional electrochemical machining. Examples include: diminish- ing the gap size to below 0.1mm, reducing the inaccuracy of the machined profile caused by internal disturbance, eliminating the macro defects on machined surface connected with the hydrodynamic flow disturbances. One primary issue in pulse electrochemical microma- chining(PECM) is using pulses of electrical current to identify and control machining depth as well as the gap size between the two electrodes. This paper presents an identification method for estimating the machined groove depth by using in-process measurement of the machining current and interelectrode gap size [6, 7, 8]. 2 Electrochemical material removal model The procedure for calculating removal rates for a pure element is as follows. If the atomic weight, A, valency of the dissolving ions, z and the applied electrical current, I, are known, and F is a Faraday’s constant, the Faraday’s law gives the rate of material removal, dm dt : dm dt ¼ AI zF ð1Þ Microsystem Technologies 9 (2002) 61–66 Ó Springer-Verlag 2002 DOI 10.1007/s00542-002-0184-8 Received: 5 July 2001/Accepted: 11 December 2001 J.-W. Park, Y.-H. Moon Mechanical Engineering Division, Pusan National University, Pusan, 609-735, Korea E.-S. Lee (&) Mechanical Engineering Division, Inha University, Incheon, 402-751, Korea E-mail: leees@inha.ac.kr C.-H. Won Yewon Tech Co., Ltd., Taejon, 306-020, Korea This work was supported by Korea Research Foundation Grant (KRF-2001-041-E00095) Paper presented at the 12th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 28–29 June, 2001. 61