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6th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING 24-26 April 2008, Tallinn, Estonia EQUIPMENT FOR MAGNETO-ABRASIVE FINISHING OF ROLLER BEARING BALLS Deaconescu, T. & Deaconescu, A. Abstract. The processing of surfaces by Abrasive erosion can be achieved with the magneto-abrasive erosion is achieved by grains fixed in a solid body (procedures working media with both magnetic and known as grinding, honing, etc.), with abrasive properties, the binder of the semi-free grains held by a magnetic field grains forming the working medium being or a viscous liquid medium (known as the magnetic field. The paper presents a magneto-abrasive finishing, lapping, device for magneto-abrasive finishing of ultrasound dimensional machining, etc.), roller bearing balls adaptable on a or with free grains set into motion by a universal milling machine. small viscosity liquid or gas (abrasive jet Keywords: magneto-abrasive finishing, cutting). roller bearing balls. Magneto-abrasive finishing falls into the category of machining by abrasive erosion, 1. INTRODUCTION the principles of which are presented in the following. Abrasive erosion holds an important weight amongst surface machining 2. MAGNETO-ABRASIVE FINISHING processes, this being a technological method of dimensional machining by The first mention of a magneto-abrasive means of cutting – scratching – plastic machining procedure is connected to the deformation of the part surface carried out name of the Russian scientist Karol, who by the abrasive grains held in a solid, in 1938 proposed the use of an alternating liquid, gas or magnetic support. This magnetic field for finishing the interior procedure is based on the destruction of surfaces of pipes, by means of magneto- the integrity and subsequent removal of abrasive powders. Further major the tooling allowance at the surface of a contributions acknowledged in literature part, by means of the dynamic action of are inter alia those of M. Baron, E.G. the erosive agents, materialized by solid Konovalov, L.M. Kozuro in Russia, Al. particles (abrasive grains). The use of Makedonski in Bulgaria, and K. Kato, abrasive grains as cutting tools aims Nakagawa, Taakeo Shinmura, Koya mainly at rendering special quality Takazawa, Eiji Hatano in Japan [3]. surfaces of high precision in regard of Machining with magneto-abrasive grains is dimensions, form and position. Achieving based on the effects generated by the relative these objectives however requires small motion and the pressure generated between values of the tooling allowance, often even the part surfaces to be machined and the under one micron [1]. abrasive particles acting as tools and Considering these requirements, Japanese sustained in the machined area by means of researcher Norio Taniguchi has introduced a magnetic field. the concept of „nanotechnology”, placing Magneto-abrasive machining implies firstly abrasive erosion within this category of the contact between the part surface and an cutting procedures [2]. abrasive grain. Maintaining the contact with the machined surface, even for a mobile the edges of the grain are sufficiently sharp, grain and/or part requires the existence of a or a process of superficial deformation for sufficiently strong magnetic field on one less sharp grain edges. Due to the existence hand, and ferromagnetic properties of the of a relative motion, as well as due to the grain, on the other. friction forces, a deformation of the hairs The existence of relative motion between the composing the “magneto-abrasive brush” “hairs of the abrasive brush” and the part can be recorded (fig. 1) [4]: surface determines a micro-cutting process if Fig. 1. Deformation of the „ magneto-abrasive brush” In order to achieve micro-cutting or micro- The machining of parts by magneto-abrasive deformation the grains need to be erosion is based on its numerous advantages: sufficiently hard and must have cutting (a) the presence of the permanent “self- edges. Secondly, the grains need to be sharpening” of the “magneto-abrasive maintained in the machining area by the brush”, as the worn edges of the grains electromagnetic field. This last requirement are continuously oriented; is achieved when the structure of the grains (b) the absence of the “clogging” contains at least one ferromagnetic phenomenon characteristic for abrasive component. The simultaneous bodies (grinding stones, etc.); materialization of these requirements was (c) the hardness of the “magneto-abrasive possible either by using particles made of brush” can be easily adjusted between sufficiently hard ferromagnetic materials, or certain limits, depending on the nature by using complex grains consisting of an of the processed material; abrasive component and a ferromagnetic (d) due to the forces generated during one. The action mode of a grain can be machining, the remaining tensions are observed in figure 1 –b. low. This type of processing has also a few The main characteristics which define disadvantages, which however do not affect magneto-abrasive erosion are: the efficiency and applicability of the (a) the energy brought to the working area procedure: generates continuous, progressive and (a) a remnant magnetic field is generated cumulative elementary processes; following to machining; (b) the 3D form of the transfer object can (b) the cost of obtaining powders with be copied onto the machined object; simultaneously abrasive and magnetic (c) low mechanical strain on the characteristics; components of the machining (c) the procedure cannot be applied to too equipment. large parts, due to the complexity of the equipment required in such cases. Due to its performance related to surface 3. MAGNETO-ABRASIVE FINISHING quality, dimensional and geometrical EQUIPMENT FOR ROLLER precision, magneto-abrasive machining falls BEARING BALLS into the category of super-finishing procedures. The obtained performances are The presented equipment belongs to the close or even superior to those achieved by category of magneto-abrasive finishing lapping or vibratory smoothing. equipment for complex surfaces, that is for Further on a variant of magneto-abrasive spherical surfaces. The device allows finishing equipment for roller bearing balls finishing roller bearing balls of diameters will be presented, designed and between 8 and 18 mm (fig. 2). It is mounted manufactured at the Transylvania University on the table of the FUS 22 universal milling of Brasov, Romania. machine by means of six screws for T grooves. Fig. 2. Roller bearing balls The finishing procedure consists in rolling The milling machine also carried out the the balls by “magneto-abrasive brushes” positioning of the device in relation to the achieved by electromagnets and magneto- superior disk, by vertically moving the abrasive powders. machine table. The kinematics of the equipment consists in In order to remove the superior disk for a driving the two disks in opposed senses of new charge of balls, the magnetic yokes rotation, the balls to be finished being are rotated by means of a straight located between these. While the inferior cylindrical gear driven by an oscillating disk will be driven by an electric motor by pneumatic motor. The cinematic diagram means of a belt transmission, the superior of the equipment is presented in figure 3. disk will be driven by the main shaft of the milling machine. The components of the cinematic diagram are: 1 – Electric motor; 2 – Belt transmission; 3 – Oscillating pneumatic motor; 4 – Gear – pinions mechanism; 5 – Inferior disk; 6 – Main shaft of the milling machine; 7 – Superior disk; Fig. 3. Kinematic diagram of the finishing 8 – Electromagnet. equipment The magneto-abrasive particles in the working space become magnetic dipoles from the moment they are subjected to the action of an exterior magnetic field. Generally the direction of the vector of the magnetic moment of a dipole does not coincide with either the exterior induction vector or the geometrical axes of the particle. For this reason the magneto- abrasive particle will carry out three Fig. 4. Magneto-abrasive grain – magnetic motions in the working space between the dipole analogy two disks (figure 4): 1. Larmor rotation; effect of the centrifugal force generated 2. Motion along the force lines with from motion 2. reflection in the polar zones; The following figures present sections and 3. Drift transversal to the force-lines by views of the developed equipment: Fig. 5. Section through the roller bearing balls finishing equipment Fig. 6: View of the finishing equipment The magneto-abrasive finishing of roller diameter balls, where the initial deviation bearing balls by means of the device from sphericity of about 70 μm was presented above allowed also the reduced by machining to only 0.15...0.20 improvement of the geometrical form of μm. the machined parts. Figure 7 shows the results obtained by finishing 12 mm Fig. 7. Deviations from sphericity corrected by magneto-abrasive finishing The materials recommended as working media can be divided into several categories: (a) ferro-magnetic grains, obtained from materials with both abrasive and magnetic properties. The most frequently used materials of this type are ferro-boron, ferro-tungsten and hard cast iron. Fig. 8 Composite grain (b) composite grains, made from a matrix with ferro-magnetic properties and holding The graining of the magneto-abrasive a multitude of abrasive particles. material is one of the performance While the dimensions of the composite determining factors of the procedure, grains are of the order D = 100...200 μm, indicated by roughness Ra and productivity those of the abrasive grains are: d = 5...30 Pr. It was observed that for obtaining the μm (figure 8). The main materials included best surface roughness and the highest by the ferro-magnetic matrix of a productivity, when using 15%TiC+ 85%Fe composite grain are: Al O (10...20 %), 2 3 magneto-abrasive material, a graining of TiC (15 %), WC (20 %), Cr C (20...30 3 2 d=125/160 μm is recommended (figure 9): %), ZrC (10...20 %), diamond. [5]. Fig. 9. Pr = f(d) and Ra = f(d) The particle shape is determined by surface finish and profile requirements and the type of contaminant to be removed. The shape of an abrasive when new is not always the same as when in the operating mix. Whereas steel shot remains round throughout its life and iron grit remain angular, steel grits may loose angularity depending upon their initial hardness. The grains preferred for magneto-abrasive finishing should have initially positive rake angles. The attached figure shows chilled iron grits, their shape being adequate for Fig. 10. Chilled iron grits this type of machining. The performance of magneto-abrasive Future Trends of Ultraprecision machining is influenced by some Machining and Ultrafine Materials technological parameters, the most Processing. Annals of CIRP, vol. important ones being cutting speed, 32/2/1983. magnetic induction, dimension and shape 3. Slatineanu, L. Tehnologii neconven- of the machining gap, the characteristics of tionale in constructia de masini. the magneto-abrasive material, the degree Editura TEHNICA INFO Chisinau, of filling of the area, etc. Moldova, 2000. On the device presented above, the 4. Baron, I. Tehnologia abrazivnoi obra- thickness of the removed material layer botki v magnitnom pole. depending on the cutting speed was: Masinostroenie, Leningrad, 1979. • v =10 m/min, h=1.27 μm, 5. Ciobanu, M. Cercetari privind optimi- • v =15 m/min, h=2.68 μm, zarea parametrilor de lucru la • v =50 m/min, h=4.11 μm, finisarea magneto-abraziva cu ferofluide. Doctoral Thesis, Iasi, • v =150 m/min, h=4.48 μm. Romania, 1993. 4. CONCLUSION 6. DATA ABOUT AUTHORS The main characteristics of the magneto- abrasive finishing device described in the 1. Prof.dr.eng. Tudor Deaconescu paper are constructive simplicity and high Transylvania University of Brasov, performance. Its launching into series Romania manufacturing and addition to the B-dul Eroilor nr. 29 accessories kit of universal milling RO-500036 Brasov, Romania machines is bound to significantly enlarge [email protected] the range of machining procedures 2. Assoc.Prof.dr.eng. Andrea Deaconescu achievable on such machines. Transylvania University of Brasov, Romania 5. REFERENCES B-dul Eroilor nr. 29 RO-500036 Brasov, Romania 1. Komanduri, R. et al. Technological [email protected] advances in fine abrasive processes. Annals of CIRP, vol. 46, no. 2/1997. 2. Taniguchi, N. Current Status in and