ap-4-12.dvi Acta Polytechnica Vol. 52 No. 4/2012 New Abrasive Materials and Their Influence on the Surface Quality of Bearing Steel After Grinding Ondrej Jusko1 1 CTU in Prague, Faculty of Mechanical Engineering, Technická 4, Prague, Czech Republic Correspondence to: ondrej.jusko@fs.cvut.cz Abstract This paper focuses on the influence of various types of abrasive grains on cutting properties during the grinding process for bearing steel. In this experiment, not only conventional super-hard abrasive materials but also a new type of abrasive material were employed in grinding wheels. The measurement results were compared, and an evaluation was made of the cutting properties of the new abrasive material. The options were then evaluated for their practical applicability. The measurement results indicated that a grinding wheel with Abral and SG grains is the most suitable for grinding hardened bearing steel in order to achieve the best roughness and geometrical accuracy. Keywords: grinding, bearing steel, abrasive material, roughness, geometric accuracy. 1 Introduction Research and development in the area of grinding wheels is a basic aspect of grinding technology. Three groups of abrasive materials are employed in grind- ing wheels. The first group consists of conventional grinding wheels made of Al2O3. Super-hard grind- ing wheel on the basis of CBN and diamond form the second group. It is less expensive to apply grind- ing wheels based on Al2O3, but there is a risk of thermal damage to the working surface. When we apply CBN and diamond-based grinding wheels, we can produce high quality surfaces, but this is an ex- pensive method. This situation provided the moti- vation to seek a new direction in the development of grinding wheels, and to investigate a type of grind- ing wheel with an innovative abrasive material. This third group of abrasive materials consists of grinding wheels with SG and Abral grains. 2 Experimental details The bearing steel was ground using the plunge-cut grinding method on a cylindrical grinding machine. We used rings of 14 109.6 bearing steel as samples. The surface roughness was measured using the Taly- surf 6 laboratory type contact profilomer, to measure the surface with the profile method with progres- sive transformation of the information on the pro- file throughout the mechanic shift of the point. The Talyrond 30 device, which uses the rotary table prin- ciple, was used for measuring the roundness. The parameters for the experimental measurements were: – speed of the wheel vc = 45 m/s – speed of the workpiece: vP = 30 m/min – grinding cycle: 1st phase: ap = 0.03 mm, vfr = 0.52 mm/min 2nd phase: ap1 = 0.02 mm, vfr1 = 0.52 mm/min, ap2 = 0.01 mm, vfr2 = 0.26 mm/min 3rd phase: ap1 = 0.02 mm, vfr1 = 0.52 mm/min, ap2 = 0.005 mm, vfr2 = 0.26 mm/min, ap3 = 0.005 mm, vfr3 = 0.11 mm/min – grinding tool: Al2O3 — white corundum (aluminium oxide), a con- ventional material. The results were used for com- parative purposes. A 99A 60J 9V grinding wheel was used. CBN — cubic boron nitride is another very hard ma- terial. Its regular grain and firm bond make it suit- able for grinding hard surfaces. A B-VII B64 K75 grinding wheel was used. SG (Seeded Gel) — is a ceramic aluminium oxide manufactured by a sintering process. Each abra- sive frit consists of sub-micron size particles which are separated from the grit under the grinding force. This keeps SG sharper than conventional abrasives, which can be dulled when flats are worn on the work- ing points. An AG 92/99 80 Hs(J) 9V grinding wheel was used. Abral — is a crystalline mixture of Al2O3 and AlON. Abral is produced by fusing aluminium oxide with AlON, followed by slow solidification. The material is created by natural crystallization and the monocrys- talline mixture is not disrupted by crushing. The crystalline mixture has a firm structure and a firm lattice. For this reason, its surface is more resilient 80 Acta Polytechnica Vol. 52 No. 4/2012 and harder than a regular corundum surface. An AG 92/99 80 Hs(J) 9V C45 grinding wheel was chosen for this experiment. 3 Results 3.1 Surface Roughness The results of the study show, that the most appro- priate material is the grinding wheel with SG and Abral grains. This conclusion is based on the rough- ness criteria that were achieved. In the grinding cycle with one or two phases, Abral seems to be a better grain option for the roughness criterion. In the three- phases grinding cycle, the Ra value results are almost identical for the abrasive materials. However, the SG material produced better profile results. The second grains roughness group consists of Al2O3 and CBN. The CBN wheel is made of sharper grains and the instrument produces better thermal conductivity, i.e. its surface is not so heavily ther- mally loaded. This results in a sharper surface than that of Al2O3 wheel, the grains of which have a bigger radius. This also affected the surface roughness mea- surement after the hardened bearing steel had been ground. An interesting finding was that when the cut was applied to this abrasive material, the best roughness results were obtained when the speed of the cross feed was highest and when the depth of the cut was greatest. Figure 1: Influence of the grinding cycle on the surface’s roughness Ra Figure 2: Influence of the grinding cycle on the surface’s roughness Rz Figure 3: Influence of the grinding cycle on the surface’s roughness Rt 81 Acta Polytechnica Vol. 52 No. 4/2012 Figure 4: Influence of the grinding cycle on the out-of-roundness 3.2 Geometric accuracy A comparison of the measurements shows, that the CBN wheel has quite a big tendency to deviate from roundness. This deviation may originate from the production of the grinding wheels, which have thin layer of the abrasive material with the binder on their metallic surface. The production process involves a certain axis deviation of the metallic body and devi- ation of the axis of the thin abrasive material layer. This eccentricity creates a change in the depth of the cut-off layer, i.e. in the frequency, due to the high speed of the wheel. Figure 4 shows, that when a conventional wheel and an innovative type of abrasive grinding wheel are used with decreasing cut depth, thus reducing the radial feed rate, the roundness deviation also de- creases. The roundness values that are achieved are comparable with the values for these abrasive mate- rials. The lowest roundness value is for the Abral grinding wheel in the 1st and 2nd grinding cycle and for the SG grinding wheel in the 3rd grinding cy- cle. 4 Conclusion Our experiment has shown that the least appropri- ate material for the grinding wheel s for the cut- ting 14 109.6 bearing steel is CBN with Al2O3 grains. Abral and SG grinding wheels are more suitable, due to the high durability of the CBN grain layer on the perimeter of the wheel, despite the high cost and the high demand on the accuracy of the ring when mounting the bearing. A comparison of the two inno- vative abrasive materials shows that the performance of Abral is slightly superior. There are several unanswered questions that re- quire further research and development on the influ- ence of the abrasive material in relation to the for- mation of the new surfaces in the cutting process. A possible application would be for the measuring the temperature in the contact spot between the abrasive tool and the workpiece, and consequently, its influ- ence in relation to the production and the distribu- tion of the residual tension in the superficial bearing steel after grinding. References [1] Gašparek, J.: Dokončovacie spôsoby obrábania. Bratislava : ALFA, 1979. ISBN 63-009-80. [2] Jusko, O.: Vplyv nových brúsiacich materiálov na obrábaćı proces. Dizertačná práca, Praha : České vysoké učeńı technické v Praze, Fakulta strojńı, 2011. [3] Mádl, J., Holešovský, F.: Integrita obrobených povrch̊u z hlediska funkčńıch vlastnost́ı. Úst́ı nad Labem : FKK Company, 2008. ISBN 978-80-7414-092-2. [4] Marinescu, I., Hitchiner, M., Rowe, B.: Handbook of Machining with Grinding Wheels. 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