Research On Vitrified Bond CBN Abrasive Tools For Ultra-precision Grinding Of Bearing

Vitrified bond CBN abrasive tools have been widely applied in high precisionand efficiency grinding field due to their advantages in comparision with otherabrasive tools. In this work, vitrified bond CBN abrasive tools used in ultra-precisiongrinding of bearing had been researched. Proceed from three elements of abrasivetools, the influence of the composition of vitrified bond, the introduction of poreforming agents, the change of abrasive formula and technics on the properties ofabrasive tools had been discussed, respectively. The refractoriness and fluidity ofvitrified bond were tested by the refractory cones and plane flowing methods. Thebending strength, Rockwell hardness and microstructures were measured by threepoint bending apparatus, Rockwell hardness instrument and scanning electronmicroscope. Archimedes principle was employed to measure the the true density ofglass powder and total porosity of abrasive tools.The results indicated that the refractoriness of basic vitrified bond increased withthe increasing of Al₂O₃ content, the fluidity took on a decreasing tendency. Therefractoriness and fluidity of basic vitrified bond were both decreased with theincreasing of B₂O₃ content. The introduction of Al₂O₃ and B₂O₃ improved the bendingstrength of the abrasive tools. The higher the content of SiO₂ introduced into the basicvitrified bond, the higher the refractoriness. When the content of SiO₂ reached 48wt.%,the lowest fluidity of basic vitrified bond and the highest bending strength of abrasivetools were obtained.After introducing with some additives, the following results were acquired. TheTiO₂ doping had little influence on the refractoriness of vitrified bond. When thecontent of TiO₂ reached 4wt.%, the fluidity of vitrified bond obtained the lowest value（159.13 %） and the bending strength of the abrasive tools was higher than othersamples （47.38 MPa）, the combination state in micro-scale was better. Therefractoriness of vitrified bond decreased gradually with the increasing of Na₃AlF₆content, and the fluidity took on the opposite tendency. When the content of Na₃AlF₆reached 5wt.%, the bending strength of abrasive tools got the highest value （46.15MPa） and the combination state in micro-scale was better. The lower the content ofalkali metals, the higher the refractoriness and the lower the fluidity of vitrified bond. Similarly, the bending strength of abrasive tools increased obviously with thedecreasing content of alkali metals.The walnut shell and activated carbon were chosen as the pore forming agents inthis work. Good pore forming effect was obtained with these two types of poreinducers. The different porosity could be got with the variation of pore inducercontent. The higher the content of pore inducer, the higher the porosity of abrasivetools. Simultaneously, the bending strength and rockwell hardness of abrasive toolsdecreased. Compared with walnut shell, activated carbon had less negative influenceon the bending strength and rockwell hardness of the abrasive tools.In this work, the molding density, content of vitrified bond, size of grains werealso investigated, the results were as follows. The porosity of the abrasive toolsdecreased with the increasing molding density, but the bending strength and rockwellhardness took on a increasing tendency. The increasing content of vitrified bondresulted in lower porosity and more excellent mechanical properties, but not the morethe better. The coarser the abrasive grains,the lower the total porosity and the higherbending strength of the abrasive tools.