Shear Stress In Magnetorheological Finishing For Glasses

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Статья. Опубликована в журнале "LLE Review". – 2008. – Vol. 1117 – P. 42-50.Название на русском языке: Сдвиговые напряжения при магнитореологическом полировании стекол.Авторы: C. Miao, S. N. Shafrir, J. C. Lambropoulos, J. Mici, and S. D. Jacobs.Аннотация к статье на английском языке: This work reports for the first time on in-situ measurements of drag and normal forces in MRF. Three optical glasses ranging in hardness and chemical composition were tested. A spot-taking machine (STM) was used as a test bed for MRF spotting experiments. We examined how the measurable drag and normal forces, and the calculated shear stress as a function of material mechanical properties, contribute to material removal in MRF. A modified Preston’s equation, combining shear stress with material mechanical properties, is proposed, which suggests that material removal is dominated by the material mechanical properties. Our main observations are summarized as follows: Normal force was measured simultaneously with drag force in MRF for the first time. Normal force was within the range of 6 to 9 N, whereas drag force was within the range of 4 to 5 N. These results are in good agreement with the literature, where either one of these forces was measured individually, but not simultaneously. It was confirmed experimentally that the hydrostatic pressure [normal force divided by the projected spot area Fn As , first term of Eq. (1) does not predict the material removal rate in MRF. It was found for the first time that the measured normal force is dependent on material hardness. It was also demonstrated for the first time how the calculated shear stress (drag force divided by the projected spot area on the part) governs the volumetric removal rate, not drag force alone. This experimentally confirms Shorey’s4 predictions that material removal in MRF is dominated by shear. For the glasses tested under the STM geometry and conditions reported here, it was found that there is a threshold for shear stress below which a removal rate becomes negligible. In order to effectively remove material in MRF, shear stress should be kept above +0.08 MPa by adjusting the process parameters. Additional work is required to identify what process parameters affect shear stress, such as the size of the projected spot area on the part. Our results show that drag force is within a range of +4 to 5 N for a range of optical glass materials, ranging from relatively soft LHG8 to hard FS. Therefore, by keeping a spot area 50 mm2 while keeping drag force in the range reported above, one should expect efficient material removal in MRF for glasses. Preston’s coefficient was calculated for MRF in terms of the hydrostatic pressure, shear stress, and a combination of a material’s figures of merit and shear stress. These calculated coefficients indicate a narrow range for our modified Preston’s coefficient when both material figures of merit and shear stress are considered for a range of optical glasses, providing predictive capabilities for new glasses. Therefore, we conclude that material removal in MRF for optical glasses is governed

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