Abstract
This article presents the results of experiments concerning a computational fluid dynamics (CFD)/numerical analysis of the flow of air in the grinding zone during the sharpening of the face surface of hob cutters while using the MQL method. The carrying out of a simulation allows one to determine the influence of various settings of the angle of the spray nozzle on the amount of air directly reaching the zone of contact of the grinding wheel with the workpiece, as well as the grinding wheel active surface (GWAS). In the numerical analysis, the ‘SST k-ω’ model available in the Ansys CFX program was used, and to which the Kato and Lander’s modification was applied. With the aim of verifying the results obtained from the basis of the numerical simulations, experimental testing was conducted. As a verification parameter, the percentage rate of grinding wheel clogging was used. The measurement of clogging was conducted by the optical method taking microscopic images of the grinding wheel active surface (GWAS) and then analysing it which the use of digital processing and image analysis. As a result of the numerical simulations, it was confirmed that the greatest effectiveness in delivering air to the contact zone of the grinding wheel with the workpiece being machined was achieved by setting the nozzle at the lowest of the angles tested (90°). At the same time, the greatest efficiency in delivering air to the grinding wheel active surface was achieved by setting the nozzle at the largest of the angles tested (90°). The experimental tests allowed one to state that the change in the inclination of the spray nozzle does not significantly influence the effectiveness of chip removal from the surface of the inter-granular spaces of the grinding wheel. By setting the nozzle at a 90° angle, wall shear stresses τw have a decisive influence on cleaning the GWAS, while at an angle of 30° the cleaning function is taken on by air being delivered directly into the contact zone of the grinding wheel with the face surface of the hob cutter being sharpened. A comparison of the percentage rates of grinding wheel clogging obtained from using the flood method (WET), as well as the MQL method, indicates the insufficient cleaning ability of the MQL method. A solution to this problem may be the application of additional cleaning nozzles employing streams of compressed air (CA) or cold compressed air (CCA).
Highlights
One of the basic functions of coolant in the grinding process is rinsing away chips in the work zone, along with cleaning the grinding wheel itself [1]
This property may be explained by analysing the results of numerical simulations and assuming that the impact of air flow delivered into the workpiece/grinding wheel contact zone on grinding wheel clogging is equivalent to the impact of wall shear stresses τw on the grinding wheel active surface (GWAS)
With the decreasing the angle of inclination of the nozzle, the impact of shear stresses τw decreases while the cleaning function is taken on by air delivered directly into the contact zone of the grinding wheel with the face surface of the hob cutter being sharpened
Summary
One of the basic functions of coolant in the grinding process is rinsing away chips in the work zone, along with cleaning the grinding wheel itself [1]. Through appropriately selected conditions for the use of water oil emulsions, such as the nominal flow rate and the angular setting of the nozzle in regard to the GWAS, the flood method facilitates the effective removal of impurities from the grinding zone, significantly limiting the occurrence and increase of clogging on the grinding wheel active surface [8,9,10]. Alongside the advantages of the MQL method related with its good lubricating properties, it possesses the disadvantage of a lack of sufficient cooling properties over a broad range of variables in the grinding process when compared with the traditional flood method [28, 29] The cause of this state of affairs is due to the small amount of coolant delivered into the grinding zone and, the low heat storage capacity of the oil and air [29, 30]. As the research studies in the field described in this article constitute something new and not dealt with until now, this work is important both for scientific and industrial reasons
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