Investigation of hole quality in rotary ultrasonic drilling of borosilicate glass using RSM

Abstract

The present article emphasizes on reducing the edge chipping and taper during rotary ultrasonic drilling of one of the most demanded ceramic glasses “BK-7.” Statistical tools of design of experiments and backing plate were adopted as two distinct approaches to curb the chipping damage. Central composite design has been conjugated with desirability function for framing the design matrix. This investigation also emphasizes to study the effect of process variables—spindle speed, ultrasonic power and feed rate—on the chipping width (CW) and taper (T). After developing the second-order regression models for the CW and T, analysis of variance was used to check the fitness of regression models and recognizing the significant model terms. Then impact of each process parameter was analyzed on responses of interest through 3-D surface plots. The feed rate came forth as the most dominating factor by having maximum influence over the qualitative aspects “CW” and “T” of the drilling process. Interactions of higher rpm and power with lower feed effectively reduced the CW and T. The backing material, employed during main experimentation, also proved its effectiveness to reduce CW when main experiments results were compared to the results of pilot experimentation, which was performed without backing plate. Scanning electron microscope (SEM) was used to analyze the different tool wear modes and microstructure of machined surfaces. Tool weight measurement revealed the dominance of bond fracture and grain fracture during the early stage of the drilling process. Apart from brittle fracture, SEM also affirmed the presence of plastically deformed regions over the machined surfaces. Little deviations between the predicted values and experimental values during the confirmatory tests validated the prediction accuracy of regression models at 95% confidence level.