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    Optimization of customized twist drill geometry for single-shot drilling of composite-metal stack
    (2020-09-01)
    Hassan, Muhammad Hafiz
    Drilling a composite-metal stack in a single shot is a challenging process, as the drill bit needs to pass through two different materials in a short time. Currently, twist drill bit is widely used in the field of drilling due to its economical state. However, the existing twist drill bits exhibits quick wear and tear upon drilling stacked up plates. This leads to frequent drill bit replacement and thus incurring additional costs to the company. This issue is further exacerbated by the lack of knowledge on suitable drilling speed and feed rate during the drilling of stacked up materials. This study focused specifically on the optimization of customized twist drill bit geometry for the production of good holes quality during a singleshot drilling process of composite-metal stack materials. First of all, an initial study was performed to identify which geometries of the drill bit and drilling parameters influence the hole quality the most. This step involved drilling a stacked-up plate at different feed rates of 0.05 mm/rev to 0.1 mm/rev and spindle speeds of 1500 rev/min to 2600 rev/min using customized twist drill bit of different geometries. The geometries of the drill bit were varied based on its helix angle, primary clearance angle, point angle, and chisel edge angle. The performance of the twist drill bit was assessed based on the drilling thrust force, hole surface roughness, burr formation and hole integrity (delamination, diameter error and hole circularity). The results were subsequently evaluated using the analysis of variance (ANOVA) to determine the influence of the drill geometry and drilling parameters on the maximum thrust force and the quality of the drilled hole. The results from initial study indicated that the any interaction of drill geometry parameters with the 2600 rev/min and 0.05 mm/rev produced a good drilled hole quality. For the drill geometry parameter, the interaction with drilling at the helix angle of 30° contributed to minimum drilling thrust force, hole surface roughness, burr formation and good hole integrity. For the point angle parameter, drilling with 130° was proven to reduce the burr formation by 89%. Other parameters like primary clearance angle and chisel edge angle were further analysed in the extended study since both parameters have interaction with the other drilling parameters. Later on, an extended investigation was carried out to study the effect of drill geometry on thrust force and hole quality comprising of chisel edge angle, primary clearance angle, and point angle at helix angle of 30°, 2600 rev/min, and 0.05 mm/rev. Twenty different drill designs with varied chisel edge angle, primary clearance angle, and point angle were fabricated and tested on the drilling of stacked up materials. The range of point angle parameters was extended from 130° to 140° in order to observe the impact of hole integrity. From the extended study, the most significant parameter affecting the drilling thrust force and hole surface roughness is the primary clearance angle followed by the chisel edge angle. For burr formation, the most significant parameter is the chisel edge angle, followed by the point angle. Through statistical analysis, the selected optimum drill geometry angles that scored the highest desirability based on the helix angle of 30° and drilling at 2600 rev/min and 0.05 mm/rev were chisel edge angle of 45°, primary clearance angle of 6°, and point angle of 130°. The new optimum drill design was fabricated and followed by a set of validating experimental works that produced errors in range of 6.64% to 9.23% for maximum thrust force, hole surface roughness, and burr formation. This confirmed its validity. For the hole integrity, the delamination factor for all trial was observed in range of 1.0038 and 1.0196 and the hole circularity achieved below 30 μm for all trials. The current study provides practical guidelines for machinists in selecting the right geometry angles to achieve suitable design for customised twist drills which are not readily available from standard drill catalogue in the market.