###### Abstract  
Drilling is one of the most performed machining operations for riveeting and assembly operations in many industrial sectors. The accuracy of the drilled holes and their surface finish play a vital role in the longevity and performance of the machined components, which, in turn, increase productivity. Therefore, this study investigated the effect of the multi-spindle drilling process on dimensional hole tolerances, such as hole size, circularity, cylindricity, and perpendicularity. In addition, the surface defects formed in the holes were examined using scanning electron microscopy. Three aluminium alloys, AA2024, AA6061, and AA5083, which are commonly used in the aerospace, automotive, and marine sectors, were chosen as the study materials. The results showed that the holes drilled in AA2024 gave less circularity error, cylindricity error, and perpendicularity error. In the case of hole size, the holes drilled in AA6061 were less deviated from the nominal size following holes drilled in AA2024 and AA5083 alloys. Surface damage in the form of metal debris adhesion, smeared material, side flow, and feed marks was found on the inner hole surface. Holes drilled in AA5083 alloy had the worst surface finish and were the most oversized, which was associated with noticeable damage and deformations in their inner surface. The ANOVA results revealed that the spindle speed was more influential than feed and mainly affected the hole size and cylindricity errors. However, in the case of circularity error and perpendicularity error, drilling parameters were found to be insignificant. multi-spindle drilling; aluminium alloys; surface defects; hole size; circularity; perpendicularity; cylindricity +
Footnote †: journal: Applied Sciences  
13 April 2021 6  
also vital because aluminium has a modulus of elasticity that is one-third of other metals, especially compared to steels; therefore, more defects are expected in aluminium, making it challenging to hold dimensional tolerances [8]. This is either due to the soft nature of aluminium or, to a lesser extent, the presence of hard particles in some of its alloys, which sticks onto the tool and results in the formation of built-up edges and associated high surface roughness [1]. Furthermore, during dry drilling of aluminium alloys, there might be more friction and heat generation where the hot chips clog the flutes, thus making the hole boundaries vulnerable due to temperature increases. Consequently, alloys with a high coefficient of thermal expansion can experience greater thermal distortion, which could adversely impact hole quality [9]. Previously, some investigations were carried out to evaluate hole quality for various aluminium alloys, metals, and composite materials. For instance, [PERSON] et al. [7] recommended HSS-5%Co for minimum hole deviations during the one-shot drilling process of AA2024. The influence of cutting parameters on the hole size was found to be minor. However, the circularity error and radial deviation increased with the increase in the cutting parameters. In another study by [PERSON] et al. [10], it was concluded that the diametric error was affected more by the feed rate, while the cutting speed had a high impact on surface roughness. The experimental analyses during the drilling of AA6061 by [PERSON] et al. [11] revealed that spindle speed and feed rate play a vital role in affecting the thrust force, torque, and circularity error. The high thrust force, torque, and circularity error were due to the high drill diameter, spindle speed, and feed rate. Furthermore, [PERSON] et al. [12] concluded that the maximum increase of axial force and torque was due to increased feed rate. Therefore, a low feed rate was recommended due to stable and jerk-free cutting in one-shot drilling of AA6061. Besides this, the roughness was minimally influenced by both spindle speed and feed rate.