Investigations on the formation of straightness deviation in MQL deep-hole drilling of thin-walled aluminium components

One of the most challenging machining operation, particularly with regard to the difficult chip evacuation, is the deep-hole drilling process. Due to the high length-to-diameter ratio of the drill holes and of the tools no complete dry machining is possible, thus minimum quantity lubrication is used to provide a lubrication film at the contact surface of the tribological partners and to ensure a reliable chip removal capability. This paper presents a fundamental comparison of the in-process heat input into the workpiece and the resulting straightness deviations for two different tool concepts-a single-lip drill and a twist drill. In particular, the deep-hole drilling of components with small wall thicknesses, down to a minimum value of s(w) = 1 mm, was analysed to gather information about the mechanisms leading to straightness deviations. Due to the conclusions, particularly using a single-lip drill, a novel experimental approach for the determination of the force distribution over the drill radius was developed. It enables the incremental force measurement along the cutting edge and has been used to generate input data for a finite element analysis of the tool deflection and the contact conditions between tool and workpiece. The results indicate that the single-lip drill is a competitive alternative to the twist drill, but the asymmetric cutting edge design and the required guide pad support limit its productivity, in particular with regard to the heat input and the straightness deviation in the manufacturing of drill holes in thin-walled workpieces.