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The research presented in this paper investigated the effects of spindle speed and feed rate on the resultant cutting forces (thrust force and torque) and temperatures while drilling SawBones biomechanical test materials and cadaveric cortical bone (bovine and porcine femur) specimens. It also investigated cortical bone anisotropy on the cutting forces, when drilling in axial and radial directions. The cutting forces are only affected by the feed rate, whereas the cutting temperature in contrast is affected by both spindle speed and feed rate. The temperature distribution indicates friction as the primary heat source, which is caused by the rubbing of the tool margins and the already cut chips over the borehole wall. Cutting forces were considerably higher when drilling animal cortical bone, in comparison to cortical test material. Drilling direction, and therewith anisotropy, appears to have a negligible effect on the cutting forces. The results suggest that this can be attributed to the osteons being cut at an angle rather than in purely axial or radial direction, as a result of a twist drill's point angle.

Interlayer burr formation during drilling of stacks is a widespread issue in the aerospace industry. The minimisation of the interlayer burr would contribute to significant time and cost savings, as it would allow for clamping, drilling and fastening to be carried out without any intermediate deburring that requires separating the layers. This paper reports about a notbefore observed phenomenon associated with the relationship between interlayer gap width and burr height when drilling aluminium-aluminium stacks with the presence of sealant at the interface. Initial experiments of the research were conducted to determine the interlayer gap widths in relation to a range of clamping forces, followed by drilling experiments to assess how the interlayer gap width affects the interlayer burr formation process and burr height. Although the presence of an interlayer gap results in larger burrs being formed, it allows upwards-travelling chips to enter the gap and erode away the newly formed burr. Larger interlayer gap widths were found to yield a more pronounced abrasion of the interlayer burrs, often leading to their complete removal; in some cases, this abrasion even resulted in a noticeable rounding of the borehole edges. This phenomenon was found to strongly affect the interlayer borehole quality and, thus, makes this research highly significant to the aerospace industry, where the quality of the borehole at the stack interface is of major interest.

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