Abstract
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 not-before 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.
Highlights
The design and assembly of aircraft components rely heavily on the use of fasteners [1]
A visual assessment of the scan profiles obtained from the contactless profilometer revealed that a burr of traditional shape, i.e. a ridge of material adjacent to the borehole wall and protruding out from the surrounding material, was only present in a very few of the samples, most commonly those which were drilled without pre-set interlayer gap
For the other experiments, the recorded maximum surface levels are quite constant and could well be a measure of the surface roughness of the shot-peened coupons as opposed to that of a burr formed on the borehole edge. 3.2 Interlayer burr removal as a result of chip flow The separation of the plates after drilling often revealed significant swarf ingress in the interlayer gap, as can be seen in Fig. 6, which shows the coupon’s bottom surface of the top layer around two boreholes drilled with an interlayer gap width of 0.3 mm
Summary
The design and assembly of aircraft components rely heavily on the use of fasteners [1]. To ensure that the boreholes for each fastener are aligned with the required accuracy, the components are assembled prior to drilling, with the regions where components overlap commonly being referred to as stacks [2]. The research on stack drilling has started to focus on the interface between layers following the desire of aircraft manufacturers to introduce one-way assembly [3, 4]. This is a combined manufacturing and assembly technique, whereby component layers are fastened together directly after drilling without any intermediate process that would require separating the layers, thereby considerably reducing assembly time. This implies that, for example, drilling burrs created between two adjacent layers—often referred to as interlayer burrs—must be kept below a certain limit
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