The effect of interlayer gap width on burr formation in drilling of aluminium-aluminium aerospace stacks
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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.
KeywordsStack drilling Interlayer gap Burr formation Burr abrasion Clamping force Stack interface
The design and assembly of aircraft components rely heavily on the use of fasteners . 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 . The drilling of such stacks, referred to as stack drilling, is therefore characterised by the provision of a borehole through a series of different layers, which can comprise of—but is not limited to—aluminium, titanium and carbon fibre reinforced plastic (CFRP). Recently, 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. To achieve this, the borehole interface between adjacent layers must be clear of imperfections that would require reworking. 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.
Various studies have focused on the reduction of interlayer burr formation through the optimisation of process parameters in drilling of aerospace stacks. Zhu et al.  investigated the impact of tool geometry on burr formation in dry drilling of aluminium-titanium stacks. They observed that double cone drills produce significantly smaller burrs when compared with drills with multipoint and step geometries. Melkote et al.  and Tian et al.  established relationships between feed rate and burr height and thickness, which is explained by the strong correlation between feed rate and thrust force . The interlayer gap width is believed to strongly influence interlayer burr formation and has thus been the subject of extensive research. Jie , using an analytical model and experiments, found that applying a pre-load clamping force in the proximity of the drilling site when drilling aluminium-titanium stacks reduces the interlayer gap width, thereby resulting in smaller interlayer burrs. This observation is in agreement with the work conducted by Li et al.  and Yin et al. , who focussed on aluminium-aluminium stacks. Gao et al.  however point out that increasing the pre-load clamping force beyond a certain threshold does not lead to a further reduction in the interlayer gap. Melkote et al. , who assessed the effect of various clamping methods on interlayer burr formation in drilling of aluminium-aluminium stacks, recorded an increase in interlayer burr height with an increase in the distance between drilling site and clamping location.
The review of the literature suggests that research on drilling of stacked materials with the view on interlayer burr formation exhibits two major shortcomings. Firstly, the actual width of the interlayer gap throughout the drilling cycle was not directly determined, thereby making it difficult to correlate a particular gap width to a certain burr height. Because of the setups used in previous research, which in many cases involved large plates as workpieces, the established relationships between gap width and burr size are specific to the particular setup rather than generic and universally applicable. Secondly, the stacks were drilled with the two layers in direct contact, and no publication was found where prior to drilling an interlayer medium was applied, such as sealants that are commonly used in wingbox assemblies to prevent fuel leakage. However, in order to achieve one-way assembly, the separation of the two layers between drilling the holes and inserting the fasteners needs to be avoided, which means that the sealant has to be applied at the stack interface prior to clamping and drilling. The presence of this sealant film most likely will affect the width of the interlayer gap, resulting, as a consequence, in the possibility that it influences burr formation.
To address these shortcomings, this paper establishes a relationship between interlayer gap width and burr height when drilling aluminium-aluminium stacks with the presence of sealant at the interface. To achieve this, initially, the correlation between the interlayer gap width and clamping load applied was determined and, subsequently, the effect of the interlayer gap on interlayer burr formation was investigated.
2 Experimental methodology
2.1 Experimental setup
Despite the fact that this setup is an artificial scenario, the fundamental information gathered with regards to the relation between interlayer gap width and burr size can be applied to the drilling of larger components, where the interlayer gap is subject to change during a drilling cycle, the more the further the hole is located away from the clamping point, as observed by Melkote et al. .
0; 0.1 mm 0.2 mm;
0.3 mm; 0.4 mm
Coolant flow rate
Pressure ring inner diameter
Pressure ring outer diameter
2.2 Interlayer gap measurements
In order to establish the relationship between clamping load and resultant interlayer gap width, a coordinate measuring machine (LK G90C) was used to determine the actual thickness of the stack, i.e. coupons plus sealant (PR1782-C12). After having clamped the stack in the vice with a layer of sealant in the interface, the distance between the stack’s top and bottom surface was determined. Clamping loads ranging from 100 to 3000 N were applied, and three different measurements were taken per clamping load, each time removing and reapplying the sealant layer. The thickness of the interlayer gap width was then determined as the difference between the measured total stack thickness (i.e. including sealant layer) and the combined thickness of the two aluminium coupons (i.e. without sealant).
In response to the above-mentioned findings, the drilling experiments were carried out using shims (i.e. thin metal strips serving as spacers) of thicknesses similar to the interlayer gap widths measured when applying clamping loads ranging from 100 N to 3000 N. These shims served three purposes. Firstly, they made sure that the interlayer gap width could be set to the pre-defined values (see Table 1). Secondly, they ensured that no sealant was squeezed out from the interface during the drilling cycle in response to the thrust force, which would have resulted in a change of the interlayer gap width. Thirdly, the presence of shims prevented changes of the interlayer gap width as a result of variations in the amount of sealant applied.
2.3 Drilling experiments
Prior to clamping, a sufficiently large quantity of sealant was applied to the bottom plate of the stack to cover 80–90% of the plate’s area, and two narrow metal shims were placed in between the two layers along two opposite edges of the stack. The stack was then placed inside the vice and the two screws connecting the vice’s rest and frame were tightened to provide the total clamping load. One hole was then drilled at the centre of each coupon. The procedure was repeated using shims of different thicknesses to represent different interlayer gap widths. Two coupons were drilled for each pre-set interlayer gap.
2.4 Burr height measurement
After separation, the plates were washed in dichloromethane for around 60 seconds, which was sufficient to dissolve the sealant without having to apply any force to the part, e.g. wiping off the sealant, which could have potentially damaged the burr. It was decided to measure the height of the exit interlayer burr, i.e. the burr on the bottom surface of the top layer, which is believed to be the main contributor to the overall interlayer burr formation in stack drilling . For each hole, two line scans using a contactless profilometer (Nanofocus μm scan) were carried out across the centre of the hole, at 0∘ and 90∘, thereby gathering data representing the borehole edges in four locations (0∘, 90∘, 180∘ and 270∘). The in-plane step size for these scans was 1 μm, and the resolution in the z-axis (i.e. height) was 0.1 μm.
3 Results and discussion
3.1 Interlayer burr formation process
3.2 Interlayer burr removal as a result of chip flow
As a result of the drilling tool geometry and the cutting conditions utilised, both of which were in-line with current industrial practice, it was noticed that the chips generated were relatively thin, which would have made it easier for the chips to enter the interlayer gap. The thin chips also resulted in extensive chip nesting, which would have constrained the chip flow along the drill flutes, thereby increasing the chance of chips becoming entrapped in the interlayer gap and consequently abrading the newly formed burr.
3.3 Impact of gap width on initial burr formation process in top layer
To further validate the assumption made in the previous section, a second set of experiments was conducted, in which the drilling cycle was stopped once the tool’s cutting edges got fully engaged with the second stack layer. The aim was to replicate the interval during which the exit interlayer burr is fully formed, i.e. drilling of top layer plus initial engagement with bottom layer, whilst preventing its potential removal when drilling through the bottom layer. This set of experiments was carried out using a stack without pre-set interlayer gap (i.e. gap width 0 mm) and one with a 0.4 mm gap width. In addition to this, a further experiment was conducted using a single layer, to allow for the comparison between a constrained (i.e. stack interlayer) and unconstrained (i.e. single layer) situation.
In this paper, the influence of interlayer gap width on interlayer burr formation when drilling aluminium-aluminium stacks with sealant applied at the interface has been experimentally investigated. The way in which the research was conducted, which differed from previous work in terms of how the interlayer gap was set and the workpiece size, revealed a phenomenon not identified before, which strongly affects the borehole quality at the interlayer gap.
Pre-load clamping of stacks causes the sealant to be squeezed out from the stack interface. Increasing the clamping load results in an increase in the amount of sealant being squeezed out and, thus, in a reduction of the interlayer gap width. For the small-sized coupons used during this research a clamping force of 3,000 N was sufficient to reduce the interlayer gap width to zero, indicating that almost all sealant had been displaced.
In drilling, the size of the burr strongly depends on the space available for the burr to protrude into. Introducing a pre-set interlayer gap to a stack results in a substantial increase in interlayer burr height as opposed to drilling a stack with no interlayer gap. Similarly, drilling single layers results in larger exit burrs being formed when compared to the interlayer burrs formed in stacks drilled with a pre-set interlayer gap.
The presence of an interlayer gap can however be detrimental to burr formation, which is due to two phenomena: the sliding action of the upwards-travelling chips over the borehole edges, in combination with some of the chips entering the gap and being spun around by the rotating tool, thereby further eroding the borehole edge as well as the newly formed burr. This abrasion is considerable at the borehole edge of the upper layer (i.e. exit interlayer burr), but less pronounced at the edge of the bottom layer (i.e. entry interlayer burr). This is attributed to the fact that the former represents a greater obstacle to the chip flow and, hence, experiences a more intimate workpiece-chip contact.
The abrasive action of the chips intensifies with an increase in the interlayer gap width, as this provides more space available for the chips to enter the interface and, thus, erode the burr. Hence, larger interlayer gap widths can result in the complete removal of the interlayer burr and potentially lead to a substantial rounding of the borehole edge. Excessive rounding as a result of a continuous and intensive chip flow across a wide interlayer gap could jeopardise the quality of the workpiece, as it affects the geometry of the borehole.
This project was financially supported by Airbus UK (SP1702924), who also provided the workpiece material, sealant, coolant and cutting tools for the experimental work.
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