The interlayer gap and non-coaxiality in stack drilling

Interlayer gap formation during through-hole drilling in stacked structures is a common problem in large assembly operations. The resulting interfacial burrs and non-coaxial stacked holes deteriorate the machining quality and increase the overall assembly time and costs. This study presents both experimental work and theoretical analysis to understand the interlayer gap formations and non-coaxiality occurrences in the drilling of stacked structures of broad skins and narrow stringers, which imitate typical structures of large assemblies. First, some stack drilling experiments are specially designed and are performed to observe that the stacked holes are non-coaxial in a regular fashion by measuring the hole diameters of the upper layer, lower layer and interface. Then, a simplified mechanical model of the stacks is built in 2-D to study how the interlayer gaps and non-coaxiality are formed during stack drilling. Through quantitative analysis based on the simplified model, it is noted that the stack stiffness, drilling thrust force and pressing force have important impact on the interlayer gaps and non-coaxiality. Finally, finite element methods are adopted to present the deformations in 3-D. The calculation results agree with the theoretical explanations for non-coaxiality given by the analytical simplified model. In addition, the beneficial effects of the fasteners and ribs are discussed based on the calculation results, and thus, they could contribute to proposals for better designs for stack drilling.