During mechanical assembly, individual parts are joined by different types of fasteners which are commonly to be installed into tightly tolerated holes. Drilling of widely used modern materials like CFRP and titanium leads to challenges in terms of tool and process development. A significant challenge is one step drilling in assemblies made from mixed material stacks. It results in deviating hole diameters making the additional reaming operation essential.”But also drilling of thick single material stacks imposes difficulties in terms of hole tolerance, chip extraction, heat accumulation and lubrication issues, leading to the necessity of drilling in several steps to achieve the required hole quality and integrity.
During orbital drilling the drive spindle rotates eccentrically in addition to tool rotation and feed movement, leading to a circular path of the cutting tool. Orbital drilling can offer advantages compared with conventional drilling and reaming. The eccentricity can be set steplessly even during the drilling process which allows to drill different hole diameters with one tool, tapered holes, change diameter within the hole or to perform a finishing cut during the tool return stroke. Tool diameter deviations also can be compensated, thus tool diameter tolerances can be enlarged and regrinding of worn tools can be applied more often, saving tool costs. The interrupted cut facilitates an extraction of the small chips, enabling drilling of closed structures without subsequent cleaning or deburring as well as one-way-assembly applications in case of blind fastener use. Damaged surfaces due to snarl chips can be avoided. Furthermore, dry drilling of aluminum material can be applied due to the reduced introduction of process heat.
Although already applied in several Airbus sites, the orbital drilling process still faces different challenges depending of the relevant application. In general, the radial loads on the cutting tool and machine present challenges in terms of hole tolerance when drilling high thickness / diameter ratio stacks or hole diameters below approximately ¼”. Bending of the cutting tool and vibration issues can lead to high tolerance widths and poor surface quality.
Due to the changed cutting kinematics compared with conventional drilling and depending of the cut material, tool and cutting parameters, the orbital drilling process introduces reduced compressive residual stresses into the hole wall, leading to a decrease of joint fatigue life. Strategies to produce orbitally drilled holes having at least the same fatigue life level compared to conventional drilling while optimizing the drilling parameters to meet engineering as well as economical requirements are going to be presented.