More than 1200 large-diameter (up to ¾-in) holes must be drilled into titanium/carbon stacks for the side-of-body joint on a particular Boeing commercial aircraft fuselage. This is the joint between the upper shell (section 44) of the center fuselage and the lower corresponding section (section 45), which includes the wingbox and the landing gear well.
The upper shell material is mainly carbon-fiber-reinforced plastic (CFRP) reinforced with a thick titanium inner structure (Boeing prefers not to disclose any more specific aircraft design details). The upper shell is secured to section 45 on both sides by a long titanium side-of-body fitting.
One of the many challenges associated with side-of-body joints is keeping debris, or burrs, from entering between the layers while drilling holes. This is especially true with titanium/CFRP material stack-ups. The burrs can cause stress concentrations, reducing the life of the aircraft.
Because manual drilling was not yielding consistent hole quality, Boeing decided to automate the side-of-body drilling process, teaming with Electroimpact to develop a solution.
Boeing required that the automated drilling process eliminate burrs between the layers to allow for one-up-assembly. One-up-assembly eliminates the need to remove the upper shell for cleaning and deburring, avoiding time- and cost-intensive operations.
Implementing an automated solution into existing assembly lines was complicated by the location of the work area, which is more than 15 ft above the factory floor.
The focus was first on stabilizing the drilling process. A support structure was needed to provide the necessary travel for a small drilling machine and to provide access to the fuselage and automated guided vehicle (AGV) system in the existing assembly lines. The resulting support structure (a long beam elevated on two columns) needed to be optimized for both stiffness as well as natural frequency due to the “inverted pendulum” effect of the layout. Electroimpact engineers used extensive FEA to optimize the structure for both low deflection and high natural frequency. As it turned out, the stiffest beam was not the most stable solution in this case.
Another key was developing an automated drilling machine that was light but that could also support the heavy drilling loads created when drilling holes in titanium of up to ¾-in diameter. Engineers had to take into account how these drilling forces would affect overall system (i.e., machine and structure) stability, which determines hole quality.
The final machine package weighs less than 6500 lb and includes a mobile interface to give Boeing the flexibility to move the machine from one side of the aircraft to the other or to different assembly lines using the existing factory crane.
The drilling machine is an all-servo system capable of storing hundreds of drilling process parameters to provide custom drilling profiles for each hole type and material stack-up configuration. Hole quality and cycle times were optimized with the different profiles to control burrs at the material interfaces or transition points.
Parameters for up to five unique material layers could be referenced along with additional processes for breakthrough and countersink operations. Parameters include spindle speeds, feed rates, clamp force, peck times, and lubrication. To ensure that parameters were switched appropriately, drill depth was determined from either the tip or from the full diameter. This was extremely important when entering or exiting titanium, which would destroy cutters if CFRP parameters were used. Drill thrust and distance drilled is monitored to increase hole quantity and maximize drill life.
To achieve a drilling process for one-up-assembly, the machine is capable of installing a temporary doweling/clamping fastener. It is programmed to automatically select a fastener based on the programmed stack thickness and install the fastener in certain holes. These fasteners provide the clamping force required to eliminate burrs from entering between the layers. Shifting of the different layers is prevented by having a close fit between the fastener and hole diameters.
The drilling and fastening processes are qualified by Boeing to achieve the one-up-assembly for the side-of-body joint.
The stability of the system (both the support structure and machine), controllability of the drilling process, and mobility of the machines provide Boeing with a flexible and reliable drilling system.
This article is based on SAE International technical paper 2013-01-2296 by Michael Assadi, Christopher Martin, and Eliot Siegel of Electroimpact Inc.; and Dennis Mathis of Boeing.