Aircraft manufacturers have always desired more flexible automated drilling solutions due to constantly changing requirements as technology advances, production rates fluctuate, and new products are introduced. This desire has become more evident in recent years as manufacturers embrace lean manufacturing techniques to streamline their processes. Traditional hole-drilling systems—i.e., drill templates and large machine tools—do not meet the desired flexibility requirements of this new lean manufacturing environment, and they are expensive.
Commonly, unique jigs and templates need to be designed and fabricated. They can be expensive to design, build, store, and maintain if used on a very limited basis. In addition, they are inflexible and do not allow engineering changes or manufacturing process changes. There are also ergonomic issues associated with continually lifting heavy templates and the associated power feed drill equipment.
Another option is large, multiple-axis machine tools for drilling. While these conventional machines provide increased quality, accuracy, and improved ergonomics, there remain a few drawbacks. This type of equipment carries high investment costs and requires long lead times. Generally, conventional equipment only has the capacity to drill one hole at a time and is inflexible to production rate changes. It uses large amounts of floor space and does not allow concurrent work while in operation, which makes it difficult for manufacturing to reconfigure build processes.
Advances in machine automation and increased component accuracy have led to additional drilling options. One new method is to use smaller part-applied tools that reference to local and accurate part features. Part-applied drilling systems that use flexible rail technology have recently begun to be implemented in production.
The initial plan for drilling Boeing 787 test fuselage sections was to use Flex Track machines, which are portable, four-axis, numerically controlled (NC) drilling machines developed by Boeing and subsequently licensed to West Coast Industries. However, there are areas of the fuselage that the Flex Track could not reach, so an alternative low-cost approach was pursued to complement the Flex Track machines.
The result is the Mini Flex Track, which is a simple two-axis NC positioning system that minimizes machine setup time, requires minimal operator training because of a simple user interface, is general-purpose enough to use in many applications, and meets strict accuracy requirements.
The X-axis of the Mini Flex Track is a flexible vacuum rail that attaches to the part and runs parallel to the fastener pattern. A positioning carriage rides along the X-axis and positions another perpendicular rail. The Y-axis rail has a quick-disconnect interface that accepts various end effectors. Several simple end effectors were built, allowing a single machine to accommodate many tasks simply by swapping out tools based on the drilling application.
The carriage consists of a baseplate on which are mounted two motor assemblies and other components. The X- and Y-axis motors are of similar construction and consist of servo motors connected through a planetary gearbox to a pinion gear. Fail-safe brakes engage the motor assemblies to stop machine motion in case of power failure. Positive air pressure is maintained inside the polyurethane housing to dissipate heat and to minimize the amount of dust and debris entering the unit. Thermistors monitor motor temperature.
The Mini Flex Track controller and servo drives are mounted onboard to increase system portability and to reduce the size of the power cable to the machine. The electronics consist of a multiple-axis controller for motion control, a microprocessor for managing the user interface and system I/O, and a pendant for operator interaction. The system has 2 MB of internal memory to store datasets and other process information.
The carriage has a mass of about 10 kg. Both the X- and Y-axis are capable of lifting approximately 20 kg per axis.
To start a process, the operator first loads the machine to the part. The operator has the option of loading each component individually or as coupled components. Precise application of the Mini Flex Track to the airplane part is not required; the machine only needs to be placed so all hole locations are within the machine travel envelope. The onboard controller will accommodate any misalignment between machine coordinates and airplane coordinates.
After all components are loaded, the user interface will prompt the operator to select a part program to run. The user will specify the part program either by using the pendant barcode scanner to scan a sticker applied to the part or by manually selecting from a list of available programs. Part programs can be stored locally on the controller or transferred to and from the machine by either ethernet or USB connection. Most applications to date have required only point-to-point positioning. This allows the part programs to be a simple comma-separated-variable file, only requiring a minimum of five columns consisting of position identifier, X and Y location, diameter, and the pattern or job number that each position is grouped into.
Eric Reid of The Boeing Co. wrote this article for Aerospace Engineering.