Pulsed assembly lines are providing enormous potential to the aviation industry, especially in terms of reduced lead times, optimizing asset utilization, and increasing the ratio of value-adding processes, according to researchers from Brötje-Automation and Airbus. (Purchase their SAE technical paper, 2011-01-2771, here.)
There are special challenges for automated drilling and fastening processes because of the unique part positions upon each pulse and because of the need for balancing work on several serialized fastening machines. The key to overcoming those challenges is versatile NC part programming that eliminates the need for any additionally written NC programs.
For the Airbus A350 project, a harmonized concept for the assembly of typical fuselage panels has been developed and realized plants in Stade, Augsburg, and Nordenham, Germany. The main idea focuses on the integration of a pulsed assembly line that saves space and supports efficient and transparent assembly of the panels.
In a conventional approach, fuselage panel assembly lines are built up by several locally separated cells (Figure 1), each dedicated to a specific production step.
Some aircraft have panels of different length for different sections of the plane or for derivatives of the base model. Thus each cell must be designed for the largest panel, with an accompanying accommodation in terms of space in the plant. Additional space has to be reserved for the transport of panels between cells.
The largest panel defines the cycle time from one cell to the next. Work may be in progress in a large cell while it is complete in smaller ones. This idle time can be addressed in a number of ways (including duplication of the overloaded cell), but typically they take up more plant space.
The pulse motion line (PML) is based on the idea of arranging automatic and manual cells successively without space between them (Figure 2). The number of work areas for each specific task can be determined based on its expected workload (Figure 3). During production the fuselage panels are located one after another within the line, separated by a minimal safety gap. When the work orders of all areas are completed, a pulse is performed; the panels simultaneously are moved by 4.5 m into the next work area.
For the A350 panel structure, up to seven frames are operated in a given work area within every cycle. When the pulse is executed, the next seven frames are worked on within this work area, regardless of whether belonging to the same or to the following panel. Thus, manufacturing large panels does not delay the completion of subsequent shorter panels.
The size of work centers for a certain task can be defined independently of the lengths of the panels to be produced. Further minimization of the required space is enabled by the elimination of transport zones between the work centers. The modularity of the PML offers a high level of flexibility: If in the course of ramp-up a certain task is revealed as a permanent bottleneck, the change of a work area from one task to the other is possible without the claim for additional space.
And with its scalability, PML allows for precise tweaks or extensions needed for a rate increase.
The NC program
Automated tasks such as clip frame riveting present challenges with PML. With a conventional clip frame fastening machine, the part to be processed is fixed in the cell before every frame of the panel is fastened. A predefined NC part program exactly defines movements, parameters, and the sequence of how the positions are processed in order to complete the panel in one go. The NC programmer needs to know the position of the part within the cell when writing the part program weeks before first production. This program can be used for processing all upcoming fuselage panels of this type.
With PML, the panels are not to be processed completely in one go; only the frames that are within the work area are machined (Figure 4). The current configuration of the frames in the work area depends on the sequence of panels of different types and lengths and depends on the flexible production planning that can be changed until shortly before the start of the production. Thus the tasks to perform within the work area are unique and require a dedicated NC program that needs to be available upon each pulse.
The Airbus and Brötje researchers investigated several NC programming approaches. Under a conventional NC approach, a single NC program that fits the predicted situation in the work area for each pulse and each fastening machine would be required. Because of the large amount of different situations that are possible within the work area, it’s improbable that the program could be re-used. Plus, lead times of several weeks would be required for planning the sequence and gap of the panels within the production line.
Researchers also looked at reusable programs but found that it would not be possible to minimize the amount of tool changes across frames.
In the end, the researchers agreed on a solution called “Versatile NC PROGRAMS.” It incorporates the idea of writing an NC part program for the whole panel.
The NC panel program can be understood as an abstract, which means it will never be operated on the whole. Instead it is used as a catalog from which partial NC program fragments are extracted and configured to a new NC program that is dedicated to the special situation within the work area.
The concept of the automated configuration of NC program fragments promises not just the shortest throughput time and the most acceptable effort on NC programming, but it also is characterized by great flexibility and provides the best process reliability.
This article is based on SAE technical paper 2011-01-2771 by Christian Hein of Brötje-Automation GmbH and Henning Schneider of Airbus.