Hydraulic and fuel lines on Cessna Aircraft Co.'s Citation X business jet comprise about 700 assemblies of small diameter tubing. In the past, the lines were flared and fastened with fittings, but to reduce the weight of the plane, Cessna wanted to use titanium, which had to be assembled by welding. About 60% required a weld on both ends and the rest needed a weld on one end. Conventional gas tungsten arc welding (GTAW) can be used on larger diameter titanium lines but generally can't produce an acceptable weld quality on the smaller diameters, especially below 0.75 in. (19 mm). Changing to titanium lines presented a challenge of fastening small-diameter titanium lines while meeting extremely high levels of quality and reliability.
- Maintain the highest quality production standards
- Ensure repeatability of weld quality
- Achieve efficient production
- Install user-friendly welding equipment
- Minimize the need for operator training
Cessna engineers turned to orbital welding, a mechanized version of the GTAW process. "When we made the decision to go with orbital welding, our number one concern was to meet the high quality standards that we require on all of our planes," said Bill Starnes, production superintendent. Cessna selected a Model 9AF-750 orbital welding head and a Model 207 welding power supply from Arc Machines Inc. (Pacoima, Calif.).
The power supply controls weld parameters such as welding current, primary and background amps, travel speed, weld bead overlap, delay of rotation at the start of the weld and current downslope at the end of the weld. The enclosed weld heads used for small diameter tubing welds provide a chamber filled with inert gas that encloses the entire joint during the weld. A timed post-purge after the arc has been extinguished allows the weld tool to cool sufficiently to prevent oxidation before the weld head is opened to remove the welded tube.
In orbital GTAW, the tungsten tube or fitting is installed in a weld head, and an independent set of clamps on each side of the weld head holds tubes and fittings in line, butted together and ready to be welded. The tube remains in place while the weld head rotor revolves around the weld joint circumference to complete the weld. The arc welding current is regulated with a control system, automating the entire process and providing a more precise and reliable method than manual welding. Once a weld program has been established, an orbital welding system can repeatedly perform the same weld in exactly the same manner, eliminating the normal variability, inconsistencies and errors of manual welding. As a result, orbital welding doesn't require as much training on the part of the operator. Productivity of orbital welding systems also generally is far superior than manual welding, making it possible to produce high quality, consistent welds at a speed close to the maximum welding speed.
Arc Machines offered a training program that provided all Cessna welding operators with a clear understanding of what was needed to produce quality welds. The setups for orbital welding are straightforward, including setting the speed, rotation, dwell and penetration parameters, which are different for each size of tubing. The operator is also responsible for selecting the right fitting. Material preparation is the key to producing good welds. During the cutoff operation, contamination or burring is avoided to provide the clean and smooth surface required for high-quality welding. Both material preparation and welding are performed in a special controlled environment to avoid possible airborne contaminants.
For more information: Arc Machines Inc.; tel: 818-896-9556; fax: 818-890-3724; Internet: www.arcmachines.com.