JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009 - 57

Waterjet system speed and angles need to be carefully programmed to maintain uniform edge quality. Trimming I-beam (stringer) composites requires special tooling such as catchers and cutting heads. Figure 8a shows a cross section of a stringer with the top flange trimmed at 90 degrees with an AWJ. The bottom flanges show the condition of the edge before trimming. Figure 8b shows a trimming overview example for the 787 aircraft stringers. The top and bottom flanges are shaped and trimmed with the ends trimmed to the centre rib or very close to it. Also, the bottom flange needs to be bevelled at about 45 degrees. Some access holes are also drilled in the rib. These stringers, especially the ones used in the wings, are relatively long, over 40 m, and thus they require special machinery to handle them during cutting. nozzle is used to cut near the bottom flange. The example of trimming and shape-cutting shown in Figure 8d is quite complex, due to the 3-D nature of the trimming in different planes. The design of a holding fixture in this example is of critical importance. While the majority of trimming applications addresses relatively thin sections (less than 20 mm), there are several thick-section (up to 76 mm) cutting applications near the wing-to-fuselage joining areas. Figure 9 shows example cuts in thick composite sections which were found to be faster than a mechanical tool by an order of magnitude using much less demanding fixtures and heavy equipment. The end trimming of composite stringers (I-beams) is another application that requires a careful 5axis manipulation strategy as not to cause the exit jet damage the opposite sides of the stringer. Figure 10 shows a trimmed section from Fig. 10: Stringer end cut-off. the end of a stringer. A rate of 25 mm/min can be used to cut through a 76-mm thick section producing an acceptable 10.16 micrometer surface finish. A most challenging cutting application is the cutting of composite honeycomb structures as shown in Figure 11. Cutting this class of materials introduces additional challenges, as the cutting path is not continuous. A cut at the bottom of a honeycomb structure appears as a series of punched holes. Cutting at a lead angle (a few degrees) has been found to be effective for minimizing this effect, as shown in Figure 12. a: Composite stringer b: Required trimming of stringer flanges and ends c: access hole drilling at the root d: AWJ trimming and cutting of of a wing-box stringer complex 3-D composite wing part Fig. 8: Composite trimming and shape-cutting examples. Fig. 11: Examples of honeycomb structures. Fig. 9: AWJ cuts in thick composite section. Fig. 12: Bottom surface of 25-mm-thick honeycomb cut with an AWJ with and without a lead angle (9°). Shape-cutting requires the piercing of a starting hole. Some systems use a mechanical drill to pre-drill the starting holes, but most commonly now, AWJ is used to drill these holes as part of the shape-cutting sequence. In this case, vacuum-assist nozzles are used to mitigate delamination risks. Figure 8c shows holes drilled at the root of a stringer rib. A special minimal-size AWJ Drilling (piercing) Delamination is the main mode of failure when drilling materials such as graphite epoxy with waterjets. This problem is resolved when vacuum assist and proper parameters are used. Thousands of holes have been drilled in carbon-fibre composites without any adverse effects. With proper process timing, holes No47 March-April 2009 / jec composites magazine 57
JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009
