Waterjet In Action

The advantages that Airbus cites—faster cutting and avoiding damage to the workpiece—are among the main benefits aircraft manufacturers seek through machining composites with waterjet. Other benefits include:

- No dust. Fine dust from machining composites can infiltrate controls and other electronic equipment in the shop. It also makes the shop grimy and unappealing. With waterjet, this dust is contained and controlled. For the most part, the dust is carried away with the water, from which it can later be removed.

Waterjet In Action

- No heat-affected zone. Delamination and fiber pullout are not the only dangers of mechanical cutting. Another is heat, which might melt the matrix of CFRP. But waterjet is inherently a cool process. Heat generation is slight, and the water transports the heat away.

- No rigid clamping. An unsupported edge of a CFRP workpiece is prone to vibrate during milling. For this reason, machining of composite structures on routers or similar machines often involves elaborate tooling designed to carefully and rigidly clamp the work at every trimmed edge. Vacuum fixturing built to the precise contours of the part is common. But with abrasive waterjet, the force of cutting is slight. The force also pushes down against the support beneath the part. Therefore, while programmable workholding is sometimes used (see below), rigid custom tooling is not required.

Water cut in Action

Aircraft Equipment
That “programmable workholding” is one of various features Airbus specified to allow its waterjet machines to be adapted to the particular needs of complex aircraft parts. Work is supported atop a flexible header system consisting of an array of effectors that resembles a bed of nails. Each effector’s vertical position is set independently, so the overall array can follow the contours of the part. Changing from one part number’s positions to the programmed effector positions for a different part takes only about 2 minutes, Mr. Saberton says. This compares very well to the hours that might be required to move one custom fixture off the machine and replace it with another hard fixture. Mr. Morazo-Perez says Airbus initially questioned this approach to workholding—worrying in particular whether the flexible tooling would stand up to the water over time. The company became convinced after visiting shops using similar programmable workholding on waterjet machines in the U.S.

Other machine features particular to aircraft abrasive waterjet machining include:

- Side-fire nozzles. Aircraft skins can include small ribs and stringers that impede access. The machine’s ability to switch to a nozzle that redirects the jet to the side can be useful for cutting within these tight spaces.

- C-catcher. The contours of aircraft structures can double back upon themselves, meaning the jet of water exiting the cut might hit some other surface of the part. To prevent this, a C-shaped catcher (see photo) can intercept the exit side of the jet. A consumable inside of this device absorbs the energy of the jet so the water can be captured and reclaimed.

- Rotary spindle. As Airbus indicated, rotary-tool machining remains useful in many composites applications. In fact, certain features must be machined in this way. While waterjet can machine a hole, for example, it can’t machine a countersink. Therefore, some abrasive waterjet machines incorporate rotary spindles for such needs as this. The rotary spindle is also useful for marking tools.