High-Pressure Work: General Industry Coverage

Components used in the manufacture of combustion hardware for a Mitsubishi gas turbine begin life as sheet or plate stock. Because our components come from large sheets, we start every manufacturing process by cutting the desired shape or profile.

When we started planning our shop, we considered the materials to be cut and evaluated cutting methods. Cutting sheet and plate is usually performed via mechanical methods such as sawing or shearing, which are economical. But mechanical methods are often limited by the complexity of the shape being cut or the thickness of the material. Other options, such as oxy-fuel, plasma arc and most laser systems, cut via thermal means.

For us, mechanical methods are not viable options because we have to cut thicknesses from 0.06″ (1.524mm) up to 1.0″ (25.4mm), and many of the shapes are too complex to be cut with a saw or a shear. Thermal methods are effective but not desirable because the heat-affected zone can cause weld defects. Removing the heat-affected zone before welding adds cost to the process. To bypass these problems, we turned to waterjet machining.

Under Pressure

Waterjet machines work by pressurizing water to high levels—our machines operate at 60,000 psi (4,137 bar)—and passing the water through a small-diameter orifice. Restricting the high-pressure water accelerates it to velocities several times the speed of sound. Garnet, the same material used for sandpaper, is injected into the stream of water. The speed of the water combined with the garnet generates an abrasive cut similar to that of a grinding wheel or abrasive cutoff wheel.

A 60″×120″ piece of steel is cut on a waterjet machine at Savannah Machinery Works. All images courtesy C.Tate.

Waterjet cutting has performed well for us. Waterjets are not like traditional chip-making machines and, when we started using the process, it was new to everyone on my team. Although we had a basic understanding of its capabilities, we were not prepared for the transition to waterjet cutting. We learned some valuable lessons during that transition.

Care and Considerations

For example, waterjet cutting generates a large volume of waste. Over time, spent garnet and swarf will fill the tank. Waste removal requires draining the tank and hiring a contractor with a special truck to vacuum out the sludge, which is similar to grinding swarf.