Slicing steel with water and sand

April 1, 2004
Tight control helps waterjet cutters slice through just about anything.
The Shark line of waterjet cutters from Calypso Waterjet Systems come in multiple sizes from a 4 X 4-ft version to a 9 X 40-ft table. Most are available in either single or dual-head mode. Operators feed material from all sides, providing more area for cutting. Material is cut on the table surface so operators can easily view the process.
The water and sand mix exits the nozzle at 2,500 fps at a pressure of 60,000 psi with only about 10 lb of force imparted on the workpiece.

The 32-bit microprocessor-based DMC-2163 multiaxis Ethernet controller from Galil is half-the-price and half-the-size of box-level Ethernet controllers. It measures just 4.25 X 10.75 in. and is easily programmable.

Associate Editor

Water can do a lot of things. Pressurize it a bit and you can clean a building facade and run power tools. But really pressurize it and water can cut through a 6-in. chunk of steel.

That's what waterjet cutting systems from Calypso Waterjet Systems, Dallas, Tex. (, do. The machines provide smooth and precise cutting of a wide range of soft and hard, thin and thick, stacked and nonstacked materials. The process uses no chemicals, gases, or noxious liquids, resulting in an environmentally safe and clean operation. And because there is no heat or work-induced stress, almost any material can be cut from foam and plastic to ceramics, glass, and granite. The only exceptions are carbide because it's too hard, and tempered glass, which shatters because it's already stressed.

The cutters work by pressurizing water to 60,000 psi and forcing it through a small orifice in the cutting head. Inside a mixing tube, abrasive material such as garnet (high-grade sand) is mixed with high-pressure water creating an abrasive waterjet stream that exits the nozzle at about 2,500 fps, roughly twice the speed of sound. Through a combination of microerosion or grinding and a shear yielding of the material, a narrow section of material, called the kerf, is removed through the thickness. The kerf is typically between 0.030 and 0.040-in. wide.

Engineering challenges in waterjet cutting are primarily related to controlling the motion of the cutting head. According to Richard Carey Sr., an engineer at Calypso, the primary challenge with waterjet cutting systems is that they tend to be "flexible" tools, making accurate cuts in straight lines and less so when cutting circles and arcs. Consequently, the controller must compensate by adjusting and controlling acceleration and velocities, something that G-code controllers couldn't handle.

Calypso uses a motion controller from Galil, Rocklin, Calif. ( The company's six-axis DMC-2163 Ethernet motion controller features built-in circular interpolation and compensates for mechanical backlash. The controller lets the cutter traverse over 500 ipm with cut accuracies of ±0.003-in. tolerance or better.

Accelerations are based on the size and the type of material to be cut. Controls that direct the cutting must consider all the requirements for the area to be cut, such as corners, and the required cut quality. It is critical that both software and controls that output directions to the cutting head quickly compensate for these elements.

The software from Calypso ensures accuracy, eliminates programming, and features a feedrate calculator that determines the required compensation. Operators need only upload a drawing with cutting parameters and click a screen to start or stop the process. Users can also choose a material and nozzle size, then let the software calculate the compensation.

"The controller's auto-tuning capability provides tight servoloops and the responsiveness necessary to get the quality, crisp circle and arc cuts required," says Carey. "This lets us run gantries with higher accelerations and velocities."

Another problem is that the bottom of the stream typically lags behind the top. Control over acceleration and deceleration of the tool is an enormous challenge. For precise cuts, Calypso runs a large amount of code through the controller, most of which is devoted to a streaming interpreter that lets Calypso make use of Galil's powerful trip points. These are needed to perform the coordination of single-axis and vector motion which requires complex tool sequencing.

The controller is programmed by two-letter intuitive commands. The connector option on the controller further simplified the interfacing, providing another cost savings.

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