Servo couplings pour accuracy into waterjet cutting

Feb. 1, 2009
Originally conceived in the 1950s as a way to slice trees into lumber, waterjet cutting has evolved into a versatile machine tool process, complementing

Originally conceived in the 1950s as a way to slice trees into lumber, waterjet cutting has evolved into a versatile machine tool process, complementing other methods including laser machining and plasma cutting. The beauty of waterjet cutting is in its simplicity, using water rather than noxious chemicals, and performing cuts with less noise and dust than other processes. Today's applications range from cutting soft materials such as food and foam stock to drilling holes into hard metals and materials like titanium and automotive glass.

Waterjet cutting can be classified into two basic methods - water by itself for softer materials and water plus abrasives for harder materials. In either case, the concept involves running ultra-high pressure water (30,000 psi or more) through a nozzle and using the sharp, penetrating stream to chisel away exposed material. Given the immense pressure and correspondingly high flow rates, supporting components — including the motion system — must be incredibly rugged and able to withstand large amounts of force.

Jet Edge Inc., St. Michael, Minn., a manufacturer of waterjet cutting systems, designs and builds some of the most versatile systems available. Since 1984, its waterjet cutters have been used in automotive, aerospace, and industrial manufacturing, as well as machine shops and job shops. The key to its products, including its new high-rail gantry waterjet cutting system, is unmatched performance achieved through advanced motion system design. A typical drive for a waterjet cutter consists of a servomotor, ballscrew, and gearbox. In the case of its latest high-rail system — which reaches cutting speeds of up to 1,000 in./min while maintaining an exacting level of precision — the drive components connect through ServoClass couplings from Zero-Max, Plymouth, Minn. Precision is essential in high-rail cutting systems, designed for virtually any material including metals, stone, composites, glass, as well as rubber and plastic. A high rail, overhead design allows full access to the work envelope and keeps critical components including the motion system safely out of the ultra-high pressure process area.

With a capacity to cut materials up to 20 in. thick, the system's cutting nozzle runs under PC-based motion control employing CAD part files to generate motion paths and profiles. The nozzle traces the optimum cutting path in the material, advancing with simultaneous x, y, and z-axis motion. Part features such as holes and beveled or contoured edges are generated with almost no hesitation at 0.05 to 500 in./min for both straight-line runs and contours. Over the entire work envelope, the system maintains bi-directional repeatability down to 0.001 in.

Achieving this level of repeatability is a design feat, pushing precision and consistency to the limit. Tool movement is a multiaxis affair, employing servomotor-driven ballscrews in the crossbeam, side beams, and z-axis assemblies. Each drive assembly operates alike, producing three planes of motion (x, y, and z axes) over which the high-rail system operates. In addition to the servomotors and ballscrew actuators, each axis is connected with a gearbox and servo coupling. The coupling is an important component because it precisely transmits motion without adding bearing loads to the actuators as the system cycles through a complex part program at top speed.

In designing the drive system for these high rail systems, Bert Olson, electrical engineer for Jet Edge, said his company decided the most effective connection between the servomotor, ballscrew, and high precision gearbox would be with Zero-Max's ServoClass coupling because it is designed to handle the specific sensitivities of servo systems. The couplings are designed to meet the precision positioning requirements and high reverse-load characteristics of servomotor applications, particularly machine tool applications such as waterjet cutting. They also offer high torsional stiffness due to a unique disc design, which provides the misalignment capacity — and coupling flexibility — to reduce reaction loads. The result is smoother system performance and longer operating life of the connected components. In the Jet Edge application, couplings operate at up to 1,000 rpm, continuously reversing direction as the system progresses through a part program, and axes travel up to 1,000 in./min with the three axes operating simultaneously. The only downtime is when a part run is complete and a new one is set up in the machine.

For more information, call (800) 533-1731 or visit

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