Through Thick and Thin

Considerations for waterjet cutting of thick materials

By Steve Szczesniak

April 11, 2011

Combating stream lag and taper in waterjet cutting.

Waterjet machining is perhaps one of the fastest-growing cutting technologies in manufacturing today. It’s grown by leaps and bounds in recent years, and manufacturers and fabricators are discovering just how versatile and efficient it can be—especially when it comes to cutting very thick materials.

Lasers typically can cut materials only up to 1 ½ in. thick, while waterjets can handle up to 14-in.-thick materials.

In addition, the variety of materials waterjet can process—metals, glass, wood, plastic, and ceramic—is unsurpassed.

Waterjet also eliminates many of the problems that come from the heat generated by other cutting processes. Some materials, for example, produce hazardous byproducts when heated, making waterjet a safe option for rubber and plastic applications.

Distortion is another heat-related issue that manufacturers can avoid with waterjets. A waterjet creates a smooth cutting edge, free of jagged edges and burrs, which eliminates time and money spent on subsequent finishing efforts.

In addition to producing a high-quality surface and providing flexibility in terms of material type and thickness, waterjets make cutting reflective and textured materials an easy task.

The possibilities of waterjet technology make it an exciting prospect for shops interested in serving a range of industries and applications — or those that may potentially diversify down the road. For waterjet operations to be as productive as possible, however, it’s important to consider several factors.

Combating Stream Lag and Taper

Intelligent Taper Control (ITC) allows the C-axis to rotate a given amount from the database and faces the lag in the direction of the cut. This compensates for the lag amount to make the part straight without reducing cutting speed.

Stream lag and taper are two of the most common issues in waterjet operations, particularly when cutting thick materials.

Stream lag, which occurs when the power of the jet decreases from the point it enters the material to the point it exits, can occur when the cutting speed is too high, and this will negatively affect the quality of the cut.

A cutting speed that is too slow on the other hand, can cause taper—meaning that the jet exits at a different angle than it enters, or causes a wider cut at the bottom, diminishing cut accuracy.

Stream lag and taper make finding the ideal speed and pressure for certain material types and thicknesses crucial. Large cutting heads and high pressures can yield a straighter, more accurate cut through thick materials.

Some waterjet machines also contain technologies that analyze cuts and provide taper control without the need to reduce speed. These technologies automatically tilt and rotate the jet to maintain maximum speed with accurate wall straightness.

Mitsubishi’s Intelligent Taper Control (ITC) system does just that. The technology uses a 1- to 2-degree mechanical tilt to compensate for the jet widening as it exits the focus tube, maintaining stream velocity and faster production of final parts.

Speed and Productivity

As waterjet technologies advance, so does the productivity of the process. As mentioned, high pressures can increase speed and cut quality. High-horsepower pumps that are available for heavy-duty waterjet applications can reduce the amount of abrasive needed for cutting.

Keep in mind that very high-pressure waterjet operations come with increased maintenance and wear on pump seals, orifices, valves, and plumbing. When operating at, or near, the maximum pressure of the waterjet machine, operators must be diligent about inspecting these components on a regular basis.

Optimizing the use of abrasives also can improve productivity. For cutting thick materials, a coarse abrasive, such as a 60-mesh garnet, can increase cutting speed. While even coarser abrasives (and those crushed from hard rock instead of alluvial abrasives) can cut even faster, they begin to compromise surface quality at a certain point.

Higher volumes of coarse abrasives also can wear components quickly. Nozzles, for example, erode more quickly at higher pressures. If the inside diameter of the nozzle become too large, the jet intensity decreases, causing problems with speed and cut quality.

Waterjets typically can handle up to 14-in.-thick materials.

The ability to stack materials is another productivity advantage unique to waterjet technology. Thick stacks, however, can result in tapering, so operators must identify the ideal speed and pressure to create an acceptable cut.

New software and control advances enhance the waterjet’s productivity on thick materials, helping to eliminate common problems, speed cutting, improve part quality, and reduce costs.

Cut Quality

While waterjet technology might be slower than other fabrication methods, it can deliver an accurate and quality cut.

Generally speaking, cut quality of thick material is more of a concern than speed. Typically, the slower the feed rate, the better the cut quality. Still, because the quality of waterjet-cut thick materials is high, most manufacturers find that they can maintain sufficient speed while still meeting their quality requirements.

Fine-grit abrasives can improve surface quality, but will greatly diminish the speed at which thick materials can be cut.

New Technologies, Greater Efficiency

As waterjet cutting becomes an increasingly popular fabrication process, advanced technologies are in even greater demand. New waterjet machines, accessories, and software are helping make waterjet technology a viable option for an even wider variety of jobs and applications.