Page 19 - MetalForming December 2009

Waterjet cutting suits nearly every material, including glass, ceramic, wood, stone, plastic and rubber. This versatil- ity can help niche suppliers branch out of small market seg- ments and appeal to a much broader customer base.
The Right Process for the Job
Laser- and waterjet-cutting machines offer a good life expectancy since they do not use dynamic machining forces and they do not make direct contact with the workpiece during cutting. So, when both technologies can do the job at hand, how does a fabricator determine which process to use? Understanding a few key factors can help simplify the decision.
• Speed and thickness—Lasers are well-suited to cutting metal-alloy parts as thick as 1⁄2 in., but they can cut steel plate to 11⁄2-in. thick. On the other hand, waterjets can cut plate to 6 in. Lasers often have a speed advantage over waterjets. For example, on 1-in.-thick carbon steel, a waterjet with a pump that develops 60,000 PSI and equipped with a 0.014-in. ori- fice and a 0.030-in. nozzle, and consuming abrasive at a rate of 1.2lb./min., cuts at 2.9 in./min. Comparatively, a 4-kW laser with a 2-mm coaxial nozzle, and using oxygen as the assist gas at 8.4 PSI, can cut more than eight times faster at 24.5 in./min. Waterjet, however, can cut through significantly thicker materials, and alternative materials, with ease. For example, cutting the sample parts shown in the photos, a waterjet machine cut the same part from 0.75-in. foam rub- ber in 45 sec., from 0.375-in. granite in 198 sec., and from 0.625-in. glass in 126 sec.
• Material—Lasers can process materials other than metal alloys, but this doesn’t mean they should. On plastics and rub- bers, for example, laser cutting releases unappealing byprod- ucts, while waterjet can easily cut these materials without releasing hazardous byproducts.
Highly reflective materials such as aluminum and copper also are particularly difficult for lasers to cut. For example, the cutting speed for laser and waterjet are nearly identical on 1⁄2- in. aluminum, but edge quality is significantly better with waterjet (see photos).
A laser performs best on smooth surfaces; textured surfaces can alter the flow of the assist gas and disrupt the beam’s focus. A waterjet, on the other hand, does not discriminate when it comes to reflectivity or texture.
• Finish—The heat from laser cutting produces a small heat-affected zone (HAZ) near the cut, which could require secondary finishing depending on finish requirements. Waterjet does not create an HAZ. In addition, waterjet cutting delivers a smooth edge finish without jagged edges, slag or burrs, eliminating the need for secondary finishing processes such as grinding. Also, because the process does not impart heat to the workpiece material, it will not distort the parts, which then can be easily stacked to increase pro- ductivity. MF
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