- May 15, 2014
Once considered niche machines, abrasive waterjet cuts a wide swath in manufacturing today
Imagine a machine so versatile it cuts hardened tool steel on Monday, titanium bar stock on Thursday, and aluminum billet all weekend long. Abrasive waterjet does just that, slicing through many materials at speeds 10 times faster than wire EDM and 10 times more accurately than high definition plasma, and offering manufacturers an easy-to-use technology that competes with traditional machining and fabricating processes. Abrasive waterjet isn't just for metal, either—glass, ceramic, plastics and composites alike give way before waterjet's hypersonic stream of abrasive slurry. If your shop needs a machine that can cut most anything, keep reading.
A nice addition
Abrasive waterjet is an improbable process. It works by injecting bits of rock—typically garnet—smaller than grains of table salt into a high velocity stream of water. The abrasive is accelerated downstream at speeds only Superman could rival, striking the workpiece at 3000 km/hr and stripping away microscopic pieces of whatever material lies in its path. The water is pressurized to 60,000 psi or more and channeled into a stream the diameter of a piano wire. Aside from carrying the abrasive through the workpiece, it serves to cool the workpiece while simultaneously removing debris from the cut. The result is fast and accurate cutting, with none of the heat-affected zone or warpage common in competing thermal processes.
Abrasive waterjet has been used in shipbuilding for decades, but has enjoyed substantial growth over the past few years in various fabrication industries. Yet the technology isn't necessarily a replacement to conventional metal cutting technologies such as CNC stamping machines or wire EDM. Rather, it complements these processes. Consider a shop with 15 wire machines—some of their work might fall within the +/- 0.1 mm tolerance band typical of abrasive waterjet. In this case, why not use waterjet to cut those parts three to five times faster than EDM?
Even in the case of part tolerances or surface finishes more demanding than that possible with abrasive waterjet, many shops save tremendous amounts of time and money by cutting parts to near net-shape using it, then finish machining with mainstream processes. Ironic as it might sound, "hogging" with what is essentially a stream of dirty water is quite effective. One generic example: depending on operating parameters, 50 mm thick tool steel can be cut at 50 mm/min with edge quality comparable to that produced by high defintion plasma.
Not impressed? Consider that the limitations of abrasive waterjet are primarily that of Z axis height and horsepower. Plodding through 250 mm thick aluminum in the morning can be done on the same machine that zips through 1 mm sheet stock in the afternoon. It's just a matter of adjusting abrasive flow, nozzle pressure, and feedrate. That's the versatility of the technology.
It's the water
Despite its apparent simplicity, there are a number of things to learn before achieving mastery. Water quality, for example, can make or break performance. Scott Wirtanen, regional sales manager of Minnesota-based abrasive waterjet manufacturer Jet Edge Inc. says most customers use plain old tap water, but it's always a good idea to test periodically. "Very hard water and high amounts of dissolved solids can negatively impact pumps and cutting heads. This situation is easily corrected, however, through the use of a water softener or reverse osmosis system."
The pump is the heart of any abrasive waterjet system. As a rule, builders offer two choices: intensifier pumps use hydraulics to oscillate a set of center-mounted plungers, forcing water at high pressure through check valves at either end of a cylinder. And direct drive pumps operate like an electrical version of the internal combustion engine in your pickup truck, utilizing a crankshaft and series of pistons to generate pressurized water. There are pros and cons to each.
"There's less maintenance with an intensifier pump—not only are there fewer moving parts, but you're stroking less often," says Wirtanen, "around 60 times per minute compared to 1,000-1,800 strokes per minute with a direct-drive pump. Yet electricity costs are slightly less with direct-drive, and they're less expensive up front." Despite the higher price tag, however, Wirtanen claims intensifier pumps provide lower operating costs long-term, and offer pressures both higher and more consistent than direct drive systems. "In the long run, intensifiers are far less expensive."
Some say modern direct drive pumps are more efficient than traditional hydraulic intensifier pumps and can provide efficiencies between 85 to 90 per cent. In recent years, improvements in seal designs and materials means fabricators using abrasive waterjets with these pumps can operate in the 60,000 psi range for longer periods of time with less maintenance intervals.
Workholding is another consideration. Despite the awesome water pressures involved, abrasive waterjet is fairly sedate in terms of cutting forces—after all, the metal removal mechanism here is one of tiny rocks peeling away even tinier pieces of material. With heavy workpieces, you can often just throw a big chunk of plate steel on the table, pick up the edge and start cutting. At worse, a few toe clamps are sufficient to restrain even the wiliest work. The dynamics change, though, when cutting small parts—these can fall through the support rails, or be deflected into the jet.
Sales manager Jeff Day of Ohio-based WARDJet Inc. says shops avoid this problem via programming. "With small parts, you can leave small tabs behind as supports, similar to wire EDM machining, and break the parts out of the skeleton after the cycle ends." As Day explains, shops can use most any 2D CAM milling program with waterjet—it's just a matter of setting the cutter diameter equal to the size of the orifice. For those shops that do nesting—multiple parts cut from the same sheet in a single operation—it's important to work from the inside out, such that the outer edges of the sheet remain unbroken and holds everything together while machining. In some cases, multiple sheets can be stacked together, increasing output tremendously. Even "common line" cutting is possible, where the straight-line edges of adjacent parts are cut simultaneously. Both techniques save huge amounts of processing time.
Last but not least are the tiny workhorses that make abrasive waterjet possible: garnet, a hard silicate mineral. For the most part, two types of garnet are available, alluvial and mined. The former is extracted from riverbeds—it costs roughly half the price of mined garnet and makes up the majority of abrasive sales. The downside is that alluvial garnet is "rounder" than it's mined counterpart, and is somewhat less effective at cutting. For this reason, mined garnet, with its sharper edges, is best for very thick or tough materials such as Inconel or hardened steel. Whichever way you go, says Day, garnet is the most expensive component of waterjet operations, and shops can expect to spend $10-$15 per hour on abrasive alone.
This cost can be reduced somewhat through recycling. "Recycling allows shops to offset the cost of new abrasive," says Day. "With Australian garnet, for example, you can recover roughly 50 per cent of the spent abrasive by pumping it through a recycling unit, where it is filtered with a fine mesh screen, dried and sent back to the hopper." With each pass through the process, a successively smaller portion of the abrasive is recovered until eventually the garnet becomes too small for reuse and passes into garnet heaven.
Not everyone agrees with abrasive recycling. Opponents point to the high operating costs of recycling machines, and suggest that shops should focus their efforts on making parts. Also, it only makes sense that used abrasive is less effective than virgin material, although for softer workpiece materials, recycled abrasive could make perfect sense. As with any capital equipment purchase, would-be owners are advised to kick the tires, and figure out which processes and features are best for them.
Waterjet manufacturers continue to push the technology to its liquid limits—Day mentions machines that cut at extreme angles of 60° or more, and five axis heads capable of tracing complex geometries. For production environments, multi-jetted monsters cut multiple parts simultaneously, and X axis travels 10 meters in length are possible. Abrasive waterjet is an evolving technology, but its advantages are clear—high productivity, very respectable accuracy, reasonable operating costs and ease of use. Maybe it's time to get your feet wet. SMT