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When a shop chooses an abrasive for a waterjet cutting system, it chooses its cutting edge. Water by itself, even at 60,000 psi, can't effectively penetrate most metals. Instead, the abrasive garnet in the waterjet stream serves as the machine tool's "cutting edge."
During the process, the water flows at the jewel, or orifice, of the cutting head in a venturi motion, accelerating before it rockets through the nozzle at about double the speed of sound. The garnet, fed into the stream from a separate tube, is sucked into this venturi, which then propels the garnet. The garnet, which never quite reaches the speed of the water, creates an erosive atmosphere, hits the metal at a very high speed and takes a tiny piece of metal with it as it penetrates through. With thousands of abrasive grit a second hitting the metal, a clean cut results.
Just how clean, however, depends in large part on the condition of that garnet.
GARNET BASICS
Garnet comes in two basic forms, crushed and alluvial, the latter being roughly similar to sand washed up on riversides. Tumbled for millennia by nature, it is taken from various deposits and collected in a single area, and as a result has relatively blunt edges that make for a dull cutting surface. This contrasts with sharp, angular crushed garnet. Alluvial garnet resembles a bead, crushed garnet a shard. Add the energy of water pushed with tens of thousands of pounds per square inch of pressure, crushed garnet acts more like a sharp cutting tool, while the alluvial performs an action analogous to a dull one. Both kinds will work, but the sharper edges tend to cut more effectively.
So why use alluvial? "It's inexpensive, and it can indeed cut most products," both soft and hard, says Alan Bennett, market manager for Barton Mines, Lake George, N.Y. "It's cheaper to get out of the ground, so hence it's cheaper to sell." If maximum efficiency and quality of cut aren't paramount, then alluvial does make business sense for some. He emphasizes that it depends upon the demands of the application and materials. "Alluvial may cut very thin aluminum just fine, but crushed garnet may cut with less of a burr," he says. "It's just a matter of what the application requires.
"There's no rule of thumb," he continues. "For some materials and applications you don't see a difference, so you use the most inexpensive abrasive you can."
Garnet is a generic term for several different minerals. The mineral most often used in waterjet is almandite, due to its properties and the fact that it happens to be one of the most plentiful garnet minerals in the world.
It also has two characteristics that make it ideal for a waterjet operation: hardness and friability; the more friable, the more easily the material breaks down under pressure. Other minerals have greater hardness than garnet and, hence, would theoretically cut through metal more efficiently, but they aren't as friable, meaning they do not break down very quickly under force—so they may cut through anything softer they pass through, including the hardware of a waterjet system. Take aluminum oxide. Being harder than garnet, it would cut through metal very efficiently, but it would literally tear up the components inside a waterjet system. Garnet, on the other hand, has sufficient hardness to cut through metal but enough friability so as to not destroy the hardware of the waterjet system itself.
JUDGING PERFORMANCE
Bennett boils down garnet performance to two factors. First involves the condition of the waterjet system. If the pumps haven't been serviced, the pressure won't be where it needs to be, so the garnet won't cut as effectively. "If the equipment isn't maintained properly, through the use of old focusing tubes or jewels, the outcomes are not going to be as good," Bennett says. "The object of the waterjet machine is to put the energy into the garnet—the system's 'cutting tool'—so you'll have effective erosion, or cutting, of the material." If the energy isn't sufficient or properly applied, the garnet can't do its job, no matter its quality.
The second factor—and perhaps the one that's most difficult to measure—involves garnet quality, something that remains somewhat of an inexact science. Most waterjet garnet grades on the market today are of high purity, being 85 to 90 percent "pure" garnet. But not all garnet is created equal. Garnet is a complex silicate, which contains one or more different variants that depend on, among other factors, where and when the raw material was formed by nature. Different mining sites offer different qualities of garnet.
CHOOSING THE RIGHT GRADE
Choosing the proper mesh, or grade, for the operation remains imperative. Each supplier has its own recommendations, but in general coarser grades cut thick materials in situations where edge quality isn't a primary concern, while finer grades cut thinner materials with better edge-finish quality, often to near-net shape (see sidebar).
Different mesh sizes of garnet are designed to pass through different-sized nozzles, and choosing the wrong grade could stop a waterjet operation completely. If the garnet grade is too large or coarse, the granules can jam inside a tube and cause a blockage. Too fine an abrasive has a tendency to "clump" together inside the cutting head and, again, have the potential to clog. Or it may inhibit the flow of garnet in the feed tube and not consistently enter the venturi of water flow between the jewel and nozzle. These effects can occur if a shop chooses the wrong mesh for the operation—or if the shop has inconsistent garnet, where the garnet size varies widely within a particular grade.
The grade "number" equates to the number of openings in one square inch of screen that's used when sizing the media. However, this only serves as a rough point of reference. For instance, consider an "80" mesh, a popular grade for waterjet. The garnet for that grade passes through multiple screens during processing with the goal of maintaining a specified range of size that follow a standard distribution, or bell, curve. An 80 mesh could have its peak at 1/80th of an inch square, then descend in both directions from that precipice, with grain sizes between, say, 48 and 100 mesh at either end.
In garnet processing, what is paramount is process repeatability, or how well the final garnet grade stays within the range of grain sizes. If an 80-mesh grade has a large percentage of extremely small and large particles (and, hence, the sizes don't follow a bell curve or normal distribution), cut quality can be affected and the possibility of clogs arises.
So how can a shop determine a garnet grade's effectiveness, since grade ranges and garnet characteristics vary from supplier to supplier? In a word, experience. Says Bennett, asking the right questions—about the screening process used, where the garnet is mined and other specifics—can help immensely. But how well a specific vendor's products have worked in past waterjet-cutting applications may be the best judge of its future performance.
Editor's Note: Artwork is courtesy of Barton Mines, www.barton.com.