Choosing the right supply partner is an important action for a metal fabricator. A reputable waterjet manufacturer or abrasive supplier should assist a fabricator with determining the right abrasive type and grade. This aids in the selection of an abrasive that will maximize the performance of the waterjet cutting machine.

Expert advice. Expert consultation by a supplier includes helping fabricators evaluate a wide variety of different product factors, including the type and grade of abrasive that is best for the job.

Hard rock or alluvial garnet. The material being cut, its thickness, and the desired surface finish requirements influence the choice of hard rock garnet or alluvial abrasives. Hard rock garnet is best for cutting hard materials and achieving a superior edge quality, while alluvial garnet is suitable for less demanding applications.

Other abrasives. Purchasing from a supplier that carries a variety of waterjet abrasives gives operators multiple options for abrasive performance and value to meet a wide range of applications.

Choosing the proper grade. Waterjet abrasives are available in a variety of grades, ranging from coarse (50 mesh) to very fine (230 mesh). Most waterjets use grades between 50 and 120 mesh, with 80 mesh being the most widely used.

Delivery options. The supply partner should be capable of delivering waterjet abrasive in paper bags, bulk bags, or even in straight bulk. It also helps if the abrasive supplier can ship within 24 hours of receiving an order and has a warehouse nearby for one-to-two-day shipping. This ensures timely deliveries and keeps freight costs as low as possible.

The waterjet cutting industry depends on sources that can produce abrasives with high purity, tight particle size distributions, and a high degree of cleanliness, all of which are attributes that are controlled during mineral processing.

Purity. As a natural mineral, almandine garnet is mined, milled, and processed to meet the producer’s final specifications. High-purity materials typically involve added processing stages and call for greater attention to detail during the refining process when compared to low-purity products. As a result, high-purity materials cost more, but they also deliver superior cutting results. Low-purity products may contain materials other than garnet that rob a waterjet cutting machine of its ability to cut well. When produced for use as a waterjet abrasive, other minerals must be processed to meet the high-purity level required for waterjet cutting.

Particle Size Distribution. Tight, consistent control of the particle size distribution (PSD) is extremely important to maximize the performance of a waterjet. Coarse or oversized particles pose a real risk of clogging the nozzle, which can bring the machining process to a standstill and potentially damage the workpiece. Conversely, excessive fines can collect in the feed line or the cutting head, causing irregular feed or sputtering in the cutting stream. Inconsistent particle size distribution can create a nightmare for operators that have to adjust the abrasive feed rate to maintain cutting speeds.

Cleanliness. While similar to particle size distribution, abrasive cleanliness stands on its own as the final important attribute for a waterjet cutting abrasive. Cleanliness refers to the amount of super-fines present in the abrasive product. These fines are so small that they often adhere to larger particles. From a technical perspective, the producers of quality garnet abrasives use a measurement called “suspended solids” to quantify how clean the product is. Using a product that has not been properly processed generates nuisance dust while loading the abrasive hopper, which may cause poor feeding and cutting over time.

Selecting the right abrasive type and size for a waterjet cutting application can make a significant difference in the performance and profitability of your waterjet equipment.

Abrasive selection begins with an examination of the material and the cutting specifications. How difficult the material is to cut and the desired surface finish are two factors important to determining the proper abrasive to use. The abrasive product needs to have hardness, density, toughness, and a consistentparticle shape.

Virtually every abrasive known to man—both natural and synthetic—has been considered for use as an abrasive in waterjet cutting. Almandine garnet has emerged as a mineral with the best characteristics for waterjet cutting. Other minerals may be harder, heavier, or lower in cost, but almandine garnet emerges as the one abrasive with the best combination of characteristics for waterjet cutting. The naturally occurring material is mined and processed for numerous industrial applications in addition to waterjet cutting, such as blasting media and water filtration applications. The common almandine garnet is brownish-red in color and opaque. The much rarer, gemstone-quality almandine garnet is marked by a deep red color and is transparent.

Other abrasives can be used in waterjet cutting. Aluminum oxide is harder than garnet and is sometimes used for cutting very hard materials such as ceramic. One drawback, however, is that using aluminum oxide dramatically shortens mixing tube life, increasing the cost of the operation.  Staurolite is a naturally occurring mineral with a density and hardness similar to garnet and is an economical general-purpose waterjet abrasive. Softer abrasives like olivine can be used for cutting less-demanding materials such as aluminum.

A closer look at abrasive characteristics

Natural attributes and mineral processing have a direct effect on how the abrasive material will perform as a waterjet abrasive. A fabricator should be aware of how these four key attributes affect the performance of the abrasive.

Hardness. Waterjet cutters need to balance cutting speed and component wear. Using a soft abrasive extends nozzle life but slows down cutting. Using an abrasive that is very hard offers fast cutting but erodes the nozzle too quickly. Nozzle erosion decreases accuracy in the cutting process, results in recurring downtime, and adds the expense of frequent nozzle replacement. Almandine garnet falls between 7 and 8 on the Mohs hardness scale, which effectively balances the need to cut quickly and provide reasonable cutting tool life.

Density. The principal cutting force of a waterjet is a function of mass multiplied by velocity. The ideal abrasive therefore has the heaviest particle that the water stream can accelerate to maximum velocity. This generates the maximum cutting force.

An abrasive that is too light won’t pack much of a punch, and an abrasive that is too heavy won’t accelerate to maximum velocity, sapping the waterjet stream of its power. As with hardness, the key is to find an abrasive that hits the sweet spot. Almandine garnet has a specific gravity of 4.0 (four times the weight of water) and falls right into the ideal range for both punch and acceleration.

Toughness. Sometimes referred to as friability, toughness plays a direct role in how well a waterjet abrasive performs. Material that is too friable breaks down in the focusing tube and ends up too fine to cut effectively. Abrasive that is too tough (think malleable like lead) rounds during the mixing process and is too dull to cut well. The ideal abrasive has a measured rate of breakdown and produces sharp, angular cutting edges. Once again, almandine garnet, with its semi-friable nature and conchoidal fracture fills the need.

Particle Shape. Abrasives are available in every particle shape imaginable, from perfect beads like steel shot, to razor-sharp, needle-like crystals found in silicon carbide, a synthetic abrasive used in high-tech applications. Recognizing that a sphere is the ideal carrier of mass projected in a high-powered water stream, a fabricator might assume that the waterjet manufacturers would look for spherical particles. However, they must keep in mind the constant balancing act involving acceleration, wear, and cutting efficiency.

Through extensive trials on numerous waterjet platforms around the world, waterjet experts have determined that the most suitable particle shape depends on two factors: the material being cut and edge-finish required. Grains with sharp, angular edges have been proven to cut more quickly and offer superior edge finishes. Sub-rounded grains are used in more general-purpose, standard cutting applications.

A waterjet cutting machine manufacturer, or an abrasive supplier can provide more information on both types and assist in trials to determine which particle shape is best for an application.