Customers often have a variety of technical and application questions about the proper selection, performance and use of BARTON waterjet abrasives.
In addition to the information found elsewhere on this website, this section is provided by the experts at BARTON regarding topics that have come up frequently in conversation with customers. We hope you find the information helpful. Of course, if you have a question that’s not addressed in this section, don’t hesitate to contact us. BARTON customer service professionals are always ready to answer your questions.
Selecting the proper orifice gemstone
The orifice assembly is a key component of the waterjet cutting process. Its function is to direct the high-pressure stream of water through the mixing tube where it draws in the abrasive and delivers it to the cutting surface. Diamond orifices are highly regarded in the industry or waterjet cutting. Ruby and sapphire orifices are also used.
Ruby and sapphire orifices are considered functional equivalents. They are commonly used in less-critical applications because they are susceptible to damage in high-pressure applications when abrasive particles impact the sides of the orifice. The impact of these particles can damage the inlet edge and compromise the quality of the waterjet stream. Ruby and sapphire orifices usually last anywhere from 1 to 40 hours.
Diamond orifices are known to produce a more cohesive stream than a ruby or a sapphire orifices. A more cohesive stream can result in longer mixing-tube life and increased cutting power. Diamond orifices are generally warrantied for 500 hours, and consistently meet or exceed the 500 hours. It is not uncommon for an operator to get 1,000 hours or more from a single diamond orifice.
Diamond orifices are recommended for use with 6,000 bar (87K to 100K PSI) equipment and are typically preferred for long-duration cuts on expensive material that might be ruined in the event of an orifice failure mid-cut. The cost of a ruined part because the cut was compromised by a damaged orifice can far exceed the initial upfront cost of purchasing a diamond orifice instead of a ruby or sapphire orifice.
Diamond orifices also result in significant cost-efficiency because they require less maintenance and wear longer, requiring fewer replacements.
Operators running multiple cutting heads can also realize the added value of using diamond orifices. Ruby and sapphire orifices have a shorter, more unpredictable usable life than diamond orifices. That shorter life cycle adds to increased downtime, especially on multi-head machines.
And while diamond orifices are preferable for a variety of reasons, there are times when less expensive ruby and sapphire orifices are often chosen. Operators who cut a wide variety of materials that do not have exacting specifications, and change orifice/mixing tube combinations often, may prefer to use less expensive ruby or sapphire orifices, since the frequent handling of the components during regular replacements creates greater potential for loss or damage. More expensive diamond orifices would present greater financial risk to the operation.
Protecting the orifice against damage is always a good strategy. Whether ruby, sapphire or diamond orifices are chosen, the useful life of any orifice assembly can be improved by adding a short-stop filter assembly in the high-pressure line before the on-off valve to eliminate the risk of contaminants entering the line and damaging the orifice. If a short-stop filter assembly is employed, periodic cleaning with an ultrasonic cleaner is recommended to remove calcium deposits and extend the life of the orifice.
Determining Optimal Abrasive Feed Rates
The optimal abrasive feed rate is essential to achieve ideal cutting efficiency, precision and reliability of any waterjet operation. The abrasive feed rate directly impacts the quality of the cut, cutting speed and the volume of abrasive used. Improper feed rates can result in clogging in the mixing tube, overuse of abrasive, slow cutting and poor edge quality.
Abrasive feed rates will vary depending on a variety of factors. The mesh size, or grade, of abrasive being used as well as the type of abrasive are the two largest factors that affect feed rate. Other factors that affect abrasive feed rates include humidity changes and the condition of the abrasive feed hoses.
Abrasive feed issues can also occur if the abrasive regulator is not grounded. Ungrounded abrasive regulators can build static in the feedline to the cutting head and slow or stop abrasive feed.
The abrasive metering charts provided by OEMs are a great starting point to ensure use of the optimal abrasive feed rate for a specific application. However, it is always good practice to perform an abrasive calibration test and physically measure the amount of abrasive being fed from the abrasive regulator. Most OEMs recommend performing this test weekly or any time there is a change to the abrasive mesh size, abrasive type, equipment change, or other factor that is known to affect feed rate and overall cutting performance.
A simple feed rate test can be performed by following these steps:
- Follow OEM-specified instructions for calibrating abrasive feed.
- Remove the abrasive feed line, or feed block if so equipped, from the bottom of the abrasive regulator.
- Using a stopwatch, or test function available in some OEM software, turn on the abrasive feed and run the abrasive for exactly one minute into a container such as a plastic sandwich bag. Weigh the container prior to feeding the material to get its tare weight.
- Weigh the abrasive sample using an accurate scale.
- Subtract the tare weight of the bag or container to get the weight of the material. The feed rate is X kgs. of abrasive per minute.
- Run this test two or three times using the same test specification and conditions to ensure you’re getting consistent weights.
- Adjust metering disk sizes up or down to get to the desired feed rate, and repeat steps 3, 4 and 5 to once again ensure that the proper feed rate has been achieved. Test until you are confident that you have an exact feed rate.
- Enter the exact feed rate into your software settings.
Choosing the right waterjet abrasive for your application
How do hardness, density, toughness, and particle shape affect the performance of a waterjet abrasive?
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 important factors in determining the proper abrasive to use. The abrasive product needs to have hardness, density, toughness, and a consistent particle 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 the edge finish required. Grains with sharp, angular edges have been proven to cut more quickly and offer superior edge finishes. Subrounded 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.
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.
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 are best for the job.
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 mesh and 120 mesh, with 80 mesh being the most widely used.
Delivery options. The supply partner should be capable of delivering waterjet abrasive in 25 kg or 1,000 kg bags. 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.