As you might expect, the type of abrasive is important. The overwhelming choice for most waterjets is garnet abrasive. Garnet is a gemstone that has been known and used by humans for many thousands of years. The type used in waterjet machining is red garnet. Garnet is fairly hard and when it fractures, it forms sharp edges. Both of these qualities are advantages in waterjet machining. Garnet is also relatively chemically inert, and won’t react with materials being cut, making its disposal simpler as well.
A boulder with crystals of garnet embedded in it (boulder courtesy of Barton Mines)
Garnet used for waterejt machining is either alluvial or mined (sometimes called “hard rock”). Alluvial garnet comes from river beds, where it has been smoothed by the constant running water. Because the grains of alluvial garnet are smooth, it’s not as effective as mined garnet at cutting. Alluvial garnet doesn’t require expensive mining, however, so it is sold at a lower price.
Mined garnet is typically found mixed with other minerals and must be blasted out of the mine. Then it is crushed, and separated from the rest of the rock. The crushing causes the edges of the garnet to be sharp, and therefore cut better with less taper and minimal burr. Waterjet garnet is sold commercially specifically for waterjetting—you can buy bags of waterjet garnet from suppliers.
A waterjet will use from about 0.25 pound (0.1 kg) per minute to 2.0 pounds (1 kg) per minute depending on the pump and nozzle you are using. The typical usage is about one pound (0.45 kg) per minute.
The flow rate of abrasive will generally be constant for a given setup. The flow rate does not vary depending on what you are making (unless you turn abrasive flow off and use water-only cutting).
Prices for abrasive varies from 15 cents per pound to 40 cents per pound, depending on the quality of the abrasive, and where you buy it. You should pay the extra money for good abrasive, especially if you are new to this technology, as quality abrasive will result in quality products. Abrasive is one of the biggest operating costs associated with running the machine.
Consider purchasing abrasive in large quantities for a discount. You might even coordinate your purchase with a nearby competitor, as you will both save money.
Garnet can be purchased in 100 lb (44 Kg) bags and in 50 Lb (22 Kg) bags and buckets
You can use other abrasive types, some of which can make your machining cheaper. For example, if you cut a lot of aluminum, you can use a softer abrasive, such as olivine, than you would use for steel. The advantage of using a softer abrasive is that you wear out your mixing tube (nozzle) slower. Garnet is a very good general purpose abrasive, which is why it is so popular.
Never use abrasives containing silica, such as beach sand. The dust generated by silica abrasives can cause silicosis, a deadly and painful lung disease.
Qualities to look for in abrasives
You should avoid purchasing abrasive on price alone., as it will often be the case that you get what you pay for. There are many factors that determine a good abrasive, and the advantage of using a high quality abrasive is that you will get faster cutting, higher precision, and less frequent nozzle plugging.
Here are some qualities to look for in an abrasive:
This means that the abrasive has the fine particles removed, as well as the big particles. Therefore, you have a consistent mesh size. Fine particles and large particles both contribute to nozzle plugging, inefficient cutting, and other problems. While there will always be a range of particle sizes in an abrasive, the narrower the range, the better.
Sharp abrasive particles cut better. Mined garnet is sharper than garnet from a beach, or alluvial garnet, that has been worn into round beads.
Look for an abrasive that is pure. An abrasive with impurities will affect your cutting performance. Softer impurities will lengthen your cutting time, while garnet with unusually hard bits in it, such as aluminum oxide, may cut marginally faster with a severe drop in nozzle life.
Of course price should be a primary concern, but not the primary concern. Understand that a higher priced abrasive may actually reduce your hourly cost of operation. This is because a good abrasive, that does a good job cutting, will allow you to cut faster. Thus, you can get more inches of cutting out per dollar spent on abrasive.
If you want maximum cutting speed, then you can choose a coarser abrasive, such as 60 mesh or 80 mesh. If you want smoother surface finish, then choose a finer abrasive such as 100, 120, or 150 mesh. Consult the manufacturer of your nozzle for recommendations. The 80 mesh abrasive is very popular, and in high demand. Therefore, it is also the most expensive. If you go with a coarser or finer abrasive, then you can save some money. The trade-off is that you may not cut as well.
When you first get your machine, use the machine with whatever brand of abrasive your equipment manufacturer recommends. Most likely, they will recommend either their own brand, or one that causes the least trouble. Later, as you gain experience with the machine, you can shop around for better deals.
Be careful of being locked into long term deals on abrasive unless you are 100% sure that the abrasive you are ordering is right for your long term needs. If you think you are getting a good deal by signing a contract for a years’ worth of 80 mesh garnet to cut your thin aluminum, at $0.25 / lb, then discover that you could have used a softer or lower grade abrasive at $0.15/lb, then you are stuck for an entire year using the wrong garnet for the job.
There are used (or “spent”) abrasive recyclers available from WardJet. According to the manufacturer, the WARD (Water Abrasive Recycling Dispenser) recovers a large percentage of used abrasive for re-use. When the grains of the abrasive strike the metal, some of them are fractured into smaller pieces, which means that the spent abrasive is not all the same size as it used to be.
This means that you’ll need to screen the spent abrasive to remove particles that are too small, as well as remove the small pieces of material from what you were cutting. Reusing abrasive is more complicated than just shoveling your tank into your abrasive hopper.
“The function of the WARD 24 is to remove the sludge from an abrasive waterjet cutting tank, separate out the sludge and all abrasive that is smaller than 100 mesh, then wash the abrasive larger than 100 mesh, dry it and screen it once more, simultaneously allowing operators to add new abrasive to the recycled abrasive at the desired ratio.”
– Quote from EasiJet web site