Back to News The type of garnet used in abrasive waterjet cutting can have an effect on the end quality of your parts.


Technical Tip: Choosing the Right Garnet Abrasive

The type of garnet used in waterjet cutting can have an effect on part quality.


Garnet as an Abrasive

Garnet has been used for decades as an abrasive in waterjet cutting and sand blasting because of its superior physical properties. When added to high pressure streams of water, garnet abrasive is capable of cutting through tough materials including steel, aluminum, stone, and Inconel.

With the variety of garnet abrasive available, how do you decide which is right for you? By understanding a little more about garnet, you are able to make an informed purchase on which type of abrasive would work best for your application.

Waterjets and Abrasive

A waterjet should be seen as an accelerated, controlled erosion process. A pure water waterjet (one without abrasives) is ideal for very soft materials such as rubber, foam, or food products. When abrasive is added, virtually anything can be cut or eroded away. By adding abrasive, you can greatly enhance the cutting capability of your waterjet and expand the variety of materials you can cut.

A waterjet will use anywhere from 0.3 lb/min to 2.0 lb/min of abrasive. This quantity is known as the abrasive flow rate and is determined by factors including the size of your pump and nozzle. The typical usage for a waterjet company is about one pound of abrasive per minute per cutting head.

Advantages of Garnet for Waterjet Cutting

Garnet is a hard mineral typically rated between 7.0 and 7.5 on the Mohs scale (where a diamond is 10 and talcum powder is 1). Also, well washed garnet abrasive does not produce a lot of dust when cutting like other processes might. Garnet is also relatively chemically inert and will not react with materials being cut, making its disposal simpler. These qualities are advantageous in waterjet machining.

Types of Garnet Abrasive

Alluvial Garnet (River Garnet)

Alluvial garnet is particularly abundant in Australia. This alluvial garnet is typically found in deposits in river beds or natural hollows and has been smoothed by a steady flow of water. Both river and beach garnet suffer from the tumbling effect of thousands of years which rounds off the edges.

Rock Garnet

Rock garnet is produced in America, China, and Western India. Here the garnet is embedded in a host rock which, after being crushed in mills and washed, is separated out. Being freshly crushed, this garnet has the sharpest edges and consequently performs far better than other types of garnet.

Some Qualities to Look for in Abrasive

It is not recommended to purchase abrasive based off of price alone. Many factors 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 abrasive:

Double Sifted

When you purchase double sifted abrasive, the distributor uses a sieve to remove particles that are too fine or too large for the desired 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.

Purity

Abrasives that have a higher purity level will contain less foreign material. Impure abrasives can contain materials other than garnet that will affect a waterjet’s performance significantly (longer cuts, more abrasive usage etc.).

Mesh Size

 A larger number represents a smaller size abrasive, so 100 mesh is smaller than 80 mesh. If cutting thin or softer materials and a good finish is needed, a finer abrasive such as 120 or 150 mesh is recommended. When cutting harder or thicker materials, choose a coarser abrasive such as 50 or 60 mesh. For a smoother surface finish, use a finer abrasive such as 100, 120, or 150 mesh. 80 mesh abrasive is the most popular abrasive size because it provides the greatest versatility for a wide variety of applications. Consult the manufacturer of your nozzle for mesh size recommendations.

Recyclability

If you have or are planning on purchasing an abrasive recycling system, you will want to be sure that the abrasive you are using can be recycled. Some garnet grains in both alluvial and hard rock categories have fracture planes in them. This causes the grains to shatter into smaller pieces when they hit the water stream prior to coming into contact with the material being cut. Even though this type of abrasive will provide a smoother edge finish, it will slow down the cutting of thicker materials and it cannot be recycled. Since recycling could allow waterjet owners to reuse approximately 50% to 80% of the original abrasive, it is advisable to use an abrasive that cuts and recycles well.

Abrasive Cost

Abrasive represents the largest consumable cost associated with running the waterjet cutting machine. Prices for abrasive can vary from around 15 cents per pound to higher than 40 cents per pound, depending on the quality of the abrasive and where it is purchased. Since crushed garnet is typically found mixed with other minerals and must be blasted out of a mine, it costs slightly more than alluvial abrasive. Although crushed rock garnet is considered ‘high performance’, some waterjet owners have seen better results using alluvial abrasive. In certain situations, the sharp grains of the crushed rock are critical to specific applications such as cutting horizontally stressed acrylic. Performing tests with both garnet types is recommended for waterjet owners to determine the ideal abrasive type.

Some distributors may offer a reduced price per pound if the abrasive is purchased in bulk. You might even coordinate your purchase with a nearby branch location to reduce shipping costs. Be wary of signing lengthy contracts for abrasive unless you are 100% sure that the abrasive you are ordering is right for your long-term needs. In some cases, using a higher priced abrasive may actually reduce your hourly cost of operation. Good abrasive that performs well will allow you to cut material faster. As a result, you could get more inches of cutting done per dollar spent on abrasive. The operator must work to maximize cutting speed and should be aware of other factors that affect cutting speed such as the condition of the orifice and nozzle.

Conclusion

Finding the right garnet abrasive is paramount to successful waterjet cutting. By considering the type of garnet abrasive to use and weighing the advantages of cost, quality, source, and recycling capabilities, you are able to determine the best fit for your waterjet cutting application. If you are still uncertain about which type of abrasive to use with your waterjet, the team at WARDJet would be happy to use their many years of experience to help you arrive at the best decision. Contact a representative today at 330-677-9100 or sales@wardjet.com.



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