Cutting With Gems – What you don’t know about your garnet can cause cutting problems

Nov./Dec, 2011


When a company purchases new garnet for its waterjet, it might not be exactly what’s expected.

To cut materials, garnet is used as a waterjet’s cutting source along with highly pressurized water. It’s like the teeth of a saw blade. However, when a new saw blade is purchased, it’s guaranteed to be sharp, while garnet might not be.

Garnet is found naturally in the earth and is extracted along seashores or in underground mines. In its best form, it’s also a gemstone that looks very much like ruby, but less costly. The gemstone can be man-made, but garnet used for waterjet cutting isn’t. Processing it for a waterjet application might include crushing it to a specific size and washing it or just screening it for size and cleaning it.

“Most garnet from around the world comes in a sand-type form. It’s run through screens to get the exact sizing needed,” says Pete Mitchell, V.P. of Sales for the for GMA Garnet Group.

On the MOHS scale that’s used to measure the hardness of minerals, diamond is 10, the hardest substance known, while garnet is in the 7.5 to 8.5 range, and quartz is about 9. Beach sand is between 4 and 4.5. Therefore, if quartz is harder and diamond too costly, why isn’t quartz used instead?

“Garnet is the best option for the price,” remarks Mitchell. “It’s very hard and much less expensive than quartz. Garnet is the hardest product at the right price.”

But not all garnet is created equal for cutting applications. Some can be far more productive in cutting than other garnet, even though they have a similar chemical makeup.

Garnet usage

Once a company purchases a waterjet, they must find the correct mesh size of garnet for what they’ll be cutting. Mesh sizes range from 50 to 200 and indicate the size of the garnet from 600 to 50 microns in size.

To find the right size of garnet for cutting, the first step is to look at what’s being cut notes Mitchell. “If they are an average job shop that processes all types of materials, then I’ll give them a garnet mesh size that will work with both thin and thick materials which is usually an 80 mesh (300 to 150 microns). This is somewhat of an all around product. If they are specifically cutting a thicker material such as steel plate or granite countertops or something along this line, I’ll have them use a coarser material like a 60 mesh (400 – 200 microns). Companies that cut very thin sheets of aluminum or plastic will use a 120 mesh (50 -150 micron) garnet.

“Then we’ll look at what type of usage the waterjet will have, such as how many cutting heads it uses, and how often the machine will be running. Most applications will use 45 pounds of garnet per hour per head. Then quantity is based on how often the waterjet will be running.”

Garnet costs typically range from about $.22 to $.28 per pound. It usually comes in a 55 pound bag, or for higher usage in a one to two-ton bulk bag.

Garnet Quality

Some garnet is better for cutting than others mentions Mitchell. “Garnets have different hardnesses and different shapes. Some garnets are a little more angular than others and tend to give a little better cut. For instance India has a lot of garnet, but it tends to be beach sand that has tumbled on the shore of an ocean and the waves keep rolling it back and forth slowly rounding its edges off. While the garnet material that we mine still has its angular shape that nature created, which is best for cutting.”

Another important factor in buying garnet is the dust level that is packaged with it. Garnet needs to be washed during processing.

“We found that some of the mines don’t get all the byproducts out of the garnet,” says Mitchell. We try to get as close to 100 percent garnet as we can after the washing cycle. We tend to be between 97 to 98 percent garnet. I’ve seen some manufacturers offering it at right around 70 percent garnet and 30 percent other materials including dust.”

Also chlorides that are a type of salts trapped around the Garnet need to be washed away says Mitchell. “You don’t want salts in your waterjet tank. If salts build up within the tank, and you’re cutting aluminum parts, the salts will get on the aluminum and will oxidize the parts.”

You’ll also find that water in the waterjet tank can evaporate but the salts won’t. So they can slowly build up in the tank and oxidize parts after they’re cut.

“Dust and chlorides in the garnet can cause problems by clogging up the water flow too,” adds Mitchell. “If a company has problems cutting, the first thing they do is blame the machine. They really don’t look at the type of garnet that they’ve purchased that could be the source of the problem. The dust in the feed lines attracts moisture and can cause the material to cake and clog the line.”

Garnet Recycling

Garnet can be taken from the waterjet’s holding tank and be recycled says Mitchell. In fact The GMA Garnet Group has four plants around the world that will take a customer’s used garnet and recycle it. “We have a $2 million plant in New Orleans that does this.”

Recycling also depends on the quality of the garnet notes Mitchell. “If you have a garnet that breaks down very easily during the cutting process, what’s left in the tank isn’t anything usable as the garnet will be too fine. If you have a good sturdy garnet where the crystals break in half during cutting, then you end up with good quality recycled garnet with sharp corners, and larger usable particles.”

Garnet supply

Another key factor for buying garnet is supply remarks Mitchell. “I started out operating my own waterjet shop and I’d say, ‘If it ain’t spraying, it ain’t paying.’ If you run out of garnet, your waterjet shuts down. I’ve seen suppliers that will have garnet one month and not the next. A shop can’t run this way. It needs a steady source of garnet.”

Points a waterjet user should consider when buying garnet:

· Cleanliness: It must be as free as possible of chlorides and dust.

· Consistency: The mesh size must be consistent every time you buy it. Once a waterjet is adjusted for a particular mesh size, you want it to stay adjusted. Inconsistent garnet mesh will have the operator continuously adjusting the machine.

· Availability: You want a supplier that has an ample supply of garnet when and where you need it.

GMA Garnet Group

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