Waterjet Cutting

Don't let consumables fool you

By Lincoln Brunner

In a metalworking market where every penny counts, abrasive waterjets have become a staple of many fabrication shops.

Because of its precision, lack of heat-affected zone and clean edge finish, abrasive waterjet can be a cost-effective cutting method for all kinds of materials, from stainless steel to composites. However, as with any machinery, users and vendors of waterjet technology say that first-time customers need to be aware of the total cost of running a waterjet before they invest.

Whatever the total hourly cost of running a waterjet--$25 or more is a safe bet--the cost of purchasing abrasive grit (usually garnet) makes up almost half of it.

Garnet comes in several different varieties and grades, depending on the edge sharpness of the granules and other properties. Typical waterjet systems use about 1 lb. of garnet per min. of cutting. At about 18 cents per lb., garnet alone costs the shop $10.80 per hr. of cutting time.

Some workpieces require harder materials, such as aluminum oxide, to cut efficiently. Because of its cost, up to 20 times that of garnet, aluminum oxide remains less prevalent. Glass and slag are also used occasionally, but garnet remains the abrasive of choice mainly because of its efficacy, availability and affordability.

That affordability can be tested when the costs of disposal are added. "Disposal varies greatly, depending on where you're located and what you're cutting," says Deron Roberts, parts and service director for Jet Edge, a manufacturer of ultra-high-pressure waterjet systems. "It's really all over the board."

In some states, shops can simply drop their spent garnet in a dumpster and let their garbage haulers carry it to the landfill. Bryon Machado, new accounts manager for KMT Waterjet Systems, says that's what KMT does for its in-house waterjet operation in Kansas.

"Typically a 5-ft.-by-10-ft. tank holds somewhere between 15,000 lbs. and 18,000 lbs. of abrasive--that's what we pull out of our tank when we clean it out," Machado says. "We just throw it into a dumpster."

Others have to pay for a third party to remove the garnet from their shops and haul it away. One company in that group is KV Technologies. KV, a precision waterjet fabricator for the aerospace industry and other markets, generally cuts stainless steel, titanium and aluminum on its waterjet table. The company was using laser and oxyfuel to cut parts, but now relies exclusively on the waterjet purchased two and a half years ago for its production-volume operation.

Based in Suffield, Conn., KV operates under slightly more stringent environmental codes than its counterparts in other areas of the country. Consequently, there are certain materials KV simply won't cut on its waterjet table, including anything containing lead or beryllium. To comply with local codes, KV does a total metals concentrate test on its material every day and submits those numbers to its hauler.

KV Vice President and CFO Vinny Camp says that while it might be feasible for some people even in rural New England to pile spent abrasive behind their buildings and level it with a backhoe when it dries out, "in the state of Connecticut, it's basically unheard of.

"It's probably the largest problem in the waterjet industry," Camp says of garnet removal. "Over the course of the years we've been in business, it has gotten to the point where we send out the spent garnet for testing to make sure there are no contaminants so that we dispose of it as clean fill.

"Do you really want to leave a company susceptible to the EPA [Environmental Protection Agency]?" Camp says. "No, you absolutely do not. Once you let them in the door, they never go away."

Nor is it advisable to make cleaning out the machine's garnet bin an in-house operation. KV did that--once. Two people had to put on hip boots, stand in the bin and shovel out the high-grade garnet--no small task considering the company has gone through 22 tons of garnet in the past six months alone.

Now KV pays an environmental company twice per year to vacuum the spent garnet out of its machine and haul it away at a cost of $1,800 to $2,200 each time. But it's either that or pay two or three employees to shovel for two days while the machine sits idle.

Water Filtration
Another item high on the consumables list is the water used to cut the workpieces and to cool the pump pushing the cutting water through the system. Include the filters and seals in the system, and the water itself can represent a significant expense.

The typical 50-hp pump used in most waterjet machines pumps about 1 gal. of water per min. through the nozzle. In addition, Jet Edge's Roberts says, intensifier pumps like the ones his company sells use 2 gal. to 3 gal. of water per min. just to cool the hydraulic system in the pump. Positive displacement pumps use less cooling water, about 1 gal. per min. Whether a waterjet user mixes city water with the abrasive for cutting or reuses it in a closed-loop system, filtering contaminants out of process water is a must. "First and foremost is the filtration of the water," Camp says. KV uses a closed-loop system with a three-step filtration process. Between the closed-loop system and the other components in its waterjet machine, KV spends about $650 every six months on new filters, Camp estimates.

In addition, because of the 70 hrs. to 90 hrs. per week that KV runs its waterjet, the shop goes through filters more often, especially during summer months. The summer heat raises the water temperature inside the machine and degrades the filters more quickly.

"We're finding that out from personal experience; when the water gets hot, the filtration doesn't work as long," Camp says. "In a closed-loop system, you have chilled water going through the pump. When that varies, it takes pump life down."

The waterjet's workload requires KV to rebuild the machine's 50-hp pump every six weeks. Rebuilding takes 4 hrs., but the company does it so often, it's become standard practice. "We know how to rebuild," KV President Ken Collins says with a laugh.

"It all depends on how many hrs. you run your waterjet," Collins adds. "If you're like us, running 12 hrs. to 15 hrs. a day, you're rebuilding a lot more. You're always running at maximum psi in a production shop."

Camp predicts that the EPA will be setting waterjet standards in the near future, which might increase costs even more. "It's eventually going to happen," says Camp, a licensed asbestos inspector who knows his environmental regulations. "There are eventually going to be stringent regulations on the waterjet industry."

In the meantime, maintaining water quality remains a key to running a successful waterjet operation, KMT's Machado says.

"When you do maintenance, the key is to keep a clean work area," Machado says. "Waterjet is not a clean application. You're going to be doing maintenance right there on the pump. It's close to the table, and you're going to have overspray. You have to maintain a clean work area."

Other components that wear out on a regular basis are nozzles, the orifices inside the nozzles and the mixing tubes where the abrasive flows into the water inside the cutting head. Roberts says the most common nozzles handle between 75 hrs. and 80 hrs. of work--a once-a-week change-out costing $85 a pop.

The orifice (or jewel) inside the nozzle, which focuses the inlet water inside the cutting head before it mixes with the abrasive in the mixing tube, can be a little cheaper or costlier, depending on the quality. A ruby (or sapphire) orifice probably costs about $15 and lasts perhaps 20 hrs. to 40 hrs.; a diamond orifice can last for 1,000 hrs. to 1,200 hrs., but costs about $425.

Then there are mixing tubes, for which KV pays $125 apiece. Because of the shop's volume, it goes through an average of two mixing tubes per week. "You just have to keep a close eye on it and change it out when you start losing your stream," Camp says.

On the less-frequent end, high-pressure seals need to be changed out every 500 hrs. to 800 hrs., depending on the demands of the operation. "There are a lot of variables that go into seal life," Machado says. "If somebody could come out with a unit with 2,000 hrs. of seal life, they'd be a multimillionaire."

Whatever prospective reason companies need a waterjet for, Collins encourages them to get the full picture up front from the vendor. He admits that procedures such as garnet removal are just part of doing business; but one of the benchmark numbers thrown out--the $25-an-hr. cost of operation--does not factor in garnet consumption.

He says, "There's no way you can run one of those machines for $25 an hr., unless you're only running it a minimal amount of hours in a week. In my eyes, the best thing is to be very honest up front [with buyers]."

Camp concludes, "When the machine runs 12 hrs. to 15 hrs. a day, your consumable rate skyrockets." FFJ

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