“The New Flexibility of Waterjet Cutting”
The Fabricator, March 2000
By Richard Ward
Ten years ago, waterjet cutting was used primarily to circumvent specific characteristics of traditional cutting methods. For instance, heat processes such as oxyfuel, plasma, and laser generate a heat-affected zone (HAZ) along with distortion and material expansion, while waterjet has no HAZ.
Later, rapid growth in the use of composites and laminates such as stainless, INCONEL ® alloy, and phoenalics separated by a honeycomb opened up another application for waterjet cutting. The machining bits that traditionally were used were not always the best for all materials, and the phoenalics often gummed up. Because abrasive waterjet cutting is an accelerated erosion process and does not follow the established rules of machining, it quickly became an option for processing these laminated materials.
Waterjet cutting was not always economically viable, however. The early years of waterjet involved high entry costs, limited accuracy in the motion control gantries, and low-life consumables. Cutting results often were unpredictable, leaving only the brave (and those who had no other option) to use abrasive waterjet.
However, just as there have been dramatic developments in laser, plasma, oxyfuel, and machining, so has abrasive waterjet technology grown as a sought-after technology. Today, abrasive waterjet is considered and often quoted even for simple parts cut from mild steel.
Over the years, as new manufacturers have entered the waterjet market, competition has increased to address this technology’s limitations. Every OEM is searching to provide a better machine with a lower operating cost at an affordable price. New technology, materials, software, and hardware have reduced operating costs by extending the life and reliability of the wearing parts.
Abrasive Removal and Recycling
Abrasive is the main consumable in waterjet cutting, excluding wearing parts, and can account for up to 50 percent of the total cost of operating a waterjet system. One specific development in waterjet machines was the recycling of this abrasive.
Before recycling became an option, companies removed the abrasive from the catcher tank for disposal. Many different methods can be used to accomplish this task, from mechanical scrapers to reticulation of the suspended solids in the tank through cyclone separators. The abrasive then is settled in a drum, ready to be dumped. A waterjet abrasive recycling system offers the additional option of washing, sorting, and drying only the good garnet for immediate reuse.
As an example, a recycling system with a 24-inch-diameter separation screen can dry a recommended 150 pounds of abrasive per hour. Because only the reusable abrasive is dried (typically, 50 to 70 percent of new abrasive can be recycled after the first use), the total capacity of the system is such that it could support all the abrasive introduced into a waterjet by a 150-horsepower pump, using 4.5 pounds of abrasive per minute.
According to some estimates, recycling abrasive can cut the overall operating cost on a single-head waterjet cutting system by 40 percent. If three or four heads are used, the cost to operate the additional heads becomes negligible, while production can increase 300 to 400 percent.
Waterjet has become an easily expandable technology, as long as the system is purchased with growth in mind. For example, as the workload grows, one, two, or three more cutting heads can be added fairly simply. This is the equivalent of purchasing a second or third machine at a greatly reduced price, because each time a head is added, the cutting capability is doubled, tripled, etc.
Each time a cutting head is added, the waterjet pump capacity must be balanced to ensure a larger volume of water is pressurized to supply the cutting heads. Both new and used waterjet intensifiers can be added to work together in parallel, giving the operator some flexibility in intensifier choice. In addition, as long as the pump added in parallel is reliable, it does not have to be of the same make as the others. All that matters is the pressure the intensifier pump generates.
Thus, a waterjet operator can start small and within a limited budget and yet expand the system later without needing to purchase another complete machine. The first system purchased, however, must have the flexibility to grow -- for instance, to add a spreader bar to take the additional cutting heads. It is also important that the machine have the flexibility to extend the cutting tank to accept the additional heads.
The parts that fabricators process are becoming increasingly complex. To handle such parts, operators typically had to purchase a custom five- or six-axis waterjet cutting unit with a single cutting head. This limited the high-cost unit to single-head cutting.
Today, five-/six-axis conversion kits are available as bolt-on attachments to turn any waterjet gantry into a single- or multiple-head, five-/six-axis cutting system. Depending on the original controller installed, the controller may need to upgraded, but flexibility is greatly increased.
The benefits of cutting even in two axes with a five-axis system can be substantial. The taper produced by a waterjet cut is directly related to the cutting speed. In many cases, operators are forced to cut more slowly to ensure the taper is vertical and higher tolerances are achieved. By allowing the five-axis cutting head to compensate for the taper, operators can cut at speeds up to twice as high as normal, accepted speeds, yet produce the same-quality part.
The introduction of just these few developments has strongly impacted waterjet cutting. For instance, combining an abrasive recycling system with a bolt-on, five-axis cutting head can help reduce operating costs by more than 60 percent, according to this author. In addition, one OEM has predicted that these two developments combined will help push waterjet’s annual growth to more than 30 percent in coming years. Only time will tell, but, clearly, abrasive waterjet is becoming a feasible alternative to traditional cutting processes.
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