“Waterjet Problems Dissolving”
By: James R. Koelsch, Contributing Editor
Forming & Fabricating, March 2002
(Please Note: to read specifically about RICHEL, Inc. and the Recycling of Abrasive, go to the end of the article –)
Builders of waterjet cutting equipment are proud of the growing acceptance of waterjet technology. “Over the past 6 years, waterjet cutting has been the fastest growing machine tool technology in the world,” claims Chip Burnham at Flow International Corp. (Kent, WA). “Most technologies are actually going down between 40 and 45% a year because of the machine tool depression.” Although waterjet cutting is still a very small piece of the machine tool pie, the cutting process is doing quite well against other cutting processes.
One reason for its apparent success is that it can cut any material accurately without producing a heat-affected zone and inducing mechanical stress. For this reason, ESAB Welding and Cutting Products (Florence, SC) integrated waterjet into its plate-cutting gantry equipment, creating combination machines. “Combining a very accurate process, like waterjet, with a much faster cutting process, like plasma, can increase productivity by quite a bit,” says ESAB product manager Jeff Defalco. For example, the combination machine might use the faster plasma torch to cut the outside when the accuracy is not critical and save the slower waterjet for bolt holes in the middle of the shape.
Although waterjets find themselves cutting materials 4” (102 mm) thick or thinner in approximately 80% of the cases, they also can cut materials as thick as 10” (254mm). In fact, waterjets are competing against chip-making equipment, not just presses and thermal processes. “You can look at waterjet equipment as a powerful band saw for doing first operations,” says John Olsen, vice president, operations, Omax Corp. (Kent, WA). “A band saw is a lot cheaper, but it generally can’t deliver a part that’s nearly as finished.”
An example is an installation producing spur gear blanks. Unlike a band saw, the waterjet produces blanks that go directly to a grinder for final finishing. By eliminating steps between initial cutting and final grinding operations, the waterjet compensates for the fact that it cuts the stock slower than a band saw would.
PCs Simplify the Process
All builders attribute waterjet cutting inroads to a variety of developments that make the technology faster, simpler, and cheaper. Foremost among these developments is sophisticated software for controlling the process. “In the last few years, PC-based controllers have been of enormous help to waterjet cutting,” says Olsen. “They let the manufacturer get into the innards so it can adapt the controller to the job.”
Olsen and his competitors avoid conventional CNC’s because the source code for the interpolators inside these controllers is proprietary and unavailable from CNC manufacturers. Without adaptation by waterjet experts, CNC’s are inadequate for waterjet cutting because the process has an extra complication that conventional machine tools do not have. Unlike an end mill, for example, the jet is not a rigid tool. As the jet enters the material and progresses through it, it loses its kinetic energy and, therefore, its shape.
The first distortion occurs as a taper that is wide at the entry and thin at the bottom. The second is the bending produced by the bottom end lagging behind the top end. When the jet begins to bend, it also begins to wiggle from side to side near the exit, which produces a rough finish at the bottom of the cut. The resulting surface resembles an edge produced by flame cutting. As the jet goes around corners, the lag also causes big geometry errors. Both the lag and jet wandering increase with cutting speed.
Because waterjet geometry depends on feed rate, users must compensate for the changes. Rather than relying on an experienced operator’s skill at adjusting the feed with an override control knob or having a programmer parse the cutting path and determine the optimum speed for each segment by trial and error, builders have developed software for changing the speed automatically. The software looks ahead along the cutting path for contours and corners, slows the cutting speed as the jet approaches them, and resumes speed as it completes the maneuver.
To determine optimum cutting speed for the jet’s parameters, the software uses a cutting process model developed by the builder. Using its internal knowledge base, the model chooses speed automatically based on the material, its thickness, and contour. “The result is much better than an expert operator can do by hand,” says Olsen. “In fact, many new controllers have no feed rate override because the only thing you do with it is make the part worse.”
PC-based controllers available today give Omax and other waterjet-equipment builders the ability to develop the necessary compensation algorithms. “The openness of the PC makes it much simpler to accommodate the fluid dynamics,” says Olsen. “The machine is just a USB device, like a printer, plotter, or mouse. All the heavy computing about the motion control is done in the PC with conventional PC software.”
Clean Cuts, No Taper
Perhaps the most revolutionary result of the software development effort among builders is Flow’s Dynamic Waterjet with active tolerance control, which the company spent four years developing and began shipping in some of its machines in January. Using a three-axis wrist controlled by proprietary software, the cutting head on a gantry machine tilts slightly to eliminate the taper that waterjets leave on the part’s edge, effectively doubling it on the cut’s waste side. It also executes a series of motions through inside corners to avoid damaging them and produce clean cuts.
Flow’s Burnham blames stream lag and taper for preventing abrasive waterjet from becoming more widespread. “Going slower minimizes deflection and taper, but it costs,” he notes. Some software can slow the cutting speed by as much as 85% to eliminate or minimize taper. Sometimes, the cost of cutting slowly is too great for waterjets to be practical for precision jobs.
Burnham expects Dynamic Waterjet to cause a surge in waterjet technology popularity. Not only can the wrist tilt slightly to change the jet’s impingement angle, but it also can articulate the jet’s point in the appropriate direction to compensate for lag in a corner or along a contour. “The ability to hold tolerances improves dramatically at speeds that were not possible before,” says Burnham. “We can cut precision parts from two to four times faster.”
This ability is the fruit of a multi-million-dollar development effort involving a team from three companies: CIS Robotics, which solved the kinematics problems; Flow Robotics, which designed the necessary mechanics and electronics; and Flow International, which wrote the software. One engineering problem the team encountered was to minimize the Abbe error, which is the displacement of the nozzle tip caused by the unwanted pivoting of the head around its attachment point. Because this error is a function of the distance between the tip and the attachment point, Flow’s engineers designed the new head with a Quick Clamp System that grips the cutting head close to the tip.
Another engineering problem was solving the complex differential equations describing the kinematics and jet distortion in real time. Flow expanded the Erichsen Models that its controllers have used to predict the jet’s shape while it is in the material and to compensate for deflection during two-dimensional motion. The equations now do so for three-dimensional motion.
Moreover, PC-based controllers now have enough computing capacity to work with large files and solve the equations in a few minutes. Consequently, the operator imports the cutting path and enters the material and its thickness. Before the machine begins cutting, the controller takes several seconds to prepare gantry and wrist motion by adjusting the impingement angle and cutting speed at the necessary points along the cutting path.
Although machining center and similar machine tool builders have used similar wrists for a while, impeding their use in waterjet cutting has been the inability to predict the jet’s deformation. “The stream is dynamic,” explains Burnham. “Every time you speed up or slow down the slightest bit, the taper changes and the stream lag changes. So figuring it out would be a monumental iterative task that would require cutting hundreds of parts, measuring them, modifying the angles, and recutting.”
An Attack on Downtime
Streamlining troubleshooting and maintenance is another way computer technology improves waterjet’s overall productivity. Ingersoll-Rand Waterjet (Baxter Spring, KS) developed an abrasive monitoring system called Sensoline that alerts operators to problems. By way of a pressure sensor and software, the controller reads fluctuations in the process. If it detects detrimental irregularities, it sounds an alarm.
“It allows an operator to walk around and do other things in the shop while the waterjet system is operating,” says Bongani Mncwango at Ingersoll-Rand. “Should the abrasive be misfed or underfed, it will shut down the operation and signal the operator to fix the problem. The automatic shutdown reduces the chances of spoiling your workpiece or creating scrap.”
Currently, the alarm is an audible one, but the manufacturer plans to develop the necessary links for a modem to page the operator and communicate with other computers. When the operator returns to the control panel, the LED display warns of the problem based on the pressure change’s signature. The monitor detects problems, such as focusing-tube wear, damaged orifices, plugged focusing tubes, and sudden leaks.
Robotic Production Technology (Auburn Hills, MI) deploys a different sort of remote computer-monitoring scheme on the six-axis robots it installs for waterjet and other cutting processes. Because the bulk of its customers are in the automobile vendor industry and carry little work-in-progress inventory, the systems provider has responded to their demand for immediate troubleshooting services 24 hours a day, 365 days a year. RPT is offering an online service called Robolink over the phone lines by way of the modem inside controllers. The service probably will migrate to the Internet once the network’s speed is fast enough to provide continuous real-time connection, no matter how many users.
“From a PC in our facility, our technicians can see customers’ systems running in real time,” says CEO Chuck Russo. “The minute an operator presses a key, we can see whether it was the right one and can determine quickly what we can do to help.” The help can come as immediate advice over the phone, logic changes downloaded directly into the PLC, or a visit by a service person already equipped to solve the problem.
As one user discovered, the online service can save a tremendous amount of time. One morning, RPT’s staff learned of engineering changes to a part to be made on a robotic cutting station being installed at the user’s facility. New PLC logic for actuating clamps holding the part in place would be necessary. “By lunchtime,” recalls Russo, “we had made the changes, downloaded the logic over the phone line, and verified it. The customer was up and running in four to five hours.” In the past, understanding the problem, solving it, and flying a technician to California to implement the solution would have taken two to three days.
Another way RPT helps recover productivity lost through downtime is by changing orifice material. Rather than using rubies or sapphires, it installs diamond orifices. Russo says his researchers have developed a proprietary technology that overcomes the past problems many others have had with diamond.
Consequently, the orifice lasts much longer than those made of other materials. For example, a diamond orifice might last between 1000 to 2000 hours for a straight water job, during which a sapphire might last only 40 to 70 hours. Also, diamonds do not shatter as easily as sapphires when stray pieces of metal left in the lines after maintenance hits them. “When a piece hits a diamond, it tends to impregnate the diamond,” says Russo. For this reason, RPT also offers a service for inspecting the diamond orifices, evaluation them, and removing fragments.
New pump designs are another important set of advancements promising to make waterjet cutting more competitive with thermal and other cutting processes. Flow, for example, has broken the 60,000-psi (414 MPa) barrier with a series of new pumps that operate at what the company calls “hyperpressure,” which is any pressure that is 75,000- psi (517 MPa) or greater. The builder is shipping 75,000-psi (517 MPa) pumps now and plans to unveil an 87,000-psi (600 MPa) model soon.
Burnham attributes the graduation to the next level in pumping pressure to a series of small, incremental advancements in seals and other components, and investing in fundamental research in high-pressure fluid mechanics. “Because of new materials and component designs, the pump can handle stresses unimaginable a few years ago,” he says. He believes pressures exceeding 100,000 (690 MPa), and even 200,000-psi (1379 MPa) is not out of the question.
“Higher pressures in the waterjet industry are inevitable,” says Burnham. “When I started in this industry about 14 years ago, abrasive waterjet pressure was at about 30,000-psi (207 MPa). When we went to 50,000-psi (345 MPa), a number of people in the industry said there was no need to go that high. Lo and behold, they are now at 50,000 (345 MPa).”
The reason for seeking greater pressures is to intensify the jet’s energy density by increasing speed and tightening the stream. Pressure turns into velocity once the water passes through the tiny orifice. As water flows past a feeding tube, it creates a vacuum that pulls the abrasive particles into the stream and accelerates them. “As speed increases to a threshold velocity, the mode of material removal changes,” explains Burnham. “A small particle can remove a tremendously large amount of material because it has so much momentum behind it.” A faster and focused jet enhances speed, efficiency, and the ability to cut intricate detail to tight tolerances.
For the more conventional range of waterjet pressures, 55,000-psi (379 MPa) and less, Flow offers direct-drive pumps that exploit triplex technology to eliminate the hydraulic intensifiers found in other waterjet pumps. Although direct-drive pumps are nothing new, they had not been able to produce the pressures required for waterjet cutting until recently. “They were primarily used for surface preparation,” notes Burnham. “Now that they can produce pressures as high as 55,000-psi (379 MPa), they are right in the heart of the highest pressure pumps that had been available for waterjet cutting in the past.”
Omax also strongly advocates direct-drive pumps. Although intensifier pumps can have two to three times longer seal life, triplex pumps typically have a much stronger advantage in operating efficiency. “A good intensifier would have an overall efficiency of 70%, whereas a crank drive can be in the high 90s,” says Olsen. “If you were running a 100-hp (75-kW) intensifier pump, then you would be throwing away more than 30-hp (22-kW).” In regions where power costs $0.10 per kW-hr, an intensifier pump wastes roughly $2.25 per hour.
A triplex pump saves this cost by replacing the hydraulic circuit found in intensifier pumps with a crankshaft, prompting many to call the triplex design a crankshaft pump. Instead of pushing the plungers hydraulically, the pump relies on a motor to turn a crank, which actuates a set of rods that move three plungers in and out, much like a crankshaft would move the pistons in a three-cylinder engine.
An intensifier pump, on the other hand, accumulates and pressurizes oil in the chamber to push on a diaphragm. Rods coming off the diaphragm’s sides push the plungers back and forth. For an intensifier with a 20:1 ratio – the ratio of the area of the oil against the diaphragm to the area of the water against the plunger – oil at 3,000-psi (207 MPa) would transmit enough force to create 60,000-psi (414 MPa) in the water. Because hydraulics can multiply force much easier than mechanics, Flow’s new hyperpressure pumps still use intensifiers.
Streamlined Abrasive Handling
Another reason Burnham might be right about waterjets becoming increasingly popular in the future is that equipment manufacturers have made handling abrasive less expensive and less of a headache. One problem is abrasive finds its way into mechanisms and creates wear. Another is it becomes much like concrete when it mixes with the chips and settles to the bottom of the tank. Agitating and cleaning critical, difficult jobs, and separating the abrasive from the chips and water afterward for recycling has been cost prohibitive.
A number of developments have taken a giant stride toward solving both problems. For robotic applications, RPT has two devices for containing abrasive. The first is a Jet Vac, a workholding device that holds parts fast with a vacuum. Besides giving users a flexible and repeatable method of holding parts at approximately half the cost of conventional fixtures, according to Russo, the device also provides a method for recovering and recycling abrasive.
RPT’s other device for containing abrasive is a rotating-wall work changer that indexes workpieces in and out of an enclosure. Users load parts outside the enclosure while the robot is cutting. When the robot finishes the current workpiece, the mechanism rotates the new workpiece into the enclosure and the completed one out of it with 2.7 seconds. Indexing time for older units is typically between 8 and 12 seconds, according to Russo. Another of the device’s properties is that its payload capacity increases if users are willing to accept longer indexing times.
***Richel Inc. (Kent, OH) helps users avoid wear on moving parts during abrasive loading by specifying a pneumatic hopper that has no moving parts. Instead of using slides and cylinders to actuate a plunger, the engineering staff at Richel designed a diaphragm that inflates like a balloon. Pumping air into the diaphragm inflates it a little and causes it to close. “You can seal the unit so you can wash it while you’re cutting,” says Richard Ward, president. Opening the chute is a matter of bleeding the air to relieve the pressure and allow the diaphragm to shrink.
Waste Not, Want Not
Another development is much more significant for the future of waterjet cutting. Ward invented a device three years ago for recycling the abrasive and is introducing a smaller version this year to make it practical for small-scale users. “A large portion of abrasive is reusable,” he says. Although he has been selling the original device for about three years now, “It’s been a struggle because most people believe that it cannot be done.”
Although many equipment builders use Ward’s recycling unit now, doubt about its cost effectiveness lingers because past devices for automating tank emptying have been expensive and required much maintenance to repair worn parts. “If you cannot keep the system operating continuously, including a couple of hours after cutting to prevent abrasive from settling, the abrasive will clog the jets,” he says. “You will have to remove it by hand, which is quite a challenge.”
Consequently, the devices have been expensive, relying on sprays and jets to circulate large volumes of water to keep the abrasive in suspension.
His new device, however, reduces agitation’s high operating costs and maintenance by not having to run during the cutting process and for several hours afterwards. Users need switch it on when 12” (305 mm) of abrasive have accumulated at the bottom of the tank. A special nozzle design and three pumps agitate the solid mass to break and transport it to the separation’s device. “The key to success is regulating the amount of abrasive transported per minute,” says Ward. “If it’s too much, it overloads. If it’s not enough, it won’t be efficient.”
Besides the widespread bias that recycling is impractical, the other inhibitor for its greater use has been cost. The original machine was a little more than $50,000. A recently introduced model, however, is a less expensive machine that is about half the size. “At the end of the day, you will save more abrasive than you will pay for the equipment,” says Ward.
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