September 2011 / Volume 63 / Issue 9|
Now in 3-D
By Bill Kennedy, Contributing Editor
| Special glasses are not required, but other considerations apply in 3-D abrasive waterjet machining.
As it has in the movies, 3-D is making quite a splash in the world of abrasive waterjet cutting. While it started as a strictly 2-D process, new waterjet technology allows the process to be applied to create more complex parts.Courtesy of WARDJet
Paul Oehler, vice president of waterjet builder and automation supplier Romeo Engineering Inc., Fort Worth, Texas, said: “The market is about 75 percent 2-axis cutting, with the rest being 3-D. Everyone is starting to think 3-D. With the benefits of working directly from solid models, shops want to know how to take a low-cost commodity piece of plate, bar stock or round, and turn it into a high-value product with a minimal amount of tooling and setup. A 5-axis waterjet machine is the perfect tool for that work.”Changes on the Scene
Abrasive waterjet (AWJ) machining was first applied to cut flat materials and was controlled with simple G code. John Olsen, co-founder of waterjet builder OMAX Corp., Kent, Wash., said, AWJ “didn’t become accepted in metalcutting until we got away from G code, which doesn’t provide enough information to the controller to know what the jet is doing. The jet is a floppy tool. When milling, you have to be careful the tool isn’t bending or breaking, but the problem is worse with waterjetting.” Because the stream of water is a “soft” tool, an unadjusted AWJ cut tends to taper, or grow wider or smaller at the top than at the bottom. The taper changes from wide at the bottom to wide at the top as cutting speed increases.
From the get-go, 3-D AWJ systems were aimed at taper control. “To make really precise parts, even flat 2-D parts, you have to move in multiple axes because you have to tilt the jet to remove the taper,” Olsen said. “To make precise parts fast, you need at least five axes of motion—X, Y, Z and two axes of tilt.”
AWJ taper-control technologies have progressed to include CNC-positioned cutting heads that tilt in precise increments to provide close-tolerance correction. The OMAX Tilt-A-Jet system, for example, adjusts the head to a position that produces a taper-free and perpendicular cut on one side of the workpiece, while doubling the taper on the waste side.
Similarly, according to waterjet builder Flow International Corp., Kent, Wash., its Dynamic Waterjet system can cut taper-free parts up to 300 percent faster than traditional flat-plate machines. Tim Fabian, Flow’s manager of global product marketing, said the first true 5-axis AWJ machines were developed for aerospace manufacturers in the 1980s. “They were programmed on G-code controllers,” he said. “That was difficult, because the way to gain true accuracy with a waterjet stream is to precisely increase and decrease cutting speeds. G-code controllers aren’t really capable of doing that. It only made sense for somebody to get a 5-axis waterjet if they were doing the same part over and over again.”
In the mid-1990s, PC-based controls were introduced for 2-D AWJ machines. The systems stored information regarding materials and cutting conditions as well as machining models and algorithms. After an operator entered the material type, thickness and desired finish, the machine adjusted cutting speeds automatically. “When that introduction was made on the 2-D side, waterjet started taking off,” Fabian said. Because of the added level of complexity of 3-D AWJ, he said, “Only in the last couple of years have we gotten to the point where we’ve been able to use a PC control to predict the stream path on a 3-D or 5-axis waterjet. A 5-axis waterjet is very complex, having to change cutting speeds at different thicknesses, angles and accelerations as it moves through a 3-D space.”Tilt-A-Whirl
As the head tilt angle increases, complexity grows. Olsen said OMAX’s Tilt-A-Jet provides up to a 10° inclination, while the company’s A-Jet tilts as much as 60° and is capable of more acute contours. “If you tilt 45°, the equivalent thickness is about 40 percent greater, so you have to slow down accordingly. If it is a 1"-thick plate, at 45° you slow down to the same speed as when cutting a 1.4"-thick plate. The Tilt-A-Jet software compensates to handle the effective thickness.”
To compensate for uneven or warped workpieces, OMAX also offers a height sensing system called the Terrain Follower. Olsen pointed out that cutting a 45° bevel on a plate that is warped 1⁄8 " upward results in a lateral error of 1⁄8 ". The Terrain Follower is linked to the machine servos and software. “If you move the Terrain Follower foot up by 1⁄8 ", then the nozzle will follow it and move up 1⁄8 ",” Olsen said.
Courtesy of OMAX
The Psy-Winder 5-axis waterjet cutting head from waterjet builder WARDJet Inc., Tallmadge, Ohio, tilts up to 90° from vertical. Sales Manager Jeff Day said the system represents “a natural progression of where things would go in terms of tilting heads. People like the concept of 5-axis cutting with the waterjet to do things like weld prep, countersink holes, trim 3-D parts or perhaps for agricultural parts where they have to cut a blade to a very severe angle.”
The Psy-Winder head provides flexibility. “Customers often say they don’t need to do anything beyond 45° and they are mechanically limited to a 50° or 60° angle,” Day said. “With the Psy-Winder, they have additional flexibility if a job comes where they do need to tilt 90°. Then we can show them what is involved as far as removing the other limits and also, of course, making the machine safe.”
Fabian said Flow’s recently introduced Dynamic Waterjet XD technology expands on the original 2-D Dynamic Waterjet concept to automatically adjust the tilt to compensate on the fly for taper in a 3-D environment. “A true 5-axis machine can predict that stream model while it is varying thicknesses on the same plane,” he said. “Our software allows a user to directly import solid files. It sees the file as a 3-D part and generates a toolpath around it automatically.”
The increased flexibility and precision of 3-D AWJ processes must be accompanied by an increased focus on safety. Day said, “When you cut at angles that are not vertical, safety becomes more of an issue. The jet could be pointing straight at somebody or straight at the side of the machine.”
Flow ships all of its 5-axis machines with light curtains, safety devices that work like the light-beam system that prevents a garage door from descending when it shouldn’t. “We also have a safety PLC program that automatically warns the operator before executing the program if the head is going to tilt in a manner that could be compromising,” Fabian said.Real-Time Compensation
Some features of traditional metal-removal methods are being adapted to 5-axis AWJ. Romeo Engineering is developing real-time 3-D cutter compensation. That technology already is available on some traditional equipment, such as the Fanuc 16i control with a RISC processor, Oehler said. However, he added that it is expensive and doesn’t work very well with waterjet machines because unlike an endmill, a waterjet stream does not have a finite tool length. And an AWJ nozzle grows from the inside out at 0.0001 " per hour; an endmill gets smaller as it wears. Programs for cutting very thick materials can take up to 48 hours to complete, during which time the nozzle diameter changes significantly. “With our PC-based controller, we have the ability to measure and perform cutter compensation in 2- and 3-axis cutting, but not in five axes,” Oehler said. “Depending on the cutting speed and the angle of attack into and out of a corner, a lot of variables are involved in that calculation.”
Courtesy of KMT Waterjet Systems
The motivating force for AWJ is water pressure generated by two-stage intensifier pumps or single-stage, direct-drive pumps. At the cutting head, the water travels through a 0.010"- to 0.018"-dia. orifice in a plug called a “jewel,” made from ruby, sapphire or diamond. The tiny opening combines with high water pressure to produce the jet’s supersonic speed.
Bob Pedrazas, marketing manager of the Americas for KMT Waterjet Systems Inc., Baxter Springs, Kan., said typical pump pressures were 55,000 to 60,000 psi as recently as 3 years ago. In 2009, KMT launched its Streamline Pro 90,000-psi intensifier pump series. Pedrazas said the ultrahigh-pressure pumps have been widely accepted because “manufacturers want to increase performance and productivity. By increasing pressure from 60,000 to 90,000 psi, you can achieve up to double the cutting speed as well as increased part thickness capability, depending on orifice size and pump horsepower. The real improvement is less taper, improved cutting of complex shapes, better cornering and piercing, and an overall better finish.”
At IMTS 2010, KMT introduced 100,000-psi pumps. Further increases in productivity on the level of 7 to 15 percent are expected, Pedrazas said, noting that “it all depends on materials.” He added that beta testing is ending and “we are almost at the point where seal life will be acceptable to end users.”
As with any productivity-boosting technology, the use of higher pressures involves tradeoffs. Although the new pumps can run at ultrahigh pressures, seals last longer under lower pressures.
“To cut 12" of steel, an operator might run at maximum pressure,” Pedrazas said. “On the other hand, when cutting ½" aluminum, it’s already blazing through and there is no need to run at maximum pressure.” He said seal life will improve at less than maximum pressure and productivity will still be excellent. The ultrahigh-pressure pumps, Pedrazas noted, are valued in production environments where they can provide flexibility via the option to run dual cutting heads.
Courtesy of KMT Waterjet Systems
Romeo Engineering’s Oehler confirmed the benefits of a high-pressure pump. “A higher-than-60,000-psi pump cuts faster and produces a better finish with less taper angle, and a 100,000-psi pump is even better. The problems of taper and speed go away at much higher pressures, or much higher flows.” Because kinetic energy equals one half mass times velocity squared, “The more mass that you throw at it, or the higher the velocity of the water, the higher the energy of the jet, which allows it to cut thicker and faster and get a better finish,” he said.Accuracy Initiatives
Rigidity is vital for 3-D AWJs. “A waterjet has to cut at a specific focal point,” Oehler said. “As you move out in volumetric space, the errors get worse. We are covering 10 '×20 ' or 20 '×100 '. That becomes very difficult, so the machine must be rigid. It also requires more care with the setup so that there is a bridge between the machine’s coordinate system and the workpiece’s coordinate system.”
Tolerance buildup is a key consideration for all 5-axis machines, including waterjets, according to Dennis DesMarais, sales manager for the industrial division of PaR Systems Inc., Shoreview, Minn., a supplier of automation systems, material handling equipment and waterjets. “Many calculations are required,” he said. “What separates the men from the boys is how you do those calculations and how accurately you can do them.”
PaR performs a routine called mechanical error correction that uses a laser tracker coupled with automated test methods to determine six position-dependent error curves for each of the machine’s five axes of motion. The axis errors are downloaded into the controller, where software computes joint axis biases to compensate at the tool tip for machine inaccuracies in real time. “To get an accurate machine you have to go to that next level of error correction,” DesMarais said.
Courtesy of Romeo Engineering
According to PaR, its Vector waterjet can produce volumetric accuracy of 0.005" in 3-D. “Volumetric means the total accuracy in space,” DesMarais said. “A lot of people quote accuracy per axis. That’s nice, but take all those axes and combine them, and what have you got? The point is how accurately it cuts that part.”
Compared to conventional machining, he said, “The AWJ process is influenced by additional variables, including the flexibility and pressure of the water stream and the size and amount of abrasive. You want to keep as many variables out of the equation as possible.”
Courtesy of PaR Systems
PaR’s Vector system also compensates for variables in part positioning. After a shop identifies three points on a part, the system automatically transforms the points so the relationship of the part’s shape to the waterjet cutting tool is the same, no matter where or how the part is positioned. DesMarais gave the example of a large, heavy nose cone movable only by crane. “It it might be sitting cockeyed and weighs a ton, making it difficult to move. However, once you have located the part in space, the machine automatically transforms that program and cuts it.”
Five-axis AWJ machining requires a different approach than 2- or 3-axis work. “In standard waterjet cutting, you can put just about anybody on the machine,” said WARDJet’s Day. “With a little bit of training, operators can be cutting parts. Five-axis machining is more complex. You have to have your lab coat on and think more scientifically about how things are going to happen.”
For example, when machining a 45° bevel down the side of one workpiece and a 45° bevel down the side of an adjacent workpiece using a 5-axis machine, the stream from one part might collide with another part.Growing Markets
OMAX’s Olsen, who began working with AWJ in its pioneering days, said acceptance of the technology is growing. “Instead of having to go and teach people that they need it, we at least occasionally get people who come here and say, ‘I want an AWJ machine!’ That change has happened over the last few years.”
The variety of AWJ applications is also growing. “We are finding the American market is very creative in using these tools,” said Oehler of Romeo Engineering, citing as an example shops that repair and upgrade military vehicles. The work involves putting an 8° to 20° taper on the top and bottom of 5 "-thick armor plate as a weld preparation. “Otherwise, they would have had to grind that bevel,” he said. “With the waterjet, they program it, cut it in one setup and weld it. What’s astounding to me is the spectrum of applications, from artistic work one day to extremely high-tech, advanced aerospace components the next day, all from the same machine.”
Flow’s Fabian said 3-D AWJ applications range from simple bevel cutting to sophisticated processes such as roughing aircraft engine turbine blades, where, beginning with a square block or round blank of material, the AWJ cuts a hole and profiles the blade shape to within 0.010" to 0.060" of final dimensions. “Then you go in with your milling cutter and make one pass on it and you’re done,” he said.A Competitive Edge
Oehler said: “There is always going to be a need for 2- and 3-axis AWJ, but companies must choose their battles: what is more profitable for them, what is going to set them apart from their competitors. The shops with 5-axis capability can charge $350 per hour, where the shop doing 2-axis work has to compete at less than $100 per hour. There are few civilian 5-axis waterjets out there, and the first kids on the block to have the new toys are going to get all the business.”
Both large and small job shops can gain a competitive advantage with 5-axis AWJ, DesMarais said. “A 5-axis machine costs more than a 2-axis machine, but with the former there are very few jobs a shop can’t handle. If a shop has a laser, a couple of machining centers and a waterjet, they pretty much have it covered.”
The waterjet industry’s goal should be to make the 5-axis AWJ as common as a 5-axis milling machine, according to Flow’s Fabian. “You don’t see a 5-axis milling machine every day, but good shops have them,” he said. “It’s no different with waterjet machines.” CTEAbout the Author: Bill Kennedy, based in Latrobe, Pa., is a contributing editor for CTE. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or by e-mail at firstname.lastname@example.org.
Courtesy of KMT Robotic Solutions
AWJ, such as cutting automotive trim components, have evolved into a wide range of applications, including machining castings, composites, aluminum vehicle frame parts and jet engine components, according to Mark Handelsman, advanced technology product manager for KMT Robotic Solutions Inc., Auburn Hills, Mich.
Handelsman said most parts processed via robotic AWJ already have three dimensions. “To process the parts typically machined with robotic 3-D AWJ, you need something more than X-Y-Z,” he said. “You need some orientation control.”
He provided an example of cutting holes in cast iron bathtubs. The manufacturer had looked at other alternatives, including conventional drilling. “The shape of the workpiece played the biggest role,” Handelsman said. “Imagine a drain hole, holes for hot and cold water, a hole for overflow, and you can visualize that it is a very 3-D application.”
Parts processed with robotic AWJ often are large enough—such as 2 m2—to require they be moved with heavy equipment.
Robotic cutting generally emphasizes repeatability over ultimate accuracy, Handelsman said. “Once we have taught the robot a path, we can repeat it.” Depending on the size of the robot, the materials and part features, robotic AWJ can hold tolerances on the order of tenths of millimeters, he said, making 6-axis robots cost-effective for many applications.
The choice of the robot itself is the easy part, Handelsman said. “We spend the most time on developing the right process, trying to dial in what features need to be cut, how quickly we can cut them, and getting the right pressures and speeds.”
The robot’s role as toolholder or workholder varies. In many cases, the robot holds the waterjet nozzle and aims it at the part. “In some cases, it is actually better for the robot to hold the part and move it under a fixed jet,” Handelsman said. “We do it both ways; it gets back to developing the right process.”
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