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Cutting with Water Comes on Strong

 

More powerful, more accurate, more versatile

 

By Robert B. Aronson
Senior Editor 

 

Initially, the waterjet was limited to cutting soft materials such as fabric, thin metal, and chicken parts. Some units were complicated to operate, and the noise could be harmfully high. Plus, there were serious maintenance considerations.

Today, waterjet cutting is a well-established machining technique that is eating the lunch of some established machining operations. It is used as a stand-alone process or as a complement to other methods. It speeds production by eliminating manufacturing steps. For example, you can rough-cut a part with a waterjet, then finish it with conventional milling or EDM.

Here's a look at the recent advances some of the major suppliers have made.

"We have had significant sales to EDM and laser job shops," says Steve Szczesniak, national waterjet product manger, Mitsubishi Waterjet, MC Machinery Systems Inc. (Wood Dale, IL). "Sales are not usually a matter of technology advances, but of educating the potential customer to what the process can do."

The MD Jet, the latest waterjet machine from Mitsubishi, is a five-axis unit with a complete enclosure and a stainless-steel tank. This design was produced at the request of medical manufacturing customers who must eliminate unwanted moisture and debris from the work area.

To give the user more capability, the Mitsubishi units have a CNC control, not a PC, that uses a number of CAD/CAM programs. Because this is not proprietary software, links to other processes are easier.

Mitsubishi waterjets have a pump that delivers water at 60,000 psi (420 MPa ). "But users must remember that, with a multiple-head system, each head will only get a portion of that 60,000 psi water," says Szczesniak. "To have all heads cutting with 60,000 psi, you have to add additional pumps."

Waterjet cutting has been one of the fastest growing machining processes in recent years. According to Wiktor Stepien, vice president, KMT Waterjet Systems (Baxter Springs, KS). The key reasons include improved reliability and simple maintenance. "In the KMT designs, this has been achieved through the company's Autoline II, as well as the KMT Integrated Diamond Eductor cutting heads.

"We specialize in supplying OEMs in several different markets, with KMT Streamline pumps rated at up to 200 hp [150 kW]," says Stepien. "The fastest-growing segments in the recent years have been metal fabrication, stone and marble, aerospace, and automotive 3-D cutting. Some end users estimate that the cost of running waterjet systems is about one-third the cost of laser cutting systems, and 10 times faster than EDM cutting."

KMT often recommends that their customers turn the pressure down as much as possible. This positively affects any high-pressure pump.

A feature just as important as operating pressure is the use of a diamond orifice. Because it lasts up to 1000 hr, it helps control cost and improve efficiency and accuracy.

KMT has improved pump service-ability and ease of maintenance with the Streamline V Plus intensifier, which features a patented Hard-Seal Endcap Technology. They were able to eliminate a complete set of seals on the check-valve side of the pump, so the end user has less than half of the seal cost of traditional intensifier pumps. The KMT Streamline SL-V 100 S has only two seals; less advanced intensifiers could have eight. The results are less chance for leakage, less material stress, and less maintenance.

   

"More affordable waterjet systems are a need we have recognized." says Angus Catterson, general manager, Knuth Machine Tools (Lincolnshire, IL). Many companies don't have the work volume required to justify the purchase price, running expenses, and space required by a traditional waterjet system. Knuth has developed the EcoJet line to fill this need. The machines have a small footprint, and run at 22 kpsi (15 MPa). Operating at this lower pressure is said to have several advantages.

Components for lower-pressure waterjets do not need to be made from exotic materials, because wear is much less than with higher-pressure systems. Therefore initial cost and maintenance costs are lower.

Less-expensive wear parts do not need to be replaced as often, reducing running costs.

Electrical costs are reduced. The EcoJet uses a 10-hp (7.5-kW) pump vs. the 50–200 hp (37.5–150 kW) pumps used on traditional machines.

"As pressures increase, the running costs also increase," explains Catterson. "At this point in time, we find that above 60 kpsi [the pressure at which their Hydro-Jet runs] the additional running costs outweigh the benefit of greater speed.

An 87 kpsi waterjet cutting system, the WMC2, is available from Flow International Corp. (Kent, WA). Currently, many waterjet systems operate at pressures from 40–60 kpsi (280–420 MPa).

These features are claimed for the new design:

  • Cutting speeds that are 30–50% faster than units operating at 60 kpsi,
  • Part cost 30% lower than can be achieved using a 60 kpsi system,
  • Abrasive consumption 30–50% less than a 60-kpsi system consumes, and
  • Simpler, faster maintenance than conventional, lower-pressure waterjet systems.

"There may be higher-pressure systems later, but you bump into the limits of material strength," explains Flow Project Manager Chris Maier.

Waterjet machines usually employ 60–200 grade grit, but a user normally sets up a system using the single grit most suitable for a given project. But, in some cases, two sizes will be used for a roughing-finishing sequence.

"We get a better finish with our system because the velocity of the grit is higher and has greater power density," says Maier.

"To get a machine that functions at 87.5 kpsi, you can't just add a bigger motor. Many elements of our new systems had to be redesigned to withstand the 40% increase in pressure. Everything from the intensifier side of the pump had to be reworked, including the seals, nozzles, and all fittings," says Maier. Maintenance also had to be simpler. One major redesign was to combine seal elements and make them easier for the user to change.

Jet Edge (St. Michael, MN) President, Jude Lague, sees waterjet pressure reaching 100 kpsi (689 MPa) in the future, which means even faster cutting speeds and part production. "As pressures go up, so do cutting speeds, and less abrasive is used," says Lague. "This results in lower costs per part."

   

One of the reasons behind the waterjet's increased popularity has been the growing use of composite materials in aerospace and other industries. "For example, the war in Iraq and post-911 security precautions have increased the demand for body armor, vehicle armor, bulletproof glass, and other energy-absorbing materials," notes Lague, "all of these items are being cut with waterjet."

As to improvements in the waterjet industry, Lague believes, "Customers want the waterjet OEMs' to standardize the definition of 'accuracy.' For example, some quote accuracy over the entire work envelope, some quote per square foot, while others quote per axis."

Jet Edge offers improved software in its Aquavision Di control. According to Brad Schwartz, Jet Edge regional manager, this control has a graphical user interface that allows the operator to process raw geometry, use standard shapes, or manipulate previously created jobs.

"In the hierarchy of cut quality, the ranking generally is EDM, waterjet, laser, plasma, and oxy-fuel," says Steve Zlotnicki, business manager for ESAB cutting systems (Florence, SC). "There was a time when there was a wide gap between laser and plasma. Now, with high-definition plasma, it's crowding laser in some cases."

One of their key features is that ESAB machines have both waterjet and plasma. Generally, the waterjet is used for precision contours, and the plasma will do where high precision is not necessary. The main advantage is that you don't have to cut the entire part with the slower, more expensive, waterjet process. This reduces overall part cost. "ESAB machines operate at pressures from 55 to 60 kpsi (400–420 MPa). "There are some cutting-speed benefits to higher pressures, but wear may be an issue," he concludes.

Abrasive waterjet is a complementary process in many cases. Often it can reduce the amount of machining needed, as in making near-net-shape parts. According to Jeff Day, product manager, waterjet, Bystronic Inc. (Hauppauge, NY), "A good rule of thumb is 'the thicker the part, the better the chance waterjet is the best way to cut it.' And there are some materials that thermal-cutting methods don't handle well. For example, lasers have trouble with thicker stainless steel, aluminum, copper, and brass.

Currently, the Bystronic waterjet units operate at 52,200 psi (365 MPa). Higher pressures are available, but the user has to consider the benefits of faster cutting against the increased cost of nozzle erosion and seal maintenance.

One of the strongest features of the Bystronic design is its ability to handle large sections. For example, the Byjet L can work with sheets 10' (3-m) wide, and as long as wanted. It carries up to four cutting heads, and can cut through an 8" (203-mm) thickness. Maximum cutting speed in thin material is 1181 ipm (30 m/min).

Building waterjets that can be upgraded to grow with the user's needs, such as expandable CNC controls and addition of cutting heads, is the specialty of WardJet, Inc. (Tallmadge OH). The latest model, their X-Series, has these features available: Standard eight-axis motion, upgradeable to 32 axes; wireless communication between controller and Z carriage; and two 15" (381-mm) touch-screen monitors. One monitor runs the machine, while the other runs the CAD/CAM software and any other Windows or Internet-accessible programs.

"Cutting speeds are automatically generated by selecting the material to be cut, the edge quality, and tolerances needed through intuitive, user-friendly software," explains company President Rich Ward.

Materials ranging from 10" (254-mm) stainless steel to 0.010" (0.25-mm) acrylics can be cut. Tolerances tighter than ±0.005" (0.127 mm) are possible, especially in thinner materials. "What many people don't understand is the wide range of cutting speeds in a specific thickness material with waterjet that are not found in laser, plasma or oxyfuel. As a rule, waterjet cutting speeds could vary by 500%, ranging from a rough separation cut at high speed, to an excellent, almost-machined part. Paradoxically, cutting cost plummets as cutting speed increases, which explains why there could be a large variance in quoted prices on cutting the same part from different companies," says Ward.

Maintenance is one of the key issues when evaluating a waterjet system. Wear has to be monitored. Ward cautions that potential buyers should be sure to consider the cost of consumables, availability of parts, maintenance issues, and the maker's guarantees before making a decision.

"The abrasive-waterjet market is growing, due chiefly to the fact that manufacturers are expanding their capabilities, not trading off a lathe or CNC for a waterjet," explains John Cheung, Omax (Kent, WA) president and CEO. "With the waterjet, they are finding both increased capacity and capability. Often it is a good supplement for a laser or EDM and is especially practical for prototype or low-volume production."

Maintenance time, maintenance frequency, and the cost of Omax units have been lowered through higher reliability and simplified design. "Maintenance issues formerly scared people away from waterjet systems," says Cheung. "Precision has also been improved. Our machines are capable of an accuracy of ±0.0003" [0.008 mm]. The key to precision is the control system."

Operating pressure of Omax equipment is 55 kpsi. "Higher pressures will give faster straight-line cuts, but not steering control, so it may not be usable with complex parts, or those requiring constant modifying", says Cheung. "Control, rather than power, is needed for these jobs." One of the newest Omax units, the V Pump, operates at a noise level of less than 75 dBa at 3' (1 m), running at 600 rpm. Direct-drive-plunger pumps deliver a smooth pressure output, eliminating defects on the cut surface. No additional accumulator is required. Seal life exceeds 500 hr.

 

This article was first published in the November 2007 edition of Manufacturing Engineering magazine. 


Published Date : 11/1/2007

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