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Waterjet vs. EDM

 

Are these two strangers really beginning to compete for work?


By James R. Koelsch
Contributing Editor
   
   
   

         

A wave of skepticism swept through me when I received the assignment to write this story. Why would anyone compare wire electrical discharge machines (EDMs) to abrasive-waterjet cutting? After all, they are different technologies suited for different kinds of work. Waterjets are for sheet fabrication, and EDMs are for prismatic and small parts, right?

As it turns out, my colleagues were on the right track, and I was stuck in the past. Not only have technological advancements made modern EDMs much faster than their predecessors, but they also have allowed abrasive waterjets to hold tighter tolerances and produce better edges than they could before. Consequently, the capabilities of both types of machine have converged to the point where they can complement one another quite nicely in selected applications.

Some users of EDMs already have recognized this fact and have either outsourced the appropriate work to waterjet service bureaus or brought waterjet machines into their shops. Consider how a toolmaker, for example, might use both technologies. "You would cut the punch and the die with the wire EDM because of the need for high accuracy, and cut the stripper plate to a looser tolerance on the waterjet," says Gisbert Ledvon, product manager at Charmilles Technologies Inc. (Lincolnshire, IL), a builder of EDMs.

In blanking-die work, a waterjet might remove the bulk of the material and relieve stresses. A wire EDM would follow in a series of skim cuts to produce the final dimensions and finish.       

Hogging pieces from large billets is another application in which the two technologies can work together. Most shops would remove the excess material on a turning or machining center, reducing it to chips as fast as they could. Because of the abilities and accessibility of waterjets, some of them are sending their billets to waterjet houses like Chicago Waterjet (Elk Grove Village, IL), and asking them to trim all that stock away. Not only can abrasive waterjets slice the extraneous sections off quickly, but they also leave the chunks of material intact for other jobs, explains Pat Hill, a former wire-EDM applications engineer who is currently president of Chicago Waterjet.

The speed of waterjet cutting is even allowing the technology to compete against wire EDMs for work. "Waterjets are displacing EDM today in applications where EDM was used because of the part shape, but ultra precision is not necessary," claims John Olsen, vice president of operations and technology at Omax Corp. (Kent, WA), a manufacturer of waterjet equipment. "In these applications, the waterjet produces the part in as little as 10% of the time that EDM would need."

Besides speed, another advantage that waterjet has over EDM is that it can cut a broader range of materials. Because EDMs cut with sparks, "all materials processed on them must be [electrically] conductive," like graphite and metals, explains Joseph Cisar, senior applications engineer at Bystronic Inc. (Hauppauge, NY), another builder of waterjet machines. "Waterjets can cut any material, regardless of composition." They can cut parts with scale on the surface and materials that are insulators, such as glass, ceramic, wood, stone, plastic, and composites. And they cut without creating heat-affected zones or causing delamination.

                 So, will waterjet cutters advance to the point of replacing EDMs altogether? Hill at Chicago Waterjet thinks not, at least for the part of the application that prompted a manufacturing engineer to specify EDM in the first place. He notes that, although waterjet is at least ten times faster than wire EDM, EDM is about ten times more accurate. The best corner radius possible by waterjet is about 0.015" (0.38 mm) but an EDM can produce radii of less than 0.001 (0.03 mm). Moreover, the EDM can create an Ra 1-µin. (0.03-µm) finish, and the waterjet cannot.

Although abrasive waterjets can produce nice-looking edges and hold better tolerances than they could in the past, they simply cannot approach the edges and tolerances made by EDMs. The reason is the physics of the jet stream itself. As the name implies, waterjet cutting relies on a narrow stream of water traveling at supersonic speeds to erode the work material in the kerf. Although pure water is enough to slice through many materials, users can add garnet or other abrasive to the jet stream. The particles then transform the process into an abrasive process capable of cutting metals, ceramics, and other hard materials.

Even so, the cutting tool is essentially a stream of water. As it flows against the work material and creates fine chips, the jet stream encounters various resistant forces and loses some of its momentum, causing it to slow down and deflect. Not only does the stream not hold together as tightly at the bottom of the cut as at the top, but it also acquires a parabolic shape in the direction of travel. The three-dimensional distortion of the stream creates a taper that is a function of material thickness and cutting speed (as well as the velocity of the water flowing in the jet).

Because slowing the cutting speed until there is no taper is unacceptable for most applications, builders of waterjet-cutting machines have taken a number of steps to limit the effects of this phenomenon. One step has been to create stiffer and sharper jets that resist deflection and cut more easily by increasing flow rates and operating pressures, which can be as high as 60,000 psi (4.14 GPa) today.

Although the cost per inch decreases as cutting speed increases, the flow rate of the abrasive also increases. "Eventually, a point is reached where more abrasive will overburden the process and reduce the cutting speed and increase cost per inch," notes Mohamed Hashish, senior vice president of technology, Flow International Corp. (Kent, WA). "Therefore, a peak-performance point exists for the abrasive flow rate." Most builders have developed software that calculates the optimal cutting parameters automatically for that point for most applications.

Another step that builders have taken to decrease distortion and increase cutting speed has been to develop software that anticipates the behavior of the jet in the cut and compensates for it by slowing the cutting speed and titling the nozzle as the machine follows contours and rounds corners. "First, the tool moves fast where it can and slow where it must in a manner that is optimized far beyond what a human can do," says Olsen at Omax. "Second, predicting the taper in the cut and tilting the nozzle to remove it has increased the precision of the process, allowing many parts to be made with little or no secondary processing."

 Besides needing the software that can look ahead and take corrective action, the ability to tilt the nozzle also demands a motion system capable of executing the software's commands. Most builders have equipped their conventional two-dimensional machines with three-dimensional wrists that can adjust the angle of the nozzle on the fly for the geometry and thickness of the part as it follows the cutting path.

 "Simply put, the cutting head is tilted by a few degrees to take all the taper off the 'good side' of the part," explains Hashish at Flow. "Inside corners improve because the entrance angle and exit point are altered to ensure the stream does whatever it must while in the material to produce an accurate corner with minimal speed reduction."               

He credits this ability with not only eliminating taper within certain operating conditions, but also increasing cutting speeds by 25 - 400%, and widening the range of material thickness that a waterjet machine can cut precisely. Today's waterjets can cut soft material such as ceramic fiber at 100 - 500 ipm (2.5 - 12.7 m/min), and common metals such as aluminum at 70 - 80 ipm (1.8 - 2.0 m/min). Although Hashish knows of machines cutting dense, rigid materials (as opposed to foam and other flexible materials) as thick as 15" (381 mm), he says that 8" (203 mm) is usually the practical limit for cutting steel and titanium in production. He and his colleagues at other builders put the sweet spot for waterjet cutting at 2" (51-mm) thick and thinner.        

Because dimensional tolerances and surface finishes depend ultimately on thickness and cutting speed, tilting the nozzle can compensate only so much. "On small parts, it is common today to hold ±0.001", and on larger parts, one can hold ±0.005" [0.13 mm] quite easily," says Olsen at Omax. "A very rough rule of thumb is that you can go from a terrible finish that just barely cuts through to a very smooth finish--the best that can be achieved--by lowering the cutting speed by a factor of five or six."

Hashish at Flow describes the typical finished smooth, sandblasted edges as usually ranging from Ra 60 - 250-µin. (0.0015 - 0.006 mm) when cut with 50, 80, or 120-grit abrasive. "The most common operating parameters are 1.0 gpm [3.8 L/min] of 60,000 psi water and approximately 1.2 lb/min [0.55 kg/min] of 80-mesh garnet abrasive," he says. Because surface quality depends ultimately on cutting speed, he stresses the need to determine how fast you can cut to produce an acceptable part at the least cost.

EDMs work differently. Their cutting tools are electric sparks, which each machine produces and controls by means of a generator. Electricity flows from the generator to an electrode which, as the name implies, is a wire for wire machines. Because the electrode is not quite touching the workpiece, the electrical charge must build to a voltage that allows it to jump the carefully controlled gap, and zap off a small piece of the work material. The electrolyte not only provides the resistance for building the spark and a path to the workpiece, but also flushes away the eroded particles to prevent them from being targets for the next spark.

Sophisticated software adjusts the gap and various parameters in the generator in real time to produce a spark with the desired characteristics. For roughing cuts, it applies more power to blast away bigger chucks of material and, therefore, cut faster. For finishing, it lowers the power to erode away smaller particles, which produces a finer finish. Less energy in the spark also means less heat; thus smaller heat-affected zones.

The most important technological advances over the last few years continue to be in the generator. "Everybody is going to higher-speed generators that cut faster and make the machines more productive," says Mike Bystrek, manager of wire research and development, Mitsubishi EDM Div., MC Machinery Systems Inc. (Wood Dale, IL). "In our latest generator, for example, the waveform itself is manipulated in such a fashion that it sustains peak amplitudes longer, which gives us faster cutting speeds." Because the sparks last longer, they can remove more material, as much as 47 in.2/hr (303 cm2/hr) in some grades of steel.

Other manufacturers are reporting similar results with their latest generators, noting that cutting speeds depend on the material being cut. In general, EDMs cut aluminum faster than steel and steel faster than carbide. "In aluminum, 172 in.2/hr [1109 cm2/hr] is now possible in 4.8" [122-mm] tall parts," says Ledvon at Charmilles.

He and others note that other innovations also have helped to boost the cutting speeds of EDMs. Ledvon points to the twin-wire machines that use two wires, each with a different diameter, to increase productivity. Various builders also supplement their new generators and adaptive controls by fitting their machines with either linear motors or the latest high-speed servomotors to make their machines more responsive and, thus, faster.

Of course, there are tradeoffs for cutting at these high speeds. According to Bystrek at Mitsubishi, surface finish is around Ra 125 µin. (3 µm) at 47 in.2/hr in steel, rather than the 90 to 95 µin. (2.3 - 2.4 µm) that was more typical at the old maximum roughing speeds. Moreover, the heat-affected zone is probably 0.001 - 0.0015" (0.03 - 0.04-mm) thick. Users wanting better finishes and smaller heat-affected zones can produce them in the same way as they did before-by taking a series of semifinishing and finishing cuts at successively lower power settings. With this technique, EDMs are capable of producing heat-affected zones of less than 0.0001" (2.5 µm) and surface finishes as fine as Ra 1 µin. (0.03 µm).

Such was the case for a kit of carbide gages that Makino Inc. (Mason, OH) made as samples to demonstrate the capability of one of its high-end machines. "A roughing pass produced an Ra 40-µin. [1-µm] finish, which is okay for a lot of general mold work," says Jeff Kiszonas, EDM product manager. "We did one skim pass to get down to 25 µin. [0.6 µm]." After five more finishing passes, the machine produced a 1-µin. finish. Although the finish is not of mirror quality, it reflects well-defined shadows.

Although parts cost more to produce on an EDM than on a waterjet, because it's so much slower, an EDM actually costs roughly the same to buy as a waterjet and less to run. Price tags on new wire EDMs start as low as $90,000 and can exceed $500,000, depending on their accuracy ratings and the amount of automation on them. "For 80 - 90% of the applications, though, the initial cost is $100,000 - $150,000," says Bystrek at Mitsubishi. Capital costs for a waterjet, on the other hand, range from $40,000 at the low end, and can exceed $250,000 for a large and fully loaded machine, according to Olsen at Omax.

Most EDM builders estimate the operating costs of their machines at approximately $4/hr. The actual figure, however, will depend on the application and the type of wire used, because wire runs from $3.50 to $14.00 a pound, depending on material, size, and quality. Moreover, Makino's Kiszonas says consumption varies on similar machines made by different manufacturers by as much as 30%.                       

Besides the wire and electricity, consumable items include the wire guides, flushing nozzles, filters, and resin for deionizing the water used as the electrolyte. Diamond guides usually cost $500 - $1000 to replace, but last a long time. "We guarantee our wire guides to last 12,000 hr," says Kiszonas. "If you're just running one shift a day and assume one shift worth of work to be 2000 hour in a year, the guides should last at least six years." Because ceramic guides are triple the price, he recommends them for few jobs.

Operating costs are quite a bit higher on a waterjet, running between $25 and $35 per hour for each nozzle. "This breaks down as roughly 50% for abrasive, 25% for nozzle wear items, and the remainder for utilities and pump maintenance," says Omax's Olsen.

Hill at Chicago Waterjet says that everything touched by the water--from the intakes on the motors to the jet nozzle--will eventually wear and need replacing. "Your biggest consumables will be water, abrasive, abrasive nozzles, sapphire [orifices], and high-pressure seals," adds Cisar at Bystronic. Other variables, such as labor, floor space, electricity, and local disposal regulations, also contribute to the cost, causing operating costs to vary by region.

Engineers at most builders have made a number of design changes to their companies' machines to keep operating costs low. For example, they have simplified connections on the cutting head to streamline the replacement of consumable items. "The seal life in intensifier pumps also has increased over the years," notes Robbie Smallwood, waterjet product manager at ESAB Welding & Cutting Products (Florence, SC). "This results in less downtime and reduced cost per hour."

Another way to contain cost per piece is to stack several sheets on one another and cut identical pieces from them at the same time. "For wire EDM, the cutting speeds [for stacks] are the same as a solid piece of material," says Ledvon at Charmilles. Consequently, the per-part time is drastically less when you divide the cycle time by the amount of plates. The only requirement is that the stack is solid and nothing in it moves.

Although waterjets can cut stacks of sheet materials, they do a good job of it only when conditions are right. For example, the material must not be brittle enough for the top layer to crack as the jet pierces it. Most importantly, "can the material be stacked so there are no air gaps between the layers?" asks Smallwood. "The jet will expand as it hits the air gap and will sand-blast and distort the bottom [side of the gap]." It also will lose energy and disperse, creating poor edge quality.

Consequently, the size of these gaps determines whether stacking will work. "The jet does not disperse significantly for air gaps a few thousandths of an inch thick [typical for materials that are stacked flat]," says Hashish. "If, however, the air gap between layers reaches a tenth of an inch [2.5 mm] or greater, inferior edge quality will be the result." One technique to solving the problem is to compress the edges with clamps, and another is to submerge the stack under water to fill the gap with water. Hashish recommends submerging stacks only if the parts will not float and interfere with the motion of the machine and cause damage.

The required accuracy and edge quality are other considerations for determining whether stacking will work in waterjet cutting. "Cutting stacks will take some of the accuracy away on the bottom piece or layer," says Smallwood at ESAB. Because the bottom layer might contain a taper or burr, experts recommend setting the cutting speed to produce the desired edge quality and tolerance on the part produced from the bottom layer of material.

They also note that the optimum height of the stacks usually falls between 0.25 and 0.375" (6.4 and 9.5 mm), but depends on the material and the shape of the part. "Parts with lots of sharp internal corners require slowing the jet in thicker parts, and therefore have a thinner optimum stack height than, for example, round disks," says Olsen at Omax. Hashish of Flow adds that the optimum thickness could be as great as 0.5" (12.7 mm) if the parts are small and loading is time-consuming.

Users of EDMs and waterjets must take special precautions to protect workers and the environment from both processes and their byproducts. In the case of EDMs, a process is necessary for disposing of the filters that remove the fines in the electrolyte to prevent the sparks from recutting the tiny chips into smaller pieces and slowing the process. Because various governmental agencies consider certain metals to be hazardous, users must follow the pertinent national and local regulations and contract a waste handling company that's certified to handle those materials.

Users of waterjets, however, must do more than follow regulations for disposing of fines and the abrasive used to cut them. They also must be more aggressive about circulating abrasive and then cleaning the machine than users of EDMs. "Removing used abrasive can be a time-consuming task," says Cisar at Bystronic. "But if it's allowed to settle and build up, it can be very difficult to remove." So he recommends cleaning the machine routinely.

Some users hire a service that can periodically send a truck to suck the sludge from the tank and haul it way. Others buy automatic equipment that removes the sludge from the cutting table continually. "These systems must be running all the time while you are cutting so the abrasive doesn't build up in the tank," notes ESAB's Smallwood. The garnet removal systems stir the water by pumping water back into the tank, thereby keeping the abrasive and kerf material in suspension. A centrifuge separates solids and dumps them into a hopper for disposal or recycling.

Besides properly disposing of the sludge, users of waterjets also must protect their workers from noise. "A waterjet running in open air can generate sound levels up to 130 dBa, a painful level," says Olsen at Omax. "For this reason, most 2-D plate cutters operate under water, so the sound is reduced to about 70 to 75 dBa. At this level, it's possible to carry on a quiet conversation right next to the machine." Cutting in air, on the other hand, requires isolating the machine from the rest of the shop, and giving the operator earplugs.

"Cutting under water helps not only with the noise but also with dust and spray caused by piercing and cutting," adds Smallwood. Another noise reduction device that he offers is bricks of corrugated plastic that dampen the noise. "The down side to the bricks is they are consumed and make a mess in the tank."

A second source of noise is the pump. According to Olsen, hydraulic pumps are loud and must be either well soundproofed or isolated in a separate room. A crankshaft-driven pump is quieter, operating at about 70 dBa.

Consequently, manufacturing engineers must put some thought into any decision to transfer work from an EDM to a waterjet. "Accuracy and surface roughness are the two primary differentiating factors between EDM and waterjet cutting," says Hashish at Flow. "If your application requires an extremely tight tolerance, EDM would be your best bet." He urges users to consider waterjet when speed and versatility are more important than accuracy because it can be as much as 50 times faster.

Although waterjets can be a perceived competitor of EDM, they are really complementary processes, and will remain so for the foreseeable future. Waterjets can free EDMs burdened with less accurate work to concentrate on work with tight tolerances.

 

This article was first published in the October 2005 edition of Manufacturing Engineering magazine.                     

 


Published Date : 10/1/2005

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