An Engineer’s Guide to Waterjet Cutting

      Parts cut using abrasive waterjets. (Images courtesy of Jet Edge, Omax and Flow International.)You don’t get much closer to the spirit of engineering than in the beginnings of waterjet cutting.

  “I got started years ago, in about ’71,” said Dr. John Olsen, one of the originators of waterjet technology and currently VP of operations at Omax Abrasive Waterjets. “I had been reading about some experiments done on rock cutting in England and a friend of mine and I thought it would be fun to try and build a pump and cut something. That was a kind of back-alley operation; it was in my garage and his garage.”

  Dr. John Olsen holding a tilting head waterjet with two linear actuators. (Image courtesy of Omax.)It might sound like many of the stories you hear about start-ups in Silicon Valley today, but the connections between waterjet and computing technologies run deeper than that, as Dr. Olsen explained:

  “Oddly enough, one of the biggest changes that made abrasivejets practical was the advent of the PC. A jet is not a very rigid tool—it bends all over the place and makes taper and what-have-you. To make precision parts, you need quite a bit of computing power to predict what the shape of the jet will be so that you can compensate for it. At the time, we were told ‘Nobody will ever accept a PC on the factory floor. Doesn’t that sound funny today?”

  Waterjet Basics

  Pure vs. Abrasive Waterjet

Abrsaive waterjet nozzle. (Image courtesy of Omax.)

  Abrasive waterjet nozzle. (Image courtesy of Omax.)In the broadest sense, the term ‘waterjet’ encompasses any cutting tool that utilizes a high-pressure stream of water. More specifically, waterjets can be divided into pure and abrasive subcategories.

  The term ‘pure waterjet’ refers to cutting tools that use only water, while the term ‘abrasive waterjet’ or sometimes just ‘abrasivejet’ refers to—as you might have guessed—waterjets that use an abrasive to accelerate the cutting process.

  Pure waterjet is used for cutting softer materials, including gasket, foam, food, paper, plastic and carpet.

  Abrasive waterjet is used to cut harder materials, such as metal, ceramic, stone, wood and glass.

  Waterjet abrasives are typically made of garnet, with grit size ranging from 50 to 220 mesh, though 80 is the most common. Many waterjet machines are capable of switching from pure waterjet cutting to abrasive waterjet cutting, making them uniquely versatile.

  Waterjet Materials

  Versatility is one of the primary strengths of waterjet technology. To illustrate the sheer number of materials that can be cut using a waterjet, Chip Burnham, vice president of global marketing for Flow International Corporation, provided a list of materials in order of cutting speed, from slowest to fastest, for any given constant material thickness:

  Sample list of materials that can be cut on an abrasive waterjet in order of cutting speed from slowest to fastest.This list is by no means exhaustive and it’s actually easier to list the things that waterjet can’t cut.

  “There’s only a few things that I haven’t been able to cut in my career,” said Burnham.

  “One of them is tempered glass—although some people still do it, the tempered glass has to be resealed after because it has stresses in it and when you cut it with waterjet you relieve the stresses. I cut beautiful things out of tempered glass and astonished glassmakers in the early years of waterjet just to come back into the lab the next day and find it in a million pieces because it shattered overnight.”

  Scott Wirtanen, regional sales manager at Jet Edge, also emphasized the versatility of waterjet cutting: “Waterjet’s going to perform almost identically in almost any material. So the versatility of waterjet, both in materials and in thicknesses is unparalleled.”

  Waterjet Pressure

  If you’ve spent even a little time with waterjet engineers, you’re probably familiar with a longstanding topic of debate: how much of a difference does pressure make?

  One way to understand the sheer amount of pressure involved in waterjet cutting is to compare it with other water sources in terms of maximum pounds per square inch (psi).

Logarithmic scale comparing four sources of water in terms of upper pressure limits.

  Logarithmic scale comparing four sources of water in terms of upper pressure limits.

  As this graph makes clear, pressure can make an enormous difference—like the one between washing your hands and cutting them off. But the pressure variation within waterjet is much narrower, typically 60,000 - 90,000 psi.

  Some engineers argue that higher pressure is the key to faster cutting, while others contend that what really matters is motor efficiency.

  In order to give both sides a fair shake, let’s take a moment to consider their respective arguments.

  90,000psi vs 60,000psi

  The 90k advocates point out that increasing the waterjet pump pressure from 60,000 psi to 90,000 psi increases the velocity of the stream, which they argue increases cutting speed by 50 percent or more, depending on the application.

流量马赫4c喷水切割碳纤维。 (图片由Flow International提供)

  A Flow Mach 4c Dynamic XD waterjet cutting carbon fiber. (Image courtesy of Flow International.)They also claim that cutting at 90,000 psi reduces abrasive consumption, since each garnet particle is imparted with more energy. Finally, the higher working pressure when piercing and cutting is supposed to reduce delamination for composite materials.

  “Cut speed is tied directly to nozzle horsepower,” said Burnham. “So horsepower consumed is a function of the amount of water that’s exiting the orifice and the pressure. A small orifice with low pressure consumes very little power and cuts slowly. In contrast, a larger orifice with high pressure will consume a lot of power and cut more quickly. It’s the amount of water and the pressure that is demanding the horsepower.”

  Wirtanen agrees, stating that if you were to use the same orifice-nozzle combination, the same amount of abrasive and the same volume of water, 90,000 psi will still outperform 60,000 psi every time.

  “However,” he added, “the area can be a bit clouded when you start to look at dual cutting heads using one pump. For example, a 60,000 psi intensifier pump at 100 horsepower will produce two gallons per minute. A hundred horsepower pump at 90,000 psi will put out approximately 1.4 or 1.5 gallons per minute. So the ability to drive dual cutting heads at that point is limited on the 90,000 psi pump because of the GPM that it’s capable of.”

双头马赫4c水刀。 (图片由Flow International提供)

  A dual-head Mach 4c waterjet. (Image courtesy of Flow International.)

  Wirtanen continued, “At 1.4 or 1.5 GPM with a 90,000 psi, 100 horsepower pump, you’re able to run at max pressure through 0.011” orifices. A 100 horsepower, 60,000 psi pump can run through 0.015” orifices. So because of that difference driven by the GPM capability on either pump, you’re almost cutting at the same cutting speed with dual heads.”

  In reply to these claims, 60k supporters point out that if you increase pressure then you need to reduce the size of the nozzle. This is based on the fact that power is proportional to pressure times volume flowrate, as represented in the formula:

  Power = kPV

  Advocates of 90k might reply that the higher pressure, smaller volume nozzle will cut faster because it has higher power density (i.e., the same amount of power in a smaller area), but 60k supporters deny that this is true outside of pure waterjet applications.

  “One of the big issues is that if you want to pump at 90,000, then you’re stuck with an intensifier and an intensifier pumping system is so inefficient that you actually get more cutting power at lower pressure with a direct-drive pump,” said Olsen. “So the efficiency trumps pressure.”

  This brings up an important distinction in waterjet and another controversial topic: intensifiers and direct drive pumps.

  Waterjet Pumps: Intensifier vs Direct Drive

  Schematic of a hydraulic intensifier pump. (Image courtesy of Jet Edge.)There are two basic types of pumps in waterjet: direct drive and intensifier. Direct drive pumps use a crankshaft to move the plungers that pressurize the water, whereas intensifiers use hydraulic rams.

  Each type has advantages and disadvantages, as Wirtanen explained:

  “Direct drive pumps are inherently a simpler design, but do require a dramatically higher amount of maintenance than an intensifier pump. Because they’re a simpler designer, they’re less expensive for an initial investment, but in the long run, the intensifier pumps have a dramatically lower cost of ownership.”

直驱泵示意图  (图片由Jet Edge提供)

  Schematic of a direct drive pump. (Image courtesy of Jet Edge.)Hence, if the initial investment is your primary concern, then a direct drive pump is the way to go. On the other hand, if you’re looking for the lowest maintenance cost, an intensifier pump is the better option. This illustrates one of the basic tenets of manufacturing: it all comes down to your particular application.

  Waterjet Costs

  As with any fabrication technology, there are multiple ways to calculate waterjet costs. However, some provide a more accurate estimate than others. For example, trying to calculate costs in terms of dollars per hour of machine time is problematic because waterjets can be configured with single or multiple cutting heads, which affects cutting times.

厚厚的泡沫部件由流动马赫4c动态XD切割。 (图片由Flow International提供)

  Thick foam parts cut by a Flow Mach 4c Dynamic XD waterjet. (Image courtesy of Flow International.)The typical price range for a waterjet with a single cutting head is approximately USD$100-135 per hour, though high-end parts can run up to $2,000 per hour depending on material type and thickness as well as part geometry. These same three factors are what makes it difficult to calculate waterjet costs in terms of dollars per square inch of material cut, since they all contribute to longer cutting times.

  The best approach to calculating waterjet costs is in terms of dollars per part. However, there is a large list of factors that go into such calculations, including:

  The time it takes to program the toolpath

  Risk of breakage with fragile material

  How many times the material needs to be pierced

  Cost of consumables

  Setup, unloading and maintenance time

  Order quantity

  Common Misconceptions about Waterjet Cutting

Cutting head on a MAXIEM 1515 JetMachining Center. (Image courtesy of Omax.)

  Cutting head on a MAXIEM 1515 JetMachining Center. (Image courtesy of Omax.)“When they began, they were kind of the tools of last resort,” explained Dr. Olsen. “The jet was not a very rigid, precise cutting tool and so early abrasive jets—say, in the ‘80s—were used if you had absolutely no other possible way to cut the material. Then you would pick an abrasive jet and expect to have a very imprecise cut, almost like oxy acetylene burning. Because waterjets started with that application, it’s been a long struggle to get them accepted as a precise machining technique.”

  Another common misconception about waterjets concerns their ability to cut thicker materials, as Burnham explained: “People say that water and abrasive can’t cut thick [sections], but we can cut through over a foot thick in pretty much any common material. We’ll have to cut slower, but it does have the ability to cut thick."

A thick gear cut with waterjet. (Image courtesy of JetEdge.)

  A thick steel gear cut with waterjet. (Image courtesy of JetEdge.)“I have people who are saving weeks of cutting by roughing out material that is 24” thick,” Burnham added. “They rough it out and then put it on a mill and by roughing it out save weeks of milling time, but that’s not common—most cutting these days is three inches or less.”

  Of course, the fact that you can save time by roughing with waterjet doesn’t mean that it’s a particularly fast cutting method compared to plasma or laser cutting.

  “A lot of customers will expect to cut at the same speeds as a laser or plasma,” Wirtanen observed. “Waterjet is definitely slower, so the importance of dual cutting heads or three of four cutting heads certainly can’t be overlooked.”

  This brings us to the inevitable comparisons between waterjet and other cutting technologies, including laser, flame and plasma cutting. Although each cutting method has its own advantages and disadvantages, waterjet has often been overlooked despite the benefits it offers.

  “That’s beginning to change now,” said Dr. Olsen, “but for at least ten years I would say that was the case. But it’s beginning to change because it’s become popular and it’s now used as a first operation for almost anything a person would want to make. A lot of shops are using waterjet that way.”

  So what are the advantages of waterjet?

  Advantages of Waterjet Cutting

  Waterjet has two main advantages compared to other cutting techniques: versatility and simplicity.

  Its versatility is illustrated by the sheer variety of materials and thicknesses that can be cut on a waterjet, but its simplicity is best illustrated by comparing waterjet to laser, plasma and flame cutting.

  Waterjet vs. Laser

Laser cutting.

  Laser cutting.In laser cutting, a focused beam of light is used to melt, burn or vaporize the material being cut. The laser can be static, with the material moving underneath it, or it can move across the material which remains fixed in place. In the latter case, additional optics are required to compensate for changes in distance from the emitting end of the laser.

  Although laser cutting is often seen as complimentary to waterjet, with many shops using both technologies, the latter does have some distinct advantages over the former.

  “In comparison to a laser, for example, when you look at titanium or stainless steel or aluminium, there’s limited capacity with regard to thickness. I think the max thickness in general is around an inch and a quarter to an inch and a half,” said Wirtanen.

  “We’ve cut through eight- or nine-inch-thick material. When you look at a laser’s capabilities, it’s commonly understood that there’s difficulty in processing reflective materials, like aluminum or any of the yellow metals like bronze or copper,” he added.

  In other words, materials and material thicknesses that are difficult or impossible to cut with a laser tend to pose little difficulty for a waterjet. This ability to cut thick materials is one reason waterjet is a popular choice for roughing out parts—surface finish is another.

  “The beautiful thing about finishing a rough cut from a waterjet on a mill is that it’s a virgin surface that has gone through an erosion process,” said Burnham. “If you try to rough cut with plasma or a laser and then finish it on a mill, you’re removing work-hardened and heat-hardened material.”

  This brings up one of the biggest advantages of waterjet cutting, which is the absence of heat-affected zones (HAZ). This a problem for laser cutting and another competing fabrication technology: plasma cutting.

  Waterjet vs. Plasma

Plasma cutting.

  Plasma cutting.Plasma cutting operates on electrically conductive materials using an accelerated stream of gas which is superheated into plasma via an arc of electricity. The plasma melts the material while moving fast enough to blow molten metal away from the cutting area.

  Although it’s typically faster than waterjet, plasma cutting suffers when it comes to surface finish.

  “Plasma can outperform waterjet with regard to speed, but the cut quality is far inferior,” said Wirtanen. “With regard to edge quality, tolerance capability, and the secondary operations that are required with plasma, you don’t see those with waterjet.”

  The HAZ makes secondary operations for plasma (and laser) cutting even more problematic, since HAZs can harden surfaces and make them more brittle. This in turn makes it more difficult to complete secondary operations, like tapping or beveling.

  Another advantage waterjet has over plasma in particular is the ability to cut materials that don’t easily melt, like granite, or materials that are destroyed by melting, like laminates.

  Waterjet vs. Flame Cutting

Flame cutting.

  Flame cutting.Flame cutting, which is only used for iron and steel, involves heating metals to high temperatures and then introducing oxygen to melt the metal for the cut. The heated metal reacts to the oxygen and forms iron oxide, which has a lower melting point than the metal itself.

  Once again, the primary advantage waterjet has over this competing cutting technology is its ability to cut more than just iron and steel. Unlike flame cutting, which creates heat-affected zones, waterjet cutting does not appreciably heat the material being cut. Although it may get as hot as 120° F (49° C) during piercing, the material is only heated a few degrees during cutting.

  Waterjet cutting is also more precise than flame cutting and the former leaves an almost sandblasted finish, which is significantly smoother than the rough edges produced with flame cutting. The smaller kerf produced using waterjet is another advantage, particularly when cutting expensive materials.

  The Future of Waterjet

  Despite its versatility and simplicity, waterjet remains a relatively niche cutting technology. There are numerous reasons for this state of affairs, but Burnham emphasized one in particular:

  “What’s the most common hard material cut? It’s mild steel, which everything can cut—plasma can cut it, laser can cut it, punches, mills, routers, saws; everyone can cut mild steel. Moving up to plate, still a lot of processes can cut mild steel in plate. Basically, as you get away from the super-common materials into the more exotic materials, and you get a thicker, like up to a foot thick, then you’re getting into the realm where waterjet shines.”

Metal gears cut on a waterjet. (Image courtesy of Flow International.)

  Metal gears cut on a waterjet. (Image courtesy of Flow International.)This suggests that as manufacturers move into more exotic materials, the demand for waterjet will increase. So if you’re looking to get into waterjet cutting, where should you begin?

  “The biggest thing is to have test cuts made to be sure it does what you think it’s going to do,” Dr. Olsen advised. “You want to know how precise the part came out. Is the surface finish okay for my application? How long did it take?

  “Those things are best known by actually making a part that’s indicative of the kind of parts you intend to make. There are lots of things to watch out for. For example, if you’re making things out of carbon fiber and you want to cut that up, then you have to watch and learn about delamination upon piercing and have a way to minimize that,” he concluded.