Things To Consider When Evaluating The Method
Over time, we’ve reviewed many different ways of cutting metal — for instance, just last month we talked about some of the pros and cons of wire EDM — but there are more cutting methods than the ones we usually address. Case in point: We recently realized we have never talked about the Grand Canyon of cutting methods, waterjet abrasive cutting.
Modern waterjet abrasive machining has been evolving over the past 30-plus years or so and includes some interesting techniques. You can certainly read more online from experts in the field — but for the moment, here are some of our observations about the waterjet method.
What Is Waterjet Abrasive Cutting?
Waterjet cutting has its roots in using water (although at comparatively low pressure) to cut soft materials such as paper and even food, going back to the 1930’s. Over the years, high-pressure technology and equipment were developed and fine-tuned, and today waterjet abrasive machining is another of the techniques used for precision metal cutting. The classic applications for waterjet cutting are complex shapes cut out of large-scale metal or composite material sheets, including very thick ones.
Basically, an abrasive waterjet cutting machine uses a high-pressure stream of water in combination with an abrasive, directing the stream onto a narrow line on the workpiece and removing material by eroding it. The addition of a granular abrasive — primarily, garnet powder is used — boosts the cutting ability of the high-PSI water stream. By adding the abrasive to the water at the nozzle, the waterjet can be switched between water-only and water with abrasive as needed; this allows the machine to be used with water-only for positioning (also known in machining parlance as the “rapid”) and then with an abrasive for the workpiece.
The waterjet abrasive method is commonly grouped together with plasma cutting, oxy-fuel cutting, and laser cutting because of their ability to make complex shapes cut in the C/Y axes of a large, flat sheet of material. Plasma cutting has the lowest operating costs but is the most brutal and makes the largest kerf; however, it is indispensable for certain cutting settings. Oxy-fuel cutting is a chemical reaction often described as rapid controlled rusting, but despite that pejorative description, it produces a smaller kerf and smoother ends and edges. Abrasive waterjet cutting has the advantages of not generating heat and the ability the cut through non-metallics with an even smaller kerf, albeit with some complications as described below. Laser cutting, the main challenger to waterjet in appropriate applications, produces the smallest kerf among these four methods and, of course, cannot be summarized in one sentence!
Are There Problems With Waterjet Abrasive Cutting?
As with all precision metal cutting methods, waterjet abrasive machining has its trade-offs — benefits and sacrifices that should be taken into consideration when you are deciding whether to choose it as your cutting method. The following, in no particular order, are some of the potential issues.
Voids, Tubes, Bundles, and Honeycombs Are Problems
As with laser beams, the waterjet stream is most effective when it is most concentrated. Voids, as when cutting through tubing, will cause the stream to diffuse, rapidly losing its accuracy and cutting ability. Similarly, stranded or fibrous materials will act like honeycombs and can also “squirm” in the stream, refusing to be cut or resulting in a very poor cut.
The abrasive waterjet cutting nozzle exit needs to be just the right distance from the work piece that is being cut. The right standoff height to ensure optimal cutting is 1 mm (0.0394”) to 1.5 mm (0.0591”), but this creates two dilemmas. The first is a matter of practicality with 3D parts that simply don’t allow such proximity. The second, more universal consequence is that for precision dimensions, the waterjet kerf always has a taper shape. Remarkably, the taper varies, but it is always there.
Tolerance and Bend
That taper mentioned above can wreak havoc on precision tolerances, with conical and barrel effects that modern machines will compensate for by “tilting” the nozzle, which sacrifices one side of the cut in favor of the other. The grain structure of the material being cut combined with the nozzle direction of travel “leaving behind” the stream of water causes the cut surface to show a bending pattern similar to waving a garden hose back and forth. This bending causes a slope in the cut.
Oh, Your Nibs!
There is another situation analogous to why waterjet struggles to cut fibrous material, with some of the stream sliding around the material to be cut instead of going through the material: When the waterjet stream gets to the end of the cut path, part of the stream is now shooting through air, and like most things, the concentrated energy favors the path of least resistance. The can cause a defective end cut that is not a clean cut and is often called the nib.
Hazing of the Surface Finish
With waterjet abrasive garnet blasting away at a rate of 60,000 PSI or more, the garnet powder can rough up or matte the finish of any surrounding, exposed material. This hazing might be strictly cosmetic — or it may be functional if it affects the surface finish and the material’s Ra value. (You can read more about surface finish and how different processes affect it in our blog Why Use a Surface Finish Chart?)
So Much Abrasive Debris In So Little Time
Here’s an amazing fact about waterjet abrasive garnet: The average waterjet setting can produce 2 pounds of wasted garnet power for every minute of cutting. Of course, the amount of waste depends on the thickness of the cut, the material, and other factors. But as a rule of thumb, that means something that takes 15 minutes to cut would produce 30 pounds of abrasive garnet debris, mixed in with a whole lot of water. Thus, it becomes a major disposal issue.
Here’s another fact: During the waterjet abrasive cutting process, only about 5% of the garnet powder is actually performing cutting action; the balance is just part of the waterjet stream path. But interestingly, studies done in about 2000 found that after ejection, even though the garnet is not dulled from cutting, some of it is fractured. That makes it problematic to recycle those many pounds of garnet powder for reuse; though not dull, the fractured media would not deliver the proper cutting action.
Not Tougher Than Tungsten
The ability of waterjet abrasive machining to cut is driven by many variables, but principal among them are the PSI and the hardness of the abrasive particles. For example, items such as paper or food are cut by waterjet alone, not by abrasive waterjet. (After all, no one wants abrasive garnet powder in their cheese.) But when you are using abrasive powders to cut materials such as metals, waterjet cutting requires the metal being cut to be softer than the abrasive that is being fed into the nozzle as part of the abrasive stream.
For us at Metal Cutting Corporation, where we cut a lot of tungsten, one of the intriguing coincidences is that on the Mohs scale, tungsten and garnet are remarkably similar. While it is possible to cut tungsten using waterjet abrasive machining, it is challenging because of the fact that tungsten and garnet are so close in hardness.
To Waterjet Or Not To Waterjet?
For cutting complex shapes from a large sheet of material, there is a logic to why one might choose waterjet abrasive cutting — or perhaps even plasmas, oxy-fuel, or laser cutting. However, we leave that discussion to the experts in these methods. For applications such as 2-axis cut-off of small precision tubes, rods, and wires, other methods are often a better choice.
For tips on how to choose the best precision metal cutting method for your metal fabrication project, download our white paper download our white paper *Choose with Confidence: Comparing 2-Axis Precision Cutting Methods*.
This week’s blogger, Josh Jablons, is the President of Metal Cutting Corporation.