April 15, 2002
Abrasive waterjet cutting may or may not be right for your particular application; but, knowing all you can about the process and its pros and cons can help you make that decision better.
"If you need a machine and don't buy it, then you will ultimately find you have paid for it but don't have it" — Henry Ford
A variety of machines are available on the market, so choosing the right machine for your particular application requires some study.
This article, the first in a series on purchasing an abrasive jet (waterjet) system, may help you decide whether the process itself is right for your application. Future articles will try to provide information to help you select among the available machines and will cover such topics as X-Y tables, pumps, accessories, and controllers.
Clean water is pumped through a fine nozzle at pressures from 40,000 to 60,000 pounds per square inch (PSI). The stream rushes through a carbide tube, where dry garnet abrasive is added. The water and garnet mix within the tube, and a high-speed, garnet-laden stream shoots out. The stream cuts by a process akin to grinding, the difference being that abrasives are moved by water rather than by a solid wheel.
The jet itself is moved by an X-Y mechanism over a table that supports the work piece. Pump power usually ranges from 20 to 100 HP, and table sizes are 2 square feet and larger.
Setup times on waterjet systems can be rapid. With this type of cutting, all shapes are made with a single tool, and no multitool qualification is required. In addition, cutting forces are low, so minimal fixturing and clamping are required. The process can be effective for short runs and one-of-a-kind prototype parts, as well as for high-volume production.
The jet can cut all metals without producing a heat-affected zone that interferes with welding or subsequent machining. Reflective metals, such as brass and copper, can be cut faster than steel. Materials with inclusions or other nonuniformities can be cut easily, too.
Appearance. The cut surface can range from a smooth sandblasted appearance to a rough, striated surface, depending on the speed at which the jet moves through the material.
At higher speeds, the jet wiggles from side to side within the cut, with greatest amplitude at the bottom of the cut. The cutting speed for a material usually is expressed in terms of the speed at which the jet can just barely sever the material. Then parts are made at various fractions of this speed, depending on the surface quality required (see Figure 1).
Jet Lag. Another complication is that the exit point of the jet on the bottom of the material lags the entry point on the top of the material. This situation produces errors when the jet executes a corner or tight radius. For straight-line cutting, the speed is limited by the side-to-side motion of the jet; for shape cutting, it is limited by the lag of the jet. With modern controllers, software handles these complications, and the only effect is that it takes longer to make the part.
Newcomers to abrasive jets often ask, "How fast can I cut this material?" intending to compare the process with something they know, such as oxyacetylene burning or sawing. As we have seen, the answer to this question is complicated and depends even on the shape being cut. The separation speed is given by the equation below:
Where: P = Stagnation pressure of the water jet in thousands of psi (Ksi), typically 50,000
d = Orifice diameter in inches, typically 0.014
Ma = Abrasive flow rate in lb./min. typically 0.8
fa = Abrasive factor (1.0 for garnet)
Q = Quality seen in Figure 1. Set to 1.0 to calculate separation speed
H = Material thickness in inches
Dm = Mixing tube diameter in inches, typically 0.030 to 0.040
V = Traverse speed in inches/min.
M = Machinability of material
|Machinability (M) of Various Materials|
|Hardened Tool Steel||
Real parts will be made at 10 percent to 50 percent of the separation speed, depending on the surface finish and corner qualities required. Note that as the thickness is doubled, the cutting speed is more than halved. In general, this means that stacking is not a good idea, because making a single part takes less than half the time of cutting a double thickness. However, for thin parts, the process may be limited by the top speed of the machine, and stacking can be effective up to a height of about 0.25 inches.
The jet diameter is from 0.020 to 0.050 in., giving a minimum part feature radius of half that amount. Very thin sections can be made, but the jet is not good at making skim cuts in which less than one jet diameter is to be removed. It also is difficult to make interrupted cuts such as those required for cutting both sides of a tube. When you are cutting one side of a tube, you will need a jet deflector to prevent damage to the other side.
Depth control is not good for making cuts that go only partway through the material. The best that can be expected is about +/- 20 percent of the depth being cut. You can make decorative grooves that do not sever the part, but precise depth control is not possible.
Even with these limitations, you still have a variety of design options. For instance, square and rectangular holes can be made to match with tabs on the mating part. This allows you to use a self-jigging assembly that requires less labor to assemble and increases precision. The tab then can be twisted, plug-welded, or drilled and tapped to make permanent or temporary assemblies.
That last design issues is heat-treating. With abrasive jet machining, it is possible to harden or heat the material first and then cut the part. Rockwell 60 tool steel can be cut at about the same rate as mild steel. Springs and flexures can be made directly from heat-treated steel.
Precision depends on the machine, the nozzle condition, and the cutting process.
In a precision machine with a perfect nozzle, the major errors come from the cutting process. The cutting process produces a slightly tapered angle in the kerf and, therefore, on the cut edge.
Surprisingly, the taper angle is greatest in thin materials. A steel part 2 in. thick may have only a 0.001-in. to 0.003-in. taper, while a 1/8-in.-thick part may have a 0.005- to 0.008- in. taper.
Five-axis machines can remove this taper with software, but these units are more expensive and more difficult to operate than two-axis machines.
As the nozzle wears, the stream diameter becomes larger, and the tool offset must be increased to compensate. This introduces the possibility of measurement and operator error. A very old nozzle may produce an elliptical stream for which you cannot compensate.
Precision errors may occur at lead-in and lead-out points, where a small projection or indentation may appear on the surface of the part. Generally, these errors are small and the lead-in can be placed at some unimportant portion of the part. For holes, the lead-in errors can be removed completely by tapping or reaming the hole. Holes can be made precisely enough to tap without secondary operations.
An exposed jet is noisy and throws a lot of abrasive dust, but these factors are eliminated by cutting underwater. An abrasive jet machine cutting under water can be placed anywhere that you might place a grinder. No noxious fumes or smoke is generated, and the part does not become contaminated with cutting oils.
The machine generates two waste streams—excess water containing very small amounts of solid fines, which usually is sent directly to a drain, and spent abrasives with metal slugs, which are sent to a landfill. If the material being cut is poisonous—lead or beryllium, for example—both waste streams must be cleaned or recycled.
An abrasive jet machine costs about $25 per hour to operate for consumables and maintenance parts. Cutting rates can be estimated from the equation given previously and. of course, vary with material and thickness. For example, -in.-thick stainless steel cuts at about 6 IPM with a good-quality edge. This translates to a cost of about 7 cents per inch. It would cost about $.70 to cut out a 3-in.-diameter disk or to form 10 holes with 5/16-in. diameter.
The higher the pressure, the faster the cutting and the more maintenance required. At pressures around 60,000 PSI maintenance costs skyrocket because these pressures cause stresses that exceed the endurance limits of the steels used for pressure containment. For this reason, jet cutting machines usually operate at 55,000 PSI or less. But even at these lower pressures, jet cutting machinery requires more maintenance than traditional machine tools. Operators tend to choose higher-pressure operation for the productivity gain and then live with the higher maintenance.
Maintenance items include all parts wetted by the high-pressure water and parts through which abrasive flows. Nozzle parts are replaced at 50- to 100-hour intervals, and pump seals are replaced at 300- to 1,000-hour intervals. New troubleshooting and maintenance techniques must be learned for successful operation of abrasive jet equipment, but the skills to be learned are not difficult, and thousands of successful machines are in operation.
Consider an abrasive jet if any of the items below are true: