Since the inception of waterjet technology nearly 50 years ago, there has been an ongoing argument concerning what combination of pressure and power results in optimal cutting performance. Do bigger numbers translate into better or faster cutting? What combination of pressure, horsepower, and nozzle assembly is best for a given application? What does all this really mean?
First, a Primer
To frame the discussion, let’s remove the intensifier versus direct-drive pumps argument. If you’ve ever investigated purchasing a waterjet system, you’ve probably been hit with an onslaught of marketing and sales data showing the benefits of each. Hydraulic intensifier pumps can deliver exceptionally high pressures at the cost of an energy-intensive hydraulic system. Others have advocated direct-drive systems that use a mechanical crankshaft pump (see Figure 1).
In previous decades a trade-off between these technologies existed. Intensifier pumps were considered easier and cheaper to maintain, especially at high pressures, while direct-drive systems offered higher energy efficiency. The technology has evolved and the trade-offs have changed over the years.
Regardless, the basic principles behind waterjet cutting haven’t changed. The nozzle/orifice combination assists in pressurizing the water as it is squeezed from the high-pressure piping through an opening measured in hundredths of an inch. Passing through a small-diameter orifice, the water forms a coherent jet of water that then passes through a venturi nozzle, where a metered amount of granular abrasive is drawn into the water stream. The mixture of water and abrasive particles passes through a special ceramic mixing tube, and the resulting abrasive/water slurry exits the nozzle as a coherent cutting stream of abrasive particles traveling at very high speed.
Abrasives cut only when they successfully reach the material. The abrasive’s mesh must be the correct size for the orifice to avoid clogging. Eighty-mesh garnet is most universal between nozzle sizes, whereas 50-mesh garnet is much coarser and is typically used with larger-diameter orifices such as 0.022 or 0.020 in. Using a narrower nozzle with 50-mesh garnet will increase the likelihood of clogs. For smaller nozzles used for high-precision applications, such as 0.014- or 0.010-in. nozzles, a mesh of 120 or higher is optimal.
Nozzle size is not the only factor that determines the ideal mesh size for a given application. Much like sandpaper, finer surface finishes require higher, more fine-grained mesh sizes. A 220-mesh garnet will provide smoother, more accurate finishes over 80 mesh, especially when cutting thin material.
Understanding Waterjet Pressure
Pressure is determined by the volume of water being pushed through a nozzle orifice by a pump (see Figure 2). The smaller the orifice, the higher the pressure. Hypothetically, with a 100-HP pump and a wide orifice, you could max out your waterjet at 30,000 PSI—but no OEM sells anything like this because it isn’t effective. On the flip side of the argument, it is possible to achieve 60,000 PSI with a 5-HP pump, but the applications are severely limited, and the orifice would be absurdly narrow.
Power is proportional to pressure times volume flow rate (P = kp × V). For a given pump power, any increase in pressure must be matched by a proportional decrease in volume flow rate. This means that a higher-pressure pump must use a nozzle with a smaller orifice. For example, a 50-HP intensifier pump with a 0.014-in. nozzle orifice at 60 KSI is constrained to a 0.010-in. orifice at 90 KSI.
For pure waterjet applications performed without abrasives, more pressure may lead to faster cutting. In fact, the smaller diameter of the jet that comes from a high-pressure system may be more effective in water-only cutting applications, such as food products or foam rubber. In abrasive waterjet cutting systems, however, the abrasive does the cutting, not the water. Instead, the water accelerates small abrasive particles in a coherent stream that can erode the material being cut.
From 10,000 to 60,000 PSI, abrasive waterjet cutting speed increases steadily. Finish and accuracy also improve because the higher PSI focuses the particles at a single point. At higher pressures, however, the direct relationship between PSI and cutting speed begins to break down.
Send us a message and one of our customer service representatives will contact you soon.Contact ×