Why Velocity Matters

Pressures above 75,000 PSI can improve waterjet cutting productivity

Flow Waterjet dynamic cutting head

Dynamic Waterjet® XD integrates 3-D functionality with waterjet taper compensation, resulting in more accurately cut parts. Photo courtesy of Flow Corp.

Waterjet cutting has proven itself as an effective cutting process and has been one of the fastest-growing, nontraditional machine tool processes in the world for the past 20 years. There are many reasons for its rapid acceptance in metal manufacturing.

It is a cold-cutting process that can cut nearly every material without adding heat or stress, and it is extremely easy to set up and operate.

These advantages allow manufacturers to produce small or large batches of parts quickly for even their toughest projects.

Advancements in pump technology have enabled the creation of ultrahigh-pressure (UHP) jet streams. This highly pressured water increases the speed at which abrasive hits the material, making higher cutting speeds and faster travel rates possible.

High Pressure vs. Lower Pressure

When UHP waterjets were first introduced, pressures in the range of 36,000 PSI were fairly common. Every decade since, pressure has increased, moving to 55,000 PSI by the end of the 1980s, and reaching a standard of 60,000 PSI by the mid-1990s.

In 2004 Flow Intl. Corp. introduced the HyperJet pump, which is rated at 94,000 PSI, and it entered into the era of what it calls hyperpressure. Hyperpressure is defined simply as pressure that 75,000 PSI or greater.

With abrasive waterjet cutting, it is actually the abrasive particles within the waterjet stream that erode the material and make the separation cut. The water is the abrasive’s accelerator. Higher-pressure water increases the kinetic energy of the abrasive particles contained within it, and as the water and abrasive particles move faster, the jet diameter becomes smaller, but the jet’s power density and efficiency increase. (see Figure 1).

Manufacturers quickly discovered that when compared to other pumps that operate at approximately 60,000 PSI, these 75,000+ PSI pumps improve productivity. Just as increasing wattage increases CO2 laser cutting productivity, increasing pressure significantly improves waterjet productivity.

Many other alternatives have been attempted to improve productivity. The four main options were:

1. Increasing horsepower.
2. Using multiple cutting heads.
3. Using more aggressive abrasives.
4. Optimizing toolpaths.

Of these, the only improvement that makes large productivity gains is optimizing the toolpaths. Today’s advanced waterjet machine tools have toolpath optimization that speeds up on straight lines and slows down on tight geometry to control finished part anomalies caused by stream lag and also to shorten part cycle times.

waterjet jet stream speed chart

Figure 1. Over the past decades, advancements in pump technology have enabled the creation of ultrahigh-pressure (UHP) jet streams.

Even more advanced systems have taper compensation in which an articulated wrist tilts the head over slightly to compensate for the naturally occurring V-shaped taper produced by waterjet cutting.

The other attempts did not produce efficiency gains for a number of reasons.

More horsepower allows the waterjet to cut faster but demands a proportional amount of additional abrasive, driving up costs. Adding heads splits the power between the heads, doing little for throughput and requiring the operator to ensure both are cutting at precisely the same level. Using more aggressive abrasive drives up operating costs and erodes the mixing tube nozzle more rapidly (five to 10 times faster).

Raising the pressure, however, will improve efficiency. Increasing pressure speeds up cutting and reduces cost per inch. At 60,000 PSI, garnet abrasive accounts for more than half of the machine’s operating cost. Running continuously at 87,000 PSI cuts the abrasive use to less than half. Pierce time—the amount of time to drill a start hole—is dramatically reduced as well.

Shorter cycle times mean more parts produced per hour and more jobs completed per day. Fixed costs such as building space, overhead, and equipment depreciation are covered faster, which adds to bottom-line profitability.

Higher pressure also enables greater cutting detail due to the smaller stream diameter.

Velocity Matters

Pressure equals productivity and efficiency because of jet velocity. Why is that so? As pressure goes up, the speed of the stream increases. Once the stream exits the orifice, it’s all about velocity. There is no pressure in the waterjet stream once it exits the cutting head; the water pressure has been converted to velocity as the water exits the waterjet orifice.

A faster and smaller waterjet stream means the abrasive particles move faster, carry more momentum, and remove more material, more aggressively. Less abrasive is used per length of cut because each grain can erode more material. The goal is to make the abrasive go as fast as possible, and stream velocity is the key to this efficiency.

Raise Pressure, Not Horsepower

The only way to make a waterjet stream go faster is to raise pressure, but not by increasing horsepower. This might seem counterintuitive, but it is true. Every pump has a maximum operating pressure, so to gain the benefits of a higher-velocity stream, the pump must be designed to operate at higher pressure.

Dr. Mohamed Hashish, who led the team that invented the abrasive waterjet in 1979, has discussed the correlation of water velocity to pressure in numerous technical papers he authored over the decades.

waterjet cutting head abrasive speed

Figure 2. Abrasive speed is governed by water speed. Increasing power by increasing the pressure—and not the flow rate—creates higher velocity.

Figure 2 illustrates that key relationship. The abrasive is pulled into the cutting head from very low velocity via a Venturi effect (a way of creating suction) and then accelerated down the mixing tube by the supersonic water. Therefore, the abrasive speed is governed by the water speed. As Hashish said, “Increasing power by increasing the pressure, and not the flow rate, gives us the benefit of increased velocity.”

Here is an example of improved efficiency through velocity: The abrasive consumption of a 60,000-PSI pump running at 50 HP is the same as an 87,000-PSI pump running at 100 HP.

The difference is that with the higher-pressure pump, the stream—and thus the abrasive—travels much faster and cuts at approximately twice the speed. This means the garnet use, which is the main cost driver, is cut in half per minute and even less per inch, making the higher-pressure system more efficient than the lower-pressure pump. If the horsepower is constant, the higher-pressure pump uses nearly half the abrasive and cuts at a slightly faster speed than the 60,000 PSI-system—a definite improvement in efficiency.

The Future of Waterjet

Pressures have risen steadily throughout the history of waterjet technology. It is expected that that trend will continue with a 25 to 30 percent increase in pump pressures in the next five to 10 years. Waterjet stream velocity will go up accordingly, further increasing cutting efficiency.

There will also be a clear split in systems that are designed for higher productivity and lower productivity. Pressure will play a significant part in that split.

Since 80 percent of manufacturers will need higher productivity—and more importantly efficiency—to stay competitive, higher-pressure systems will continue to proliferate.