The challenge in making an accurate part with an Abrasive Waterjet has always been the fact that the jet stream bends as it moves through the material being cut.   As shown in the figure below, the lower part of the jet lags behind and moves differently than the upper part and that movement is a complex function of the jet parameters (such as orifice size, water flow rate, pressure and abrasive feed rate), the material being cut (what it is and how thick it is) and the geometry of the desired part.

For example, to create an accurate inside corner careful attention must be paid to the rate of deceleration of the nozzle as it approaches the corner in order to assure that the lower part of the jet has caught up with the upper part of the jet at the exact instant that the corner point is reached.  Otherwise the corner will tend to be washed out.  Even more important, the acceleration of the nozzle as it leaves the corner must be carefully controlled to avoid the “kickback” effect that can leave a notch in the lower edge of the part and ruin its accuracy.  Similar issues apply to outside corners, holes and curved sections of a part.  In general, the more complex the part geometry and the thicker the material being cut, the more important the motion of the jet and the less important the straight-line cutting speed in determining part production time.  Indeed, in many complex parts the nozzle spends all of its time accelerating and decelerating and never even approaches its maximum straight-line speed.

At OMAX we have spent years studying and modeling the motion of the abrasivejet and have an ongoing research effort to improve that modeling and to develop control strategies and algorithms that allow our control software to optimize part production time.  Our software is continually being improved and upgraded to take advantage of this research.  As a result we have seen production times for many representative parts cut in half over the years by machines that have the same straight-line cutting speed that they have always had.  The improvement has come from they way we move the jet, not by using expensive power to increase its maximum speed.  When evaluating different AWJ systems it is critical that potential buyers do more than just compare maximum cutting speed.  You really owe it to yourself to have test parts made that represent the complexity and desired accuracy of the parts that you plan to make.  Then compare the production times to determine which machine is truly faster.

Best regards,

Dr. John Olsen

Fatigue is a term used to describe the failure mechanism of metals that are subjected to cyclical stress. Cyclical stresses cause very small cracks to form and then grow until the resulting large crack weakens the structure so much that a sudden failure occurs. It is as if the metal became fatigued and gave up. You can witness such a failure in just a few cycles by bending a paper clip back and forth until it breaks.

Every time a pressure-loaded component in a waterjet system is pressurized and then un-pressured it goes through a stress cycle.  If a high pressure component is subjected to its full rated pressure and allowed to remain at that pressure, its life will be virtually infinite.  However, if its pressure is cycled on and off over and over again, it will most certainly fail. The number of cycles required for failure is determined by how close the cyclical stress is to the ultimate strength of the material. The S-N diagram (Stress vs Number of cycles before failure) shown below illustrates this for a typical steel.   When the stress level for each cycle is close to the rated maximum stress of the material, the component may fail after just a few thousand cycles.  However, as the cyclical stress is reduced, the average number of allowable stress cycles before failure increases dramatically.  Indeed if the cyclical stress level is reduced to a level below what is known as the Endurance Limit, the allowable stress cycles before failure in steels become virtually infinite and the fatigue problem goes away.

Generalized S-N Curve for Steels

What can we do to minimize the problem of fatigue?  One trick used by manufacturers of ultra-high pressure components is a manufacturing process known as autofrettage.  This technique was originally developed to strengthen large gun barrels on battleships (an application in which each shot fired represents a stress cycle and where a fatigue failure can be particularly catastrophic!).  Under this technique residual compressive stresses are intentionally created in the inside surface of a hollow cylinder (such as a pump barrel or a length of ultra-high pressure tubing).  These residual stresses have the effect of reducing the effective net cyclical loading and thus increasing the number of allowable stress cycles before failure.  Unfortunately this technique also increases the cost of these components.  In addition, it is not really practical for the complex shapes found in such components as fittings, valves, nozzle bodies, etc.

By far the most effective way to reduce fatigue stress and increase component life is to simply operate the system at lower pressure.  As one can see on the S-N curve, a 30% reduction in cyclical stress can increase the allowable number of stress cycles by a factor of 8 or more.  The primary reason that fatigue has become such an issue in ultra-high pressure waterjet systems is that users want to increase their productivity and are doing this by raising the operating pressures to increase the power at the cutting nozzle.  However, as I have discussed in previous entries to this blog, power at the nozzle can also be increased by simply increasing the flow rate. With greater flow rate, as opposed to greater pressure, higher cutting power can still be realized without the drawback of decreased component fatigue life.  This results in lower maintenance costs and greater system reliability, which in turn means more cost-effective operation and less unexpected downtime.  Indeed, unexpected downtime is the worst part of fatigue.  That nice smooth S-N diagram is statistically correct and can accurately predict the average cycles to failure for a large sample of components.  However, for an individual component the average doesn’t have much meaning.  The slightest scratch or imperfection in a component can speed the initiation of cracks and reduce the fatigue life below the average.  You can be sure the component will fail from fatigue, but you never really know quite when.  That can really mess up your production schedule.

Best regards,

John Olsen

Reference

Elements of Material Science, Second Edition by Lawrence H. Van Vlack,
published by Addison-Wesley Publishing Company in 1964

The abrasive waterjet cutting mechanism is similar to a grinding wheel.  The abrasive particles are moved by the water jet rather than the wheel and chip out small pieces just like the abrasive particles in the wheel does.  By using a hard abrasive such as garnet, an abrasive waterjet can cut a very wide range of metals and other hard materials and thus become a useful technology for machine and fabricating shops.

The key thing to remember is that in an abrasive waterjet the abrasive particles are doing the cutting, not the water.  The role of the water is to entrain the abrasive particles that are introduced at the nozzle and accelerate enough of them to a high enough speed to cut effectively.  Faster cutting is achieved by increasing the speed of the particles and/or by increasing the number of particles entrained in the water stream.

Interestingly enough, this gets us right back to the two basic ways of increasing hydraulic horsepower at the nozzle discussed in my entry of last November– increase pressure or increase water flow rate.  Increased pressure increases the speed of the water passing through the nozzle and has the potential to increase the speed of the entrained abrasive particles.  Increased water flow rate increases the ability of the water stream to entrain more abrasive and thus create a cutting jet with more “teeth”.

Either approach can increase cutting speed and either approach has associated costs.  The “more flow” approach results in an increase in consumption of water and abrasive, but at a known and predictable cost.  The “more pressure” approach results in higher maintenance costs, increased unpredictability of high-pressure component life and reduced system reliability.

Best regards,

John Olsen

1. The pump is of low efficiency and a big motor is really required.
2. The motor is larger than required for marketing or manufacturing reasons.

For the first case, I have written some fairly detailed articles about the difference in efficiency between direct-drive pumps and hydraulic intensifier pumps and how a direct-drive pump with a smaller motor can often outperform a hydraulic intensifier with a larger motor. (Refer to such an article at www.omax.com under “products”/ “direct-drive pumps” if you really want to get into it!). However the source of confusion goes even deeper than just the difference in these two types of pumps. Indeed, pumps of the same basic type can have different levels of efficiency depending on the details of how they are designed and manufactured. When the pump designer determined what size motor he would need to drive his pump he had to take that efficiency into account. He (or she!) started by knowing the desired effective power at the nozzle, based on pressure and flow rate. Then he had to add the power wasted by the inefficiency of his pump design and drive system (mechanical or hydraulic). Finally he had to select a motor that would do the job.

For the second case, the Manufacturing Manager has some input on motor selection because he wants a motor size that is readily available from suppliers or perhaps one that is already stocked for some other purpose. It is not uncommon for a pump manufacturer to use a drive motor that is actually larger than really needed. Moreover, the Marketing folks love it because it gives them “power bragging rights” over other pump manufacturers. Unfortunately the actual user may pay for those bragging rights with added electric utility demand charges and no real performance benefit.

The end result of all of this is that the final selected motor size may be substantially larger than the net effective power delivered to the nozzle. As I mentioned in my last entry, output pressure and flow rate are the true indicators of pump power, not the size of the motor.

Best regards,

John Olsen

Regardless of the size or type of pump or the size of its drive motor, the real measure of power output is the power at the waterjet nozzle. This is a direct function of the nozzle pressure and the volume flow rate through the nozzle, which can be expressed by the following formula:

HP = 0.58PQ

Where:

1. HP equals the hydraulic power actually delivered through the nozzle in units of horsepower

2. P is the water pressure at the nozzle in units of thousands of pounds per square inch  (for example, use 55 for 55,000 psi). This can usually be closely approximated by the pump output pressure, but watch out for systems that try to operate relatively large nozzle orifices (say greater that 0.014”) using relatively long runs of ¼” ultra-high pressure tubing with many fittings.  The pressure drop between the pump and the nozzle for such systems can be several thousand psi.
3. Q is the volume flow rate through the nozzle, in units of gallons per minute
4. The constant of 0.58 accounts for the units of measure being used in the equation.

This simple equation makes two things very clear:

1. The size of pump motor and the exact design and brand of pump are not in the equation.  All that really matters in determining true nozzle power are the nozzle pressure and the volume flow rate
2. Both pressure and volume flow rate are in the equation and have equal effect.  Power at the nozzle can be increased by increasing pressure or increasing volume flow rate or a combination of both.

So the next time you are trying to compare ultra-high pressure pumps, ignore the size of the drive motor shown on the manufacturer’s spec sheet.  Go further down the spec sheet and find the values of the recommended continuous operating pressure and the corresponding output volume flow rate.  Then grab your calculator and determine for yourself the actual effective output power.

Best regards,

John Olsen