In the first place, the listed X-Y accuracy of a cutting table is very often based on static measurements of X and Y displacement—the actual change in location of X or Y carriage along the X or Y axis compared to a change directed by the machine controller.  This may be an actual comparison made using a precise measuring device such as a laser to determine the true distance moved, or it may merely be the stated linear accuracy of the ball screw or other linear motion device used to move the cutting nozzle.  In either case, this approach fails to take into account any variation in location of the nozzle due to squareness, twist or straightness errors in the machine.  Other measurements may track the actual nozzle position vs. the commanded position for point to point moves. Such measurements of accuracy based on static conditions really do not give a good indication of the accuracy of the system while it is actually cutting and thus do not really predict the accuracy of the part being produced.  Additional errors occur when the machine is moving due to servo following errors, vibration and other motion related sources. A far better way to evaluate X-Y accuracy is to measure it using a dynamic test, such as the Renishaw Ball Bar ™.  This approach electronically measures the deviation of actual nozzle motion from that of a perfect programmed circle while the nozzle is moving at a rapid speed typical of an actual cut.  By performing such tests in a series of locations around the cutting table and system manufacturer (and the potential purchaser) can get a much more realistic evaluation of the true accuracy of the X-Y motion system in actual operation.

However it must be realized that even an accurate specification of X-Y positioning accuracy while in motion will not fully predict the accuracy of a part produced on an AWJ system.  The reason for this lies in the nature of the cutting jet itself.  Simply put, it is not a rigid inflexible cutting tool.  The jet bends as it moves.  The lower part of the jet does not behave exactly like the upper part.  The width of the jet changes and deforms.  The jet tapers as it cuts.  And all of these things change with different materials, different thicknesses, different nozzle parameters, different cutting speeds, different rates of acceleration and a host of other factors.  Thus the accuracy of the motion system only sets the lower limit on the error seen in a part.  By moving very slowly everywhere along the part path a user can approach this accuracy. However, no user wants to cut that slowly.

When cutting quickly, the speed must be accurately controlled to manage the jet deflections so as to minimize part error.  This task is very difficult to do with manual programming. It is the control software that sets speeds, accelerations, rates of change of acceleration, actual cutting path, taper compensation angles and other variables based on its own embedded algorithms and cutting strategies.  The most precise cutting table in the world will not make an accurate and repeatable part quickly without the right software.

We at OMAX are proud of the design of our cutting tables and their inherent precision and accuracy of motion.  However, it is our control software that allows them to make accurate and repeatable parts.  That is why we tell potential OMAX users not to judge our system (or any other AWJ system, for that matter) on the specifications of its X-Y accuracy.  Make some actual test parts and measure them and compare.  The measured accuracy of actual parts produced on an AWJ system is the one and only true measure of accuracy.

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

The heart of any abrasive waterjet nozzle is the jewel orifice that forms the waterjet stream.  These are generally made from a synthetic jewel—usually sapphire, ruby or diamond.  Traditionally these jewels have a square-edged orifice, but some nozzle manufacturers offer round-edged jewel orifices for various manufacturing and service reasons.  In all cases the actual effective diameter of the waterjet stream is slightly smaller than that of the orifice itself because of the physics of fluid flow through a hole. You can see this effect in the drawing below, which shows three different diameter orifices resulting the exact same diameter jet streams.

The Same Jet Formed by Three Different Diameter Orifices

This effective reduction in stream cross-sectional area is expressed by something we call the “discharge coefficient” or “orifice coefficient”.  A round-edge orifice has a slightly higher “discharge coefficient” than a square-edged orifice.  This means that the jet stream leaving a round-edged orifice is slightly larger than that leaving a square-edged orifice of the same diameter, which in turn means that the volume flow rate is also slightly greater and so the effective hydraulic horsepower is greater.  However, please do not think that this means that a round-edged orifice is more efficient or that it magically produces more power from the same pump. In the world of physics you never get something for nothing.  If a pump is operating at maximum output using a square-edged orifice and you wish to change to a round-edged orifice, you will need to go to a slightly smaller orifice size to avoid overloading the pump. Similarly, if you are currently using a round-edged orifice and desire to change to a square-edged orifice, you will find that you can operate a slightly larger orifice without overloading the pump.  In either case, the effective power at the nozzle will be the same.  Alas, there is no free lunch!

Best regards,

John Olsen

The first element is the actual traction drive mechanism. This mechanism uses the time-tested steel-on-steel traction capability found in the railroad locomotive. A hardened steel drive wheel mounted to a movable carriage assembly is pressed against a hardened steel rail by the action of a spring flexure mechanism. Rotation of the drive wheel by a brushless synchronous motor causes the carriage to move along the rail. This drive system is extremely robust and operates well in the worst imaginable wet and abrasive-laden environment, as demonstrated by the millions of miles traveled by railroad locomotives in conditions ranging from heavy rain to drifting sand. It provides smooth movement over a wide range of speeds and accelerations and can be designed to provide only the forces actually needed to smoothly move the cutting nozzle, resulting in a high level of operator safety. No lubricants are required for the drive surfaces, which further enhances reliability and minimizes wear in a contaminated environment. The drive is low in cost compared to traditional linear drive systems and can be easily expanded in size at very little cost by merely lengthening the steel guide rail. In addition, off-the-shelf extruded rail guides can be used to minimize manufacturing costs.

The potential drawback of the traction drive mechanism is that it is what we engineers refer to as a “loosely coupled” drive system. There is no continuous positive mechanical linkage between the moving carriage and the rail, as found between a ballscrew and ballnut or a rack and pinion. Thus the need for the second element—a precision magnetic linear encoder system. Not long ago, long-length linear encoders were both exotic and expensive. This is no longer the case. Now these precision magnetic devices are readily available in virtually limitless lengths at very reasonable cost. They are reliable and quite insensitive to contaminants and dirty conditions. Moreover, they are highly accurate and have a resolution down to 1 micron (0.00004”), more than adequate for the most precise abrasive waterjet applications. Indeed the inherent accuracy of this system is on par with the accuracy of a much more expensive linear motor drive, which also relies on a linear encoder for determining position.

The final element of the traction drive system is the electronic Drive Control. The traction drive mechanism provides the motive power to smoothly move the carriage along the rail. The linear encoder provides the data to determine precisely where the carriage is. The Drive Control links these, along with input from the machine control computer in a unique feedback/feedforward control loop. Conceptually, the system functions as follows: The Drive Control first takes input from the machine control computer to determine the desired direction, acceleration, speed and distance of motion based on the tool path and pre-computed velocity profile associated with the part being made. The Drive Control then instructs the traction drive motor to provide the desired carriage acceleration, speed and distance based on an assumed coupling between the drive wheel and the rail. The Drive Control monitors the actual carriage motion based on the input from the linear encoder sensor and adjusts its instructions to the traction drive motor based on what is actually happening. This is the “feedback” part of the control. In addition the Drive Control modifies its intended future commands to the traction drive based on the currently measured drive wheel/rail coupling. This is the “feedforward” part of the control. The end result of this feedback/feedforward control approach is that the mechanically “loosely coupled” drive system becomes a “close coupled” drive that automatically adjusts for changes in conditions, such as possible contaminants on the rails and long-term wear.

Both extensive laboratory tests and real operations have shown the drive to be robust and reliable. It is also accurate: capable of producing parts with tolerances in the range of +/-0.001” or better. Finally, actual experience in manufacturing the Traction Drive shows that it can indeed be very cost-effective compared to more traditional machine tool drive systems. It should help point the way to lower overall costs for precision abrasive waterjet cutting systems. That is indeed good news because it will broaden the market for this technology, thus putting the flexibility and efficiency of the abrasive waterjet cutting process into the hands of a greater number of manufacturers, large and small. As readers of this blog already know, my personal goal is an abrasive waterjet cutting system in every machine shop. The Traction Drive is another step toward that goal.

Best regards,

Dr. John Olsen