Theoretically there is a cutting speed for any particular material and thickness that is “just right” and will result in zero taper.  As our ongoing R & D efforts improved our understanding of cutting jet behavior we were able to develop a computer model that accurately predicts the amount of taper based on the material being cut, its thickness, the speed of cut and the nozzle parameters of water pressure, orifice size, mixing tube size, abrasive characteristics and abrasive flow rate.  That model permitted us to accurately predict the speed at which there would be no taper.  This meant that our control software could be enhanced to permit the user to specify a cut quality that we call “zero taper quality” along any particular cutting segment.  By doing this the user could create a part that was taper-free.

The drawback to this rather elegant solution to taper control is that the cutting speed for “zero taper quality” tends to be slow.  This slow speed was acceptable for making parts where just a few key elements needed to be taper-free, but it was a not an optimum solution for making complete parts with the highest possible accuracy in the least amount of time.  Our customers want to cut precise parts as quickly as possible, so we decided to take another approach to the problem.  We developed the Tilt-A-Jet.  This retro-fittable accessory has the ability to tilt the cutting nozzle very precisely (to an accuracy of better than +/-0.06 degrees) in a direction perpendicular to the direction of cut.  Our advanced computer model is used to control the tilt so that it automatically compensates for the natural taper of the cutting jet, regardless of the cutting speed.  This provides a fast cut, based on the user’s desired cut quality, that is also taper-free.  It has the additional benefit of maintaining the taper-free cut even as the nozzle must slow for sharp corners or tight curves.  No special programming is required.  The OMAX Intelli-Max control software receives geometric input directly from a standard 2-D drawing file (such as a DXF or DWG file) and the computer model automatically controls the degree of tilt needed for taper compensation along each increment of the cutting path.  The result is a precise taper-free part that can be made quickly and economically without 3-D programming.

As our computer model of jet behavior continues to evolve we have added an additional feature to the Tilt-A-Jet system.  We know that even faster cuts are possible, particularly in thicker material, if we tilt the jet slightly forward in the direction of cut.   The mechanism of the Tilt-A-Jet enables us to do this quite easily with a simple free control software upgrade.  No 3-D programming is required.  Once activated the tilt-forward feature is programmed automatically. This has meant even faster taper-free parts for our customers at no additional cost.  In addition, software upgrades have allowed our customers to use the same Tilt-A-Jet system to put intentional taper into elements of a part.  Once again this requires no 3-D programming and is fully automated.  Intentional taper capability is now widely used by our tool and die customers for providing relief in die components.

The Tilt-A-Jet accessory has greatly expanded the capabilities of abrasive waterjet cutting by permitting the rapid production of precise taper-free parts and parts with intentional precision taper.  This means more applications are now suitable for our technology and more shops are finding a need for our system.

Remember—our goal is an abrasive waterjet system in every shop.

Best regards,

John Olsen

Watch Video

OMAX Tilt-A-Jet Nozzle and OMAX A-Jet Nozzle

I have often been asked why OMAX doesn’t just make a single tilting head to accomplish both bevel cutting and taper elimination.  The answer is simple:  A system designed to accomplish both tasks is not really very good at either.  Bevel cutting requires a very wide range of angular motion while taper elimination requires very precise angular control over a much smaller range of motion.  A little bit of trigonometry shows the inherent problem in providing both in the same system.  A claimed angular accuracy of “better than +/- 1 degree” sounds pretty good and might be fine for cutting beveled surfaces.  However the dimensional offset caused by taper angle can be calculated using the equation

Taper offset = (material thickness) X (tangent of taper angle)

Using this equation we can compute that an angular accuracy of +/- 1 degree means +/-0.017 inches of taper in a piece of 1″ thick material.  This level of angular control is totally unsuitable for taper compensation.  Indeed, one is better off with no compensation at all!  Similarly a claimed accuracy of “20 minutes of arc” also sound pretty good until you realize that 20 minutes of arc is 0.33 degrees, which translates into 0.006″ of taper in a piece of 1″ thick material–still not nearly good enough for taper control.  When you are trying to design a tilting system with the accuracy needed for +/-0.001” of taper in 1” thick material, you are looking at a necessary angular accuracy of better than +/-0.06 degrees!  This is virtually impossible in a bevel-cutting mechanism that is also required to quickly move +/- 60 degrees.  Thus either taper-control accuracy or bevel-cutting speed or both must be compromised.

A Swiss Army knife can be an OK thing.  It can be used as a knife or a can opener.  However it is most certainly not the best available knife nor is it the best available can opener.   For optimum performance one needs two different tools for two different applications.  So it is with tilting abrasive waterjet cutting heads.  That is why OMAX offers two options—the A-Jet for bevel cutting and the Tilt-A-Jet for precision taper control.

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