Waterjet Dictionary

Mesh values do not represent particles of an exact dimension, but represent a distribution of particle sizes. An 80 mesh abrasive will have some particles larger and smaller than exactly 80 mesh. Mesh sizes are usually determined by allowing abrasive to fall through a series of screens – each screen smaller in mesh size from top to bottom. Typical mesh sizes used in abrasive waterjet machining are 220 to 50 mesh, most common are 80 and 120. The larger the mesh number, the smaller the particle size.

The abrasive waterjet (sometimes called the abrasivejet) stream accelerates abrasive particles and those particles, not the water, erode the material. The abrasive waterjet is much more powerful than a pure waterjet and is capable of cutting hard materials such as metals, glass, stone, and composites: none of which can be cut with a pure waterjet. Abrasive waterjets using standard parameters can cut materials with hardness up to and slightly beyond aluminum oxide ceramic (often called alumina, AD 99.9). 
 

Abrasive Waterjet Attributes

• Extremely versatile process
• No Heat Affected Zones
• No mechanical stresses
• Easy to program
• Thin stream (0.020to 0.050 inch in diameter)
• Extremely detailed geometry
• Thin material cutting
• Over 12+E7 inch thick cutting
• Stack cutting
• Little material loss due to cutting
• Simple to fixture
• Low cutting forces (under 1 lb. while cutting)
• One jet setup for nearly all abrasive jet jobs
• Easily switched from single to multi-head use
• Quickly switch from pure waterjet to abrasive waterjet
• Reduced secondary operations
• Little or no burr

Check valves are found in waterjet pumps. They are one way doorways that allow something, in this case water, to pass in only one direction.

As an example, low pressure water comes in through a typical low pressure hose and enters the pump awaiting pressurization. Once pressurized, that water is not allowed to exit through the low pressure check valve because it would immediately burst the low pressure hose. Instead, another check valve opens to allow the high pressure water to be safely routed out the high pressure stainless steel lines on its way to the cutting head.

CFRP stands for Carbon Fiber Reinforced Plastic. Carbon fiber composites are used in tennis racquets, golf clubs, prosthetics, and modern aircraft. For this definition we will use the example of commercial aircraft. Boeing and Airbus composite wings, spars, struts, tail sections as a superior material to aluminum. Cutting of composites with traditional milling or routing processes can leave delamination, micro-cracks, whiskers, and fiber pull-out. Cutting with waterjet does not exhibit these problems.

Today's lightweight advanced composites can be as hard and rigid as steel, or as flexible as rubber, and still hold up to the stresses of supersonic flight. The same properties that make these materials so tough also make them extremely difficult to cut. Composite technologists continue to introduce new material combinations that defy the capabilities of traditional machining methods.

Until recently, conventional cutting methods, diamond or carbide-tipped mills or routers, band saws, cutoff saws and abrasive wheels were used to cut these unconventional materials. Due to the composition and fiber orientation of advanced composites, conventional cutting methods damaged the composites either by heating them up or by leaving frayed or delaminated edges. In addition, these methods were often slow, frequently leaving behind delamination and other issues requiring costly rework.

Composites can come in many forms. High temperature engines use metals reinforced with ceramic fibers (metal matrix composite). Typically, engineers are seeking to reduce weight while delivering higher strength, greater flexibility, or temperature resistance. These materials give production shops fits, yet they can be cut with speed, precision, and material integrity on a Flow waterjet.

On a motion system, the control system takes the part program, speeds, and jet on/off commands and turns it into language that the electrical system can understand. Typically a CNC (computer numerical control) control system, a PC-based control system, or a hybrid of both is used.  

To further explain; an engineer or designer might draw a square to be cut on a waterjet within a CAD (computer aided design) program such as AutoCAD®. A programmer (might be the same person) then takes that square drawing as a .dxf or .dwg file type and brings it into a CAM (computer aided manufacturing) software package.  

Here, the programmer adds in the waterjet start stop locations, direction of travel, cutter compensation, and travel velocities necessary. This file is then sent to the control system where the operator (again, could be the same person) opens up the file in the machine tool control system, locates the cutting head in the start position over the target material, and presses cycle start to cut the part.

The control system then turns the cutting file into electrical current going from the control system drives to the machine tool motors to move the machine around.  The control system also fires off digital outputs to start and stop the water and the abrasive automatically.  

A waterjet cutting head is where the water pressure is converted to water velocity when the water passes through the jewel orifice.  

In abrasive waterjet cutting, the cutting head also includes the mixing chamber and mixing tube.  Sometimes you will hear of the cutting head also including the on/off valve.  This valve resides just above the orifice and utilizes some type of poppet and seat configuration to allow the operator or the CNC controller to start and stop the waterjet stream.

In waterjet cutting cutting power density relates to how much energy you can put onto what size of area. A smaller stream at higher pressure means the stream is moving at higher velocity and will have greater power density than a wider stream at lower pressure/velocity.  

Rotary direct drive pumps are used on over 20% of waterjet systems installed worldwide. Unlike intensifier based pumps, the direct drive rotary pump has no hydraulic pump. Sometimes called a triplex pump, the electric motor rotates a crank with three pistons to generate the ultrahigh-pressure water.

On an motion system the drive motors take a plus/minus current from the CNC drive amplifiers to provide a clockwise or counterclockwise rotation. This rotation moves the machine.

Dynamic Waterjet® is patented Flow technology that improves cut speed by 2 to 4 times allowing, simultaneously, for far better finished part tolerance.

In waterjet cutting, the jet creates two errors when cutting a part at high speed:  stream lag and taper. Stream lag is where the stream exits the workpiece behind the entrance point. Taper is V-shaped. Both stream lag and taper can be minimized by slowing down (usually to 15 to 20% of maximum cut speed) but cannot be eliminated.

To enable high speed cutting, Dynamic Waterjet automatically angles the head to one side so that all the taper goes to the scrap side, and slightly tilts the head forward to compensate for stream lag. This taper and stream lag compensation takes place automatically in a behind-the-scenes operation. The operator or programmer do not need to program the angles, the control system takes care of it.  The angle even changes automatically with cutting speed, so precision corners and arcs can be created as the cutting head changes speed to negotiate corners.

Dynamic XD® is patented Flow technology where the automatic articulation of the cutting head impingement angle is used not only for flat part cutting, but also for bevel and 3D cutting. See Dynamic Waterjet® for more explanation.

The Emergency Stop is a feature where an operator can stop the machine tool and put it into a safe, non-threatening mode at any instant. E-stop buttons are always red and prominently displayed. In waterjet, the E-stop will stop the cutting process and motion, and if so designed will also turn off the pump and evacuate the high-pressure lines of pressure.

On a motion system, the feedback system provides positional and possibly velocity feedback to the CNC control system. Telling the control that the machine has done what it was told to do.

The higher the drive, motor, and feedback resolution the more precise the waterjet cutting head motion will be. Feedback systems can be encoders attached to the motors, tape scale or glass scale attached to the machine frame in direction of travel, or other means.

Garnet abrasive is used on 99% of all waterjet machines due to it's cutting capability, consistency, cost, cutting head wear rate, and non-hazardous characteristics. The size of the garnet abrasive typically used today for waterjet cutting ranges from 50 mesh to 220 mesh, with the most common being 80 mesh. The higher the mesh number, the finer the grain size. 320 mesh is similar to dust.

In waterjet, the high-pressure plumbing safely transports the water from the pressure generating waterjet pump to the cutting head. Plumbing can consist of somewhat flexible stainless steel lines of 1/4 inch, 3/8 inch, or 9/16 inch outside diameter, T's, elbows, and swivels. HyperPressure™ plumbing is of different design and rating than normal ultrahigh-pressure plumbing.

HyperPressure™ describes a waterjet pump delivering 75,000 psi or higher. Ultrahigh-pressure is usually 40,000 to 75,000 psi, and HyperPressure is 75,000 psi and over. In general, standard pressure waterjet systems run at 55,000 to 60,000 ultrahigh-pressure, and more advanced systems run with pumps rated at 94,000 psi.

The linear intensifier pump is the original, and most common, technology used in waterjet cutting. Intensifier pumps use the “intensification principle” to pressurize water.

The “intensification principle”, or ratio, uses the difference in biscuit/plunger area to intensify, or increase the pressure. Hydraulic oil is pressurized and the low pressure oil pushes against a biscuit, which has a face area 20 times greater than the face of the high pressure plunger that pushes against the water. Therefore, the pressure is “intensified” twenty times. E.g., 3,000 psi of oil pressure will generate 60,000 psi of water pressure due to the 20:1 ratio of biscuit area to plunger area.

To create a pure waterjet stream the water pressure must be converted to velocity. This conversion takes place when the water is passed through a tiny jewel orifice. A hole in the sapphire, ruby, or diamond orifice ranges from 0.003" to 0.020" (most common 0.014"). The larger the orifice the more water and horsepower is required to maintain pressure.

Orifice size does not dictate maximum water pressure - only horsepower and pump design determine max pressure. 

The top of the orifice has a very sharp edge so that the waterjet stream is coherent. A rough or rounded edge will create a fuzzy, turbulent jet and may exhibit an angular trajectory that is not desired.  

An orfice blows out in waterjet from two primary causes. First, calcium can build up on the orifice and break off, causing instant orifice failure. Second, the orifice edge can become rounded or break from particle impact. In waterjet, an orifice is usually either good or bad - gradual degredation is less common. Sapphire and ruby orifice can last 40 to 200 hours, depending on application and pressure, with good water. A diamond might be 8 to 10 times more expensive but will last 8 to 10 times longer.

Kerf is defined as the width of the cut, or a groove or slit caused by cutting. In abrasive waterjet cutting the width of the Kerf is directly affected by the mixing tube diameter. The kerf is about 10% larger than the mixing tube diameter.

So, for a 0.030" mixing tube, the kerf will be 0.033" Of course, the kerf will increase as the mixing tube grows. Tube growth is about 0.001" per 8 hours of jet-on time.

A waterjet's thin cutting width is a major attribute, allowing intricate detail. A pure waterjet stream ranges from 0.003" to 0.015", and an abrasive waterjet from 0.015 to 0.070" (typically 0.040").

Used in abrasive waterjet cutting, the mixing tube is the final component in the cutting head. Water pressure is converted to water velocity when the water passes through the jewel orifice.

The supersonic waterjet stream then enters the mixing chamber where the abrasive is pulled into the head via a venturi effect. Then the water and the abrasive pass through the mixing tube and exit as a mixture of water, abrasive and some air.

The mixing tube can range from 0.015" to 0.070" in inside diameter, and 1.5 to 6" long. It has an internal entrance cone. The most common mixing tube is 0.040" inside diameter and 4 inches long. This tube would typically use 80 mesh garnet abrasive. In normal cutting a mixing tube of high quality material (nano-grained composite carbide with very little binder to maximize erosion resistance) wears at approximately 0.001" diameter growth per 6 to 8 hours of run time, and also wears concentrically.

Finished part accuracy is a combination of process error (the waterjet) + machine error (the XY velocity, smoothness, and path accuracy) + workpiece stability (fixturing, flatness, homogeneity, stable with temperature).

The waterjet beam is not rigid, but can bend and move in the material.  The waterjet stream parameters and non-rigid characteristics can affect part accuracy, such as stream lag, V-shaped taper, abrasive flow rate, etc. Controlling these characteristics has been the focus of waterjet suppliers for many years.

Dynamic Waterjet® automatically compensates for stream lag and taper allowing for 2 to 4 times faster cutting and finished part tolerances of 1 to 3 thousandths of an inch.

Programming software is also called CAM software (computer aided manufacturing). Programming software is usually on a PC, though machine tools can also be programmed right on the machine. A programmer imports a previously created CAD drawing as a .dxf or .dwg file type (or other format) or creates the pattern anew in the CAM software package.

A programmer uses the programming software in waterjet to add start and stop locations, direction of travel, cutter compensation, and travel velocities necessary. This file is then sent to the control system to be executed in cutting the part.

Pure waterjet is the original water cutting method. The first commercial applications were in the early to mid 1970s, and involved the cutting of corrugated cardboard. The largest uses for pure waterjet cutting are disposable diapers, tissue paper, and automotive interiors. In the cases of tissue paper and disposable diapers the waterjet process creates less moisture on the material than touching or breathing on it.

Pure waterjet can also be used to remove coatings, such as paint off ships, if you are interested in learning more about these applications click here.

Pure Waterjet Attributes

• Very thin stream (0.003 to 0.010 inch in diameter is the common range)
• Extremely detailed geometry
• Very little material loss due to cutting
• Non-heat cutting
• Cut very thick
• Cut very thin
• Usually cuts very quickly
• Able to cut soft, light materials
• (e.g., fiberglass insulation up to 24" thick)
• Extremely low cutting forces
• Simple fixturing
• 24 hour per day operation

Stream lag is a deflection of a pure waterjet or abrasive waterjet caused by cutting material; the exit point of the stream at the bottom of the material trails behind the entrance point of the stream at the top of the material.  The faster material is cut, the greater the stream lag.  Typically stream lag is exhibited whenever cutting occurs at 25% of maximum cut speed or greater.  Below 25% the stream is very close to vertical.

With waterjet cutting, as pressure goes up, the speed of the waterjet stream increases.

Once the stream exits the orifice, it's all about velocity. There is no pressure in the stream after the water passes through the orifice.

In abrasive waterjet cutting the faster the stream, the quicker the abrasive moves, the faster the cutting, the smaller the diameter of the stream, and the less abrasive is required. 

Taper in waterjet cutting is pertaining to non-straight kerf walls. Kerf taper is most common in abrasive waterjet cutting, as opposed to pure waterjet cutting. When cutting material, the top of the cut is often wider than the bottom of the cut, yielding a V-shaped taper. Slow down cut speed to minimize taper. Even below 20% of maximum cut speed the kerf will usually still exhibit some taper.

When cutting with a waterjet machine where the motion of the cutting head is smooth and precise, a smooth edge can be created. Once the cutting speed exceeds approximately 50% of maximum cut speed waviness can usually be seen at the bottom of the cut surface. These are called striations. The transition zone is the depth where the smooth turns to striated. At 70% of maximum cut speed the transition zone will be higher up the cut surface than at 60%.

The waterjet industry has different definitions for differences in pressure levels. Ultrahigh-pressure is between 40,000 psi (276 Mpa) to 75,000 psi (517 Mpa). For waterjet cutting, most pumps operate between 55-60,000 psi (379-412 MPa).

Vacuum created by a high speed fluid or media passing from a larger tube area to a smaller tube area. In abrasive waterjet cutting, a venturi effect is created by the pure waterjet stream passing through the wider mixing chamber and then into the narrow mixing tube.

Abrasive is pulled in by a venturi effect into the mixing chamber and accelerated like a bullet out of a rifle, or shot gun pellets in a shot gun, out of the mixing tube, thereby creating the abrasive waterjet.

Extremely easy to set up and operate, waterjet cutting technology is a cold-cutting process that can quickly produce small or large batches of parts, even for difficult projects. Waterjet cutting is a supersonic erosion process.  Pure waterjet cuts materials that can be cut with a knife, and the abrasive waterjet can cut anything harder.

Waterjet slitting systems quickly and efficiently cut paper products. Operations that employ waterjet slitting systems enjoy profitability and cost savings. Waterjets produce no airborne dust. Dust free cutting will improve your working conditions, safety, and produce a higher quality product. In many tissue and towel applications, waterjet slitting eliminates rewinding so you save on capital equipment expense by slitting on-line.  A waterjet slitter can be installed directly on the machine, and the edge meets all requirements for converting operations..