Waterjet cutting may be a simpler processing method, but it is equipped with a powerful punch and requires the operator to maintain awareness of the wear and accuracy of multiple parts.
The simplest water jet cutting is the process of cutting high-pressure water jets into materials. This technology is usually complementary to other processing technologies, such as milling, laser, EDM, and plasma. In the water jet process, no harmful substances or steam are formed, and no heat-affected zone or mechanical stress is formed. Water jets can cut ultra-thin details on stone, glass and metal; quickly drill holes in titanium; cut food; and even kill pathogens in beverages and dips.
All waterjet machines have a pump that can pressurize the water for delivery to the cutting head, where it is converted to a supersonic flow. There are two main types of pumps: direct drive based pumps and booster based pumps.
The role of the direct drive pump is similar to that of a high-pressure cleaner, and the three-cylinder pump drives three plungers directly from the electric motor. The maximum continuous working pressure is 10% to 25% lower than similar booster pumps, but this still keeps them between 20,000 and 50,000 psi.
Intensifier-based pumps make up the majority of ultra-high pressure pumps (that is, pumps over 30,000 psi). These pumps contain two fluid circuits, one for water and the other for hydraulics. The water inlet filter first passes through a 1 micron cartridge filter and then a 0.45 micron filter to suck in ordinary tap water. This water enters the booster pump. Before it enters the booster pump, the pressure of the booster pump is maintained at about 90 psi. Here, the pressure is increased to 60,000 psi. Before the water finally leaves the pump set and reaches the cutting head through the pipeline, the water passes through the shock absorber. The device can suppress pressure fluctuations to improve consistency and eliminate pulses that leave marks on the workpiece.
In the hydraulic circuit, the electric motor between the electric motors draws oil from the oil tank and pressurizes it. The pressurized oil flows to the manifold, and the valve of the manifold alternately injects hydraulic oil on both sides of the biscuit and plunger assembly to generate the stroke action of the booster. Since the surface of the plunger is smaller than that of the biscuit, the oil pressure “enhances” the water pressure.
The booster is a reciprocating pump, which means that the biscuit and plunger assembly delivers high-pressure water from one side of the booster, while low-pressure water fills the other side. Recirculation also allows the hydraulic oil to cool when it returns to the tank. The check valve ensures that low-pressure and high-pressure water can only flow in one direction. The high-pressure cylinders and end caps that encapsulate the plunger and biscuit components must meet special requirements to withstand the forces of the process and constant pressure cycles. The entire system is designed to gradually fail, and leakage will flow to special “drain holes”, which can be monitored by the operator in order to better schedule regular maintenance.
A special high-pressure pipe transports the water to the cutting head. The pipe can also provide freedom of movement for the cutting head, depending on the size of the pipe. Stainless steel is the material of choice for these pipes, and there are three common sizes. Steel pipes with a diameter of 1/4 inch are flexible enough to connect to sports equipment, but are not recommended for long-distance transportation of high-pressure water. Since this tube is easy to bend, even into a roll, a length of 10 to 20 feet can achieve X, Y, and Z motion. Larger 3/8-inch pipes 3/8-inches usually carry water from the pump to the bottom of the moving equipment. Although it can be bent, it is generally not suitable for pipeline motion equipment. The largest pipe, measuring 9/16 inches, is best for transporting high-pressure water over long distances. A larger diameter helps reduce pressure loss. Pipes of this size are very compatible with large pumps, because a large amount of high-pressure water also has a greater risk of potential pressure loss. However, pipes of this size cannot be bent, and fittings need to be installed at the corners.
The pure water jet cutting machine is the earliest water jet cutting machine, and its history can be traced back to the early 1970s. Compared with contact or inhalation of materials, they produce less water on the materials, so they are suitable for the production of products such as automotive interiors and disposable diapers. The fluid is very thin-0.004 inches to 0.010 inches in diameter-and provides extremely detailed geometries with very little material loss. The cutting force is extremely low, and the fixing is usually simple. These machines are best suited for 24-hour operation.
When considering a cutting head for a pure waterjet machine, it is important to remember that the flow velocity is the microscopic fragments or particles of the tearing material, not the pressure. To achieve this high speed, pressurized water flows through a small hole in a gem (usually a sapphire, ruby or diamond) fixed at the end of the nozzle. Typical cutting uses an orifice diameter of 0.004 inches to 0.010 inches, while special applications (such as sprayed concrete) can use sizes up to 0.10 inches. At 40,000 psi, the flow from the orifice travels at a speed of approximately Mach 2, and at 60,000 psi, the flow exceeds Mach 3.
Different jewelry has different expertise in waterjet cutting. Sapphire is the most common general-purpose material. They last approximately 50 to 100 hours of cutting time, although the abrasive waterjet application halves these times. Rubies are not suitable for pure waterjet cutting, but the water flow they produce is very suitable for abrasive cutting. In the abrasive cutting process, the cutting time for rubies is about 50 to 100 hours. Diamonds are much more expensive than sapphires and rubies, but the cutting time is between 800 and 2,000 hours. This makes the diamond particularly suitable for 24-hour operation. In some cases, the diamond orifice can also be ultrasonically cleaned and reused.
In the abrasive waterjet machine, the mechanism of material removal is not the water flow itself. Conversely, the flow accelerates abrasive particles to corrode the material. These machines are thousands of times more powerful than pure waterjet cutting machines, and can cut hard materials such as metal, stone, composite materials, and ceramics.
The abrasive stream is larger than the pure water jet stream, with a diameter between 0.020 inches and 0.050 inches. They can cut stacks and materials up to 10 inches thick without creating heat-affected zones or mechanical stress. Although their strength has increased, the cutting force of the abrasive stream is still less than one pound. Almost all abrasive jetting operations use a jetting device, and can easily switch from single-head use to multi-head use, and even the abrasive water jet can be converted to a pure water jet.
The abrasive is hard, specially selected and sized sand-usually garnet. Different grid sizes are suitable for different jobs. A smooth surface can be obtained with 120 mesh abrasives, while 80 mesh abrasives have proven to be more suitable for general-purpose applications. 50 mesh abrasive cutting speed is faster, but the surface is slightly rougher.
Although water jets are easier to operate than many other machines, the mixing tube requires operator attention. The acceleration potential of this tube is like a rifle barrel, with different sizes and different replacement life. The long-lasting mixing tube is a revolutionary innovation in abrasive water jet cutting, but the tube is still very fragile-if the cutting head comes in contact with a fixture, a heavy object, or the target material, the tube may brake. Damaged pipes cannot be repaired, so keeping costs down requires minimizing replacement. Modern machines usually have an automatic collision detection function to prevent collisions with the mixing tube.
The separation distance between the mixing tube and the target material is usually 0.010 inches to 0.200 inches, but the operator must keep in mind that a separation greater than 0.080 inches will cause frosting on the top of the cut edge of the part. Underwater cutting and other techniques can reduce or eliminate this frosting.
Initially, the mixing tube was made of tungsten carbide and only had a service life of four to six cutting hours. Today’s low-cost composite pipes can reach a cutting life of 35 to 60 hours and are recommended for rough cutting or training new operators. The composite cemented carbide tube extends its service life to 80 to 90 cutting hours. The high-quality composite cemented carbide tube has a cutting life of 100 to 150 hours, is suitable for precision and daily work, and exhibits the most predictable concentric wear.
In addition to providing motion, waterjet machine tools must also include a method of securing the workpiece and a system for collecting and collecting water and debris from machining operations.
Stationary and one-dimensional machines are the simplest waterjets. Stationary water jets are commonly used in aerospace to trim composite materials. The operator feeds the material into the creek like a band saw, while the catcher collects the creek and debris. Most stationary waterjets are pure waterjets, but not all. The slitting machine is a variant of the stationary machine, in which products such as paper are fed through the machine, and the water jet cuts the product into a specific width. A crosscutting machine is a machine that moves along an axis. They often work with slitting machines to make grid-like patterns on products such as vending machines such as brownies. The slitting machine cuts the product into a specific width, while the cross-cutting machine cross-cuts the product fed below it.
Operators should not manually use this type of abrasive waterjet. It is difficult to move the cut object at a specific and consistent speed, and it is extremely dangerous. Many manufacturers will not even quote machines for these settings.
The XY table, also called a flatbed cutting machine, is the most common two-dimensional waterjet cutting machine. Pure water jets cut gaskets, plastics, rubber, and foam, while abrasive models cut metals, composites, glass, stone, and ceramics. The workbench can be as small as 2 × 4 feet or as large as 30 × 100 feet. Usually, the control of these machine tools is handled by CNC or PC. Servo motors, usually with closed-loop feedback, ensure the integrity of position and speed. The basic unit includes linear guides, bearing housings and ball screw drives, while the bridge unit also includes these technologies, and the collection tank includes material support.
XY workbenches usually come in two styles: the mid-rail gantry workbench includes two base guide rails and a bridge, while the cantilever workbench uses a base and a rigid bridge. Both machine types include some form of head height adjustability. This Z-axis adjustability can take the form of a manual crank, an electric screw, or a fully programmable servo screw.
The sump on the XY workbench is usually a water tank filled with water, which is equipped with grilles or slats to support the workpiece. The cutting process consumes these supports slowly. The trap can be cleaned automatically, the waste is stored in the container, or it can be manual, and the operator regularly shovels the can.
As the proportion of items with almost no flat surfaces increases, five-axis (or more) capabilities are essential for modern waterjet cutting. Fortunately, the lightweight cutter head and low recoil force during the cutting process provide design engineers with freedom that high-load milling does not have. Five-axis waterjet cutting initially used a template system, but users soon turned to programmable five-axis to get rid of the cost of template.
However, even with dedicated software, 3D cutting is more complicated than 2D cutting. The composite tail part of the Boeing 777 is an extreme example. First, the operator uploads the program and programs the flexible “pogostick” staff. The overhead crane transports the material of the parts, and the spring bar is unscrewed to an appropriate height and the parts are fixed. The special non-cutting Z axis uses a contact probe to accurately position the part in space, and sample points to obtain the correct part elevation and direction. After that, the program is redirected to the actual position of the part; the probe retracts to make room for the Z-axis of the cutting head; the program runs to control all five axes to keep the cutting head perpendicular to the surface to be cut, and to operate as required Travel at precise speed.
Abrasives are required to cut composite materials or any metal larger than 0.05 inches, which means that the ejector needs to be prevented from cutting the spring bar and tool bed after cutting. Special point capture is the best way to achieve five-axis waterjet cutting. Tests have shown that this technology can stop a 50-horsepower jet aircraft below 6 inches. The C-shaped frame connects the catcher to the Z-axis wrist to correctly catch the ball when the head trims the entire circumference of the part. The point catcher also stops abrasion and consumes steel balls at a rate of about 0.5 to 1 pound per hour. In this system, the jet is stopped by the dispersion of kinetic energy: after the jet enters the trap, it encounters the contained steel ball, and the steel ball rotates to consume the energy of the jet. Even when horizontally and (in some cases) upside down, the spot catcher can work.
Not all five-axis parts are equally complex. As the size of the part increases, program adjustment and verification of part position and cutting accuracy become more complicated. Many shops use 3D machines for simple 2D cutting and complex 3D cutting every day.
Operators should be aware that there is a big difference between part accuracy and machine motion accuracy. Even a machine with near-perfect accuracy, dynamic motion, speed control, and excellent repeatability may not be able to produce “perfect” parts. The accuracy of the finished part is a combination of process error, machine error (XY performance) and workpiece stability (fixture, flatness and temperature stability).
When cutting materials with a thickness of less than 1 inch, the accuracy of the water jet is usually between ±0.003 to 0.015 inches (0.07 to 0.4 mm). The accuracy of materials more than 1 inch thick is within ±0.005 to 0.100 inches (0.12 to 2.5 mm). The high-performance XY table is designed for linear positioning accuracy of 0.005 inches or higher.
Potential errors that affect accuracy include tool compensation errors, programming errors, and machine movement. Tool compensation is the value input into the control system to take into account the cutting width of the jet-that is, the amount of cutting path that must be expanded in order for the final part to get the correct size. To avoid potential errors in high-precision work, operators should perform trial cuts and understand that tool compensation must be adjusted to match the frequency of mixing tube wear.
Programming errors most often occur because some XY controls do not display the dimensions on the part program, making it difficult to detect the lack of dimensional matching between the part program and the CAD drawing. Important aspects of machine motion that can introduce errors are the gap and repeatability in the mechanical unit. Servo adjustment is also important, because improper servo adjustment can cause errors in gaps, repeatability, verticality, and chatter. Small parts with a length and width of less than 12 inches do not require as many XY tables as large parts, so the possibility of machine motion errors is less.
Abrasives account for two-thirds of the operating costs of waterjet systems. Others include power, water, air, seals, check valves, orifices, mixing pipes, water inlet filters, and spare parts for hydraulic pumps and high-pressure cylinders.
Full power operation seemed more expensive at first, but the increase in productivity exceeded the cost. As the abrasive flow rate increases, the cutting speed will increase and the cost per inch will decrease until it reaches the optimal point. For maximum productivity, the operator should run the cutting head at the fastest cutting speed and maximum horsepower for optimal use. If a 100-horsepower system can only run a 50-horsepower head, then running two heads on the system can achieve this efficiency.
Optimizing abrasive waterjet cutting requires attention to the specific situation at hand, but can provide excellent productivity increases.
It is unwise to cut an air gap larger than 0.020 inches because the jet opens up in the gap and roughly cuts lower levels. Stacking the material sheets closely together can prevent this.
Measure productivity in terms of cost per inch (that is, the number of parts manufactured by the system), not cost per hour. In fact, rapid production is necessary to amortize indirect costs.
Waterjets that often pierce composite materials, glass, and stones should be equipped with a controller that can reduce and increase water pressure. Vacuum assist and other technologies increase the likelihood of successfully piercing fragile or laminated materials without damaging the target material.
Material handling automation makes sense only when material handling accounts for a large part of the production cost of parts. Abrasive waterjet machines usually use manual unloading, while plate cutting mainly uses automation.
Most waterjet systems use ordinary tap water, and 90% of waterjet operators do not make any preparations other than softening the water before sending the water to the inlet filter. Using reverse osmosis and deionizers to purify water may be tempting, but removing ions makes it easier for the water to absorb ions from metals in pumps and high-pressure pipes. It can extend the life of the orifice, but the cost of replacing the high-pressure cylinder, check valve and end cover is much higher.
Underwater cutting reduces surface frosting (also known as “fogging”) on the top edge of abrasive waterjet cutting, while also greatly reducing jet noise and workplace chaos. However, this does reduce the visibility of the jet, so it is recommended to use electronic performance monitoring to detect deviations from peak conditions and stop the system before any component damage.
For systems that use different abrasive screen sizes for different jobs, please use additional storage and metering for common sizes. Small (100 lb) or large (500 to 2,000 lb) bulk conveying and related metering valves allow rapid switching between screen mesh sizes, reducing downtime and hassle, while increasing productivity.
The separator can effectively cut materials with a thickness of less than 0.3 inches. Although these lugs can usually ensure a second grinding of the tap, they can achieve faster material handling. Harder materials will have smaller labels.
Machine with abrasive water jet and control the cutting depth. For the right parts, this nascent process may provide a compelling alternative.
Sunlight-Tech Inc. has used GF Machining Solutions’ Microlution laser micromachining and micromilling centers to produce parts with tolerances less than 1 micron.
Waterjet cutting occupies a place in the field of material manufacturing. This article looks at how waterjets work for your store and looks at the process.
Post time: Sep-04-2021