(2002-7-2)
Cutting with high-pressure liquid jet is an advanced material processing method. The United States obtained a patent at 196 1. However, this theme was first published in Russia in the mid-1940s. However, in the late 1980s, Moscow Institute of Experimental Science of Metal Cutting Machine Tool began to conduct in-depth research. The EMO exhibitions in Hanover 1997 and Paris 1999 have proved that this method has made great progress, and the technical progress in this area is very remarkable. In Paris alone, three kinds of large-scale working devices were exhibited, which were owned by Flow (Germany), Bystronic (Switzerland) and Digital Control (France). Trumpf (Germany), which specializes in mechanical drilling and laser drilling, has also begun to develop water jet cutting equipment.
Technical characteristics of water jet cutting method
Water jet cutting is a cutting process in which water jet or abrasive water mixture is ejected from a nozzle with a diameter of 0. 1mm~0.2mm at a speed of 100m/s under the pressure of 400MPa. When abrasive water is cut, the jet is ejected from the nozzle through a small-diameter (about 1mm) cemented carbide nozzle, and abrasive powder is sucked out of the special hopper through the injection at the nozzle entrance. The kinetic energy of the water jet will be transferred to the abrasive particles, so as to remove micro-fragments from the cutting blank. At this time, there is actually no heat and force acting on the blank, so there is no deformation, burr and material structure distortion on the cutting surface.
When the cutting power reaches above 30kW, the working head can be freely clamped by hand or robot clamping device. The microprocessor numerical control device of the robot can realize cutting at any angle in space.
The water spray device can cut metal plates, accurately make right angles, drill small holes, open narrow slots and complete very close profiles (including non-thermal conductive materials).
The incision can start from any point on the surface of the workpiece. Notch width is small (water cutting 0. 1mm~0.3mm, abrasive water cutting 1mm~ 1.5mm), which can ensure material saving and reduce energy consumption. This process has the characteristics of flexibility and ecological cleanliness, and can usually be used to complete a single order that needs to be quickly readjusted. Table 1 lists the evaluation results of various sheet metal cutting methods (according to the data of Flow company).
Table 1 Proportion of various cutting methods
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Evaluation criteria Evaluation of various cutting methods (5 points)
Abrasive water cutting laser cutting plasma cutting oxygen cutting
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Slot width 5 2 4 5
Incision quality 5 5 2 2
Cutting speed 4 5 4 4
Cutting accuracy 5 5 4 3
Possibility of grinding the surface 5 4 4 2
Form sludge 4 2 2 4
Total processing time 4 4 3 2
Process flexibility 5 4 2 3
Universality 5 3 4 2
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The main process parameter is productivity or cutting speed V. For most water jet equipment, V = 0.001m/min ~12m/min, and the cutting accuracy is 0.1mm.. Table 2 lists the data of Flow Company on the cutting speed and blank thickness of abrasive water for various materials. Table 3 lists the data of Ingersoll Rand (Germany) about the cutting speed of water jet.
Table 2 Cutting speed of different blank thicknesses
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Thickness of cutting material blank (mm)
5 10 20 50
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Aluminum cutting 2.28 1.04 0.47 0. 16
Titanium cutting 1.09 0.49 0.22 0.08
The speed of corrosion-resistant steel is 0.62 0.28 0. 12 0.04.
Glass degree 6.45 2.91.1.30 0.46
Granite (m/3.55 1.60 0.72 0.25)
Marble minimum 4.59 2.06 0.93 0.32
)
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Note: the working pressure is 350MPa；; Abrasive supply 680g/min; The abrasive is 80 # granularity granite.
Table 3 Cutting speed adopted by Ingersoll Rand Company
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Thickness of cutting material blank (mm)
2 5 10
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Rubber ① 25- 10
Polyurethane cutting 20 8 3
The speed of PTFE is 6 2 0.8.
Plywood (m/min) 25 4 0.5
Foam ②-25
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Note: ① When the blank thickness is 20mm, v=2m/min.
② When the blank thickness is 100mm, v=5m/min.
Because modern computers are equipped with water jet cutting devices, a large number of databases can be used to optimize the cutting process (including real-time optimization) and solve the optimization task of cutting materials, so the manufacturability of this method can be significantly expanded.
Layout scheme
At present, there are many schemes for the layout of water jet equipment, including the form of robot matching and the form with turntable, but the layout of moving gantry or fixed gantry is the most widely used.
Auxiliary angular coordinates [such as Bystronic, Waterjet (Italy)] are considered in advance in a series of devices for moving the frame. When the working head moves along the Z axis, the tube blank can be processed, including the non-circular surface.
Eliminating the residual energy of jet is carried out in the open groove under the workbench, which makes the ecological environment worse. The sludge in the tank must be cleaned regularly with the help of scraper conveyor, screw conveyor and hydraulic scroll machine.
The worktable used for blank positioning can be made of stainless steel plate, wire mesh and sharp nails into a grid shape; The surface of the table can be formed of granular materials. Similar workbench is expensive and worn by jet, so its service life is short.
The workbench used in the device of Watejet Company has the advantages of a solid frame consisting of a square (120mm× 1200mm) net, a depth sedimentation tank made of corrosion-resistant steel, a transmission device arranged in the outer dimension of the frame, and an end switch for controlling zero point. The maintenance area is set at one side of the frame length, the loading area can be completely opened, the network elements can be replaced melodiously, and the frame is very high.
Equipment with movable gantry is best used to process heavy parts that are not easy to move when cutting. The typical representative is the Tll-200 equipment of Flow Company, and its workbench size is 2m×4m.
An example of fixed gantry layout is the equipment of Ingersoll Rand Company, and its workbench size is 2m×3m.
When the residual jet energy in the collector is eliminated, the formed sludge will be discharged into the sedimentation tank along the special hose. Because the area near the outlet hole of the working head is covered with a special cover, and the cover is connected with the vacuum cleaner, and the collector is arranged several millimeters away from the blank, the cutting process can achieve the maximum ecological cleanliness. Without the waste water tank, the weight of the equipment can be significantly reduced.
The above-mentioned layout is more complicated in structure than that of the movable portal frame, and there are parts that wear rapidly in the collector. In addition, there are some restrictions on the cutting speed in order to avoid the jet bending too much at the exit of the blank and deviating from the collector.
Typical size of workbench (taking the equipment of flow company as an example): 2m×1.5m; ; 2m×3m； 2m×4m； 2m×6m； 3m×2m； 3m×4m and 3m×6m (the first number is the dimension along the X axis). In order to make the working head move along each axis, most electromechanical servo drives adopt ball screw drive. Generally, the movement along the Z axis shall not exceed 180mm~200mm.
When manufacturing the device, special attention should be paid to the selection of materials and the protection of the transmission device to prevent water and dust from falling into it. On Waterjet's device, there is a corrosion-resistant steel trolley and an aluminum slider supported by preloaded balls, and the slider moves along polished corrosion-resistant steel guide rails. The transmission is realized by brushless AC motor through epicycloidal gear and toothed belt with sufficient tensile strength. The transmission relationship along Y coordinate is realized by aluminum shaft. The double protection of rubber belt and corrugated curtain is considered in advance for the transmission device.
Main parameters of water spraying device
One of the main components of water jet device is high pressure device. It consists of a hydraulic device box, a variable pump with a driving motor, one or more superchargers, accumulators, hydraulic components, an oil regulating system (filtering and cooling), an auxiliary pump for process water, a fine water filter and an electric control box.
In the double-acting supercharger, the oil is supplied from 1 pump to the low-pressure (generally 12MPa~ 16PMa) hydraulic cylinder, forcing its piston to move together with the piston of the high-pressure hydraulic cylinder. Because the ratio of working areas of low-pressure and high-pressure hydraulic cylinder pistons is (20 ~ 30): 1, the water pressure can be increased accordingly. Therefore, the process water under the pressure of 0.8MPa~ 1MPa can fill the high-pressure hydraulic cylinder through another one-way valve and enter the accumulator and working head.
The accumulator can buffer the pressure fluctuation when the piston of the hydraulic cylinder is reversed. The pressure fluctuation at the inlet of the working head should not exceed 5%. Therefore, when selecting the capacity of the accumulator, it must be considered that the compression of water in the high-pressure hydraulic cylinder is close to 20% when the pressure is 400MPa. Because the accumulator is a high-pressure container with a lot of metal in it, it is not used in modern equipment as much as possible. In order to reduce the pressure fluctuation, an accumulator with a capacity of about 2.5L is generally installed on the booster oil path of the oil pump.
The efficiency of water jet cutting process is largely related to the working pressure P, so the device designer tried to ensure its maximum value, but it has not been successfully applied after exceeding the limit of 400MPa. Generally P=350MPa~380MPa. The problem is that when P=300MPa, the stress on the inner surface of high-pressure hydraulic cylinder has reached the yield limit of most structural steels (even without local stress concentration factor). This requires the use of high-strength corrosion-resistant steel. In addition, the service life of elastomer seal has not exceeded 500h at present. In order to reduce the axial load acting on the hydraulic cylinder fasteners, the radial size of the seal should be the smallest.
The main parameters of the water jet device (process water flow Q and transmission power P) are determined according to the nozzle diameter D and the efficiency of the hydraulic system (Figure 1), and D is generally less than 0.65 mm In order to improve the water flow, it is realized by installing an auxiliary (second) standard supercharger. In fact, all superchargers are designed with double-acting superchargers and controllable hydraulic directional valves. The high-pressure hydraulic cylinder has a smooth inner hole (no stress concentration source) and a cover with a built-in one-way valve.
The flow company produces more than 30 kinds of superchargers with a pressure of 380MPa, q =1.93l/min ~12l/min.
The high-pressure device used in Bystronic's device has the following main advantages: there are two double-acting superchargers, which can keep the parameters P and Q constant without using an accumulator; Stepless adjustment of stamping pressure and working pressure; There are three-stage fine water filters; Can observe the situation of high-pressure seal, and quickly replace the seal; Enough power to supply several working heads.