Abrasive jet system

Abstract

An abrasive jet system for producing an abrasive cutting jet is provided. The system includes a source of abrasive suspension connected via a conduit to a cutting head. A source of displacement fluid is provided to flush abrasive suspension in the conduit towards the source of abrasive suspension, when the pressure at the source of abrasive suspension is below the pressure at the source of displacement fluid and no water jet is present in the cutting head. Also provided is a method for suspending a settled or partly settled bed of abrasive.

Claims

1. An abrasive jet system for producing an abrasive cutting jet, the abrasive jet system comprising a cutting head having at least two connections and adapted to receive pressurised water at a first connection of the cutting head to generate a high velocity waterjet, a source of abrasive suspension, a conduit, at a second connection of the cutting head, for providing the source of abrasive suspension in fluid communication with the cutting head, an abrasive suspension on/off valve provided in said conduit and having an open state and a closed state, a source of displacement fluid in fluid communication with a portion of said conduit located between the cutting head and the abrasive suspension on/off valve, wherein the pressure at said source of abrasive suspension is controllable to be below the pressure at said source of displacement fluid, wherein when said abrasive suspension on/off valve is open and said high velocity waterjet is present in the cutting head, the abrasive suspension flows via said conduit into the cutting head and is entrained by the high velocity waterjet in the cutting head to produce an abrasive cutting jet, and wherein when the cutting head is void of high velocity waterjet and said pressure at the source of abrasive suspension is below the pressure at said source of displacement fluid, and said abrasive suspension on/off valve is opened, flow of abrasive suspension present in the conduit is reversed.

2. The abrasive jet system as claimed in claim 1, wherein said pressure at said source of abrasive suspension is a subatmospheric pressure.

3. The abrasive jet system of claim 1, further comprising a control system configured to control the opening and closing of said abrasive suspension on/off valve, wherein when said abrasive suspension on/off valve is closed, and the cutting head is void of high velocity waterjet, the control system is adapted to, within a time period, open said abrasive suspension on/off valve to allow displacement fluid to displace abrasive suspension present in the conduit towards said source of abrasive suspension.

4. The abrasive jet system of claim 1, further comprising a hopper containing a bed of abrasive particles which, in operation of the abrasive jet system, is suspended, and a flow circuit having an inlet end and an outlet end which are in fluid communication with said hopper, wherein said conduit is connected to said flow circuit at a junction point, wherein said junction point represents said source of abrasive suspension.

5. The abrasive jet system of claim 4, further comprising a control unit configured to control the pressure at said junction point, and a pressure sensing device adapted to monitor the pressure in the flow circuit in the vicinity of said junction point to provide a signal indicative of said pressure to the control unit.

6. The abrasive jet system of claim 5, further comprising a pump in the flow circuit downstream of said junction point, wherein the pressure at the junction point is controlled by controlling the flow through the pump.

7. The abrasive jet system as claimed in claim 6, wherein said pump is in the form of a peristaltic pump with variable speed drive, wherein the pressure at the junction point is controlled by controlling the speed of the pump.

8. The abrasive jet system of claim 6, wherein said control unit is operatively connected to the pump and adapted to control the speed of a fluid flow through the pump based on said signal from the pressure sensing device, thereby controlling the pressure at said junction point.

9. The abrasive jet system of claim 6, wherein said hopper includes cover water above the bed of abrasive particles, the abrasive jet system further comprising a control system, and an agitator for agitating said bed of abrasive particles, the agitator being controlled by the control system, wherein the dimensions of said hopper is such that the agitated bed of abrasive particles are maintained with a depth of cover water above the bed such that the cover water surface is quiescent.

10. The abrasive jet system of claim 9, wherein the inlet end of the flow circuit is positioned at or positionable to a location in the agitated bed of abrasive particles where the abrasive concentration is that required at the cutting head.

11. The abrasive jet system of claim 9, wherein water is fed into said hopper either directly or into the flow circuit downstream of said junction point.

12. The abrasive jet system of claim 9, wherein said hopper is provided with an overflow pipe.

13. The abrasive jet system of claim 9, wherein said hopper is mounted on a weight sensor for determining the amount of abrasive in said hopper.

14. The abrasive jet system of claim 13, wherein a source of dry abrasive particles is provided above the surface of the cover water.

15. The abrasive jet system of claim 14, wherein said control system is operatively connected to the weight sensor and the source of dry abrasive particles in order to control the discharging of dry abrasive particles from said source of dry abrasive particles onto the surface of the cover water based on an input signal from the weight sensor.

16. The abrasive jet system of claim 13, wherein said agitator is driven by a variable speed motor and provides a signal of the torque on the agitator.

17. The abrasive jet system of claim 16, wherein said control system is configured to start up an abrasive feed system with the agitator positioned in the cover water starting up the agitator, measuring the torque on the agitator, and repositioning the agitator into the abrasive bed in the hopper at such a rate that a predetermined load change on the weight sensor is not exceeded.

18. The abrasive jet system of claim 16, wherein said control system is configured to shut down an abrasive feed system in such a manner as to be able to automatically re-start the abrasive feed system, wherein when shutting down the abrasive feed system the control system stops the agitator, re-positions the agitator and the inlet end of the flow circuit into the cover water, opens the abrasive suspension on/off valve to clear abrasive from said conduit followed by closing the abrasive suspension on/off valve, and after a sufficient time period to clear abrasive from the flow circuit stops the pump.

19. The abrasive jet system of claim 9, wherein said agitator is movable between said bed of abrasive particles and said cover water.

20. The abrasive jet system of claim 9, wherein said inlet end of the flow circuit is movable between said bed of abrasive particles and said cover water.

21. The abrasive jet system of claim 9, wherein said control system is configured to start up an abrasive feed system with the agitator positioned in the cover water start up the agitator, measuring the torque on the agitator, and repositioning the agitator into the abrasive bed in the hopper at such a rate that a predetermined torque on the agitator is not exceeded.

22. The abrasive jet system of claim 21, wherein during said start up, said control system is further configured to with the inlet end of the flow circuit positioned in the cover water start up the pump, and reposition the inlet end of the flow circuit into the abrasive bed.

23. The abrasive jet system of claim 5, further comprising a variable restriction in the flow circuit upstream of said junction point, wherein the pressure at the junction point is controlled by controlling the flow through the restriction.

24. The abrasive jet system of claim 23, wherein said variable restriction is in the form of an actuator adapted to act on flexible tubing to cause a pressure drop to lower the pressure at said junction point.

25. The abrasive jet system of claim 23, wherein said control unit is operatively connected to the variable restriction and adapted to vary the restriction based on said signal from the pressure sensing device, thereby controlling the pressure at said junction point.

26. The abrasive jet system of claim 4, wherein a rate of flow in said flow circuit is greater than a rate of flow in said conduit to the cutting head.

27. The abrasive jet system of claim 1, further comprising a focus tube having an outlet from which said cutting jet is discharged, the focus tube outlet being submerged in ambient fluid, thereby allowing ambient fluid to enter through the focus tube outlet when said cutting jet is not discharged, wherein, when said cutting jet is not discharged, said source of displacement fluid is the ambient fluid entered in the focus tube.

28. The abrasive jet system of claim 1, wherein a valved source of displacement fluid is connected to the cutting head via a valved connection such that when there is no waterjet present in the cutting head, opening the source of displacement fluid valve and opening the abrasive suspension on/off valve in the conduit causes abrasive suspension to be displaced from the conduit towards the source of abrasive suspension, and when said waterjet is present in the cutting head, opening the source of displacement fluid valve causes displacement fluid to flow to the cutting head to be entrained by the waterjet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will now be described with reference to the enclosed figures, where

(2) FIG. 1 shows an entrainment cutting head.

(3) FIGS. 2 and 3 show abrasive suspension feed systems for a cutting head.

(4) FIG. 4 shows an abrasive suspension hopper.

(5) FIG. 5 shows part of an abrasive suspension feed system.

(6) FIG. 6 shows an abrasive suspension feed system.

DETAILED DESCRIPTION OF THE DRAWINGS

(7) Referring first to FIG. 1 that shows a cutting head 14 that generates a cutting jet 108. Pressurised water from a source 15 flows via pressurised water shut off valve 110 and collimation tube 102 to a waterjet nozzle 103 to produce a high velocity waterjet 104. The waterjet 104 traverses an entrainment chamber 105 and enters the bore 106 of a focus tube 107. In traversing entrainment chamber 105 the waterjet 104 entrains abrasive suspension from source 100 through conduit 19, with abrasive suspension on/off valve 13 to passageway 111 and into entrainment chamber 105 and focus tube bore 106. In focus tube bore 106 momentum is exchanged between the waterjet 104 and abrasive particles and water in abrasive suspension to generate the cutting jet 108 at a focus tube outlet 118.

(8) A source of displacement fluid 16 at above atmospheric pressure may be provided through valve 17, tubing 117 and passageway 112 to entrainment chamber 105 or a source of displacement fluid 16 may be provided through valve 18 to conduit 19 between abrasive suspension on/off valve 13 and cutting head passageway 111. The displacement fluid will usually be water although in some circumstances pressurised air may be preferable.

(9) To cut effectively with cutting heads described in EP 2 097 223 B1 and WO 2011/070154 A1 the pressure of abrasive suspension from source 100 needs to be at a pressure below atmospheric to achieve desired abrasive concentrations in cutting jet 108. If pressurised water shut off valve 110 is closed and abrasive suspension on/off valve 13 is open and if separate source of displacement fluid 16 is not provided or valves 17 and 18 (FIG. 1) are closed, ambient fluid 26 (FIG. 6) at 118 is drawn in focus tube bore 106. The ambient fluid drawn in at 118 is air unless focus tube 107 is submerged when water is drawn in. The fluid drawn into focus tube bore 106 flows through entrainment chamber 105, passageway 111 and conduit 19 towards the abrasive suspension source 100.

(10) With pressurised water shut off valve 110 closed and abrasive suspension on/off valve 13 open and either valve 17 or 18 open displacement fluid from source 16 flows towards the abrasive suspension source 100. In the case of pressurised displacement fluid flowing through valve 17 it passes through tubing 117, passageway 112, entrainment chamber 105, passageway 111, and conduit 19 with abrasive suspension on/off valve 13 displacing abrasive suspension towards the source of abrasive suspension 100. In passing through entrainment chamber 105 part of the displacement fluid also flows through focus tube bore 106 to discharge at focus tube outlet 118. When displacement fluid from a source 16 enters through valve 18 to conduit 19 it flows towards the source of abrasive suspension 100 through valve 13 and also towards the focus tube outlet 118 through passageway 111, entrainment chamber 105 and focus tube bore 106.

(11) With pressurised water shut off valve 110 and abrasive suspension shut off valve 13 open, and if either a valve 17 or 18 is open to a source of displacement fluid 16 at a pressure above that of the source of abrasive suspension 100, displacement fluid from source 16 is entrained by a waterjet 104. Displacement fluid also flows towards the abrasive suspension source 100.

(12) With pressurised water shut off valve 110 open and abrasive suspension shut off valve 13 closed and a displacement fluid valve 17 or 18 open the flow of displacement water from a source 16 may be varied so as to change the characteristics of the waterjet at the focus tube 107 outlet 118. The amount of water flowing from source 16 can increase or decrease the intensity of cavitation on a workpiece close to the focus tube 107 outlet 118. The ability to vary cavitation intensity can be useful in marking and etching workpiece surfaces.

(13) When an abrasive waterjet 104 is entraining abrasive suspension the concentration of abrasive in cutting jet 108 can be reduced by metering displacement water from source 16 through valve 17 or 18. In the case of displacement water metered through valve 17 rapid changes in abrasive concentration in cutting jet 108 are possible to provide control of material removal during etching and milling.

(14) Referring now to FIG. 2 that shows an abrasive suspension feed circuit 1 for an entrainment cutting head 14. Hopper 2 has an agitator 3 with stirrer 5 and is partially filled with abrasive/water mixture 4 with an abrasive content of typically 60 to 70 wt %. Hopper 2 is topped up with abrasive suspension from source 10 through pump 22 and conduit 9. A pump 22, which will usually be a reversible peristaltic pump, transfers abrasive suspension 4 into and out of hopper 2 through conduit 9. Hopper 2 is sealed from the environment and air space 20 is connected via conduit 23 to a vacuum source 21.

(15) Abrasive suspension 4 enters conduit 19 with abrasive suspension on/off valve 13 at point 7. With valve 110 open to pressurised water source 15, and valve 13 open a waterjet 104 in cutting head 14 entrains abrasive suspension from hopper 2 at 7 via conduit 19 to entrainment chamber 104 of cutting head 14. The rate of abrasive suspension flow is dependent on the abrasive suspension rheological properties at point 7, on the pressure loss characteristics of the flow line between point 7 and entrainment chamber 104 and on the pressure differential between point 7 and entrainment chamber 104. The pressure at point 7 is controlled by the vacuum source 21. Thus, in this exemplary embodiment, point 7 may be regarded as a source of abrasive suspension which may be arranged in fluid communication with the cutting head 14 via the conduit 19.

(16) A source of displacement fluid 16 may be connected via valve 17 to entrainment chamber 105 of cutting head 14 or connected via valve 18 to conduit 19.

(17) With pressurised water valve 110 closed conduit 19 can be essentially emptied of abrasive suspension when abrasive suspension shut off valve 13 is open and valve 18 or 17 are open to a source of displacement fluid 16 at a pressure above atmospheric pressure. Part of the displacement fluid also flows through focus tube 107.

(18) When a source of displacement fluid 16 is not provided or valve 17 or 18 if present are closed, displacement fluid from the environment at focus tube 107 outlet 118 enters a focus tube when valve 110 is closed and valve 13 open and there is a vacuum in space 20 in hopper 2. The displacement fluid is air unless a focus tube 107 outlet is submerged in water when the displacement fluid is water. The displacement fluid flows through conduit 19 to discharge into hopper 2 at point 7.

(19) Displacement air entering hopper 2 at point 7 bubbles up through abrasive suspension 4 and is exhausted through conduit 23. To limit the amount of air that vacuum source 21 has to pump valve 13 is desirably scheduled to be open for a time just sufficient to allow abrasive suspension to be cleared from conduit 19. In order to avoid changing the abrasive concentration in abrasive suspension 4 in vessel 2 when the displacement fluid is water, valve 13 is desirably scheduled to be open for a time just sufficient for displacement water to clear abrasive suspension from conduit 19.

(20) Valve 13 may be located at point 7 to conduit 19 to avoid programming periodic opening and closing of valve 13 to flush migrating and dewatering abrasive that enters conduit 19 at point 7 after initial clearing of abrasive suspension from conduit 19.

(21) Depending on abrasive suspension characteristics and the addition of additives, abrasive suspension 4 in hopper 2 may be maintained in a suitable condition without agitator 3 and stirrer 5. Instead it can be arranged that abrasive suspension 4 is cyclically pumped into and out of hopper 2 by pump 22, whilst maintaining sufficient abrasive suspension 4 in hopper 2 to supply abrasive suspension 4 to cutting head 14.

(22) Referring now to FIG. 3 that shows an abrasive suspension feed system 30 in which substantially more abrasive suspension is pumped in a flow circuit passing close to a cutting head 14 than is required by a cutting head. The flow circuit is formed by conduit 6 with inlet 77 and variable flow restriction 31, conduit 8, a junction at point 7, conduit 32, pump 33 and conduit 34 with outlet 35 from the flow circuit in hopper 2. In this exemplary embodiment, junction point 7 may be regarded as a source of abrasive suspension which may be arranged in fluid communication with the cutting head 14 via conduit 19.

(23) The flow direction in the flow circuit may be reversed so that the pressure at junction 7 can be above or below the pressure acting on the surface 23 in hopper 2. Reversal of flow direction may be necessary if a blockage occurs in the flow circuit.

(24) The pressure at junction point 7 is controlled by varying the speed of pump 33, by the setting of restriction 31, by the abrasive suspension surface level 23 in hopper 2 relative to cutting head 14 and by the pressure in air space 20 acting on surface 23. The pressure in air space 20 is the air source pressure 24 connected through conduit 11 and is preferably atmospheric pressure.

(25) A pressure sensor 38 located close to junction point 7 provides a signal for setting the pressure at junction point 7.

(26) Operation of hopper 2 of FIG. 3 is generally as for hopper 2 of FIG. 2 as is the function of valves 13, 17 and 18.

(27) Referring now to FIG. 4 that shows part of an abrasive suspension feed system 70 that with part of an abrasive suspension feed system 40 of FIG. 5 forms a complete abrasive suspension feed system.

(28) FIG. 4 shows that a layer of cover water 73 is provided above a suspended abrasive bed 72. An agitator 51 with impeller 50 is adapted to stir the bed 72. A weight sensor 55 comprising a load cell 25 is mounted to the hopper 71, and is vertically movable (double arrow 56) depending on the change of load. The vertical movement may be used as an output to a control system 27 for adding dry abrasive powder particles, or controlling the rate at which the impeller 50 is lowered into the bed 72 during start up.

(29) Dry abrasive powder particles are contained in a silo 60 which is mounted in a holder 61. When powder is to be discharged, a valve 69 in a discharge pipe 63 is opened and a vibrator 62 is actuated to provide vibrations to the holder 61, whereby the powder particles fall down to the surface 54 of the cover water 73 and descend in a density current to the agitated abrasive bed 72 with little mixing with the cover water 73. Water from a water source 64 through connection 65 may be bled into the abrasive bed 72 at a controlled rate and appropriate location to help maintain the desired abrasive concentration at inlet 77 where abrasive suspension is withdrawn from the bed 72.

(30) FIG. 4 also shows an overflow 52 at the top portion of the hopper 71. Excess water flowing up through the bed 72 over flows at the cover water surface 54 to maintain a set water level allowing continuous calculation of the weight of abrasive in the hopper 71 and to carry away material in the abrasive feed that does not wet and floats on the cover water surface 54.

(31) Referring now to FIG. 5 that shows part of abrasive suspension feed system 30 of FIG. 3 but with additional features that makes it particularly suitable for use with the abrasive suspension hopper 71 of FIG. 4.

(32) To prevent abrasive accumulating and dewatering at junction point 7 when there is no flow through conduit 19 to cutting head 14, abrasive suspension valve 13 of FIG. 3 is replaced by valve 45 located at junction point 7.

(33) When the hopper 71 of FIG. 4 is operating it requires a flow of topping up water to maintain the cover water level 54. Topping up water beneficially flows into conduit 32 to reduce the abrasive concentration at entry to pump 33.

(34) Direct measurement of abrasive suspension vacuum pressures that can fall below 400 mm Hg is difficult because abrasive accumulates in connections to and the space within a pressure gauge or pressure transducer. By measuring the pressure of topping up water entering conduit 32 the pressure at junction point 7 can be inferred without abrasive suspension being in direct contact with a pressure transducer. Topping up water from source 47 flows through valve 46, conduit 39 to a non-return valve 48 that prevents abrasive suspension entering conduit 39 from conduit 32. Non-return valve 48 may be of a duckbill or similar valve made of polymeric material with a low pressure drop. Pressure sensor 49 measures the pressure in conduit 39, which is essentially the pressure at junction point 7 that controls the flow of abrasive suspension to cutting head 14 and provides a signal indicative of the pressure to the control unit 28.

(35) To provide a measure of abrasive concentration the weight of abrasive suspension in a section of conduit 44 can be determined. Conduit 44 is attached by flexible joints 41 and 42 to conduit 8. A load cell 43 is used to determine the change in weight between water flow through conduit 44 and abrasive suspension flow allowing a calculation of suspension density.

(36) Although in FIGS. 3-5 one cutting head 14 is connected to flow circuit 8, 32 between restrictor 31 and pump 33 two or more cutting heads of an abrasive waterjet cutting apparatus could be connected to flow circuit 8, 32.

(37) Referring to FIG. 6 that shows a suspension feed system 80 that has features of feed system 1 of FIG. 1 and of hopper 71 of FIG. 4. A dry abrasive powder silo 81 is connected to hopper 89 to allow topping up of hopper 89 with dry abrasive powder 83 in a manner similar to topping up hopper 71 of FIG. 4 but with a vacuum in air space 20 above the cover water 73. Silo 81 has sealable lid 82 to allow filling up silo 81 with abrasive powder 83. Conduit 86 with isolation valve 85 allow pressures in air space 90 in silo 81 to be equalised with the pressure in airspace 20 in hopper 89 provided by vacuum source 21 through conduit 23. Powder valve 87 is opened when pressures in air spaces 20 and 90 are equalised. With powder valve 87 open actuation of vibrator 62 cause abrasive powder 83 to flow from silo 81 through connection 88 and fall onto cover water 73 surface 54.

(38) With valves 85 and 87 closed and valve 84 open the pressure across sealable lid 82 is equalised and the lid 82 can be opened to top up silo 81 with abrasive powder 83.

(39) When rotation of stirrer 5 is to be stopped for a time period sufficiently long that abrasive bed 72 could settle the majority of the bed 72 may be removed through conduit 92 with inlet at 93 by pump 10 to storage source 91. Alternatively an additive that prevents firm settling of abrasive particles may be introduced into bed 72 from source 91 or by other means to allow stirrer 5 to start rotation in a settled bed 72 without excessive torque.

(40) A signal from level sensor 94 along with the output from load cell 25 can be used to maintain the cover water level 54 from water source 96 through connection 97 and to control topping up abrasive bed 72 with abrasive from silo 81.

(41) An abrasive waterjet apparatus for machining forms part of a machine tool. The machine tool is required to be highly automated and this includes the abrasive feed system. Except for the filling of a bulk hopper with dry abrasive powder all abrasive suspension feed system functions need to be automated and controlled by the machine tool control system through the control system of the abrasive waterjet apparatus. Automation extends from initiating start up actions that make abrasive suspension of a specified concentration available close to a cutting head to the shutting down of a feed system for an extended period in such a manner as to allow automated start-up of the system at a later date. The preferred abrasive suspension feed system embodiment of this invention, shown in FIGS. 4 and 5, is particularly suited to automated operation.

(42) The exception to full automation is loss of electrical power for a period sufficiently long that troublesome settling of abrasive particles occurs when manual intervention is required using a source of pressurised water or compressed air to clear abrasive suspension for tubing and passageways to prevent blockages on restarting a feed system. Abrasive feed system tubing is arranged to have quick connect and disconnect connections including the tubing for the peristaltic pump to allow easy access to flush abrasive suspension from a flow circuit following loss of power.