EP3787837B1

EP3787837B1 - Fluid jet processing

Info

Publication number: EP3787837B1
Application number: EP19718840.2A
Authority: EP
Inventors: Anthony Beaucamp; Tomoya KATSUURA
Original assignee: Zeeko Innovations Ltd
Current assignee: Zeeko Innovations Ltd
Priority date: 2018-04-20
Filing date: 2019-04-15
Publication date: 2022-10-12
Anticipated expiration: 2039-04-15
Also published as: TWI818980B; GB2573012A; EP3787837A1; GB201806475D0; WO2019202299A1; TW201945127A

Description

The present invention relates to methods and apparatus for processing workpieces using a fluid jet. In particular the invention relates to a fluid jet treatment apparatus according to the preamble of independent claim 1, a fluid jet treatment tool according to the preamble of independent claim 5 and a treatment process comprising using such a fluid jet treatment apparatus. A fluid jet treatment apparatus according to the preamble of claim 1 and a fluid jet treatment tool according to the preamble of claim 5 are known from document US 5,154,347 . The workpiece may be subjected to shaping and polishing workpieces using a jet of fluid in which abrasive particles are suspended. Alternatively the workplace may be subjected to cleaning or heat transfer processes (heating or cooling) by directing a fluid jet onto the workpiece surface.
Fluid jet polishing (FJP) is a versatile polishing process mainly used for ultra-precision finishing of small and complex surfaces such as optical components and artificial joints. In this technique, a pre-mixed slurry of carrier fluid and abrasive particles is pressurized and ejected as a jet from a nozzle onto a workpiece. The typical pressure of the inlet slurry is from 0.2 to 2.0 MPa, and the outlet diameter of the nozzle from 0.1 to 2.0 mm. Impingement of the jet onto a workpiece surface results in material being removed from a "footprint" in a time-dependent fashion, i.e the longer the jet impinges on a particular footprint, the more material is removed from that footprint.
This process has some important advantages. First, the slurry jet can generate sub-millimetre polishing footprints, which makes it possible to reach difficult areas such as corners and cavities. Second, the process may be used to remove machining marks on a workpiece caused by earlier treatment processes without generating additional traces. Finally, the process accuracy can be predictively controlled because tool wear is usually limited to the slow and progressive breakdown of abrasive particles. Due to these advantages, the FJP process is mainly used for super-smooth finishing of small and complex components in various industries such as optics and the medical field (for example the surfaces of artificial joints).
The conventional FJP process provides a controllable sub-millimetre polishing footprint and almost no tool wear, but a disadvantage of the technique is its low material removal rate.
In recent years, the demand has grown for a technique to efficiently finish larger workpieces or hard materials down to a super-smooth surface. Also, Additive manufacturing has become an important technology for prototyping precision components, but the surfaces of additively manufactured components are so rough that the components cannot be used without undergoing a finishing process. Typical surface roughness of some additively manufactured components made by Selective Laser Melting (SLM) method can be about 10 µm Ra, so a processing method for efficiently finishing such a rough surface is required.
To meet these needs, improvement in material removal rate is required, and a number of process enhancements have been proposed in order to improve material removal rate of FJP processes.
In an article entitled "A novel multi-jet polishing process and tool for high-efficiency polishing" published in the International Journal of Machine Tools and Manufacture in 2017, Wang et a1. proposed parallel processing using a nozzle array to provide a plurality of fluid jets directed at the workpiece. Material removal rate improves by jetting simultaneously from multiple outlets. This process is suitable for the efficient finishing of precise surfaces consisting of repetitive elements. However, interference between the individual jets often results in inferior uniformity of material removal rate as compared with FJP using a single jet. In addition, adaptability to complex shapes is also reduced.
Another variant of the FJP technique is magnetorheological jet finishing, in which the fluid contains micrometre-scale ferromagnetic particles. The apparent viscosity of the fluid greatly increases when exposed to an external magnetic field. When this fluid is used as a carrier fluid in an FJP process, the jet is stabilized and stiffened by the application of a magnetic field. The lower likelihood of the jet collapsing enables precise processing of more remote and difficult areas, while increase of the apparent viscosity improves removal rate. However, the handling and recycling of magnetorheological fluid requires highly specialized equipment and know-how, which makes the implementation of a magnetorheological fluid assisted FJP inconvenient and costly.
Another proposal has been the injection of air bubbles into the slurry jet, in order to energize the motion of abrasive particles within the carrier fluid. Messelink et al. devised a method of intermittently introducing compressed air into the slurry using a mixing valve (Exploiting the process stability of fluid jet polishing, Optical Fabrication and Testing, OSA Technical Digest 2008, pp. OThD3). The method succeeded in improving material removal rate of FJP, but the slurry jet became unstable because the introduced bubbles were too large, and were irregular in size. Although removal rates were improved, there was a reduction in machining accuracy and a significant deterioration in surface roughness (more than 10,000%).
The present invention seeks to provide a FJP process in which small footprints of the workpiece may be treated accurately, with an efficacious treatment process. The treatment process may be a shaping or polishing process using a treatment fluid containing abrasive particles and providing an improved material removal rate without compromising the surface finish, or may be a cleaning or a heat transfer process using a treatment fluid without abrasive particles with improved cleaning performance and heat transfer rates, respectively.
The present disclosure in one aspect provides fluid jet treatment processes in which a jet of fluid is directed at the workpiece, the fluid jet including discrete regions of fluid having different density and/or viscosity properties from the remaining treatment fluid. In some examples, regions of reduced fluid density are produced by generating or injecting micro-bubbles in the fluid. In other examples, regions of different fluid density are produced by injecting micro-droplets of a second fluid into the treatment fluid. In a treatment process which is intended to shape or the workpiece surface by removing material from the workpiece, the material removal rate is improved as compared to conventional FJP, and the accuracy and surface finish obtainable by the process is maintained or improved.
In another aspect, the present disclosure seeks to provide a fluid jet treatment process in which heat transfer between a workpiece and the fluid is improved, to provide an effective cooling or heating of a workpiece by the application of a fluid jet.
In a third aspect, the present disclosure seeks to provide a fluid jet treatment process in which a workpiece can be effectively cleaned by using a fluid jet to remove surface impurities.
According to a first aspect of the invention, there is provided a fluid jet treatment process in accordance with claim 8 hereto. A jet of treatment fluid is directed at a workpiece to impact on the workpiece at a treatment footprint, the process being characterised by the jet of fluid having regions of different density and/or viscosity. These regions are produced by applying ultrasonic vibrations to the fluid such that cavitation bubbles are periodically formed in the fluid and are entrained in the fluid jet to form regions of reduced density. Most preferably, the cavitation bubbles are transported by the fluid jet to the point of impact with the workpiece.
In comparative examples, the regions of different density and/or viscosity within the jet may be produced by periodically generating or injecting micro-bubbles into the treatment fluid, such that groups of micro-bubbles are entrained in discrete regions of the jet.
In yet a further comparative example, regions of different viscosity may be produced by periodically injecting into the treatment fluid micro-droplets of a second fluid, having a different density from the treatment fluid, so that the micro-droplets are entrained in discrete regions of the jet.
The application of ultrasonic vibrations to the treatment fluid may be performed in combination with the injection of micro-bubbles or micro-droplets into the treatment fluid.
A second aspect of the invention provides a fluid jet treatment apparatus in accordance with claim 1 hereto. The apparatus comprises:

a support for a workpiece;
a treatment head for producing a jet of fluid; and
means for directing the jet of fluid at successive locations on the surface of the workpiece;
wherein the treatment head includes means for forming regions of different density and/or viscosity in the jet.

The means for forming the regions of different density and/or viscosity comprises an ultrasonic vibration generator for generating ultrasonic pressure waves within the fluid to cause cavitation bubbles to form periodically within the treatment fluid and pass in groups along the fluid jet, forming regions of reduced density. Most preferably, the treatment head is configured to be held sufficiently close to the workpiece that cavitation bubbles impact on the workpiece surface.
The treatment head includes a cavity having an inlet duct through which fluid may be supplied to the cavity and an outlet passage from which fluid can exit the cavity in the form of a jet. The cavity has one or more resonant frequencies at which "standing" acoustic waves may be generated by driving the vibration generator at or near those frequencies. The ultrasonic vibration generator is preferably operable at a number of frequencies, including one or more resonant frequencies of the cavity.
The shape of the cavity is conical, with the outlet passage arranged axially at the narrower end of the conical cavity and the vibration generator positioned at the larger axial end of the cavity. The vibration generator comprises a concentrator plate having a concave face directed towards the passageway, to concentrate the waves at a focus point adjacent the passageway. The intensity of pressure waves at the focus point should exceed the pressure of the fluid in the cavity.
The travel time of the fluid from the focus point (cavitation area) to the workpiece surface is preferably lower than the average lifetime of the cavitation bubbles. The frequency of the pressure waves maybe adjusted to provide a sufficient average lifetime for the cavitation bubbles for them to reach the workpiece surface. The shape and size of the cavity within the polishing tool may be arranged such that it has a resonant frequency which, when the fluid is vibrated at the resonant frequency, result in the formation, at a cavitation area within the cavity, of cavitation bubbles having an average lifetime equal to or greater than the time taken by the fluid to travel from the cavitation area to the workpiece.
In other embodiments of this aspect, the means for forming the regions of different density and/or viscosity also comprises means for periodically injecting or generating micro-bubbles within the fluid such that groups of micro-bubbles are entrained in the jet to form the discrete regions. In yet a further alternative embodiment, the means for forming regions of different viscosity may also comprise means for periodically injecting into the treatment fluid micro-droplets of a second fluid, having a different density from the treatment fluid, so that the micro-droplets are entrained in discrete regions of the jet.
A third aspect of the invention provides a fluid jet treatment tool for generating a fluid jet including regions of different density and/or viscosity, in accordance with claim 5 hereto. The tool comprises:

a treatment head having an exit passage for producing a jet of fluid; and
means for forming regions of different density and/or viscosity in the jet.

The means for forming regions of different density and/or viscosity in the jet comprises an ultrasonic actuator for applying ultrasonic vibrations to the fluid in the polishing head, such that cavitation bubbles are generated periodically within the fluid and are entrained in the jet.
The treatment head includes a cavity into which the pressurised fluid is supplied, and a passageway leading from the cavity through which the fluid exits to produce the jet. The ultrasonic actuator is preferably operable at one or more frequencies which produce resonant standing waves within the cavity.
In other embodiments of this aspect, the means for forming the regions of different density and/or viscosity may also comprise means for periodically injecting or generating micro-bubbles within the fluid such that groups of micro-bubbles are entrained in the jet to form the discrete regions, or means for periodically injecting into the treatment fluid micro-droplets of a second fluid, having a different density from the treatment fluid, so that the micro-droplets are entrained in discrete regions of the jet.
A pump means may be used to supply a pressurised fluid to the treatment head. Treatment fluid which has impacted on the workpiece may be collected and recirculated.
The treatment applied using the apparatus and process of the present invention may be a shaping treatment to shape the workpiece, a polishing treatment to polish the surface of the workpiece, or a cleaning treatment to remove impurities from the surface of the workpiece.
Embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a fluid jet polishing apparatus according to the invention;
Figure 2 is a schematic sectional view of a fluid jet polishing tool according to the invention;
Figure 2A is an partial enlarged view of the fluid jet of Figure 2;
Figure 2B is an partial enlarged view of the fluid jet similar to Figure 2A, showing a second embodiment of the invention;
Figure 2C is an partial enlarged view of the fluid jet of Figure 2A, showing a third embodiment of the invention;
Figure 3 is a schematic view illustrating the fluid jet polishing apparatus of Figure 1;
Figure 4 is a block diagram of the control system of the apparatus of Figure 1;
Figure 5 shows the data stored in the tool path memory;.

Figure 1 is a perspective diagram of a polishing machine which is the treatment apparatus according to one embodiment of the present invention. The polishing machine comprises a table 1, on which there is mounted an X-slide mechanism 2 for movement in the x-direction. On the X-slide mechanism 2 there is mounted a Y-slide mechanism 3 for movement in the y-direction. On the Y-slide mechanism 3 there is mounted a turntable 4 for rotation in the direction indicated by arrow c about a vertical axis (as seen in the Figure).
The turntable 4 is mounted on the Y-slide mechanism 3 via a Z-movement mechanism (not shown) for movement of the turntable 4 in the z direction (the vertical direction as seen in the Figure). The turntable 4 has a support surface onto which a workpiece 5 is mounted and held. Thus, this arrangement provides for motion of the workpiece 5 in four axes - namely x, y, z and c.
The polishing machine is also provided with a back member 6 on which is mounted a pivot arrangement for pivotally moving a polishing head 7. The polishing head 7 is arranged to direct a jet 8j of polishing fluid 8 on to the workpiece 5 from a lower axial end (as seen in the Figure) of the polishing head 7, for polishing or abrading the workpiece 5. After impinging on the workpiece 5, the polishing fluid 8 runs under gravity off the workpiece 5 and is collected for recirculation.
The pivot mechanism mounted on the back member 6 comprises a first pivot member 700 mounted in an arm for pivoting the polishing head 7 about a pivot point in a first plane, so that the jet 8j may be directed onto the workpiece. The first pivot mechanism 700 is mounted on a second pivot mechanism 800 which provides for the pivoting of the polishing head 7 about a pivot point in a plane perpendicular to the plane of pivoting of the first pivot mechanism 700 in the arm. Thus these two orthogonal pivoting mechanisms provide two further axes of control, namely a and b, for controlling the angle at which the jet 8j is directed towards the workpiece 5.
The back member 6 of the polishing machine also houses a computer control system 9 which includes a display 10 and control inputs 11. This allows a user to input data or commands to control the motion of the workpiece 5 and of the polishing head 7 and to view displayed information regarding the abrading, polishing or cleaning process.
Each of the axes of motion x, y, z, c, a and b is driven by respective drive actuator (not shown). Sensors (not shown) are also provided for sensing the positions of the actuators to provide position information for use by the computer control system 9 to control the polishing or abrading process.
The computer control system 9 is also provided with further axes of control related to the polishing head 7. These include the pressure of the fluid, and control of an ultrasonic actuator acting on the fluid within the polishing head 7. These additional axes of control will be described below in more detail, in relation to the polishing head 7.
Thus, the computer control system 9 operates an algorithm to control these axes (or parameters) in order to clean, abrade or polish the workpiece 5 mounted on the turntable 4, to achieve the desired surface profile and/or surface quality (such as smoothness).
As will be apparent to those skilled in the art, the polishing machine can be used to treat any desired surface profile including a surface profile containing both concave and convex areas.
As will be described in more detail below, in this embodiment, the computer control system 9 is arranged to control the relative movement of the polishing head 7 and the workpiece 5 so that the jet 8j is arranged to impinge on the workpiece at a treatment region or "footprint" which is moved relative to the workpiece so as to follow a tool-path over the surface of the workpiece 5, and controls the amount of polishing over the surface by: (i) varying the time spent at each point along the tool-path (the dwell time), (ii) varying pressure of the jet 8j at points along the tool-path, and/or (iii) varying ultrasonic energy (frequency and/or power) applied to the fluid forming the jet 8j at different points along the tool path. The relative movement may be produced by keeping the workpiece 5 stationary and moving the polishing head 7, by keeping the polishing head 7 stationary and moving the workpiece 5, or by a combination of movement by the polishing head 7 and the workpiece 5. The tool path may be a raster, a spiral, or any other convenient tool path. In some embodiments, the use of a pseudorandom tool-path can avoid the formation of polishing artefacts that may be generated using a periodic spiral or raster tool-path.
Figure 2 is a schematic sectional view of the polishing tool 7. The tool comprises a tool body 70 formed from steel or other suitable material, within which is formed a conical cavity 71. A narrow outlet passage 72 leads axially from the narrower end of the conical cavity 71 to the front face 73 of the tool. The diameter of the passageway is typically about 1 mm, but may be from 0.5 mm to 2 mm or more.
The passageway may be formed in a an insert 70a of hard, wear-resistant material, such as sapphire, ruby, or a wear-resistant metal, set into the leading end of the tool body 70. With regard to the aforementioned ultrasonic vibration generator, the wider end of the conical cavity is closed by a vibrator plate 74 which can be caused to vibrate in the axial direction of the conical cavity 71 by an actuator 75. The vibration frequencies of the actuator 75 are in the ultrasonic frequency range, from about 15 to about 150 kHz. On the face of the vibrator plate 74 which faces toward the cavity, a concave-faced concentrator plate 76 is mounted. An entry duct 77 leads radially into the cavity, for the supply of polishing fluid 8 to the interior of the cavity 71. When sufficient pressure is applied to the fluid, by pumping fluid into the entry or inlet duct 77, a jet of polishing fluid 8j issues from the passageway 72.
In the illustrated embodiment, a pressure sensor 79 and a temperature sensor 80 detect the pressure and temperature, respectively, of the fluid in the entry duct 77, and relay this information to the control system 9. The pressure and/or temperature may alternatively be sensed at points within the cavity 71.
When the cavity 71 is filled with the polishing fluid, the vibrations induced in the vibration plate 74 by the actuator 75 are transmitted to the fluid by the concentrator plate 76 as a series of curved wave fronts W, which travel through the fluid in the axial direction of the cavity, and are further concentrated by the converging conical sides of the cavity 71 so that at a focus point F within the cavity 71 adjacent the entrance of the passage 72, the pressure waves caused by the vibrator plate 74 are of sufficient magnitude as to cause cavitation bubbles 78 to form within the polishing fluid. The actuator must be driven with sufficient power that the maximum negative pressure caused by the ultrasonic pressure waves in the fluid is sufficiently greater than the fluid pressure generated by the pump 17, such that regions within the fluid experience sufficient negative pressure to form cavitation bubbles.
As the vibration plate 74 vibrates, alternating waves of increased and decreased pressure propagate through the fluid within the cavity 71. The concentrator plate 76 causes these pressure waves to be curved, and the conical shape of the cavity concentrates these pressure waves so that at a point on the axis of the cavity adjacent the entrance to the passage 72, the fluid experiences large periodic variations of pressure. The power applied to the ultrasonic vibrator 75 is such as to produce a variation in pressure within the fluid which causes cavitation bubbles 78 to be formed periodically in the fluid 8. By operating the vibrator 75 at a resonant frequency of the cavity 71 a standing wave may be produced within the cavity, which results in the formation of cavitation bubbles but at reduced power levels to the transducer as compared to non-resonant frequencies. The frequency at which the ultrasonic vibrator 75 is driven determines the periodicity of formation of the cavitation bubbles 78.
The fluid 8 in the cavity 71, pressurised by the operation of the pump 17, exits through the passage 72 to form the jet 8j. The cavitation bubbles 78, which are periodically formed adjacent the inlet to the passage 72, are entrained in the fluid and pass out of the cavity through the passage 72 and into the jet 8j. The bubbles 78 are formed periodically, resulting in the jet 8j being formed from alternate sections of fluid with cavitation bubbles 78, and sections where there are no bubbles 78. The presence of the bubbles in some sections of the jet reduces the overall density at these sections in the jet, and leads to a "hammering" effect on the workpiece where the jet impinges, which is thought to enhance the material removal rate even if the cavitation bubbles collapse before reaching the point of impact with the workpiece material.
The cavitation bubbles are a transient phenomenon, and the size of each bubble and its "lifetime" are dependent on the frequency of the ultrasonic vibrations. The average lifetime of the cavitation bubbles is about 200 pressure cycles of the ultrasonic vibrations, when the frequency is 20 - 200 kHz. The bubbles have a longer lifetime when the frequency of vibration is lower, lasting for about 7.2ms when the acoustic frequency is 26kHz, and lasting for about 1.4ms when the acoustic frequency is about 130kHz. The bubbles 78 are formed within the fluid 8 and are immediately entrained in the fluid passing down the passage 72 and into the jet 8j. The speed of the fluid exiting in the jet 8j is controlled by controlling the pressure exerted by the pump 17 supplying the fluid 8 to the entry duct 77. In the preferred embodiment, the speed is controlled such that cavitation bubbles 78 pass through the passageway 72 and are emitted within the fluid jet 8j. Most preferably, the front face 73 of the polishing tool 7 is positioned sufficiently close to the workpiece 5 that the cavitation bubbles 78 are still present in the jet when the jet strikes the surface of the workpiece 5. This "stand-off distance" between the front face 73 of the polishing tool 7 and the surface of the workpiece 5 being treated is typically 1.5 mm but may be in the range from 1 to 3 mm or more. However, the cavitation bubbles 78 are not required to persist in the jet 8j long enough to reach the surface of the workpiece, in order for the improvement of material removal to be achieved.
The generation of cavitation bubbles 78 depends on the frequency and power of the ultrasonic vibrations applied by the actuator 75, the angle of the conical cavity 71, the diameter of the passageway 72, and the pressure of the fluid supplied to the entry duct 77. In the illustrated embodiment, the conical angle and the diameter of the passageway are fixed by the structure of the tool, and thus control of the generation of cavitation bubbles and their delivery in the jet 8j is effected by controlling the pressure of the fluid and the frequency and power of the vibrations applied by the vibrator plate 74 and the concentrator plate 76.
The treatment fluid 8 is preferably water, but may be a mixture of water and a solvent such as alcohol, or water and a detergent or surfactant. In further alternatives, a solvent such as alcohol may be used alone. For shaping and polishing processes, the treatment fluid contains abrasive particles such as Al₂O₃ whose average particle size ranges from 0.1 to 5 µm, in the amount of approximately 20 g of particles per litre of fluid. The frequency of the actuator 75 is preferably modulated so that the cavitation bubbles produced in the polishing fluid are of a similar size to the abrasive particles, in order to maximise material removal. The polishing head 7 may be configured such that the cavity 71 has a resonant frequency which corresponds to the frequency required to produce cavitation bubbles of a particular size, the polishing fluid may include abrasive particles of a similar size, and the pump pressure may be arranged such that the speed of the fluid in the jet is sufficient for the cavitation bubbles to travel from their creation point to the point of impact of the jet on the workpiece within their lifetime.
Figure 2A is an enlarged view showing the formation and transportation of the cavitation bubbles. The ultrasonic vibrations in the fluid give rise to periodic generation of cavitation bubbles, as the concentrated pressure waves alternately produce high and low pressures at the entrance to the passageway 72. The cavitation bubbles 78 are thus produced in "clouds" at a frequency roughly equal to the frequency at which the transducer 75 is driven. These clouds of bubbles are then carried by the fluid 8 into the passageway 72, and exit the passageway as the jet 8j.
In the illustrated embodiment, the cavity has an axial height of approximately 35 mm, and a diameter at its large end of about 30 mm. The concentrator plate 76 may have a diameter of about 25 mm and a radius of curvature of 35 mm, corresponding to the axial length of the conical cavity 71. The illustrated embodiment provides improved material removal rates when operated with pump pressures of from 0.1 to 1.5 MPa, and at ultrasonic frequencies of from 20 to 130 kHz using an input power to the ultrasonic transducer 75 of 50W. Lower or higher input power may be used, provided that the combination of input power and frequency to the transducer 75, the pump pressure, and the distance from the tool 7 to the workpiece 5 results in the formation of cavitation bubbles 78 and their transportation through the jet 8j to impinge on the workpiece 5. Using the tool of the illustrated embodiment with a pump pressure of 0.8 MPa, and a distance of 1.5 mm between the tool and the workpiece, improved material removal rates, as compared to using the fluid jet alone, were obtained when the transducer 75 was driven with a frequency of 78kHz and a power of 50W.
Figure 2B is a partial sectional view of the polishing head 7, and illustrates an alternative embodiment of the invention. In this embodiment, a hollow injection needle 90 is supplied with pressurised gas, and injects micro bubbles 78 of pressurised gas into the treatment fluid within the cavity 71. The micro-bubbles preferably have a diameter of from 10 to 100 µm The injection needle 90 is provided in addition to the ultrasonic transducer 75, and the tip of the injection needle 90 may be positioned at or near the focus of the ultrasonic waves produced by the transducer 75. When the micro-bubbles are injected simultaneously with the application of ultrasonic waves, the micro-bubbles are broken down so that they have a diameter of between 1 and 10 µm.
The gas pressure applied to the injection needle 90 may be controlled by a valve 92 such that groups of micro bubbles 78 are injected into the treatment fluid at intervals, to form discreet clouds of micro bubbles adjacent the tip of the injection needle 90. These clouds are entrained in the treatment fluid and carried through the jet 8j is the treatment fluid flows out of the cavity 71 through the passageway 72. The gas injected through the injection needle 90 may be air, supplied either from a high-pressure reservoir 91, or directly from a compressor fed from the atmosphere, through the valve 92. Alternatively, the injected gas may be an inert gas such as nitrogen, injected from the high-pressure reservoir 91 and controlled by the valve 92. In this embodiment, the valve 92 may be controlled by the control system 9 of the apparatus, and may be synchronised with the operation of the ultrasonic transducer. For example, the valve 92 may be periodically opened and closed, and ultrasonic vibrations may be applied during the intervals where the valve 92 is open. Alternatively, the valve 92 may be periodically opened and closed, and the ultrasonic vibrations may be applied all of the time. A third alternative is to open and close the valve 92 periodically, and apply vibrations when the valve is closed.
While only one injection needle 90 is shown, it is foreseen that a plurality of micro-needles may be provided, for example extending radially relative to the conical cavity 71 and having their discharge ends arranged in a circle surrounding the focal point F of the cavity.
As an alternative to the reservoir 91 and valve 92, the gas may be supplied to the injection needle 90 via a positive-displacement pump which can deliver a predetermined quantity of gas.
Figure 2C is a partial sectional view of the polishing head 7, and illustrates a further alternative embodiment of the invention. In this embodiment, the body 70 of the polishing tool 7, which is provided with the ultrasonic transducer 75, is formed from an insulating material, and a metallic bead 93 is mounted within the cavity 71, supported by one or more struts 94. The metallic bead 93 is preferably positioned immediately upstream of the entrance to the passageway 72. An induction coil 95 is mounted within the body 70 adjacent the bead 93, and a power supply 96 supplies power to the induction coil 95. The power supply is controlled by the control system 9.
In this embodiment, when power is supplied to the induction coil 95 by the power supply 96, the metallic bead 93 is heated and the treatment fluid adjacent the surface of the bead 78 is caused to boil, forming micro-bubbles of vapour on the surface of the bead. These micro bubbles are entrained in the treatment fluid and are carried through and out of the passageway 72 to form regions in the jet 8j of reduced density, due to the presence of the clouds of micro bubbles in discreet regions of the jet.
As an alternative to heating the bead 93 by an induction coil, a beam 97 from a laser 98 may be directed at the bead 93, and pulsed to provide a localised heating to generate micro-bubbles. The laser 98 may be mounted within the tool body 70, as shown in Figure 2C, so that the beam 97 enters the cavity 71 through a window 99 in the wall of the cavity 71. Alternatively, the laser 98 may be mounted coaxially with the conical cavity, and the laser beam 97 may enter the cavity through a window formed in the centre of the ultrasonic transducer, and travel axially through the cavity to strike the bead 93. In a further alternative, the bead 93 may be omitted, and the laser 98 may be mounted coaxially with the conical cavity with the laser beam 97 entering the cavity through a window formed in the centre of the ultrasonic transducer, and travelling axially through the cavity to exit the cavity coaxially with the passageway 72, and impinge on the workpiece surface at the point of contact of the jet 8j. Pulsing the laser will cause localised heating of the workpiece surface, and micro-bubbles may be generated by localised boiling of the fluid adjacent the impact point of the laser. The pulsed laser may be operated in conjunction with the ultrasonic transducer, so that micro-bubbles are generated at the workpiece surface in addition to cavitation bubbles generated at the upstream end of the passage 72 by the ultrasonic transducer.
In a yet further alternative, an internal heating element may be provided within the bead 93, in order to heat the bead 93 and cause micro-bubbles of vapour to be formed. Other suitable means for heating the bead 93 will be apparent to the skilled man.
The heated bead may be used in conjunction with the ultrasonic transducer, in which case the bead 93 is preferably positioned at or close to the focus position F of the ultrasonic vibrations produced by the transducer 75. The heating of the bead may be synchronised with the application of ultrasonic vibrations.
The bead 93 may have a smooth polished surface, or may have a textured surface to provide nucleation points for boiling of the liquid.
Figure 3 schematically illustrates the polishing apparatus of Figure 1, showing the main components of the fluid jet polishing apparatus.
As can be seen in Figure 3, the workpiece 5 is mounted on the turntable 4, which is itself mounted on the Y-slide mechanism 3. The Y-slide mechanism 3 is mounted on the X-slide mechanism 2. The polishing head 7 is positioned above the workpiece 5 and aligned so that the fluid jet 8j issuing from the polishing head 7 impinges on the workpiece 5. After impacting on the workpiece, the polishing fluid 8 flows down to a collection vessel 15. In the Figure, for illustration purposes, the fluid simply flows down over the turntable 4 and slide mechanisms 3 and 2 to the collection vessel 15. In practice, suitable means may be provided to collect the polishing fluid 8 so that it does not flow over the turntable and slides.
From the collection vessel 15, the polishing fluid flows through a filter 16 to a pump 17 which returns the fluid to the polishing head 7. Within the polishing head 7, the ultrasonic actuator 75 drives the vibrator plate which is in contact with the polishing fluid. An agitator 18 in the collection vessel 15, driven by a motor 19, stirs the polishing fluid in order to keep the abrasive particles evenly suspended in the fluid.
The movements of the turntable 4, the X and Y slides 3 and 2, and the movements of the first and second pivot mechanism is 700 and 800 are controlled by a processor unit 12 within the control system 9, as are the operation of the pump 17, the ultrasonic actuator 14 and the motor 19 driving the agitator 18.
Figures 4 and 5 schematically illustrate the control system 9. The processor unit 12 includes a processor 121 which provides output signals to controllers for the individual elements of the system, namely a pump controller 122, an ultrasound controller 123, an agitator controller 124, a turntable controller 125, X and Y slide controllers 126 and 127, and controllers for the first pivot mechanism 128 and the second pivot mechanism 129. The processor unit may also include controllers for controlling the operation of the valves 92 for injecting air or gas into the treatment fluid, and a controller for controlling the power supply 96 to the induction coil 95.. The processor unit 12 also includes a tool path memory 130 which stores the parameters for the tool path to be followed when moving the polishing head 7 over the workpiece 5.
The pump controller 122 controls the operation of the pump 17 to deliver the required pressure to the fluid 8 within the cavity 71 of the polishing head 7.
The ultrasound controller 123 controls the operation of the ultrasound generator 75 to produce vibrations at a particular frequency and power.
The agitator controller 124 controls the motor 19 of the agitator to agitate the polishing fluid as required, in order to keep the abrasive particles in suspension.
The turntable controller 125, and the X and Y slide controllers 126 and 127, and the first and second pivot mechanism controllers 128 and 129 control the relative movement of the polishing head 7 and the workpiece 5 in order to move the polishing footprint over the workpiece surface.
The tool path memory 130 stores data relating to the tool path which will be followed by the polishing head 7.
Figure 5 illustrates the form of data stored in the tool path memory, which is in the form of a table. The first column of data headed L represents each successive point on the tool path which is to be followed by the polishing head 7. For each point on the tool path, the remaining columns of the table define the pump pressure P, the ultrasonic power and frequency U, the on/off state of the agitator motor A, the turntable position T , the X slide position X , the Y slide position Y, the position of the first pivot mechanism M1 and the position of the second pivot mechanism M2 which are required at that point on the tool path. The tool path data may also include data for setting the valve 92 and for operating the power supply 96. The successive points on the tool path may be visited at a constant rate, the speed of the jet over the surface of the workpiece being controlled by the spacing of the points on the tool path. Alternatively, the tool path may be based on a number of equally spaced points, and may include a further column indicating the amount of time to be spent at each of the tool path points.
As can be seen from the first three lines of data in the table of Figure 5, the operation of the various elements is constant except that the X slide is moved from position 42 to position 44. The processor 121 reads each line of data in succession, and sends the required setting to each of the controllers 122 to 129 so that each element is operated in accordance with the tool path data as the tool is moved along the tool path from the first to the last position.
To perform a shaping or polishing operation, the tool path data is compiled by measuring the workpiece to determine its actual surface form, comparing this with the required surface form to develop a map of the surface showing how much material is to be removed from each point, determining the material removal profile of the tool footprint based on parameters including the polishing fluid to be used and the material of the workpiece, determining a tool path which is a series of locations to be visited by the tool to treat the entire surface, and then compiling a tool path table which provides, for each location, a control parameter for each element of the apparatus. The moves the fluid jet over the surface of the workpiece, removing an appropriate amount of material at each point to bring the surface of the workpiece to the desired profile.
The parameters of pump pressure and ultrasound power may be varied, keeping the ultrasound frequency constant, to ensure that cavitation bubbles formed within the cavity 71 are entrained in the jet and impinge on the surface of the workpiece.
At locations where increased material removal is required, the pump pressure may be increased and simultaneously the ultrasound power may be increased to ensure that cavitation bubbles are still formed within the cavity, even at the increased pump pressure. The ultrasound frequency may be kept constant, and may be set at a resonant frequency of the cavity, in order to generate standing waves in the cavity which ensure that cavitation bubbles are generated for the least amount of power applied to the ultrasound generator 75.
Alternatively, both the frequency and power of the ultrasound generator 75 may be varied, as well as the pump pressure, at points along the tool path.
At locations where the tool is spaced further away from the workpiece surface, the frequency of the ultrasound generator 75 may be lowered in order to produce cavitation bubbles having a longer lifetime, so that they persist within the jet of fluid until the fluid reaches the workpiece surface. In embodiments where micro-bubbles are injected into the treatment fluid, the pressure of the injected gas and/or the operation of the valve 92 may be arranged so that the injected bubbles are larger or formed,, so that they persist within the jet. Likewise, in embodiments where micro-bubbles are formed by boiling on the surface of a bead 93, the power supply 96 may be controlled so that the power provided by the induction coil 95 is greater, and micro-bubbles of larger diameter are formed to be entrained into the jet.
As an alternative to injecting micro-bubbles of gas using the injection needle 90, the needle may be used to inject micro-droplets of a liquid having different properties from the treatment fluid. For example the needle 90 may inject microdroplets of a denser liquid such as a saline solution, or may inject a less dense liquid such as a light oil, in order to produce within the jet 8j regions of micro-droplets of the injected fluid.
Inputting of the data for the tool path memory is preferably performed using the input means 11, which may be a keyboard, a disk drive or a connection to a communications network such as a LAN or the Internet. The tool path data may be computed or compiled at a remote location and sent to the apparatus via a communications link, to be stored in the tool path memory 130.
While the processes above have been described in relation to the use of a single jet to perform a treatment process on a workpiece, it is foreseen that a treatment head may provide an array of fluid jets which impinge on the workpiece at the same position, or at a number of adjacent positions, to effect a treatment process on the workpiece. Where the treatment process is a heat transfer process, the fluid jets may impinge on the surface at spaced locations.
As an alternative to performing the treatment process in air, it is foreseen that both the treatment head 7 and the workpiece 5 may be immersed in a bath of the treatment fluid. In these embodiments, additional drag on the fluid exiting the passageway 72 reduces the speed in the jet 8j, and consequently the workpiece may have to be positioned closer to the tool head.
In addition to the uses of the fluid jet for shaping and cleaning a surface, the inventors have discovered that by directing a jet including regions of different fluid properties onto a workpiece, the liquid as it spreads out over the surface of the workpiece is a more effective heat transfer medium for drawing heat out of the surface into the fluid, or transferring heat to the surface from the fluid. The regions of different fluid properties within the jet give rise to the formation of vortices in the regions of the liquid near to the workpiece surface, and this mechanism increases the heat transfer between the fluid and the workpiece surface. The apparatus illustrated in the Figures may thus be used for directing a jet of liquid onto the surface of a workpiece in order to transfer heat into or out of the workpiece. The treatment fluid may in these cases be water, glycol, or a mixture of water and glycol or a dielectric solution.

Experimental and Theoretical Background

When compared to conventional FJP, the novel process was found to significantly increase material removal rate by a factor of up to 380%, as shown in the histogram of Table 1 below. Furthermore, the final surface roughness was maintained or even slightly improved, a result in sharp contrast with other FJP enhancement systems such as airbubble injection.
In the macro-scale hypothesis, the increase in material removal rate is theorized to issue from additional erosive action of the abrasive particles due to vibrations of the body of slurry in the UFJP impigement zone. Such vibrations may come from two related sources: (1) a fi:action of the acoustic pressure waves escaping the nozzle cavity through the outlet instead of being reflected, and (2) the generation of shock waves at the nozzle outlet due to the periodic switching between disparate densities and dynamic viscosities of the ejected fluid, in the form of slurry and slurry/micro-bubble mixture.

3.2 Experimental verification

To verify the macro-scale hypothesis, experiments were carried out in which pure water only was injected into the cavitating nozzle (i.e.: no abrasives). The jet was impinged onto an aluminium block fixed to a 3-axis dynamometer, and the force was recorded with a 1 MHz data logger. A control experiment at 0 MPa was carried out to establish the influence of structural vibration from the experimental jig. Experimental conditions are summarized in Table 2.

Table 2. Parameters of vibration experiment

Workpiece
Dimensions	40 x 40x 15 mm
Substrate material	Aluminium
Surface roughness	Ra 0.18 µm

Ultrasonic FJP
Operating frequency

	22,71, 127 kHz
Input power	50W
Nozzle diameter	lmm
Nozzle distance	3mm
Pump pressure
	0,0.2. 0.4. 0.8 MPa
Working fluid	Pure water

Dynamometer
Device 3-axis	(Kistler 9027C)
Pre-load	14kN
Sampling rate
	1 MHz

Control experiments with a water pressure of 0 MPa showed that a very low amplitude signal is transferred from the nozzle to the workpiece through structural vibration of the experimental jig. However, this signal was negligible when compared to signals transferred through the fluid jet.
Table 6 below shows typical results of force measurements in the lateral and normal directions, after subtraction of the DC component. While almost no vibration of the force was detected in standard FJP, vibrations of magnitude 0.5 - 2.0 N were detected at all operational frequencies of the ultrasonic FJP system. The strongest vibrations were recorded at 22 kHz, which is also the frequency producing the strongest material removal boost (see Fig. 2). Spectral analysis of the signals by Fourier transform revealed a strong peak at 22 kHz when using the associated frequency. Peaks at 71 and 127 kHz were less pronounced, as these frequencies are beyond the specification range of the dynamometer used in this experiment. A low frequency peak at 50 Hz was detected in all measurements, including standard FJP, and is assumed to relate to pulsations of the slurry pressuring system.
Figure 6. Workpiece vibration force in the lateral and normal directions, when subjected to standard and ultrasonic FJP at 0.8 MPa pressure
From these experiments, compelling evidence of fluid vibration being transferred to the workpiece surface was detected.

Claims

A fluid jet treatment apparatus comprising:
a support (1) for a workpiece (5);

a treatment head (7) for producing a jet of fluid (8j); and

means for directing the jet of fluid at successive locations on the surface of the workpiece;

wherein the treatment head includes means to produce in the fluid jet regions of fluid having different density and/or viscosity properties, including:
a cavity (71) having an inlet duct (77) through which fluid (8) may be supplied to the cavity;

an outlet passage (72) from which fluid can exit the cavity in the form of a jet; and

an ultrasonic vibration generator (75) for generating ultrasonic pressure waves (W) within the fluid;

characterised in that the cavity is conical, and has the outlet passage (72) arranged axially at the narrower end of the conical cavity (71) and the ultrasonic vibration generator positioned at the larger axial end of the cavity, the ultrasonic vibration generator comprising a concentrator plate (76) having a concave face directed towards the outlet passage, to concentrate the waves at a focus point (F) adjacent the outlet passage (72).
A fluid jet treatment apparatus according to claim 1, wherein the ultrasonic vibration generator is operable to cause cavitation bubbles (78) to form within the fluid and pass along the fluid jet (8j).
A fluid jet treatment apparatus according to claim 1, wherein the cavity has one or more resonant frequencies, and the ultrasonic vibration generator is adapted to operate at frequencies at which "standing" acoustic waves are generated within the cavity.
A fluid jet treatment apparatus according to any of claims 1 to 3, wherein the treatment head is configured to be held sufficiently close to the workpiece that cavitation bubbles in the fluid jet impact on the workpiece surface.
A fluid jet treatment tool for generating a fluid jet including regions of fluid having different density and/or viscosity properties, comprising:
a treatment head for producing a jet of fluid (8j); and

means to produce in the fluid jet regions of fluid having different density and/or viscosity properties, including:
a cavity (71) having an inlet duct (77) through which fluid (8) may be supplied to the cavity;

an outlet passage (72) from which fluid can exit the cavity in the form of a jet; and

an ultrasonic vibration generator (75) for generating ultrasonic pressure waves within the fluid;

characterised in that the cavity is conical, and has the outlet passage (72) arranged axially at the narrower end of the conical cavity (71) and the ultrasonic vibration generator (75) positioned at the larger axial end of the cavity, the ultrasonic vibration generator comprising a concentrator plate (76) having a concave face directed towards the outlet passage, to concentrate the waves at a focus point (F) adjacent the outlet passage.
A fluid jet treatment tool according to claim 5, wherein the ultrasonic vibration generator is operable to apply ultrasonic vibrations to a fluid in the treatment head, such that cavitation bubbles (78) are periodically generated within the fluid and are entrained in regions of the jet.
A fluid jet treatment tool according to claim 6, wherein the ultrasonic vibration generator is operable at one or more frequencies which produce resonant standing waves within the cavity.
A treatment process comprising using a fluid jet treatment apparatus in accordance with any of claims 1 to 4 to direct a jet of fluid at a workpiece to impact on the workpiece at a treatment footprint, the fluid jet including regions of fluid having different density and/or viscosity properties.
A treatment process according to claim 8, wherein regions of reduced fluid density are produced by generating or injecting micro-bubbles in the fluid.
A treatment process according to claim 8 or claim 9, wherein ultrasonic vibrations are applied to the fluid.
A treatment process according to claim 10 wherein ultrasonic vibrations are applied at a frequency and power such that micro-bubbles (78) are formed by cavitation in the fluid (8) and are entrained in the fluid jet (8j).
A treatment process according to claim 11, wherein the micro-bubbles are transported by the fluid jet to the point of impact with the workpiece.
A treatment process according to any of claims 8 to 12 wherein the fluid contains abrasive particles, and wherein the micro-bubbles are similar in size to the abrasive particles.
A treatment process according to any of claims 10 to 13 wherein the jet of fluid is directed from the outlet passage leading from the cavity within the tool, and wherein the frequency of the ultrasonic vibrations corresponds to a resonant frequency of the cavity.