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Micromachining with abrasive waterjets
Journal of Materials Processing Technology 149 (2004) 37–42
Micromachining with abrasive waterjets D.S. Miller∗ Miller Innovations, Brookside Church Walk, Harrold, Bedford, UK
Abstract Micromachining abrasive waterjet systems have been developed to cut with jet diameters from 30 to 70 m. The jets cut, profile, drill, slot and mill metals, glass, ceramics, polymers and composite materials. Profiling trials with 40 m jets carrying nanometre sized particles of aluminium oxide indicate that cutting should be possible with jet diameters less than 10 m. As jet diameters are reduced the ability to quickly stop and start cutting jets becomes increasingly important, particularly when drilling holes. Diamond seated valves are described that have the potential to be developed to carry out tens of jet on/off cycles per second. © 2004 Elsevier B.V. All rights reserved. Keywords: Waterjet; Micromachine; Abrasive
1. Introduction Over the past 10 years abrasive waterjets has been one of the fastest growing non-conventional machining methods. Abrasive waterjet machining systems are both competitive with and complementary to laser machining systems. Operators of both types of system exploit the cutting speed of lasers on materials such as steel and use abrasive waterjets to cut materials that are difficult or impossible to cut with their laser systems. Abrasive waterjets are preferred for: • • • • •
cutting thick materials beyond the capability of lasers; highly reflective materials such as copper; brittle materials such as stone and glass; parts that require a good edge quality; parts that cannot have heat affected areas.
Lasers are rapidly evolving with: • sustained improvements in the efficiency of cutting beam generation; • higher cutting beam powers and power densities; • development of smaller beam diameters for micromachining; • discovery and exploitation of new beam generation modes. In comparison to lasers, abrasive waterjet developments have been very limited. Only one method of abrasive waterjet ∗ Tel.: +44-1234-721089; fax: +44-1234-721089. E-mail address: [email protected] (D.S. Miller).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.041
generation has been exploited for precision machining. This method involves entraining abrasive particles carried in air into a high velocity waterjet. The method is very inefficient with less than 3% of a waterjet’s energy being transferred to abrasive particles. The process of entraining abrasive carried in air becomes increasingly ineffective at jet diameters under 500 m and ceases to operate at jet diameters of 300 m. As jet diameters less than 100 m are required for micromachining the current generation of abrasive waterjets cannot be used to micromachine. The author has looked recently at the fundamental fluid dynamic process that could be used to generate abrasive waterjets for micromachining and at whether any of these processes would be suitable for commercial exploitation [2]. It was concluded that the process known as the abrasive suspension method is the best means for producing fine cutting jets, although at least one new entrainment method could operate down to jet diameters of 40 m or so. This paper is about micromachining jets formed by the suspension method. It is predicted that by 2010 the revenues from the sale of micromachining laser systems will be similar to the revenues from the sale of laser machining systems for general machining [1]. Because of the substantial benefits abrasive waterjets bring to micromachining it could be expected that the longer term revenues from micromachining waterjets would be of the same order as those for abrasive waterjets for general machining. First, however, it is necessary to establish micromachining abrasive waterjets as effective and reliable machine tools.
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D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
2. Suspension abrasive waterjets The obvious and most efficient way of generating abrasive waterjets is to pass a pressurised suspension of abrasive particles in water through a ceramic or diamond cutting nozzle. The suspension method of jet generation can produce five times more cut surface area per minute than entraining abrasive particles carried in air into a high velocity waterjet [3]. However, all the cutting capabilities of abrasive suspensions can be released within suspension abrasive waterjet equipment. For instance, when a substantial pressure difference occurs across a valve during opening or closing, an abrasive suspension flow will cut metal valve seats. Catastrophic seat failure usually occurs after a few opening and closing operations. Attempts have been made to develop suspension systems for precision machining but adequate reliability has not been achieved because they need valves that pass abrasive suspensions. Suspension systems for on-site and munitions disposal have been more successful because operating periods are usually short and high levels of maintenance are tolerable. On-site and munitions disposal systems operate at pressures under 700 bar compared to the 3000–4000 bar of entrainment systems. The higher effectiveness of the suspension method of generating abrasive waterjets gives similar jet cutting energy densities as entrainment systems at a quarter of the water pressures needed by entrainment jets. Micromachining abrasive waterjets use water flow rates from 10 l/h down to 1 l/h or so, depending on jet diameter. Corresponding abrasive flow rates are 2 kg/h too less than 100 g/h. An abrasive storage vessel volume of about 300 ml is required to feed a 50 m diameter jet for an hour when the abrasive concentration in a jet is 20 wt.%. The small volume of abrasive used per hour by micromachining abrasive waterjets makes it practical to operate systems in a batch mode. A cartridge containing sufficient abrasive for an hour or more of cutting is loaded into a storage vessel of a micromachining system. Using a cartridge allows flow circuits to be designed so that they only have one valve subjected to abrasive suspension flows. The small physical size of this valve makes it practical to make the valve seats from diamond to withstand erosion.
Fig. 1. Development micromachining system.
Water compressibility gives rise to both beneficial and adverse flow phenomena. The relatively large volume of abrasive storage vessels, compared to the water volume flow rate, results in compressed water volumes in systems being the equivalent to several seconds of pump flow. It, therefore, takes seconds to pressurise systems. When pressures vary, water compressibility can cause the flow direction in parts of flow circuits to reverse and carry abrasive particles into clean water parts of flow circuits. Flow circuits and control strategies have been devised to prevent abrasive being carried into the clean water parts of flow circuits [4]. Fig. 1 shows one of the two systems used for the profiling and drilling studies. The system consists of a pump module and an X–Y cutting table module. The cutting table has a 100 mm × 100 mm profiling area and is designed to sit on a workbench. A removable work piece support/jet catcher tank allows recovery of small cut parts by removing and washing the support/catcher tank to collect cut parts on a sieve. The catcher tank is moved in the X–Y directions by ball screw stages with a repeatability of 2 m. Referring to Fig. 1, the abrasive storage vessel is located over the support/catcher tank. The cutting nozzle is mounted directly below the abrasive storage vessel to minimise the length of passageways carrying abrasive suspensions. Abrasive storage vessels may be filled with pre-mixed abrasive suspensions or an abrasive bed may be formed in storage vessels; in the latter case, the abrasive mixture is diluted before it reaches the nozzle.
3. Micromachining abrasive waterjet development
3.2. Flow circuits
3.1. Background
A basic flow circuit for micromachining abrasive waterjets is shown in Fig. 2 and described in detail in [4]. Pressurised water from a pumping unit is fed to a flow controller. The flow controller has a number of functions:
An assessment of market potential, technical risks, development costs and ease of manufacture resulted in the decision to base prototype micromachining systems on 50 m diameter jets. Such systems have the flexibility to operate with jet diameters from 30 to 70 m. A water pressure of 700 bar was chosen to limit water compressibility to 3% whilst achieving a jet energy density similar to an entrainment abrasive waterjet operating with a water pressure of 3000 bar.
• When abrasive is turned off the flow controller directs all of the water from the pump towards the cutting nozzle. At the same time it causes a small amount of water to flow into the base of the abrasive storage vessel displacing water from the top of the vessel towards the flow controller.
D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
Abrasive Storage Vessel
Flow Controller
Pump
Abrasive Suspension Valve Nozzle
Fig. 2. Flow circuit.
This function gives a clean cut-off of the abrasive supply to the nozzle when abrasive flow is stopped. • When abrasive is turned on the flow controller directs part of the water flow from the pump to the top of the abrasive storage vessel to displace abrasive mixture out of the vessel to be diluted by water that by-passes the storage vessel. The percentage of water flowing to the storage vessel depends on the abrasive mixture concentration in the storage vessel and the desired abrasive concentration at the nozzle. • To de-pressurise the system the flow controller vents compressed water from the top of the abrasive storage vessel.
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ide. Because of the small size of micromachining abrasive waterjet nozzles it is essential to prevent contamination of the abrasive once it is prepared for use. This is achieved by loading the abrasive into cartridges. 3.6. Abrasive storage vessel The abrasive storage vessel shown in Fig. 1 has a barrel with an interrupted thread that allows it to be opened by rotating the barrel through an eighth of a turn and lifting the barrel off the vessel base. With the barrel removed cartridges can be inserted and removed from the vessel base. The cartridges have connections that mate with connections in the vessel base for water flow into and out of cartridges and for abrasive to flow out of cartridges. 3.7. Cutting nozzles Cutting nozzles are made from industrial diamond or high wear resistance silicon carbide. Nozzle life has not been a problem during development cutting trials. Further studies are required to establish the best materials and bore shapes for nozzles.
4. On/off valves for abrasive suspensions 3.3. Water supply With water consumption of a few litres per hour the water feed can be from a small reservoir that is replenished periodically. By pressurising the reservoir the potential for cavitation at the pump inlet can be avoided. A proprietary 0.5 m filter is appropriate to protect the pumping unit from contamination in the water supply. 3.4. Pumping module The pumping duty for a 50 m bore nozzle operating at 700 bar is 2.5 l/h of water. A cost-effective way of meeting this duty is with intensifier plunger pumps powered by compressed air. The pair of pumps can be seen in Fig. 1. A plunger pump is located within compact pneumatic cylinders. Compressed air is supplied to the pneumatic cylinders via solenoid valves under the control of a programmable logic controller (PLC). The PLC synchronises the operation of the two plunger pumps to minimise water pressure fluctuations. The PLC also controls pneumatically actuated valves in the flow circuits. A 1.2 kW, single phase, compressor provides an adequate supply of air for the pumps and for operating valves in the abrasive waterjet circuit. 3.5. Abrasives Garnet is the most widely used abrasive for abrasive waterjets and it is available in the particle size ranges needed for micromachining. Trials are also run with aluminium ox-
Micromachining abrasive waterjets can machine several features per second. For instance, when drilling holes in metal foils they can theoretically produce tens of holes per second. If the water flow has to be stopped before moving a cutting head to a new cutting position, and there is no valve before a cutting nozzle, a suspension abrasive waterjet system has to be de-pressurised. After moving the cutting head to the new position the system is then re-pressurised. When reproducing small features that take fractions of a second to cut, up to 99% of machining time and pumping energy could be wasted in de-pressurising and re-pressurising systems. A valve that can start and stop abrasive suspensions is essential to minimise cutting cycle times and energy waste. Without a suspension on/off valve the capabilities and market for micromachining abrasive waterjets would be greatly reduced. The valve should: • be able to open and close several times per second, with valves for system feeding nozzles under 50 m diameter being capable of future developments to cycle tens of times per second; • open and close reliably on abrasive suspensions for tens of millions of cycles. Ten million cycles represents machining 1000 components with 10 000 holes, a not unrealistic requirement for components requiring many features such as screens. Valves for highly erosive conditions need mechanisms involving a valve element moving at right angles to a seat in
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D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
Fig. 3. Prototype suspension valve.
such a way that abrasive particles cannot get between contacting surfaces. The valve developed for starting and stopping abrasive suspensions has two lapped diamond seats, one of which slides relative to the other to cover or uncover a flow passage. A pneumatically operated version of a prototype valve is shown in Fig. 3. When a flow circuit is pressurised, fluid forces act through a tube feeding abrasive suspension to a moving seat to force the seat onto the stationary seat. In effect the tube acts as a strut under buckling load to transmit fluid pressure loads on the end of the tube to the valve seat. Because of the low coefficient of friction of diamond on diamond, actuating forces for the valve are modest. Further information on various versions of the valve is given in [5].
5. Profiling and drilling trials Profiling and drilling studies have been directed at demonstrating the feasibility and capability of microabrasive waterjets and to the development of components and systems. Diamond nozzles, with bore diameters between 40 and 60 m were used with water pressures of 700 bar. The abrasive was garnet powder with a mean particle diameter of 8 m.
Fig. 4. Examples of materials cut with 40–60 m jets.
A range of composite materials has been successfully profiled, including circuit boards and carbon fibre composites, but a ceramic metal composite that conventional abrasive waterjets would cut could not be cut. This was probably due to the ceramic particle size, relative to the jet size, causing the jet to deflect and fail to destroy the matrix material. Fig. 5 shows part of a butterfly motif that illustrates the fine nature of features that can be reproduced. Web widths can be less the material thickness. Maximum metal thickness profiled with a 50 m diameter jet is 9 mm but only at 1–2 mm/min. Cut depths to width ratios are much greater than are possible with micromachining lasers. The evidence to date points to scale effects reducing cutting performance from that predicted assuming that with decreasing jet diameters there is a linear decrease in cut surface area generated per minute. For instance, a 50 m diameter jet would be expected to generate 10% of the cut surface area per minute as a 500 m diameter jet, when operating at the same pressure and abrasive weight concentration. Evidence to date is that a 50 m diameter jet only produces about 60% of the predicted cut surface area expected from studies on larger suspension jets.
5.1. Profiling studies The test piece shown in Fig. 4 has been used extensively for profiling, drilling and slotting trials. The test piece takes the form of a 28 mm diameter paper clip with a profile, holes or slots in the central area. The dragon motif is particularly good for showing the detail that can be machined. Cutting out the motif involves some 1600 straight-line nozzle moves. The test pieces shown were cut from a variety of materials including metals, polymers, glass and plywood. The test piece thickness ranges from 50 m to 4 mm. Cutting speeds varied from up to 400 mm/min for 50 m thick materials to 15 mm/min for 3 mm thick titanium.
Fig. 5. Example of profiling with thin sections (smallest division 100 m).
D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
Fig. 6. Eighty-five micron diameter holes on 250 m pitch through 50 m stainless steel.
5.2. Drilling studies Fig. 6 shows a 33 × 33 array of holes, with mean diameters of 85 m, drilled on a 250 m pitch, in 50 m thick stainless steel using a 58 m diameter nozzle. The drilling rate was 2.5 holes/s, which was limited by the valve actuator response rather than the abrasive jet drilling capability. For a range of materials and material thickness hole diameters were about 1.5 times the jet diameter. Hole cross-sectional areas are about twice the jet area as a consequence of jets turning back on themselves to escape from holes as they were drilled. In carrying out the profiling studies, most cuts were started by holding the jet stationary to pierce the material. This is not an effective technique for materials more than a few jet diameters thick. Once a jet has penetrated sufficiently far into the material, the water and abrasive escaping from the hole interact strongly with the jet. A near stable situation arises where the jet is forced to spread and turn back on itself, leaving the deepening of the hole to secondary flow processes. A 50 m diameter stationary jet drills through over 6 mm of steel, but it takes a minute or two. Moving the jet is a much more effective strategy as an escape path is provided for abrasive and water leaving the cutting zone but it means holes are trepanned and have diameters several times that of the jet. From drilling studies with stationary jets it was evident that hole shapes were effected by the nozzle bore shapes. Nozzle bore ovality was amplified in drilled holes. Achieving a high degree of nozzle hole roundness is therefore important during the drilling of nozzle bores.
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nanometre diameter abrasive could be used for abrasive waterjets. The smallest diameter nozzles available for the trial had 40 m diameter bores. There is a nozzle bore to particle diameter ratio above which cutting performance deteriorates with decreasing particle size. When the nozzle bore to particle diameter ratio is too high it is probable that particles entering the cutting zone interfere with each other and high drag forces cause particles to begin to follow water flow streamlines before impacting on the material being cut. Deterioration in cutting performance is observed with nozzle bore to abrasive diameter ratios above 10–1. Abrasive mean diameters of 3 m, 300 and 50 nm were used giving nozzle bore to abrasive diameter ratios of 13, 130 and 800. Aluminium oxide abrasive was used with the 300 and 50 nm abrasive classed as de-agglomerated. The author could only sieve the abrasive through a 20 m screen, so it is highly likely that the abrasive contained a significant percentage of agglomerated particles. Profiling with 3 m mean diameter aluminium oxide gave similar results to profiling with 8 m mean diameter garnet. Lower traverse rates were needed when profiling with 300 nm mean diameter aluminium oxide, compared to 8 m garnet. Fig. 7 shows cut edge definition was very good with 300 nm abrasive particles. Cutting rates with 50 nm abrasive were extremely low and the cutting mode very different from any previously observed mode. When cutting 50 thick stainless steel it was observed that on the jet entry side the surface was worn and highly polished over a width equivalent to the jet diameter. Once the jet broke through, at a particular location, it appeared that cutting virtually stopped to leave polished shelves on either side of the breakthrough. The breakthrough wandered across the jet impact area, leaving a very jagged outline to profiles. The cutting behaviour was consistent with the abrasive diameter being much too small for the jet diameter. The trials indicated that, as expected, sub-micron abrasive particles cut materials such as stainless steel. There is a need for trials with smaller diameter nozzles and with better control over abrasive agglomeration and size distribution.
6. Trials with nanometre diameter abrasive Nanometre diameter abrasive is extensively used in the electronics, optics and other industries for material removal and polishing, and could be expected to cut materials if suspended in high velocity fluid jets. Trials were carried out to get an indication of whether
Fig. 7. Profile from 50 m thick stainless steel, cut with 300 nm abrasive.
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D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
7. Discussion
8. Conclusions
Micromachining abrasive waterjets generated by the suspension method cut the same range of materials as abrasive waterjets for general machining, albeit through thinner materials. The author is not aware of any fluid dynamic phenomena that would make it impossible to produce suspension jets down to a few microns. The critical factors in determining minimum jet diameters are likely to be the ability to manufacture cutting nozzles, to control abrasive quality and system cleanliness. Nozzle bore diameters down to 10 m can be drilled in diamond by lasers and the bores polished. Developments in the manufacture of components in diamond for microelectronic mechanical systems (MEMS) are likely to lead to methods of producing nozzles with micron and sub-micron bore diameters. Good housekeeping and abrasive preparation is sufficient to minimise nozzle blockages down to nozzle diameters of 50 m or so. For nozzle diameters below 50 m the level of cleanliness and abrasive preparation requirements becomes increasingly stringent. Below nozzle diameters of 10 m it will probably be necessary to use cartridge assemblies that include the abrasive suspension storage vessel and cutting head. Cartridges would need to be prepared and filled in clean room environments. The flow through a 10 m diameter nozzle would be about 100 ml of abrasive suspension per hour so reusable cartridges, including nozzle and valve sub-assemblies, would be small enough to be readily installed onto machining systems.
• Micromachining abrasive waterjets provide the unique machining capabilities of conventional abrasive waterjets but at the microlevel. • The development of valves to operate with abrasive suspension allows abrasive waterjets to drill several holes per second and should make practical micromilling and marking with abrasive waterjets. • Abrasive waterjets generated by the suspension method have many more variables that can be controlled than abrasive waterjets generated by the entrainment method. Extensive research is required to understand and control suspension jet generation and to optimise operating parameters. References [1] Optech Consulting AG, Market Report, Laser Material Processing-Edition, 2003. [2] D.S. Miller, Developments in abrasive waterjets for micromachining, in: Proceedings of the 2003 WJTA American Waterjet Conference, Houston, 2003. [3] R. Kovacevic, et al., State of the art research and development in abrasive waterjet machining, J. Manuf. Sci. Eng. Trans. ASME 119 (1997) 776. [4] D.S. Miller, Fluid abrasive jets for machining, European Patent Application PTC/GB02/01835 (2003). [5] D.S. Miller, Abrasive fluid jet machining apparatus, International Patent Application PTC/GB02/01835 (2003).
Micromachining with abrasive waterjets D.S. Miller∗ Miller Innovations, Brookside Church Walk, Harrold, Bedford, UK
Abstract Micromachining abrasive waterjet systems have been developed to cut with jet diameters from 30 to 70 m. The jets cut, profile, drill, slot and mill metals, glass, ceramics, polymers and composite materials. Profiling trials with 40 m jets carrying nanometre sized particles of aluminium oxide indicate that cutting should be possible with jet diameters less than 10 m. As jet diameters are reduced the ability to quickly stop and start cutting jets becomes increasingly important, particularly when drilling holes. Diamond seated valves are described that have the potential to be developed to carry out tens of jet on/off cycles per second. © 2004 Elsevier B.V. All rights reserved. Keywords: Waterjet; Micromachine; Abrasive
1. Introduction Over the past 10 years abrasive waterjets has been one of the fastest growing non-conventional machining methods. Abrasive waterjet machining systems are both competitive with and complementary to laser machining systems. Operators of both types of system exploit the cutting speed of lasers on materials such as steel and use abrasive waterjets to cut materials that are difficult or impossible to cut with their laser systems. Abrasive waterjets are preferred for: • • • • •
cutting thick materials beyond the capability of lasers; highly reflective materials such as copper; brittle materials such as stone and glass; parts that require a good edge quality; parts that cannot have heat affected areas.
Lasers are rapidly evolving with: • sustained improvements in the efficiency of cutting beam generation; • higher cutting beam powers and power densities; • development of smaller beam diameters for micromachining; • discovery and exploitation of new beam generation modes. In comparison to lasers, abrasive waterjet developments have been very limited. Only one method of abrasive waterjet ∗ Tel.: +44-1234-721089; fax: +44-1234-721089. E-mail address: [email protected] (D.S. Miller).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.041
generation has been exploited for precision machining. This method involves entraining abrasive particles carried in air into a high velocity waterjet. The method is very inefficient with less than 3% of a waterjet’s energy being transferred to abrasive particles. The process of entraining abrasive carried in air becomes increasingly ineffective at jet diameters under 500 m and ceases to operate at jet diameters of 300 m. As jet diameters less than 100 m are required for micromachining the current generation of abrasive waterjets cannot be used to micromachine. The author has looked recently at the fundamental fluid dynamic process that could be used to generate abrasive waterjets for micromachining and at whether any of these processes would be suitable for commercial exploitation [2]. It was concluded that the process known as the abrasive suspension method is the best means for producing fine cutting jets, although at least one new entrainment method could operate down to jet diameters of 40 m or so. This paper is about micromachining jets formed by the suspension method. It is predicted that by 2010 the revenues from the sale of micromachining laser systems will be similar to the revenues from the sale of laser machining systems for general machining [1]. Because of the substantial benefits abrasive waterjets bring to micromachining it could be expected that the longer term revenues from micromachining waterjets would be of the same order as those for abrasive waterjets for general machining. First, however, it is necessary to establish micromachining abrasive waterjets as effective and reliable machine tools.
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D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
2. Suspension abrasive waterjets The obvious and most efficient way of generating abrasive waterjets is to pass a pressurised suspension of abrasive particles in water through a ceramic or diamond cutting nozzle. The suspension method of jet generation can produce five times more cut surface area per minute than entraining abrasive particles carried in air into a high velocity waterjet [3]. However, all the cutting capabilities of abrasive suspensions can be released within suspension abrasive waterjet equipment. For instance, when a substantial pressure difference occurs across a valve during opening or closing, an abrasive suspension flow will cut metal valve seats. Catastrophic seat failure usually occurs after a few opening and closing operations. Attempts have been made to develop suspension systems for precision machining but adequate reliability has not been achieved because they need valves that pass abrasive suspensions. Suspension systems for on-site and munitions disposal have been more successful because operating periods are usually short and high levels of maintenance are tolerable. On-site and munitions disposal systems operate at pressures under 700 bar compared to the 3000–4000 bar of entrainment systems. The higher effectiveness of the suspension method of generating abrasive waterjets gives similar jet cutting energy densities as entrainment systems at a quarter of the water pressures needed by entrainment jets. Micromachining abrasive waterjets use water flow rates from 10 l/h down to 1 l/h or so, depending on jet diameter. Corresponding abrasive flow rates are 2 kg/h too less than 100 g/h. An abrasive storage vessel volume of about 300 ml is required to feed a 50 m diameter jet for an hour when the abrasive concentration in a jet is 20 wt.%. The small volume of abrasive used per hour by micromachining abrasive waterjets makes it practical to operate systems in a batch mode. A cartridge containing sufficient abrasive for an hour or more of cutting is loaded into a storage vessel of a micromachining system. Using a cartridge allows flow circuits to be designed so that they only have one valve subjected to abrasive suspension flows. The small physical size of this valve makes it practical to make the valve seats from diamond to withstand erosion.
Fig. 1. Development micromachining system.
Water compressibility gives rise to both beneficial and adverse flow phenomena. The relatively large volume of abrasive storage vessels, compared to the water volume flow rate, results in compressed water volumes in systems being the equivalent to several seconds of pump flow. It, therefore, takes seconds to pressurise systems. When pressures vary, water compressibility can cause the flow direction in parts of flow circuits to reverse and carry abrasive particles into clean water parts of flow circuits. Flow circuits and control strategies have been devised to prevent abrasive being carried into the clean water parts of flow circuits [4]. Fig. 1 shows one of the two systems used for the profiling and drilling studies. The system consists of a pump module and an X–Y cutting table module. The cutting table has a 100 mm × 100 mm profiling area and is designed to sit on a workbench. A removable work piece support/jet catcher tank allows recovery of small cut parts by removing and washing the support/catcher tank to collect cut parts on a sieve. The catcher tank is moved in the X–Y directions by ball screw stages with a repeatability of 2 m. Referring to Fig. 1, the abrasive storage vessel is located over the support/catcher tank. The cutting nozzle is mounted directly below the abrasive storage vessel to minimise the length of passageways carrying abrasive suspensions. Abrasive storage vessels may be filled with pre-mixed abrasive suspensions or an abrasive bed may be formed in storage vessels; in the latter case, the abrasive mixture is diluted before it reaches the nozzle.
3. Micromachining abrasive waterjet development
3.2. Flow circuits
3.1. Background
A basic flow circuit for micromachining abrasive waterjets is shown in Fig. 2 and described in detail in [4]. Pressurised water from a pumping unit is fed to a flow controller. The flow controller has a number of functions:
An assessment of market potential, technical risks, development costs and ease of manufacture resulted in the decision to base prototype micromachining systems on 50 m diameter jets. Such systems have the flexibility to operate with jet diameters from 30 to 70 m. A water pressure of 700 bar was chosen to limit water compressibility to 3% whilst achieving a jet energy density similar to an entrainment abrasive waterjet operating with a water pressure of 3000 bar.
• When abrasive is turned off the flow controller directs all of the water from the pump towards the cutting nozzle. At the same time it causes a small amount of water to flow into the base of the abrasive storage vessel displacing water from the top of the vessel towards the flow controller.
D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
Abrasive Storage Vessel
Flow Controller
Pump
Abrasive Suspension Valve Nozzle
Fig. 2. Flow circuit.
This function gives a clean cut-off of the abrasive supply to the nozzle when abrasive flow is stopped. • When abrasive is turned on the flow controller directs part of the water flow from the pump to the top of the abrasive storage vessel to displace abrasive mixture out of the vessel to be diluted by water that by-passes the storage vessel. The percentage of water flowing to the storage vessel depends on the abrasive mixture concentration in the storage vessel and the desired abrasive concentration at the nozzle. • To de-pressurise the system the flow controller vents compressed water from the top of the abrasive storage vessel.
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ide. Because of the small size of micromachining abrasive waterjet nozzles it is essential to prevent contamination of the abrasive once it is prepared for use. This is achieved by loading the abrasive into cartridges. 3.6. Abrasive storage vessel The abrasive storage vessel shown in Fig. 1 has a barrel with an interrupted thread that allows it to be opened by rotating the barrel through an eighth of a turn and lifting the barrel off the vessel base. With the barrel removed cartridges can be inserted and removed from the vessel base. The cartridges have connections that mate with connections in the vessel base for water flow into and out of cartridges and for abrasive to flow out of cartridges. 3.7. Cutting nozzles Cutting nozzles are made from industrial diamond or high wear resistance silicon carbide. Nozzle life has not been a problem during development cutting trials. Further studies are required to establish the best materials and bore shapes for nozzles.
4. On/off valves for abrasive suspensions 3.3. Water supply With water consumption of a few litres per hour the water feed can be from a small reservoir that is replenished periodically. By pressurising the reservoir the potential for cavitation at the pump inlet can be avoided. A proprietary 0.5 m filter is appropriate to protect the pumping unit from contamination in the water supply. 3.4. Pumping module The pumping duty for a 50 m bore nozzle operating at 700 bar is 2.5 l/h of water. A cost-effective way of meeting this duty is with intensifier plunger pumps powered by compressed air. The pair of pumps can be seen in Fig. 1. A plunger pump is located within compact pneumatic cylinders. Compressed air is supplied to the pneumatic cylinders via solenoid valves under the control of a programmable logic controller (PLC). The PLC synchronises the operation of the two plunger pumps to minimise water pressure fluctuations. The PLC also controls pneumatically actuated valves in the flow circuits. A 1.2 kW, single phase, compressor provides an adequate supply of air for the pumps and for operating valves in the abrasive waterjet circuit. 3.5. Abrasives Garnet is the most widely used abrasive for abrasive waterjets and it is available in the particle size ranges needed for micromachining. Trials are also run with aluminium ox-
Micromachining abrasive waterjets can machine several features per second. For instance, when drilling holes in metal foils they can theoretically produce tens of holes per second. If the water flow has to be stopped before moving a cutting head to a new cutting position, and there is no valve before a cutting nozzle, a suspension abrasive waterjet system has to be de-pressurised. After moving the cutting head to the new position the system is then re-pressurised. When reproducing small features that take fractions of a second to cut, up to 99% of machining time and pumping energy could be wasted in de-pressurising and re-pressurising systems. A valve that can start and stop abrasive suspensions is essential to minimise cutting cycle times and energy waste. Without a suspension on/off valve the capabilities and market for micromachining abrasive waterjets would be greatly reduced. The valve should: • be able to open and close several times per second, with valves for system feeding nozzles under 50 m diameter being capable of future developments to cycle tens of times per second; • open and close reliably on abrasive suspensions for tens of millions of cycles. Ten million cycles represents machining 1000 components with 10 000 holes, a not unrealistic requirement for components requiring many features such as screens. Valves for highly erosive conditions need mechanisms involving a valve element moving at right angles to a seat in
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D.S. Miller / Journal of Materials Processing Technology 149 (2004) 37–42
Fig. 3. Prototype suspension valve.
such a way that abrasive particles cannot get between contacting surfaces. The valve developed for starting and stopping abrasive suspensions has two lapped diamond seats, one of which slides relative to the other to cover or uncover a flow passage. A pneumatically operated version of a prototype valve is shown in Fig. 3. When a flow circuit is pressurised, fluid forces act through a tube feeding abrasive suspension to a moving seat to force the seat onto the stationary seat. In effect the tube acts as a strut under buckling load to transmit fluid pressure loads on the end of the tube to the valve seat. Because of the low coefficient of friction of diamond on diamond, actuating forces for the valve are modest. Further information on various versions of the valve is given in [5].
5. Profiling and drilling trials Profiling and drilling studies have been directed at demonstrating the feasibility and capability of microabrasive waterjets and to the development of components and systems. Diamond nozzles, with bore diameters between 40 and 60 m were used with water pressures of 700 bar. The abrasive was garnet powder with a mean particle diameter of 8 m.
Fig. 4. Examples of materials cut with 40–60 m jets.
A range of composite materials has been successfully profiled, including circuit boards and carbon fibre composites, but a ceramic metal composite that conventional abrasive waterjets would cut could not be cut. This was probably due to the ceramic particle size, relative to the jet size, causing the jet to deflect and fail to destroy the matrix material. Fig. 5 shows part of a butterfly motif that illustrates the fine nature of features that can be reproduced. Web widths can be less the material thickness. Maximum metal thickness profiled with a 50 m diameter jet is 9 mm but only at 1–2 mm/min. Cut depths to width ratios are much greater than are possible with micromachining lasers. The evidence to date points to scale effects reducing cutting performance from that predicted assuming that with decreasing jet diameters there is a linear decrease in cut surface area generated per minute. For instance, a 50 m diameter jet would be expected to generate 10% of the cut surface area per minute as a 500 m diameter jet, when operating at the same pressure and abrasive weight concentration. Evidence to date is that a 50 m diameter jet only produces about 60% of the predicted cut surface area expected from studies on larger suspension jets.
5.1. Profiling studies The test piece shown in Fig. 4 has been used extensively for profiling, drilling and slotting trials. The test piece takes the form of a 28 mm diameter paper clip with a profile, holes or slots in the central area. The dragon motif is particularly good for showing the detail that can be machined. Cutting out the motif involves some 1600 straight-line nozzle moves. The test pieces shown were cut from a variety of materials including metals, polymers, glass and plywood. The test piece thickness ranges from 50 m to 4 mm. Cutting speeds varied from up to 400 mm/min for 50 m thick materials to 15 mm/min for 3 mm thick titanium.
Fig. 5. Example of profiling with thin sections (smallest division 100 m).
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Fig. 6. Eighty-five micron diameter holes on 250 m pitch through 50 m stainless steel.
5.2. Drilling studies Fig. 6 shows a 33 × 33 array of holes, with mean diameters of 85 m, drilled on a 250 m pitch, in 50 m thick stainless steel using a 58 m diameter nozzle. The drilling rate was 2.5 holes/s, which was limited by the valve actuator response rather than the abrasive jet drilling capability. For a range of materials and material thickness hole diameters were about 1.5 times the jet diameter. Hole cross-sectional areas are about twice the jet area as a consequence of jets turning back on themselves to escape from holes as they were drilled. In carrying out the profiling studies, most cuts were started by holding the jet stationary to pierce the material. This is not an effective technique for materials more than a few jet diameters thick. Once a jet has penetrated sufficiently far into the material, the water and abrasive escaping from the hole interact strongly with the jet. A near stable situation arises where the jet is forced to spread and turn back on itself, leaving the deepening of the hole to secondary flow processes. A 50 m diameter stationary jet drills through over 6 mm of steel, but it takes a minute or two. Moving the jet is a much more effective strategy as an escape path is provided for abrasive and water leaving the cutting zone but it means holes are trepanned and have diameters several times that of the jet. From drilling studies with stationary jets it was evident that hole shapes were effected by the nozzle bore shapes. Nozzle bore ovality was amplified in drilled holes. Achieving a high degree of nozzle hole roundness is therefore important during the drilling of nozzle bores.
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nanometre diameter abrasive could be used for abrasive waterjets. The smallest diameter nozzles available for the trial had 40 m diameter bores. There is a nozzle bore to particle diameter ratio above which cutting performance deteriorates with decreasing particle size. When the nozzle bore to particle diameter ratio is too high it is probable that particles entering the cutting zone interfere with each other and high drag forces cause particles to begin to follow water flow streamlines before impacting on the material being cut. Deterioration in cutting performance is observed with nozzle bore to abrasive diameter ratios above 10–1. Abrasive mean diameters of 3 m, 300 and 50 nm were used giving nozzle bore to abrasive diameter ratios of 13, 130 and 800. Aluminium oxide abrasive was used with the 300 and 50 nm abrasive classed as de-agglomerated. The author could only sieve the abrasive through a 20 m screen, so it is highly likely that the abrasive contained a significant percentage of agglomerated particles. Profiling with 3 m mean diameter aluminium oxide gave similar results to profiling with 8 m mean diameter garnet. Lower traverse rates were needed when profiling with 300 nm mean diameter aluminium oxide, compared to 8 m garnet. Fig. 7 shows cut edge definition was very good with 300 nm abrasive particles. Cutting rates with 50 nm abrasive were extremely low and the cutting mode very different from any previously observed mode. When cutting 50 thick stainless steel it was observed that on the jet entry side the surface was worn and highly polished over a width equivalent to the jet diameter. Once the jet broke through, at a particular location, it appeared that cutting virtually stopped to leave polished shelves on either side of the breakthrough. The breakthrough wandered across the jet impact area, leaving a very jagged outline to profiles. The cutting behaviour was consistent with the abrasive diameter being much too small for the jet diameter. The trials indicated that, as expected, sub-micron abrasive particles cut materials such as stainless steel. There is a need for trials with smaller diameter nozzles and with better control over abrasive agglomeration and size distribution.
6. Trials with nanometre diameter abrasive Nanometre diameter abrasive is extensively used in the electronics, optics and other industries for material removal and polishing, and could be expected to cut materials if suspended in high velocity fluid jets. Trials were carried out to get an indication of whether
Fig. 7. Profile from 50 m thick stainless steel, cut with 300 nm abrasive.
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7. Discussion
8. Conclusions
Micromachining abrasive waterjets generated by the suspension method cut the same range of materials as abrasive waterjets for general machining, albeit through thinner materials. The author is not aware of any fluid dynamic phenomena that would make it impossible to produce suspension jets down to a few microns. The critical factors in determining minimum jet diameters are likely to be the ability to manufacture cutting nozzles, to control abrasive quality and system cleanliness. Nozzle bore diameters down to 10 m can be drilled in diamond by lasers and the bores polished. Developments in the manufacture of components in diamond for microelectronic mechanical systems (MEMS) are likely to lead to methods of producing nozzles with micron and sub-micron bore diameters. Good housekeeping and abrasive preparation is sufficient to minimise nozzle blockages down to nozzle diameters of 50 m or so. For nozzle diameters below 50 m the level of cleanliness and abrasive preparation requirements becomes increasingly stringent. Below nozzle diameters of 10 m it will probably be necessary to use cartridge assemblies that include the abrasive suspension storage vessel and cutting head. Cartridges would need to be prepared and filled in clean room environments. The flow through a 10 m diameter nozzle would be about 100 ml of abrasive suspension per hour so reusable cartridges, including nozzle and valve sub-assemblies, would be small enough to be readily installed onto machining systems.
• Micromachining abrasive waterjets provide the unique machining capabilities of conventional abrasive waterjets but at the microlevel. • The development of valves to operate with abrasive suspension allows abrasive waterjets to drill several holes per second and should make practical micromilling and marking with abrasive waterjets. • Abrasive waterjets generated by the suspension method have many more variables that can be controlled than abrasive waterjets generated by the entrainment method. Extensive research is required to understand and control suspension jet generation and to optimise operating parameters. References [1] Optech Consulting AG, Market Report, Laser Material Processing-Edition, 2003. [2] D.S. Miller, Developments in abrasive waterjets for micromachining, in: Proceedings of the 2003 WJTA American Waterjet Conference, Houston, 2003. [3] R. Kovacevic, et al., State of the art research and development in abrasive waterjet machining, J. Manuf. Sci. Eng. Trans. ASME 119 (1997) 776. [4] D.S. Miller, Fluid abrasive jets for machining, European Patent Application PTC/GB02/01835 (2003). [5] D.S. Miller, Abrasive fluid jet machining apparatus, International Patent Application PTC/GB02/01835 (2003).