Patent Publication Number: US-2010120332-A1

Title: Waterjet device

Description:
TECHNICAL FIELD 
     The present invention relates to an abrasive waterjet apparatus, and to a pressure fluctuation equalising device for an abrasive waterjet apparatus. 
     BACKGROUND ART 
     Advances in abrasive waterjet apparatus capabilities extend their operation from general machining with cutting jets greater than 300 microns in diameter to fine and micro machining with cutting jet diameters down to 50 microns or so. Low flow ultra high pressure pumps are required for such apparatuses. 
     Abrasive waterjets for fine and micro machining require water flows from under 1 litre per hour up to 10 litres or so per hour and at pressures from 500 bar to 4000 bar or so. Water pressures are similar to those generated by intensifier and crank pumps that power general machining abrasive waterjets (AWJs), but flows are 1 to 10 percent or so of the output of these pumps. Scaled down versions of intensifier pumps used for AWJs are complex and relatively expensive. In scaling down direct driven plunger pumps it is desirable to simplify pump design and to take advantage of electronically controlled, high response drives. 
     To commence cutting with an AWJ, a valve is opened before a cutting head to allow ultra high pressure water to flow to a waterjet generating orifice. Air entrainment by a waterjet commences and an abrasive on/off valve is opened in the base of an abrasive hopper, allowing abrasive to be carried by airflow to a cutting head through a tube that is usually more than 300 mm in length. To stop cutting abrasive is turned off and after a delay to clear abrasive from a tube and cutting head, the water shut off valve is closed. 
     The accumulation of time delays due to actuation of valves and in establishing and stopping abrasive flow to a cutting head results in cycle times of seconds between turning off a cutting jet, repositioning a cutting head and restarting cutting in a new location. In terms of cutting cycle times, AWJs are not dynamic machining tools. 
     Abrasive waterjet machining systems compete most strongly with laser machining systems. A laser can go through a thousand or so cutting cycles in the time that an AWJ goes through one cycle. This means that AWJs cannot compete with lasers when dynamic cutting is required, such as for marking and scribing, and in profiling, drilling and slotting of thin materials. 
     Valves that start and stop ultra high pressure water flow at AWJ cutting head operate for about 150,000 cycles before valve seats fail due to seat to seat impact loads, high water velocities and cavitation. This is not a serious problem for AWJs because they cannot operate in dynamic machining mode, so 150,000 valve cycles are typically reached after a thousand hours or so of cutting time. 
     Fine and micro abrasive waterjets can operate in dynamic machining mode with many starting and stopping cutting cycles per second. This means 150,000 valve on/off cycles can be reached in less than 10 hours of cutting operations. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to provide an abrasive waterjet apparatus that is able to operate more dynamically, i.e. an apparatus of long life that allows rapid cutting cycles. 
     This objective has been achieved through an abrasive waterjet apparatus comprising a waterjet fluid flow circuit, a positive displacement pump connected to the waterjet fluid flow circuit, a pressure fluctuation equalising device for said circuit, and an abrasive cutting nozzle assembly. The pressure fluctuation equalising device comprises a high pressure chamber connected to the waterjet fluid flow circuit, a low pressure chamber, and a moveable pressure equalising member that separates said chambers. In order to equalise pressure fluctuations in the waterjet fluid flow circuit, the moveable pressure equalising member is arranged to alter the volume of the high pressure chamber. 
     Said high pressure chamber can be designed so that fluid contained therein exerts a first force on a first part of the moveable pressure equalising member, and the low pressure chamber can be designed so that fluid contained therein exerts a second force on a second part of the moveable pressure equalising member. In order to change the volume of the high pressure chamber, the pressure equalising member can be displaced in accordance with the first and the second force. 
     The cross section area of the first part of the moveable pressure equalising member can be smaller than the cross section area of the second part of the moveable pressure equalising member. Hereby, a relatively low pressure can be used in the low pressure chamber to balance the high pressure of the high pressure chamber. The relationship of the high pressure and the low pressure may essentially be the same as the relationship of the area of the second part and the first part. 
     Sensor devices, which are adapted to detect the position and/or motion of the moveable pressure equalising member, can be used to monitor and control the abrasive waterjet apparatus. Further, a motor controller may be provided for controlling the drive of the positive displacement pump. In this connection, a control system can be used to control the pump on the basis of the position or motion of the moveable pressure equalising member. Now, it is possible to optimise the operation of the abrasive waterjet apparatus in accordance with the pressure of the waterjet fluid flow circuit. E.g., the pump motor may be operated with varying torque and/or speed over one pump revolution to thereby minimise pressure fluctuations. 
     A plunger of the positive displacement pump can be arranged to be returned by the fluid pressure of a fluid conduit. The flow of said fluid conduit can be affected by the position of the pressure fluctuation equalising device. Thereby, the position of the moveable pressure equalising member can stop the pumping action. In detail, a predetermined displacement of the moveable pressure equalising member may release the pressure of the fluid conduit. As a consequence, the plunger of the pump is no longer returned whereby the pumping action is interrupted. The fluid conduit may be connected to the pressure fluctuation equalising device by a connection that can be closed by the moveable pressure equalising member so that a predetermined displacement of the moveable pressure equalising member opens the fluid conduit. 
     In order to allow prompt interruption of the waterjet apparatus, the volume of the high pressure chamber can be greater than the volume of waterjet fluid discharged through the nozzle assembly of the abrasive waterjet apparatus during one pump rotation. 
     The object is also achieved by a pressure fluctuation equalising device for an abrasive waterjet apparatus with at least one positive displacement pump connected to a waterjet fluid flow circuit. Said pressure fluctuation equalising device comprises a high pressure chamber connected to the waterjet fluid flow circuit, a low pressure chamber, and a moveable pressure equalising member that separates the chambers. The moveable pressure equalising member is arranged to alter the volume of the high pressure chamber in order to equalise pressure fluctuations in the waterjet fluid flow circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross section through a cam driven plunger pump 
         FIG. 2  shows a plunger head 
         FIG. 3  shows a pressure fluctuation equalising device 
         FIG. 4  shows circuits for a plunger pump feeding an abrasive waterjet cutting head 
         FIG. 5  shows an alternative plunger actuation arrangement 
         FIG. 6  shows an alternative plunger head 
     
    
    
     DETAILED DESCRIPTION 
     When a direct driven plunger pump feeds ultra high pressure water to a cutting head, closure of a valve at a cutting head requires actions that stop excessive water pressures occurring that could damage a pump and an abrasive waterjet flow circuit. In taking actions to prevent excessive water pressures it is desirable that a pump is driven dynamically by a servo or other high response motor. Once a pump is operated dynamically it can be desirable to use the pump rather than a valve at a cutting head to start and stop cutting operations. Using a pump dynamically is practical for up to 10 or so cutting cycles per second with a 100 micron cutting jet diameter and at higher frequencies for cutting jets less than 100 microns in diameter. 
     For a pump to carry out the duties associated with an on/off valve located at an abrasive waterjet cutting head, the compressible water volume between a pump and a cutting head must be minimised, whereas it is usual to increase the water volume in abrasive waterjet flow circuits by the use of a water filled accumulator to dampen variations in pump delivery pressure. With little compressible water volume in a circuit between a direct driven plunger pump and a cutting head, water pressure can rise precipitously resulting in serious damage to a pump and other components if:
     1. A waterjet nozzle becomes blocked   2. A valve is closed   3. Errors in a control program result in too high a pump motor speed or torque   4. A pump controller or a sensor malfunctions   

     Pressure relief devices for small water flows at ultra high pressures are unreliable and suffer rapid wear and are not suitable for preventing excessive water pressures in fine and micro machining abrasive waterjet apparatus when operating in dynamic machining mode. 
     Ideally, when pump delivery pressure goes above a predetermined value an ultra high pressure water circuit should act as if it has infinite compressibility whilst actions are taken to reduce or to maintain pressure at a predetermined level. Although this ideal cannot be realised, it has been found that adequate pseudo compressibility is provided by what is effectively a piston/plunger intensifier working in reverse mode. Pump outlet pressure acts on an intensifier plunger to initiate motion of a plunger/piston combination, rather than fluid pressure acting on a piston to initiate plunger motion as occurs in an intensifier pump. Also said intensifier or pressure fluctuation equalising device has no non-return valves so does not operate as a pump. The pressurised fluid acting on an intensifier piston is usually air for the application under consideration. 
     Pumps for abrasive waterjets operate at the limits of sealing and material technologies. Water leakage from plunger seals can become unacceptable after a few hundred hours of running. Extreme cleanliness is required when replacing plunger seals because of the risk of contaminants blocking waterjet nozzles that, in the case of micro abrasive waterjets, can be less than 20 microns in diameter. It is, therefore, desirable to remove as a unit a pump plunger head including its seals. A replacement unit being fitted and the removed unit serviced in a clean environment. 
     Multi plunger roller cam pumps can generate the pressures and flows required for fine and micro abrasive waterjets. Normally, a cam driven plunger is returned by a spring on a suction stroke. However, using a spring to return a plunger of an ultra high pressure pump leads to design and assembly complexity and difficulties in removing a plunger head as a unit. Also, when a spring is used to return a plunger it is not practical to stop a plunger returning for a new pumping stroke. 
     Pressurised fluid acting on a piston connected to a plunger can hold a plunger against a cam or a crank drive. Using pressurised fluid, preferably air, to return a plunger simplifies pump design and the removal and replacement of plunger units. Importantly, by removing air pressure a plunger remains at the end of its delivery stroke. This means that, for control and safety reasons, pumping action can be stopped independent of a pump drive motor. 
     When pump plungers are returned by air pressure, movement of a pressure fluctuation equalising device can be used to cause air pressure on pump pistons to be removed so as to leave pump plungers at the end of their delivery stroke and thereby stop pumping action within one pump rotation. It is beneficial for a pressure fluctuation equalising device plunger stroke volume to be greater than the volume of water discharged during one pump rotation. Alternatively it can be arranged that a pressure fluctuation equalising device plunger be allowed to withdraw through its seals to vent fluid pressure if other control actions to limit excessive water pressures fail. This option allows commonality of plunger assemblies between a pump and a pressure fluctuation equalising device. 
     Interfacing a servo or other high response pump drive and control system with a pressure fluctuation equalising device monitoring and control system allows considerable flexibility in carrying out abrasive waterjet machining operations. The displacement or motion of a pressure fluctuation equalising device piston can be readily measured and fluid pressure acting on a piston accurately controlled. 
     Power input to a pump motor to achieve desired water pressures could be established from an electronically controlled power supply for a motor. Knowing the area/pressure ratio across a pressure fluctuation equalising device and varying the air pressure acting on a pressure fluctuation equalising device piston, whilst measuring pump speed and input power to just displace a pressure fluctuation equalising device plunger, a pump/flow system characteristic curve can be established. By establishing the characteristic curve a pressure sensor subjected to extreme pressure and fatigue loading is not needed in a water circuit. By routinely monitoring the power input to achieve given water pressures, waterjet nozzle and seal wear and the operation of pump inlet and outlet valves can be monitored. 
     Importantly, the knowledge that excessive water pressures cannot occur provides confidence in programming machining operations that require near instantaneous ramping up of pump speed/torque. Also self-learning capabilities can be built into a control program to predict if too high a pump pressure will be reached if programmed actions were carried out and, if necessary, to over ride or modify such actions. 
     With a finite number of plungers, the pressure/flow output of a pump operating at constant speed varies and this causes faults on cut surfaces. Such faults are particularly noticeable on cut surfaces when the compressible water volume between a pump and a waterjet nozzle is minimised to provide dynamic control of machining operations. It is, therefore, desirable and practical to vary pump rotational speed during one pump rotation, using servo or other control, so as to minimise pressure/flow variations and hence achieve high quality machined surfaces. 
     A pressure fluctuation equalising device can be used in a dynamic mode to damp out variations in pump pressure during steady state running. A pressure fluctuation equalising device operates in a dynamic mode when the frequency of operation of a valve is such that a pump cannot respond sufficiently fast to maintain pressures below a set level. 
     Using an abrasive waterjet to profile miniature parts, drill thin and fragile materials, mill, mark and to etch, requires an ultra high pressure pump with a drive and associated water pressure control system that is highly responsive under the supervision of an electronic control system. Such a pumping system has not previously been known in the art. 
     According to the first aspect of the present invention there is provided a method of generating liquid flows at ultra high pressures. The method comprises:
     a) A multi cylinder plunger pump driven by a servo or other high response motor.   b) Said pump having first plungers driven by a cam/s or crank/s which may be returned by pressurised fluid acting on a first piston connected to each said first plunger.   c) Each cylinder on said pump having an inlet non-return valve through which water flows on a return stroke of said first plungers and an outlet non-return valve through which water flows on a delivery stroke of said first plungers.   d) A pressure fluctuation equalising device with pressurised fluid acting on a second piston connected to a second plunger that is acted on by the outlet water pressure from said first plungers.   e) A mechanism for varying the fluid pressure acting on said second piston.   f) A control system that:
       i. receives inputs from sensors on said pump and drive motor   ii. receives inputs from sensors on said pressure fluctuation equalising device   iii. receives inputs from a cutting head motion controller and varies pump drive speed and/or torque to meet defined cutting and safety objectives.   
       

     When a first plunger is returned by fluid acting on first piston the pressurised fluid is preferably air. 
     Pressurised air acting on a first piston may be vented to leave a first plunger at the end of its delivery stroke. 
     Venting of pressurised air acting on first pistons may be initiated by movement of said second piston. 
     Advantageously the displacement volume of second plunger is greater than the combined displacement volume of first plunger times the number of first plungers on said pump. 
     Advantageously a second plunger can withdraw through its seals if other actions to avoid too high water pressure fail. 
     The pressurised fluid acting on second piston is preferably air. 
     Pumps preferably have three to five plungers. 
     The pump drive motor may be a high response electric, air or hydraulic motor. 
     Preferably a pump is driven by an electric servomotor. 
     The motor drive control system may be capable of limiting the pressure generated by first plungers. 
     The pump motor drive systems may be capable of varying the instantaneous pump motor rotational speed/torque to provide near constant water delivery pressure. 
     Movement of second plunger/piston may be monitored by a linear displacement transducer, or sensor/s or switch/s and signals fed to a pump motor drive and control system. 
     According to a second aspect of the present invention, there is provided a cam or crank driven, multi plunger pump characterised in that during suction strokes first plungers are returned by fluid pressure acting on first piston connected to first plunger. Fluid pressure acting on said first pistons could be removed to leave first plungers at the end of their delivery stroke and thereby stop pumping action in one pump rotation. 
     Preferably the pressurising fluid acting on said first pistons is air. First pistons may act as linear bearings to carry side loads. 
     First plungers and first pistons are advantageously machined as a single unit from zirconia, tungsten carbide or other ceramic. 
     According to the third aspect of the present invention, there is provided a plunger pump as described in the first and second aspect that has plunger heads that include the plunger cylinder, plunger seal housing and seal, inlet and outlet non-return valves and a mechanism/s to allow removal of said plunger head from the pump body by quick release. 
     The inlet and outlet non-return valves for a plunger head may be in a manifold that connects and seals to the plunger head through the closure of a clamp or other device and separates from the plunger head on opening the clamp or other device. 
     According to a fourth aspect of the present invention there is provided a pressure fluctuation equalising device connected to the outlet from a pump described in the first and second aspect such that:
     a) When water pressure generated by first plungers on said pump acting on second plunger of said pressure fluctuation equalising device exceed fluid restraining forces acting on second piston attached to second plunger the second plunger/piston moves to open a port to vent fluid pressure acting on first pistons in said pump, causing pumping action to stop within one pump rotation
 
and/or
   b) Movement of said second plunger/piston assembly is detected and signal/s sent to a pump drive controller to reduce pump speed/torque and hence water pressure.   

     The pressurised fluid acting on second piston is preferably air. 
     The pressure fluctuation equalising device may act as a pressure pulsation damper for pump delivery. 
     A plunger may be withdrawn through its high pressure sealing assembly to vent pump pressure if pumping action does not cease or to vent pressure in a flow circuit. 
     Fluid pressure acting on a piston of a pressure fluctuation equalising device may be vented when a pump is stopped to ensure depressurisation of abrasive waterjet apparatus. 
     According to a fifth aspect of the present invention there is provided a waterjet cutting apparatus with a cutting head fed with pressurised water by a pump flow circuit described in proceeding aspects. 
     Pressurised water may flow directly from a pump to said cutting head and pass through a waterjet generating device to create a high velocity waterjet that entrains abrasive particles to create an abrasive waterjet. 
     Pressurised water may flow to part of an apparatus where it generates an abrasive/water mixture that flows to a nozzle in a cutting head to generate an abrasive waterjet. 
     Pressurised water may flow to part of an apparatus where it causes abrasive/water mixture to flow to a nozzle in a cutting head to generate an abrasive waterjet. 
     Referring to  FIG. 1  that shows a cross section through a multi cylinder cam driven plunger pump. Shaft  1  rotates in bearings  2  and  3 , carried in housing  4 , and has a cam bearing  5  located off the centre line of rotation of shaft  1 . The cam bearing  5  acts on plunger  6  that reciprocates in plunger head  7  retained to pump body  8 . Pump body  8  is attached to bearing housing  4 . 
     The end of plunger  6  in contact with cam bearing  5  forms a piston  9  that reciprocates in cylinder housing  10  located in pump body  8 . The piston  9  moving in cylinder housing  10  acts as a bearing to take side loads from the cam  5  and plunger  6  interaction. Plunger  6  is advantageously made from ceramic material, either as a single item with piston  9  or piston section  9  may be made of a material such as hardened steel and shrunk fit or attached by other manner to plunger  6 . The piston section  9  of plunger  6  may have a polymeric or other bearing liner and the bore in plunger head  7  may have a polymeric or other bearing liner. 
     Cylinder housing  10  has a flange  11  retained between pump body  8  and plunger head  7  with an cylindrical extension  12  that locates plunger head  7  and retains plunger seal assembly  13  in plunger head  7 . Passageways  18  in cylinder housing  10  vent any water leaking through plunge seal assembly  13  and/or allow a flow of cooling water for seal assembly  13 . 
     Pressurised air from source  16  enters through a passageway  19  in cylinder housing  10  to space  17  in cylinder  10  that is formed between seal  15  on piston  9  and seal  14  in cylinder housing  10 . When air in space  17  is pressurised plunger  6  is held against cam  5  during suction strokes causing water from source  40  to flow throw inlet non-return valve assembly  41 , passage  44  and into plunger bore  46  in plunger head  7 . On a delivery stroke cam  5  causes plunger  6  to displace pressurised water through passage  45 , non-return valve assembly  42  into connection  43 . When air in space  17  is not pressurised plunger  6  is left at the end of its delivery stroke. 
     Shaft  1  may be driven by a motor, with or without a gearbox  70  with a shaft  71  that transmits power to pump shaft  1  through key  72  or splines. Sensor  80  may detect rotational position of shaft  1  for use by a motor control system or a drive motor may have an encoder to provide rotational position. A pump may be driven by a servo or other high response motor and have a control system that operates in torque limiting mode to minimise the risk of pump pressures exceeding safe levels. Servo motors allow the instantaneous pump rotational speed to be varied over a pump revolution to compensate for non steady pressure/flow discharge characteristics that result from pumps having a finite number of plungers. The fluctuations in pump delivery pressure can be reduced by increasing the pump speed in order to counteract the fluid pressure valleys of the pump, i.e. during the phases when pump pressure falls below the average working pressure. 
     Plunger head  7  may be retained to pump body  8  by bolts or studs (not shown) or by a quick release mechanism.  FIG. 2  shows a side view of a plunger assembly that differs from that in  FIG. 1  in that plunger head  7  is attached by threaded connection to cylinder housing  10  and has a feature  48  that locates in a mating feature  47  in pump body  8 . By arranging for features  47  and  48  to be interrupted around their circumference a plunger assembly may be rotated an eighth of a turn or so to allow plunger head  7 , cylinder housing  10  and plunger  6  to be withdrawn as a unit. To further simplify the removal and replacement of a plunger assembly the inlet  41  and outlet  42  non-return valves may be combined in a separate assembly  80  that connects by passage  82  to plunge cylinder bore  45 . A metal to metal seal  83  between plunger head  7  and no-return valve assembly  80  may seal passageway  82  or alternatively a polymeric sealing arrangement can be used. Non-return valve assembly  80  may be retained by a quick release clamp (not shown) to plunger head  7 . 
     When it is not required to remove plunger  6 , plunger head  7  and cylinder housing  10  as a complete assembly the cylinder part of cylinder housing  10  may not be necessary. Piston section  9  of plunger  6  may reciprocate in a cylinder bore formed in pump body  8 . 
       FIG. 3  shows a pressure fluctuation equalising device  24  that has a plunger  20  connected to or acting on a piston  21  moving in chamber  29 . Pressurised water from connection  27  connected to pump outlet  43  acts on water in high-pressure chamber  27 ′. Pressurised air from a controlled source  28  connected to chamber  29  acts on piston  21 . When the pressure in chamber  27 ′ times the area of plunger  20  exceeds the pressure in chamber  29  times the area of piston  21  plunger/piston  20 / 21  is displaced. 
     Sensors may be used to detect movement/displacement of piston  21  for use by a pump motor controller and/or abrasive waterjet controller. Sensor signals can be used to stop a pump, to vary the speed of a pump, and to set pump pressure in conjunction with a pressure controller for air source  28 . The ability to sense movement of piston  21  is particularly useful when a pump is operating in dynamic mode, such as machining multiple features per second that require a pump to cycle from near atmospheric pressure to water pressures that can exceed 3000 bar. Under steady state operation the air pressure in chamber  29  can be set so that the water pressure at a cutting nozzle is slightly above that desired before piston  21  moves. The pump motor controller can then be set so that the motor speed or torque is slightly below that needed to move piston  21 . 
     When a positive displacement pump has plungers returned by air pressure, pressure fluctuation equalising device  24  can be used to stop pumping action by venting the air pressure from conduit  16  and space  17  to leave plunger/pistons  6 / 9  at the end of their stroke. One arrangement for venting conduit  16  through a connection  30  to atmosphere is shown on  FIG. 3 . Seal  22  on piston  21  prevents air from venting from connection  16 ′ to connection  30  when piston  21  is held against seat  26  and seal  23  isolates air in volume  29  from connection  16 ′. When excess water pressure in space  27 ′ causes displacement of plunger/piston  20 / 21  so that air from connection  16 ′ can flow to connection  30  the pressure acting on pump pistons  9  is removed. High-pressure seal assembly  25  provides a seal on plunger  20 . The length of chamber  29  may be made sufficient for plunger  20  to withdraw through high-pressure seal assembly  25  if pump water pressure at connection  27  does not decrease. The volume swept by displacement of plunger  20  is preferentially greater than the water discharge during one pump revolution. 
     Alternative arrangements for venting air from conduit  16  may involve mechanical connection from plunger/piston  20 / 21  to an air release valve located outside of the envelope of pressure fluctuation equalising device  24 . 
     Sensing the velocity or acceleration of piston  21  could provide information to determine whether a pump motor should be stopped, such as when a nozzle blocks, or when pump speed should be reduced, such as when a cutting program calls for too high a pump speed or a valve on a multi headed cutting system is closed without instructions to a pump controller to reduce pump speed. 
     Such movement/displacement sensors for the piston  21  may be arranged in the low pressure chamber  29  wall or preferably located between high-pressure seal assembly  25  and piston  21  where sensors are effectively at atmospheric pressure and line of sight sensing is easily achieved. 
     Chamber  29  may be sized to accommodate compression of air in the chamber  29  when piston  21  moves or it may be connected through connection  28  to another air volume or to a controlled fluid pressure source. 
     By having sufficient distance between seal  22  on piston  21  and connection  16 ′ and adjusting the pressure in volume  29 , pressure fluctuation equalising device  24  can act to damp out pump generated pressure fluctuations in connection  27 . However, this will adversely effect the life of seal  25  making it desirable to use a servo motor drive with varying torque or speed control over one pump revolution to minimise pressure fluctuations. 
       FIG. 4  shows a pump with 3 plunger assemblies feeding a water circuit for an abrasive cutting head  50 . Also shown are connections  16  between cylinder volume  17  and the pressure fluctuation equalising device  24  of  FIG. 3 . Discharge from a plunger head  7  passes through non-return valves  42  into conduit  43  which is connected via conduit  53  to abrasive cutting nozzle assembly  50  and to pressure fluctuation equalising device  24  via conduit  27 . Nozzle assembly  50  is fed with abrasive via conduit  52  from abrasive feed hopper  51 . Conduits  16  connect the cylinder volumes  17  to the pressure fluctuation equalising device  24  and to fluid control unit  60 . Fluid control unit  60  may have provision for rapidly increasing and decreasing the pressure in cylinder volumes  17 , and in maintaining a bleed flow of air to compensate for air losses during normal pumping operations. 
     On the initial start of pumping action pressure fluctuation equalising device  24  can be used to supplement pump action and rapidly increase pressure in connection  27 . With plunger  20  partially withdrawn, fluid pressure applied through connection  28  causes flow through connection  27  until pump pressure increases to the desired level or piston  21  reaches piston seat  26 . Similarly water pressure at nozzle assembly  50  can be rapidly reduced by venting pressure in connection  28  at the same time as stopping the pump motor. 
     Pumps may be driven by servomotors with encoders thereby allowing the rotational position of the pump to be continuously monitored. Forward prediction can be made as to when to load and off load plungers  6  to start and stop flows from cutting nozzles and also to vary power input to motors to minimise fluctuations in pump delivery pressure over one pump revolution. Pumps with multiple cylinders may have individual or sets of cylinder return piston spaces  17  connected to pressure fluctuation equalising devices  24  and/or fluid control units  60 . 
     The diameter of piston  9  on pump plunger  6  and piston  21  on pressure fluctuation equalising device  24  depends on the pressure of air sources  16  and  28 . The diameter of pistons  9  and  21  can be minimised by operating at as high an air pressure as practical and this air pressure is generally above the 6 bar or so generally used in workshops. Under normal operation the demand for air for sources  16  and  28  is small and may be met by a single cylinder compressor  61  driven by cam  5  as shown in  FIG. 5 . Piston  57  with seal  58  slides in cylinder  56  that has air inlet  59  and air outlet  60  non-return valves. Plunger  62  attached to piston  57  and reciprocating in bearing  54  is driven by cam  5  on a delivery stroke and returned by spring  55 . If inlet non-return valve  59  is connected to a source of pressurised air spring  55  is not required. 
     An alternative method of providing air at pressures above those of source  16  is for air from source  16  to flow through a non return valve to space  17  in cylinder housing  10  for the return stroke of plunger  6  and to discharge air from space  17  through a non return valve to an accumulator that acts as source  28 . A mechanism to vent air from space  17  can be provided to stop pumping action. 
     Although the description of pumps has been in relation to the radial disposition of plungers  6  around a single cam  5 , two or more cams and sets of plungers may be arranged axially. A pump plunger may be driven by a reciprocating member  70  as shown on  FIG. 6 . Member  70  may be driven by a cam or a crank  75  and onto which the piston end  71  of plunger  80  can be held by pressurised air in space  17  in manner described in relation to the pump of  FIG. 1 . Contact between plunger  76  and reciprocating member  70  can take the form of spherical seat and socket  74 . Using an intermediary reciprocating member  70  between a cam or crank  75  and a plunger  76  is particularly beneficial in minimising bending loads on plunger  76 . 
     Although the description of pumps has been in relation to pumping water said pumps can be used for other fluids.