Patent Publication Number: US-7584546-B2

Title: Alignment control for a water-jet cutting system

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a device and a method for checking the alignment of a water-jet cutting system. 
   DESCRIPTION OF THE PRIOR ART 
   Water-jet cutting systems are widely used for cutting all sorts of materials. Pure water may be sufficient if the materials to be cut are relatively soft or thin, but for cutting harder and thicker materials, the water must however be mixed with an abrasive powder so as to obtain a more powerful jet. 
   Cutting systems using a jet of abrasive-laden water comprise a pressurized-water intake connected to the inlet of a cutting head, which generally comprises a collimating tube intended to conduct the pressurized water to a water-jet-forming nozzle which is housed in the outlet of this collimating tube and emerges in a mixing chamber where the abrasive powder is introduced by suction. The abrasive-laden water jet leaving the mixing chamber then passes into the cylindrical channel of a focusing tube. The downstream end of the collimating tube, the jet-forming nozzle and the upstream end of the focusing tube are supported by a body in which the mixing chamber is also formed. 
   For high-performance cutting of good quality, including for parts of relatively large thickness, such cutting systems must deliver a water jet that is stable over time and the energy of which is as high as possible. 
   An unstable or insufficiently powerful water jet runs the risk of compromising the speed and precision of the cutting, with the risk of causing a loss of quality in the cut parts. 
   However, in these systems, the energy and the stability of the water jet may be greatly affected by the quality of alignment of the axis of the jet-forming nozzle with the axis of the focusing tube, especially because a misalignment is liable to cause substantial pressure drops in the focusing tube and cause divergence or dispersion of the jet. 
   The quality of alignment of the jet-forming nozzle with the focusing tube may also have an impact on the lifetime of the tube, a misalignment running the risk of causing premature wear of the tube owing to larger collisions between the jet and the internal wall of this tube. 
   Correct alignment of the axis of the jet-forming nozzle with the axis of the focusing tube depends mainly on the dimensional quality of the parts making up the cutting head, in particular the nozzle, the tube and the head body, but also on the quality of the fitting of the nozzle in its housing, this fitting generally consisting in pressing this nozzle with great pressure into this housing. The quality of such a fitting may be substantially compromised by the presence of undesirable bodies, such as abrasive powder grains which could get between the nozzle and the walls against which this nozzle is pressed. 
   Now, at the present time there is no means for checking the quality of alignment of the jet-forming nozzle with the focusing tube, except by carrying out timed drillings in order to evaluate the energy of the abrasive water jet, which are lengthy and somewhat imprecise trials. 
   The object of the invention is to provide a simple, inexpensive and effective solution to this problem. 
   SUMMARY OF THE INVENTION 
   The subject of the invention is a device and a method that make it possible to check the alignment of the water jet, and therefore the alignment of the axis of the jet-forming nozzle with the axis of the focusing tube, with means that are simple, inexpensive and quickly implemented. 
   For this purpose, the invention provides a device for checking the alignment of a water jet in a water-jet cutting system which comprises a pressurized water inflow, a nozzle for forming the water jet, said nozzle opening into an abrasive powder intake chamber, and a focusing tube mounted on the outlet of this chamber, this focusing tube having an internal cylindrical channel of constant diameter for passage of the water jet, which device includes means for positioning an annular element on the outlet of the focusing tube, at a certain distance from this outlet and aligned with respect to the axis of the focusing tube, this annular element having an internal diameter equal to that of the internal channel of the focusing tube. 
   Owing to its positioning and its internal diameter, this annular element makes it possible to test the alignment of the water jet output by the focusing tube. This is because if the water jet is correctly aligned, it must pass through the center of the annular element without coming into contact with this element. However, in the event of misalignment, the annular element constitutes an obstacle for at least part of the water jet, which will therefore rebound upon contact with this annular element. The presence or absence of a rebound therefore provides information about the quality of the water jet, and consequently about the quality of alignment of the axis of the jet-forming nozzle with the axis of the focusing tube. 
   According to a preferred embodiment, the annular element is formed by the downstream end of a cylindrical tubular end-piece fixed, for example screwed or fitted, to the downstream end of the focusing tube. 
   This embodiment offers a simple means of fitting and removing the annular element, which is incorporated into the tubular end-piece, while still guaranteeing that it is correctly positioned thanks to the rigidity of this tubular end-piece. 
   The tubular end-piece advantageously includes an internal cylindrical channel for passage of the water jet, this channel having a diameter greater than that of the internal channel of the focusing tube and terminating at its downstream end in an internal shoulder forming the annular element. 
   The larger diameter of the channel of the end-piece prevents this channel from acting as an extension of the focusing tube and makes the annular element intended to test for the alignment of the water jet inoperable. 
   According to another feature of the invention, the tubular end-piece includes at least one window formed in its cylindrical wall near its downstream end. 
   This window makes the checking of the alignment more reliable, by providing a lateral discharge path for at least part of the water jet in the event of misalignment. 
   The device according to the invention also includes means for reducing the water feed pressure and means for reducing the water feed flow rate, used for checking the alignment of the water jet. 
   This is because it is preferable to greatly reduce the water feed flow rate and pressure when checking the alignment of the jet, so as to have a thin coherent jet, that is to say substantially non-divergent, output by the focusing tube, thereby greatly facilitating the alignment check by reducing in particular the risks of observation areas. 
   The invention also provides a method for checking the alignment of the jet in a water-jet coupling system, which method consists:
         in fixing to the outlet of the focusing tube a cylindrical tubular end-piece having, at its downstream end, an annular element centered on the axis of the tube and having an internal diameter equal to that of the tube;   in supplying the cutting system with water at a reduced pressure and with a reduced flow rate, without addition of abrasive powder, in order to obtain a coherent and substantially non-divergent water jet at the outlet of the tube; and   in observing any water rebound on the annular element in the event of a misalignment of the water jet.       

   In one example of the implementation of the method according to the invention, in which the system is fed at a pressure of about 2500 bar for cutting and with a flow rate of about 4.7 liters per minute, the feed pressure is reduced to about 700 bar and the flow rate to about 0.22 liters per minute in order to check the alignment of the jet. 
   Such pressure and flow rate values make it possible, in this example, to obtain at the outlet of the tube a water jet that is fine enough and non-divergent. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and features of the invention will become apparent on reading the following description given by way of nonlimiting example and with reference to the appended drawings in which: 
       FIG. 1  is a schematic view in axial cross section of a water-jet cutting system using an abrasive-laden water jet; 
       FIG. 2  is a schematic view in axial cross section of a device according to the invention, in the case of misalignment of the water jet; and 
       FIG. 3  is a partial schematic view in axial cross section on a larger scale of an exemplary embodiment of the device according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring firstly to  FIG. 1 , this shows the head of an abrasive water-jet cutting system, comprising a body  10  supporting the downstream end of a collimating tube  12  that has an internal channel  14 , and the upstream end of a focusing tube  16 , which is fitted onto this body  10  thanks to a clamping nut  18  and includes an internal channel  20 , the inlet  32  of which has a funnel shape. The body  10  also includes a lateral inlet  22  provided with a coupler  24  to which an abrasive powder feed  26  is connected. A water-jet-forming nozzle  28  is housed in the body  10  at the outlet of the collimating tube  12  and emerges in a mixing chamber  30 , which is formed in the body  10  and communicates with the inlet  32  of the internal channel  20  of the focusing tube  16  and with the lateral inlet  22 . 
   In operation, pressurized water, shown symbolically by the arrow  34 , flows into the internal channel  14  of the collimating tube  12  until it encounters the nozzle  28 . The water leaves this nozzle in the form of a very powerful jet, the passage of which through the mixing chamber at high velocity causes abrasive powder to be sucked in by the venturi effect, the flow of powder being shown symbolically by the arrow  36 . The water/abrasive powder mixture then passes into the inlet  32  of the internal channel  20  of the focusing tube, where the jet is accelerated before being blasted out via the outlet  38  of this focusing tube. 
   The jet alignment checking device according to the invention comprises ( FIG. 2 ) a tubular end-piece  40  that is fitted onto the downstream part of the focusing tube  16  of the cutting system shown in  FIG. 1  and includes an internal cylindrical channel  42  of larger diameter than the diameter of the internal channel  20  of this focusing tube. An internal annular shoulder  44  is formed on the downstream end of the tubular end-piece  40  and defines a cylindrical outlet orifice  48  of the internal channel  42 . The internal diameter of this outlet orifice  48  is equal to the diameter of the internal channel  20  of the focusing tube  16 . Furthermore, the internal channels  20  and  42  and the cylindrical outlet  48  lie on the same axis. Moreover, the tubular end-piece  40  also includes an opening or window  46  formed in its cylindrical wall, near its downstream end, this opening emerging in the channel  42  of the end-piece and having dimensions that are larger than the diameter of this channel. 
   The principle of operation and use of this checking device is as follows: the abrasive powder feed is closed off and the water feed pressure and flow rate of the cutting system are reduced so as to obtain a sufficiently fine and coherent water jet, that is to say one in which there is substantially no divergence on leaving the focusing tube  16 . To reduce the water flow rate, the entire water feed flow may simply be passed into a duct equipped with a flow control valve, set to the desired flow rate, this duct being branched off the feed duct of the cutting system and being connected to it via a shutoff valve operating in on/off mode, which may for example be controlled by an air pressure. The end-piece  40  is fitted onto the focusing tube in accordance with  FIG. 2 , and then this cutting system is fed with water at the reduced flow rate. The water jet  54  then flows through the internal channel  20  of the focusing tube  16  and then through the internal channel  42  of the tubular-end piece  40  as far as its outlet  48 . 
   If the water jet is correctly aligned, it passes through the outlet orifice  48  without appreciably rebounding on the shoulder  44 . 
   However, if the water jet  54  is misaligned, as illustrated in  FIG. 2 , a relatively large portion of the water jet inevitably comes into contact with the annular shoulder  44 , giving rise to water rebound  50  at the outlet  48  and emerging via the window  46 . 
   If the misalignment of the water jet is very considerable, it may happen that this jet is reconstructed by repeatedly rebounding off the wall of the internal channel  42  and may end up resembling a correctly aligned jet arriving at the outlet  48 . The window  46  makes it possible to distinguish this situation from the case in which the jet is correctly aligned owing to a larger expulsion of water via this window. 
   The alignment check using the device according to the invention therefore consists in observing possible rebounds  50  and  52  characterizing poor alignment. 
   If the check demonstrates that the jet is poorly aligned, and therefore that the axis of the jet-forming nozzle  28  is misaligned with the axis of the focusing tube  16 , the nozzle  28  may be removed from the system and replaced with a new nozzle, or possibly refitted after being cleaned, if the misalignment was caused by the presence of undesirable bodies in the housing for this nozzle  28 . 
     FIG. 3  shows an exemplary embodiment of the end-piece  40  designed to cooperate with a clamping nut (not shown) which is screwed onto a threaded part  56  of the outer surface of the end-piece, near its upstream end. Apart from the internal channel  42 , the annular shoulder  44  and the window  46  already mentioned, the end-piece  40  includes an upstream part  58  with an axial slot, which defines two substantially semicylindrical and symmetrical jaws  60 . 
   The end-piece  40  is fitted onto the cutting head by inserting the downstream part of the focusing tube  16  between the two jaws  60  and then tightening the clamping nut on the part  56  of the end-piece until the focusing tube is clamped sufficiently by the jaws  60  to ensure that the device is properly held on the head of the cutting system. 
   In general, the tubular end-piece  40  of  FIGS. 2 and 3  constitutes one example of a means for positioning an annular element  44  at a certain distance from and along the axis of the focusing tube  16 . Other means may be used for placing the annular element along the axis of the tube  16 , these means being technical equivalents of the means described and shown.