Patent Publication Number: US-7585201-B2

Title: Water jet processing method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to water jet processing methods of cutting a platelike workpiece made of a relatively brittler material by producing a jet of high-pressure processing water directed to the workpiece. In particular, the invention relates to such a water jet processing method in the case where cutting lines are intersected with each other. 
   2. Description of the Related Art 
   Substrates mounted thereon with e.g. a semiconductor device are frequently made of a brittle material such as glass epoxy, glass composite or the like. The glass epoxy is made by impregnating an epoxy resin into a stack of glass fiber-made fabrics. The glass composite is made by impregnating an epoxy resin into a stack of cut and trimmed glass fibers. Such brittle substrates are obtained by cutting one large material in a lattice pattern. The water jet mentioned above in which high-pressure processing water is jetted to a workpiece is used as a cutting method in some cases. See Japanese Patent Laid-open No. Hei 5-253843. 
   Referring to  FIGS. 6A ,  6 B and  6 C, when a substrate material is cut in a lattice pattern by water jet, there arises a situation as below. A linear cut line (the second cut line)  2   a  is made to orthogonally intersect another linear cut line (the first cut line)  1   a  for cutting, the first cut line having initially been cut to extend in one direction. The substrate is previously formed with predetermined cutting lines each serving as an index for a corresponding cut line. 
     FIG. 7  illustrates a state where water jet is directed from a traveling nozzle  13  to a substrate  5  to form a second cut line  2   a  toward a first cut line  1   a , a shaded portion being a cut portion of a material. As shown in the figure, a front edge  2   b  of the second cut line  2   a  against which the water jet collides is formed to have a delay rearward of the advancing direction as it goes toward the back (underside) from the front surface subjected to the jet. In short, “delay-inclination” is occurring. The second cut line  2   a  reaches the first cut line  1   a  while forming such delay-inclination. In that instant, a phenomenon arises in which most pressure of the water jet cutting the material escapes at one burst toward the first cut line  1   a  which is a cavity. This phenomenon causes an insufficient cut area, i.e., insufficient processing area  2   c  under the delay-inclination. This leads to a problem in that projections  9  are formed to narrow the second cut line  2   a  as shown in  FIGS. 5A and 8B . The insufficient processing area  2   c  is more liable to occur as the travel speed of the nozzle  13  is higher and the angle θ of the delay-inclination shown in  FIG. 7  becomes larger to more increase the degree of the delay-inclination. 
   At the time when the escape of water jet mentioned above occurs, the first cut line  1   a  and the second cut line  2   a  intersect with each other in a T-shape as shown in  FIG. 6B . In order to subsequently form the second cut line  2   a  from such a position, the water jet that has traversed the first cut line  1   a  orthogonally collides against a wall (a portion indicated with reference numeral  8  of  FIG. 6B ) of the material to be cut therefrom and then the second cut line is made to crisscross intersect the first cut line  1   a . However, when the collision occurs, the processing water colliding against and bouncing off the wall hits the vicinity of the opening of the already cut second cut line  2   a  with respect to the first cut line  1   a . Thus, a disadvantage occurs that inadvertent processing is done such as damage to the vicinity of the opening (e.g., a portion surrounded by a broken line of  FIG. 8A ). 
   As described above, if cutting is performed by water jet to cause the second cut line to intersect the first cut line, a disadvantage occurs that an insufficient processing area is formed on the underside of the workpiece and in the front of the intersection of the second cut line with the first cut line and the vicinity of the opening to the first cut line is unnecessarily damaged. These disadvantages tend to be more remarkable as the travel speed of the water jet is higher. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a water jet processing method that can prevent abnormal processing and damage occurring when cut lines are made by water jet to intersect with each other, thereby providing satisfactory products. 
   In accordance with an aspect of the present invention, there is provided a water jet processing method using a water jet processing machine including: holding means for holding a platelike workpiece on which a first predetermined cutting line and a second predetermined cutting line intersecting the first predetermined cutting line are set; processing water supplying means for supplying high-pressure processing water; a nozzle adapted to emit the processing water supplied from the processing water supply means, to the workpiece held by the holding means; and emission-position moving means for relatively moving the nozzle and the holding means to change a position where the processing water is emitted from the nozzle to the workpiece; wherein while the nozzle and the holding means are relatively moved by the emission-position moving means, the processing water emitted from the nozzle is directed to the workpiece held by the holding means along the first predetermined cutting line and along the second predetermined cutting line to cut the first and second predetermined cutting lines, the water jet processing method comprising: a first cutting step for cutting the first predetermined cutting line to form a first cut line at a relative travel speed between the nozzle and the holding means set to a first speed; and a second cutting step for cutting the second predetermined cutting line to form a second cut line, the second predetermined cutting line being cut in a section anteroposterior to an intersection between the first cut line and the second predetermined cutting line at the relative travel speed set to a second speed lower than the first speed. 
   Preferably, in the second cutting step relative movement between the nozzle and the holding means is once lowered when water jet enters and cuts the anteroposterior section at the second speed. First, the relative travel speed of the water jet in front of the intersection of the second predetermined cutting line with the first cut line is made lower; therefore, the angle θ of the delay-inclination shown in  FIG. 7  is reduced to a near-zero angle. Thus, the escape of the water jet does not occur, which can reduce an insufficient processing area. In addition, the occurrence of the insufficient processing area can be suppressed depending on the travel speed. As a result, the occurrence of the projections  9  shown in  FIGS. 8A and 8B  can be prevented. 
   Next, when the second cut line is formed after it has traversed the first cut line, that is, the relative travel speed is lowered when the water jet is moved rearward of (to the leading side of the moving direction) the intersection of the cut lines, the water jet does not collide with the wall of the material to be cut, but slowly hits it to gradually carve the wall, forming the second cut line. Thus, the bounce of the processing water is unlikely to occur. That is to say, it can be prevented that the bounce of the processing water damages the vicinity of the opening of the second cut line to the first cut line. 
   In the present invention, the relative travel speed of the water jet in the second cut step is made low (lower than the travel speed during the formation of the first cut line) at least in the section or portion anteroposterior to the intersection of the cut lines. The water jet may be moved in the second cutting step at the travel speed (the first speed) equal to the travel speed during the formation of the first cut line other than the above section. In other words, for example, the following operating mode may be applicable. The second cut line is formed at the first speed and the relative travel speed of the water jet is reduced to the second speed immediately anterior to the first cut line. The water jet is relatively moved at the second speed until the second cut line traverses the first cut line, that is, cuts into it in a cross shape. Thereafter, the second speed is again returned to the first speed. The second speed is e.g. about ⅕ to 1/20, preferably, 1/10, of the first speed. 
   The present invention prevents the existence of the projections  9  shown in  FIGS. 8A and 8B . The lower the second speed is, or the more the deceleration point where the first speed is switched to the second speed is spaced from the intersection with the first cut line, the more the size of the projection  9  can be reduced. If these conditions are controlled to conform to the thickness of the workpiece, it is possible not to leave the projections  9  or to bring the size of the projections  9  to an acceptable value or less. 
   In the present invention, when the water jet to form the second cut line is immediately anterior to the intersection of the cut lines and enters the low-speed travel period, the relative movement of the jet water, i.e., the relative movement of the nozzle and the holding means, may be stopped once. If such operating mode is taken, the inclination angle θ formed at the front edge of the water jet is eliminated immediately anterior to the interval. In other words, the delay-inclination is eliminated and then the water jet is moved to the intersection with the first cut line at a low speed. Consequently, it is more ensured to produce an effect of preventing the occurrence of the insufficient processing area, compared with the case where the water jet is moved at a low speed with the delay-inclination left. 
   According to the water jet processing method of the present invention, abnormal processing or damage can be prevented that has otherwise been produced when the second cut line is made to intersect the first cut line, thereby producing an effect of providing satisfactory products. 
   The above and other object, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       FIG. 1A  is a perspective view of a substrate in a state where a first predetermined cutting line is being cut by a processing method of the present invention; 
       FIG. 1B  is a perspective view of the substrate in a state where a second predetermined cutting line is being cut; 
       FIG. 2  is a perspective view of a water jet processing machine according to an embodiment of the present invention; 
       FIGS. 3A to 3D  are cross-sectional views illustrating processes for forming a second cut line intersecting a first cut line by a method according to an embodiment of the present invention; 
       FIG. 4A  is a photograph showing a material cut by the method of the present invention as viewed from the direction opposed to the advancing direction of the second cut line; 
       FIG. 4B  is a photograph showing the back of the material in  FIG. 4A ; 
       FIG. 5A  is a photograph showing a material cut by the method of a relative example as viewed from the direction opposed to the advancing direction of the second cut line; 
       FIG. 5B  is a photograph showing the back of the material; 
       FIGS. 6A ,  6 B and  6 C illustrate processes for forming the second cut line crisscross intersecting the first cut line; 
       FIG. 7  is a cross-sectional view for assistance in explaining the occurrence principles of an insufficient processing area; 
       FIG. 8A  is a front view illustrating abnormal processing such as a projection occurring at an insufficient processing area as viewed from the direction opposed to the advancing direction of the second cut line; and 
       FIG. 8B  is a rear view of the insufficient processing area. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of the present invention will hereinafter be described with reference to the drawings. 
   [1] Substrate (Workpiece) 
   There is shown in  FIGS. 1A and 1B  a rectangular substrate  5  made of a brittle material such as glass epoxy or the like. This substrate  5  is used as a material for a mounting board mounted thereon with e.g. a semiconductor device and has an appropriate thickness. Referring to  FIG. 1A , the substrate  5  is formed on a front surface with a plurality of first predetermined cutting lines  1 A that longitudinally extend and with a plurality of second predetermined cutting lines  2 A that extend perpendicularly to the first predetermined cutting lines  1 A. The first predetermined cutting lines  1 A as well as the second predetermined cutting lines  1 B are formed parallel to each other and equally spaced apart from each other. Both ends of each of the first and second predetermined cutting lines  1 A,  2 A do not terminate at the corresponding edges of the substrate  5 , that is, are located at corresponding positions close to the associated edges thereof. Rectangular areas sectioned by the cutting lines  1 A,  2 A are divided along the cutting lines  1 A,  2 A into individual mounting substrates or the like. 
     FIG. 1A  illustrates a state where the first predetermined cutting lines  1 A are cut with water jet emitted from a nozzle  13 .  FIG. 1B  illustrates a state where the second predetermined cutting lines  2 A are cut with water jet emitted from the nozzle  13 . Next, a description is below given of a water jet processing machine suitable to cut the substrate  5  with water jet as described above by way of example. 
   [2] Configuration of Water Jet Processing Machine 
     FIG. 2  illustrates the whole of a water jet processing machine  10  according to an embodiment. The machine  10  is such that the substrate  5  is held on a holding table (holding means) adapted to be movable in X-, Y- and Z-directions and subjected to cutting by water jet, high-pressure processing water, emitted generally perpendicularly to the front surface of the substrate  5  from a nozzle  13 . The processing water used is e.g. water mixed with proper abrasive grains. Examples of the abrasive grains include metallic oxide grains such as alumina or the like. 
   The holding table  11  is mounted to a rectangular parallelepipedic stationary base  20  extending in the Y-direction via an X-Y-Z moving mechanism  60  (jet position moving means), which is composed of a Y-axial movable base  30 , a Z-axial movable base  40  and an X-axial movable base  50 . The stationary base  20  is formed on a lateral surface with a pair of guide rails  21  which extend in the longitudinal direction (the Y-direction). The Y-axial movable base  30  is mounted to the guide rails  21  so as to be slidable in the Y-direction. 
   The Y-axial movable base  30  is moved in the Y-direction along the guide rails  21  by the Y-axial moving mechanism  31 . The Y-axial moving mechanism  31  is disposed between the guide rails  21  and includes a thread rod  32  turnably supported by the stationary base  20  to extend in the Y-direction and a pulse motor  33  adapted to normally and reversely turn the thread rod  32 . The thread rod  32  is threadedly engaged with and is passed through the Y-axial movable base  30 . In addition, the thread rod  32  is turnable but axially immovable. When the pulse motor  33  of the Y-axial moving mechanism  31  is actuated to turn the thread rod  32 , the Y-axial movable base  30  moves along the guide rails  21  in the Y-direction in response to the turning direction. 
   The Z-axial movable base  40  is mounted to the Y-axial movable base  30  and the X-axial movable base  50  is mounted to the Z-axial movable base  40 . Their mounting structures are the same as the mounting structure in which the Y-axial base  30  is mounted to the stationary base  20 . Their moving mechanisms each have the same configuration as that of the Y-axial moving mechanism. 
   The Z-axial movable base  40  is slidably mounted to a pair of guide rails  34  formed on the Y-axial movable base  30  to extend in the Z-direction. In addition, the Z-axial movable base  40  is lifted and lowered along the guide rails  34  in the Z-direction by the Z-axial moving mechanism  41 . The Z-axial moving mechanism  41  is turnably supported by the Y-axial movement base  30  and includes a Z-axially extending thread rod  42  threadedly engaged with and passed through the Z-axial movable base  40  and a pulse motor  43  adapted to normally and reversely turn the thread rod  42 . When the thread rod  42  is turned by the pulse motor  43 , the Z-axial movable base  40  is moved (lifted or lowered) in the Z-direction in response to the turning direction thereof. 
   The X-axial movable base  50  is slidably mounted to a pair of guide rails  44  formed on the Z-axial movable base  40  to extend in the X-direction and is moved along the guide rails  44  in the X-direction by the X-axial moving mechanism  51 . The X-axial moving mechanism  51  is turnably supported by the Z-axial movable base  40  and includes a thread rod (not shown) threadedly engaged with and passed through the X-axial movable base  50  and extending in the Y-direction and a pulse motor  53  adapted to normally and reversely turn the thread rod. When the thread rod is turned by the pulse motor  53 , the X-axial movable base  50  is moved in the X-direction in response to the turning direction. 
   The Y-axially extending flat-platelike holding table  11  is attached to a surface of the X-axial movable base  50  opposite to the side mounted to the Z-axial movable base  40 . A Y-axially elongate rectangular attachment opening  11   a  is opened at the leading end side of the holding table  11 . Upwardly projecting positioning pins  12  are provided on the periphery of the attachment opening  11   a  at respective positions corresponding to the four corners thereof. The substrate  5  is positioned on the holding table  11  using the positioning pins  12 . The substrate  5  is moved in the X-, Y- or Z-direction along with the holding table  11  by the X-Y-X moving mechanism  60 . The substrate  5  is positioned using a jig, not shown, adapted to hold the substrate  5  and removably attached to the positioning pins  12 . 
   The nozzle  13  is disposed above the holding table  11  so as to orient the emitting direction of water jet vertically downward. The nozzle  13  is connected to processing water supplying means  14 , from which high-pressure processing water is supplied to the nozzle  13 . The water jet is emitted from the nozzle  13  toward the substrate  5  held by the holding table  11 . A duct  15  is arranged on the periphery of the nozzle  13  so as to open toward the holding table  11  in a horizontally elongate dome-shape. A duct pipe  16  is disposed in the duct  15  to be juxtaposed to the nozzle  13 . Suction means  17  is connected to the duct pipe  16 . The suction means  17  sucks mist-like processing water resulting from circumferentially diffusing water jet, from the duct  15  via the duct pipe  16 . The nozzle  13  and the duct  15  are secured to the stationary base  20  via a bracket  18  and via a nozzle support arm  19 . 
   The holding table  11  is moved in the X-, and Y-directions by the operation of the X-axial moving mechanism  51  and the Y-axial moving mechanism  31 , respectively. In this way, the emitting position of water jet from the nozzle  13  is moved with respect to the substrate  5  held by the holding table  11 . In addition, the holding table  11  is lifted or lowered in the Z-direction by the operation of the Z-axial moving mechanism  41 . In this way, the emitting distance of water jet from the nozzle  13  is adjusted with respect to the substrate  5  held on the holding table  11 . 
   A buffer tank  80  is disposed below the holding table  11  to catch water jet emitted from the nozzle  13 . The buffer tank  80  stores therein water adapted to reduce the force of water jet. The water in the buffer tank  80  is constantly regulated to a constant quantity by being drained by draining means not shown. 
   [3] Cutting of the Substrate by the Water Jet Processing Machine 
   The water jet processing machine  10  is configured as described above. Subsequently, a description is given of a method of cutting and dividing the substrate  5  using the machine  10 . The substrate  5  is set on the holding table  11  by means of the jig mentioned above in such a manner that the longitudinal direction of the substrate  5  is parallel to the longitudinal direction of the attachment opening  11   a.    
   (1) First Cutting Step 
   In a first cutting step, all the first predetermined cutting lines  1 A of the substrate  5  are cut. The first predetermined cutting lines  1 A extend in the Y-direction with the substrate  5  set on the holding table  11 . The holding table  11  is moved in the Y-direction at a constant speed (the first speed) via the Y-axial movable base  30  of the X-Y-Z moving mechanism  60  while water jet is emitted from the nozzle  13  toward the first predetermined cutting line  1 A. Thus, the water jet is passed through the substrate along the first predetermined cutting line  1 A to form a first cut line  1   a.    
   The water jet that has been passed through the substrate  5  collides with the water in the buffer tank  80  to reduce the force of the water jet. The emission-distance of water jet from the nozzle  13  to the substrate  5  is constant. The emission-distance is adjusted to an arbitrary value (e.g. about 3 mm) by lifting or lowering the Z-axial movable base  40 . 
   The first cut line  1   a  is formed as below. The target of the nozzle  13  is aligned with one end of the first predetermined cutting line  1   a  and water jet is emitted thereto. The holding table  11  is moved therefrom, i.e., from such a starting point, in the Y-direction at the first speed. When the other end, i.e., a terminal, is reached, the emission of water jet is suspended. In this way, one of the first cut lines  1   a  is formed. Next, the X-axial movable base  50  is moved to a position corresponding to the next first predetermined cutting line  1 A. The target of the nozzle  13  is aligned with one end of such a next first predetermined cutting line  1 A. Water jet is emitted again while the holding table  11  is moved this time in the revere Y-direction at a constant speed (the first speed). In this way, such a next first predetermined cutting line  1 A is cut to form a first cut line  1   a . Such operation is repeated to cut all the first predetermined cutting lines  1 A to form a plurality of the first cut lines  1   a.    
   (2) Second Cutting Step 
   In a second cutting step, all the second predetermined cutting lines  2 A extending in the X-direction are cut to form second cut lines  2   a . The second predetermined cutting lines  2 A are cut in the X-direction instead of the Y-direction in the same cutting procedure as the first predetermined cutting lines  1 A. The second cut line  2   a  intersects in a cross shape the first cut lines  1   a  that have already been cut. The holding table  11  is moved at the same speed (the first speed) as when the first cut line  1   a  is formed, except for a predetermined short section adjacent to or anteroposterior to the intersection. The formation speed for the second cut line  2 , i.e., the X-directional travel speed of the holding table  11  is set at a second speed lower than the first speed in the predetermined section or portion intersecting the first cut line  1   a.    
   Principles and procedures encountered when the second cut line  2   a  is formed are described in detail with reference to  FIGS. 3A through 3D .  FIGS. 3A and 3B  illustrate a state where while the nozzle  13  is moved in the X-direction (the left direction in the figures), water jet is emitted from the nozzle  13  onto the substrate  5  to form the second cut line  2   a .  FIG. 3C  illustrates a state where a second cut line  2   a  is subsequently formed. Although the holding table  11  is moved in the water jet processing machine  10 , a clear description is given assuming that only the nozzle  13  is moved. 
   The time S taken for the water jet directed to the front surface of the substrate  5  to reach the back surface thereof depends on conditions: the material and thickness t of the substrate  5 , the pressure of the water jet, and the diameter and quantity of the abrasive grains mixed with water. If the nozzle  13  is moved while water jet is emitted, it is moved by distance d during the time S as shown in  FIG. 3B . The distance d depends on the travel speed v 1  of the nozzle  13  and the time S. 
   In short, d=v 1 ×S. The longer the time S, or the greater the travel speed v 1 , the more an angle θ of delay-inclination, i.e., the degree of delay-inclination is increased. The delay inclination is formed by the front edge (an oblique line formed by a position of the substrate surface on which the water jet impinges and a position of the rear surface where the water jet is passed through)  2   b  of the second cut line  2   a . The problem resulting from the occurrence of the delay-inclination is as described earlier. In the present embodiment, as shown in  FIG. 3D , the nozzle  13  is moved at the first speed v 1  until it reaches a position p 1  immediately anterior to the first cut line  1   a . The nozzle  13  is moved at a second speed v 2  (e.g. about ⅕ to 1/20 of v 1 , preferably, about 1/10 of v 1 ) lower than the first speed v 1  as described above in the section from the position p 1  to a position p 2  immediately posterior to the traverse of the first cut line  1   a . The distance between the position p 1  which is a deceleration start position and the first cut line  1   a  is set to a distance exceeding at least the distance d in view of preventing the occurrence of an insufficient processing area. In addition, the second speed v 2  is set to a speed in excess of 0 and lower than the first speed v 1  according to the deceleration start position p. 
   After the nozzle  13  reaches the position P 2 , it is moved again at the first speed v 1  to the position p 1  immediately anterior to a first cut line  1   a  the nozzle  13  next intersects. Then, the nozzle  13  is moved at the second speed in the section p 1  to p 2  anteroposterior to the next intersection. Such speed control is repeated. 
   As described above, the water jet is emitted from the nozzle  13  that is moved in the section p 1  to p 2  anteroposterior to the intersection of the second cut line  2   a  with the first cut line  1   a  at a travel speed lower than in the other major section. This provides the following function. 
   First, the nozzle  13  is decelerated to the second speed v 2  from the position p 1  anterior to the intersection of the second cut line  2   a  with the first cut line  1   a . The angle θ of delay-inclination becomes a near zero (0) angle. A phenomenon called “the escape of jet water” mentioned earlier is unlikely to occur. The insufficient processing area  2   c  shown in  FIG. 7  can be reduced or the occurrence of the insufficient processing area  2   c  can be prevented depending on the speed. Thus, the projections  9  shown in  FIGS. 8A and 8B  will not occur. 
   Next, the water jet traverses the first cut line  1   a  still at the second speed v 2  to form the second cut line  2   a  to the position p 2 , that is, intersects the first line  1   a  in a cross shape. At this time, the water jet does not collide with a wall (a portion indicated with reference numeral  8  in  FIG. 6B ) of the substrate  5  but slowly hits and gradually carves the wall to form the second cut line  2   a . If the travel speed of the water jet remains unchanged at the first speed v 1 , the water jet collides with the wall and the processing water bounces off the wall and hits the periphery (a portion surrounded by a broken line of  FIG. 8A ) of an opening of the second cut line  2   a  to the first cut line  1   a , causing damage in some cases. However, since the speed encountered when the water jet traverses the first cut line  1   a  and hits the wall is as low as the second speed v 2 , the bounce of the processing water is unlikely to occur. Consequently, it is avoided that the periphery of the opening of the second cut line  2   a  to the first cut line  1   a  is damaged by the bounce of the processing water. 
   Incidentally, the second cut line  2   a  does not traverse the outermost first cut line  1   a  but comes into a T-shaped intersection therewith. In this case, the nozzle  13  is moved at the second speed from a position spaced by the distance p 1  from the first cut line  1   a . When the second cut line  2   a  reaches the first cut line  1   a , the emission of the water jet is suspended. 
   As described above, when the second cut line  2   a  is formed by intersecting the first cut line  1   a , the travel speed of the nozzle  13  is made relatively low in the section p 1  to p 2  anteroposterior to such an intersection. Thus, it is possible to prevent damage or abnormal processing in which an insufficient processing area is produced to form projections or processing water is bounced to cause damage. As a result, a plurality of individual mounting substrates can be obtained as satisfactory products. 
   In the embodiment described above, when the nozzle  13  reaches the deceleration position p 1 , it may be stopped once at the position p 1 . The angle θ of the delay-inclination is eliminated by stopping the nozzle  13  once like this and continuing emitting water jet. As a result, it is more ensured to provide an effect of preventing the occurrence of an insufficient processing area compared with the case where the nozzle  13  is moved at a low speed with the delay-inclination left without one-stop. 
   Next, an effect of the present invention is exemplified by exhibiting a working example of the present invention. 
   WORKING EXAMPLE 
   A glass epoxy plate with a thickness of 3 mm is cut in a cross shape by the same water jet processing machine as that illustrated in  FIG. 2 . Initially, a first predetermined cutting line to be cut first was cut by water jet emitted from a nozzle at a relative travel speed of 35 mm/sec. Incidentally, it was grasped that an angle θ of delay-inclination occurring at this time as shown in  FIG. 3B  was about 14.5° and the distance d is about 0.8 mm. Next, a second predetermined cutting line crisscross intersects the first cut lines thus formed was cut at 35 mm/sec at a portion thereof immediately anterior to the intersection. When a position to within 0.9 mm of the first cut line is reached, the relative travel speed of the nozzle is decelerated to 3 mm/sec and the intersection is traversed to form a second cut line. 
   COMPARATIVE EXAMPLE 
   A second cut line is made to intersect first cut lines in the same procedures as the working example except that the second cut line was made to traverse the intersection at a constant speed of 10.0 mm/sec unlike the working example without the deceleration of the second cut line immediately anterior to the intersection with the first cut line. 
   The periphery of the intersection of the second cut line with the first cut line was observed to check each of the cut states of the above-example and comparative example.  FIG. 4A  is a photograph showing a cut material in the working example as viewed from the direction opposed to the advancing direction of the second cut line and  FIG. 4B  is a photograph sowing the back of the cut material in  FIG. 4A . As shown in  FIG. 4A , a wall portion formed along the second cut line so as to be immediately anterior to the intersection (the corner in the photograph) is vertically linear and flat. In addition, as shown in  FIG. 4B , no projection occurs on the wall portion formed along the second cut line and near the intersection. 
   In contrast, in the comparative example shown in  FIGS. 5A and 5B , a downward projection occurs on the wall portion formed along the second cut line and near the intersection.  FIG. 5A  is a photograph showing a cut material as viewed from the direction opposed to the advancing direction of the second cut line and  FIG. 5B  is a photograph showing the back of the cut material. In the comparative example, since the intersecting speed of the second cut line relative to the first cut line is relatively high, when the second cut line intersects the first cut line, the escape of water jet occurs, which produces an insufficient processing area. 
   The above-results proved that the occurrence of an insufficient processing area is prevented to provide a satisfactory cut surface by setting the intersection speed of the second cut line relative to the first cut line at a relatively low level. 
   The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.