Patent Publication Number: US-2012037604-A1

Title: Laser beam machining device

Description:
TECHNICAL FIELD 
     The present invention relates to a laser machining apparatus. 
     BACKGROUND ART 
     There is a laser machining apparatus broadly used for laser beam machining such as laser cutting or laser welding by emitting a laser beam to a workpiece. 
     For instance, the conventional laser machining apparatus includes a laser oscillator, a transmitting path and a machining head, in which the laser oscillator generates a laser beam, the laser beam is transmitted through the transmitting path and the machining head emits the laser beam to the workpiece, thereby melting the surface of the workpiece. 
       FIG. 18  illustrates such machining head  100  containing a condenser lens  110 , a protection glass  120 , a lens housing  130  and a torch  140 . 
     The condenser lens  110  focuses the laser beam generated by the laser oscillator and condenses the energy thereof. The protection glass  120  prevents metal foreign matters  150 , such as high temperature spatters and fumes, from attaching to the condenser lens  110 . 
     The lens housing  130  houses the condenser lens  110  and the protection glass  120 , and protects these optical systems. The torch  140  is disposed at the tip of the machining head  100 , prevents the laser beam focused through the condenser lens  110  from dispersing and avoids the external influences on the laser beam. 
     In the above-described machining head  100 , the laser beam focused through the condenser lens  110  in the lens housing  130  is emitted to the workpiece out of the torch  140  passing through the protection glass  120 . 
     To maintain the stability of welding quality with preventing the oxidation of the surface of the workpiece and the penetration of the foreign matter, there is a common method of blowing the assist gas such as nitrogen or argon gas to the surface of the workpiece so as to generate inert atmosphere around the welding spot. 
     As to the machining head  100  illustrated in  FIG. 18 , when the foreign matters  150  enter into the torch  140  through the opening of the torch  140  and directly reach the protection glass  120  or indirectly get to the glass  120  reflecting on the inner surface of the torch  140 , and then the foreign matters adhere to the protection glass  120 . Further, if the temperature of the foreign matter is higher than the melting point of the protection glass, the foreign matter may penetrate into the glass and strongly adhere to the surface of the protection glass  120 . In such case, the surface of the protection glass  120  is degraded and damaged. 
     As a result, the transmittance of the protection glass  120  is reduced, so that the laser output is lowered and the machining quality is deteriorated. In order to keep the laser output, the protection glass  120  is required to be changed frequently, but it costs too much. 
     The machining head disclosed in JP H11-245077 A proposes a solution to solve above-mentioned problem. The torch of the machining head is formed with a spiral passage for the assist gas at the inside thereof. Due to this structure, the assist gas blows along the spiral passage to the machining point, thereby preventing the high temperature foreign matters from entering into the machining head. 
     Unfortunately, the machining head disclosed in JP H11-24507 A cannot avoid an approach of the spatter moving faster than the blowing speed of the assist gas. Therefore, there remains the problem that the adhesion to the protection glass and penetration (welding) thereinto of the foreign matters having higher temperature than the melting point of the protection glass, such as the spatter that is higher temperature than the melting point of the workpiece. Especially, in the case that the torch is close to the workpiece, the opening area of the torch where the spatters are enterable is relatively large, so that it is difficult to prevent the entry of the spatters. 
     When using the laser machining apparatus to weld the workpiece, the assist gas should be blown gently to generate inert atmosphere in the welding spot. 
     Thus, the machining head of JP H11-245077 A does not work well, because the blowing speed of the assist gas should be increased to prevent the foreign matters of high temperature from entering into the torch, which makes the quality of welding lowered. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP H11-245077 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The objective of the present invention is to provide an unexpected laser machining apparatus capable of preventing the reduction of transmittance of the protection glass caused by the adhesion of the foreign matters of high temperature such as the spatters and fumes generated in the laser machining, thereby enhancing the lifetime of the protection glass. 
     Technical Solution 
     A laser machining apparatus according to the present invention is a laser machining apparatus that emits a laser beam to the surface of a workpiece to melt the surface, thereby performing a laser beam machining. The one embodiment of the present invention includes a condenser lens for focusing the laser beam to the surface of the workpiece; a protection glass disposed nearer to the workpiece than to the condenser lens, protecting the condenser lens; a torch disposed facing the workpiece, emitting the laser beam; and cooling means for cooling the protection glass. 
     In the preferable embodiment, the cooling means blows a cooling gas to the protection glass to cool the protection glass and the atmosphere in the torch. 
     Advantageously, the cooling gas is an assist gas blown to the surface of the workpiece during the laser machining to form an inert atmosphere around the surface of the workpiece. 
     It is preferable that the cooling means includes multiple openings for blowing the assist gas, and the assist gas is blown to the surface of the protection glass from the blowing openings. 
     Preferably, the multiple openings are disposed facing the inside of the torch and spaced each other in the inner circumference of the torch. 
     In the alternative embodiment of the present invention, the laser machining apparatus further includes an exhausting means for exhausting the atmosphere in the torch to the outside thereof. 
     The exhausting means preferably includes an exhausting opening through which the atmosphere in the torch is exhausted, and adjusting means for adjusting the flow amount of the atmosphere to be exhausted. 
     In the advantageous embodiment of the present invention, the inner surface of the torch is formed in a multi-step surface. 
     The multi-step surface advantageously includes a face opposing to the surface of the workpiece, and the face opposing to the surface of the workpiece is formed such that a line reflected on the face, that is a radially incident line from the surface of the workpiece, does not direct the protection glass. 
     Furthermore, the multi-step surface of the torch is configured by multiple grooves formed depressed from the inner surface to the outer surface of the torch and formed continuously from the base end to the tip end of the torch. 
     In the other embodiment of the present invention, the tip end of the torch is provided with a tip extended toward the surface of the workpiece, and the opening area of the tip is set smaller than that of the torch. 
     Preferably, the inner surface of the tip is configured continuously from the inner surface of the torch. 
     Advantageously, the tip is separated from the torch. 
     Advantageous Effects of Invention 
     According to the embodiment of the present invention, a laser machining apparatus is provided that is capable of preventing the reduction of transmittance of the protection glass caused by the adhesion of the foreign matters of high temperature such as the spatters and fumes generated in the laser machining, thereby enhancing the lifetime of the protection glass. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically illustrates a laser machining apparatus. 
         FIG. 2  is a section view of a machining head. 
         FIG. 3  depicts a cooling means of the laser machining apparatus. 
         FIG. 4  shows a flow of an assist gas in a torch. 
         FIG. 5  is an A-A section view of the  FIG. 4 , showing an arrangement of the cooling means. 
         FIG. 6  illustrates a feeding passage of the assist gas. 
         FIG. 7  depicts an exhausting means of the laser machining apparatus. 
         FIG. 8  shows an alternative embodiment of the exhausting means of the laser machining apparatus. 
         FIG. 9  depicts a multi-step surface formed in the inner surface of the torch. 
         FIG. 10  is an enlarged view of the multi-step surface. 
         FIG. 11  shows an alternative embodiment of the multi-step surface. 
         FIG. 12  shows another embodiment of the multi-step surface. 
         FIG. 13  depicts a tip of the laser machining apparatus and is a B-B section view of  FIG. 14 . 
         FIG. 14  is a section view of the tip. 
         FIG. 15  is C-C section view of  FIG. 14  showing the opening of the tip. 
         FIG. 16  shows comparison between the opening areas of the tip and that of the torch, (a) is a tip end of the torch, and (b) is an E-E section view of the  FIG. 15  showing the tip end of the tip. 
         FIG. 17  shows the flow of assist gas around the tip end of the machining head. 
         FIG. 18  schematically illustrates a conventional laser machining apparatus including a machining head. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : laser machining apparatus 
               2 : workpiece 
               30 : machining head 
               31 : condenser lens 
               32 : protection glass 
               34 : torch 
               40 : assist-gas feeding unit (cooling means) 
               45 : assist gas (cooling gas) 
               50 : exhausting unit (exhausting means) 
           
         
       
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the attached FIGS., a laser machining apparatus  1  is described that is an embodiment of the present invention. The laser machining apparatus  1  emits the laser beam to the surface of a workpiece  2  and melts the surface of the workpiece  2 , whereby laser beam machining such as laser cutting or laser welding is performed. The workpiece  2  is made of metal, e.g., aluminum or iron, and is supported movably with respect to the laser machining apparatus  1 . 
     As illustrated in  FIG. 1 , the laser machining apparatus  1  includes a laser oscillator  10 , an optical fiber  20 , and a machining head  30 . 
     The laser oscillator  10  generates a YAG (Yttrium Aluminum Garnet) laser beam (hereinafter simply called: laser beam) and outputs it. The laser oscillator  10  generates the laser beam with a predetermined output. 
     The optical fiber  20  is a transmitting path for the laser beam generated in the laser oscillator  10 , and connects the output of the laser oscillator  10  to the base of the machining head  30 . The laser beam is transmitted from the laser oscillator  10  to the machining head  30  through the optical fiber  20 . 
     The machining head  30  of the laser machining apparatus  1  emits the laser beam and faces to the workpiece  2 . The laser beam is applied to the surface of the workpiece  2  from the tip of the machining head  30 . 
     The schematic way of machining the workpiece  2  by using the laser machining apparatus  1  is explained as follows. The laser beam generated by the laser oscillator  10  is transmitted through the optical fiber  20  to the machining head  30 , and the machining head  30  emits the laser beam to the surface of the workpiece  2 . 
     Referring to  FIG. 2 , the structure of the machining head  30  is described. 
     As depicted in  FIG. 2 , the machining head  30  contains a condenser lens  31 , a protection glass  32 , a lens housing  33  and a torch  34 . 
     The condenser lens  31  is a convex lens to focus the laser beam to the surface of the workpiece  2  (machining spot) and to condense the energy thereof. The laser beam condensed through the condenser lens  31  is focused to the machining point of the workpiece  2 . 
     The protection glass  32  is a plate glass to protect the condenser lens  31  from the foreign matters of high temperature such as spatters or fumes generated at the surface of the workpiece  2  during the machining or from grit and dust in the torch  34 . The protection glass  32  is arranged between the condenser lens  31  and the workpiece  2 . That is, the protection glass  32  is disposed at the side of workpiece  2  with respect to the condenser lens  31  so that the protection glass  32  divides the machining head  30  into two areas where the condenser lens  31  is disposed and the opposite side (where the torch  34  is disposed). 
     The laser beam passed through the condenser lens  31  is emitted to the workpiece  2  transmitting through the protection glass  32 . 
     The lens housing  33  houses the optical systems including the condenser lens  31  and the protection glass  32 , and has an opening  33   a  formed at the end thereof. The base end of the housing  33  is connected with the optical fiber  20 , whereby the laser beam is emitted into the housing  33 . The laser beam emitted into the housing  33  passes through the condenser lens  31  such that the laser beam is focused to the machining point of the workpiece  2 , and is led into the torch  34  passing through the protecting lens  32  and the opening  33   a.    
     The torch  34  is arranged continuously to the lens housing  33  and formed in a conical shape and tapered toward the tip. The torch  34  has an opening  34   a  formed at the base end corresponding to the opening  33   a  of the housing  33  and an emitting opening  34   b  formed at the tip end. The opening  34   a  of the torch  34  is connected to the opening  33   a  of the torch  34  and the inside of the torch  34  is communicated with that of the hosing  33 . The laser beam led into the torch  34  through the housing  33  is emitted through the emitting opening  34   b.    
     The torch  34  works as a housing which prevents the laser beam from dispersing and avoids the external influences on the laser beam. 
     As described above, in the machining head  30 , the protection glass  32  is arranged between the condenser lens  31  and the torch  34 , and the protection glass  32  protects the condenser lens  31  in such a way that the foreign matters such as spatters and fumes entering into the torch  34  do not reach the condenser lens. 
     As shown in  FIGS. 3 to 6 , the machining head  30  contains an assist-gas feeding unit  40 . 
     The assist-gas feeding unit  40  is a cooling means for feeding an assist gas  45  to the protection glass  32  during the laser machining is performed, and is arranged at the base end of the torch  34 . In other words, the unit  40  blows the assist gas  45  to the tip side of the protection glass  32  (especially to the space defined by the protection glass  32  and the inside of the torch  34 ). 
     The assist gas  45  is an inert gas, which is used for prevention of the oxidation of the surface of the workpiece  2  and the penetration of the foreign matters into the machining point during the laser machining apparatus  1  performs the laser machining. For instance, the assist gas  45  is a nitrogen gas or an argon gas. 
     Blowing the assist gas  45  to the machining point in the workpiece  2  forms the inert atmosphere on the surface of the workpiece  2 . 
     Moreover, the assist gas  45  is used at room temperature (about 25° C.), so that the assist gas works as the cooling gas for cooling the inside of the torch  34 . 
     As shown in  FIGS. 3 to 6 , the assist-gas feeding unit  40  includes a single or multiple (in this embodiment, four) feeding pipes  41  and a feeding source  42  for feeding the assist gas. 
     Each feeding pipe  41  faces to the inside of the torch  34  through a blowing opening  41   a , and is connected to the feeding source  42 . 
     The feeding source  42  stores the assist gas  45  and feeds the assist gas  45  to the feeding pipes  41 . 
     As depicted in  FIG. 3 , each blowing opening  41   a  of the feeding pipe  41  is arranged projecting toward the protection glass  32  from the inner wall of the torch  34 . All of the blowing openings  41   a  are disposed outside of the focusing area of the laser beam. 
     More specifically, the feeding pipes  41  are arranged such that the assist gas  45  is evenly blown to the all area of the surface of the protection glass  32  and the assist gas  45  is blown to the center portion of the glass  32 . 
     In other words, the assist gas  45  that is blown from the feeding pipes  41  to the protection glass  32  is blown around the center and gathered to the center on the surface of the glass  32 . Thus, all area of the surface of the glass  32  is cooled by the forced convection heat transfer of the assist gas  45 . Further, the assist gas  45  is continuously fed to the glass  32 , so that the atmosphere around the glass  32  is kept in low temperature. 
     As described above, when the laser machining apparatus  1  performs the laser machining to the workpiece  2 , the assist-gas feeding unit  40  blows the assist gas  45  of room temperature to the protection glass  32  through the feeding pipes  41 , so that the surface of the glass  32  is forcedly cooled and the atmosphere around the glass  32  is also cooled. Furthermore, if the metal foreign matters of high temperature, e.g. spatters and fumes, enter into the torch  34  when performing the laser machining, the assist gas  45  blows and directly cools the foreign matters of high temperature. 
     Therefore, the foreign matters are cooled till they reach the protection glass  32  and the temperature of the foreign matters become lower than the melting point of the metal composing the foreign matters (e.g., 600° C.) or than the melting point of the glass (e.g., 400° C.). Especially, if the foreign matters are cooled enough and become solidified before they reach the glass  32 , the foreign maters do not adhere to the glass  32 . 
     As a result, the foreign matters are prevented from attaching to the glass  32  with their temperature are high and from penetrating (welding) into the glass  32 , and thus the lifetime of the glass is improved and the transmittance of the glass  32  is maintained, thereby keeping the laser output. 
     The protection glass  32 , which is easy to become high temperature when performing the laser machining, is directly cooled, so that the glass  32  avoids thermal expansion. Further, cooling the glass  32  that is arranged in the vicinity of the condenser lens  31  and the torch  34  makes the machining head  30  wholly cooled. Therefore, there are small thermal influences occurred in laser-machining such as the shift of focusing point caused by the thermal expansion of the condenser lens  31  or the change of the reflection index of the laser beam in the torch  34 . 
     The laser machining apparatus  1  utilizes the assist gas  45  that is used for the purpose of good quality machining as the cooling gas for the protection glass  32 , whereby there is no need to prepare the alternative cooling means and the existing equipment can be efficiently used. 
     As depicted in  FIG. 4 , the assist gas  45  blown to the protection glass  32  from each feeding pipes  41  is impinged on each other at the center of the glass  32 , and then the blowing direction of the assist gas is changed the direction from the glass  32  to the emitting opening  34   a  of the torch  34 . Thus, the assist gas  45  blown from the each pipe  41  is guided toward the workpiece  2  so that the machining point on the workpiece  2  is surrounded with the inert atmosphere. 
     As illustrated in  FIG. 5 , the number of the feeding pipes  41  is four, and the pipes are arranged with 90° apart on the circle of the torch  34 . That is, the blowing openings  41   a  of the pipes  41  are equally spaced each other on the inner circumference of the torch  34 . 
     Due to the structure, the assist gas  45  impinges equally on the protection glass  32 , and the stable blowing is formed from the glass  32  to the emitting opening  34   a  of the torch  34 . Therefore, when the assist gas  45  is used for cooling the protection glass  32 , the assist gas  45  still provides the original performance so that the quality of laser machining of the laser machining apparatus  1  is maintained. 
     Furthermore, the assist gas  45  is blown from the multiple feeding pipes  41 , and therefore the surface of the glass  32  is broadly cooled and the glass  32  and the peripheral atmosphere are efficiently cooled. 
     The arrangement of the feeding pipes  41  is not limited to this embodiment, in which the four pipes  41  are arranged in the circumferential direction of the torch  34  with 90° spaced each other. The feeding pipes may be arranged such that the assist gas from each pipe  41  is evenly impinged each other on the surface of the protection glass  32 , and that the constant flow of the assist gas is formed from the glass  32  to the emitting opening  34   a  of the torch  34 . 
     If the number of the feeding pipes  41  is increased, the blowing amounts from the pipes  41  are decreased. In view of cooling inside of the torch  34 , the assist gas  45  should be blown to the protection glass  32  with the predetermined speed. There is a way to increase the blowing speed of the assist gas  45  by narrowing the blowing opening  41   a  of the pipe  41 , however, in order to provide the original purpose of the assist gas  45 , the blowing speed has to be set such that the oxygen existing around the openings  41   a  in the torch  34  is not mixed into the assist gas  45 . Consequently, the number of the pipes  41  is set to satisfy these conditions. 
     As shown in  FIG. 6 , the assist-gas source  42  includes a tank  42   a  storing the assist gas  45 , multiple pipes  42   b  for feeding the assist gas from the tank, and a valve  42   c  for adjusting the flow amount of the assist gas fed through the pipes  42   b.    
     In the assist-gas source  42 , the assist gas  45  fed from the single tank  42   a  is equally divided into the multiple pipes  42   b  via the valve  42   c , and finally fed to the feeding pipes  41 . The assist gas  45  is led into the torch  34  through the blowing openings  41   a  of the feeding pipes  41  and blown to the machining point of the workpiece  2  passing through the emitting opening  34   b  of the torch  34 . 
     In the valve  42   c , the flow amount of the assist gas  45  fed to the each pipe  42   b  is detected with a flow sensor so that the flow amounts of the assist gas  45  passing through the pipes  42   b  are even. That is, the flow amount of the assist gas  45  blown through the each feeding pipe  41  is constant. 
     It should be noted that the flow amount of the assist gas  45  fed from the assist-gas source  42  is adjustable in accordance with the machining condition of the workpiece  2  (laser welding, laser cutting and the like). 
     In this embodiment, from the viewpoints of preventing the facility from becoming large or complex and of making the best of existing facility, the single gas tank  42   a  provides the assist gas  45  with the each feeding pipe  41  via the valve  42   c  and the pipes  42   b . However, each feeding pipe  41  may be provided with the tank  42   a , the pipe  42   b  and the valve  42   c , in this case, the flow condition (pressure, flow amount or the like) of the assist gas  45  blown through the each feeding pipe  41  is easily adjusted. 
     As illustrated in  FIG. 7 , the machining head  30  further includes an exhausting unit  50 . 
     The exhausting unit  50  is an exhausting means for exhausting the atmosphere (involving a part of assist gas  45  reflected on the protection glass  32  and a part of the foreign matter of high temperature) in the torch  34  to the outside of the machining head  30  during the laser machining. The exhausting unit is disposed in the middle portion of the torch  34 . 
     The exhausting unit  50  includes single or multiple (in this embodiment, four) suction pipes  51 , a suction pump and an adjusting valve (both not shown). 
     The suction pipes  51  have inner ends facing inside of the torch  34  via suction ports  51   a , and the outer ends are connected to the suction pump. 
     The suction pump sucks and exhausts the atmosphere in the torch  34  through the suction ports  51   a  of the suction pipes  51 . The suction pump is connected to the suction pipes  51  via the adjusting valve, and the adjusting valve adjusts the suction amount by the suction pump. 
     As shown in  FIG. 7 , the suction ports  51   a  of the suction pipes  51  are arranged in the middle portion of the torch  34 , and arranged in the nearer side to the emitting opening  34   b  of the torch  34  than the blowing openings  41   a  of the feeding pipes  41 . The suction ports  51   a  are disposed outside of the focusing area of the laser beam. 
     Through the suction pipes  51 , the atmosphere in the torch  34  is sucked. That is, the part of the assist gas  45  and the part of the foreign matters (especially, the fumes) entered into the torch  34  are sucked through the suction pipes. 
     The suction pump is a metering pump and creates a negative pressure in the suction pipes  51  to draw the atmosphere in the torch  34  through the suction ports  51   a . The adjusting valve is disposed between the suction pipes  51  and the suction pump. 
     The adjusting valve is a variable valve to adjust the suction amount by the suction pump. That is, the exhausting amount of the atmosphere to the outside of the torch  34  is controlled by the adjusting valve. 
     Further, in the adjusting valve, the flow amounts sucked through the suction pipes  51  are detected by sensors so that the pressure in the torch  34  is constant, namely the flow amount of the assist gas  45  is constant that is fed to the machining point of the workpiece  2  through the emitting opening  34   b  of the torch  34 . 
     As described above, when the laser machining apparatus  1  performs the laser machining to the workpiece  2 , the exhausting unit  50  exhausts the atmosphere in the torch  34  to the outside of the torch  34 , the atmosphere involving the assist gas  45  and the foreign matters of high temperature. 
     Thus, the torch  34  avoids being high pressure because of the excessive feeding of the assist gas  45  from the assist-gas feeding unit  40 . So, even if the assist gas  45  is used for a coolant, the cooling performances for the protection glass  32  and the inside of the torch  34  are maintained, and the original performance as the generator of the inert atmosphere of the assist gas  45  is secured. Also, exhausting the atmosphere in the torch  34  that is easy to be high temperature during the laser machining makes the cooling performance in the torch  34  improved. 
     Moreover, the flow (feeding) amount of the assist-gas feeding unit  40  and the suction (exhausting) amount of the exhausting unit  50  are separately controlled. So, the best mode of feeding the assist gas  45  is achieved, thereby obtaining the good quality of the laser machining. 
     The exhausting means for exhausting the atmosphere in the torch  34  is not limited to the above-described embodiment. For example,  FIG. 8  depicts the alternative embodiment, in which the torch  34  has an exhausting unit  55  including a single or multiple windows  56  for communicating the inside to the outside of the torch  34  and shutters  57  for adjusting the opening areas of the windows  56 . 
     In such embodiment, when the inside of the torch  34  becomes high pressure due to the feeding of the assist gas  45 , the inside and the outside of the torch  34  are communicated through the windows  56  so that the atmosphere in the torch  34  is exhausted to the outside with the differential pressure. Through the windows  56  with the opening areas adjusted by the shutters  57 , the atmosphere in the torch  34  is communicated to the outside, so that the adjusted amount of atmosphere in the torch  34  is exhausted outside of the torch  34 . Thus, the exhausting unit  55  has the same effect as the exhausting unit  50  does, and further obtains an additional effect that there is no need to prepare the suction means, which reduces costs. 
     As depicted in  FIGS. 9 and 10 , the torch  34  has an inner surface  60  formed with grooves  61  that are configured continuously from the base end to the tip end thereof, and the inner surface  60  has a multi-step surface by the grooves  61 . 
     The inner surface  60  defines the inside space of the conical torch  34 , and the distance between the laser beam passing through the torch  34  and the surface is set larger than a predetermined length (e.g., preventing the laser beam from dispersing and from being affected by the disturbance). 
     The grooves  61  are formed on the inner surface  60  continuously in the axis direction of the torch. The grooves  61  are coved portions forming the multi-step surface on the inner surface  60 , and the multi-step configuration reflects the foreign matters entering into the torch  34  through the emitting opening  34   b  toward the direction except toward the protection glass  32 . 
     In other words, due to the configuration of the grooves  61  on the inner surface  60  of the torch  34 , the foreign matters hitting the inner surface  60  reflect diffusely on the grooves  61 , thereby reducing the arrival rate to the protection glass  32 . 
     As depicted in  FIG. 9 , the grooves  61  are formed in the circumferential direction in the inner surface  60  around the axis of the torch  34  and continuously in the axis direction. 
       FIG. 10  shows that the grooves  61  are formed as recesses depressed inside from the inner surface  60  toward the outside of the torch  34 . In detail, the groove  61  has a reflection face  62  that is perpendicular to the inner surface and facing the machining point of the workpiece  2 . When the foreign matters such as the spatters flown from the machining point of the workpiece  2  hit and reflect on the reflection face  62 , the foreign matters reflect toward the direction except toward the protection glass  32  (such that the foreign matters do not reach the glass  32  with one reflection on the inner surface  60 ). 
     That is, the grooves  61  formed in the inner surface  60  of the torch  34  prevent the foreign matters scattered over from the machining point of the workpiece  2  in the laser-machining from reflecting toward the protection glass  32  on the inner surface  60 , and each of which has the reflection face  62  that has a plane where the incident angle of the radial line from the machining point of the workpiece  2  is more than a predetermined angle (the reflected line of the incident line does not direct the protection glass  32 ). 
     As described above, the inner surface  60  of the torch  34  has the grooves  61  formed continuously from the base end to the tip end. Each groove  61  reflects the foreign matters of high temperature that is scattered from the machining point of the workpiece  2  toward the direction except toward the protection glass  32 . 
     When the foreign matters enter into the torch  34  through the emitting opening  34   b  and hit the groove  61  of the inner surface  60 , the foreign matters reflect toward the other direction than toward the protection glass  32 , thereby lowering the possibility for the foreign matters to reach the protection glass  32 . Moreover, if the foreign matters reflect in the groove  61  in plural times until they reach the protection glass  32 , the temperature of the foreign matters is lowered in response to the reflection number and accelerated to become solidified, thereby prevented from adhering to the protection glass  32 . 
     Therefore, the protection glass  32  is prevented from adhesion and penetration of the foreign matters of high temperature, and the protection glass  32  keeps its transmittance, thereby increasing the life thereof. Especially, the present embodiment provides the unexpected effect compared with the conventional structure that has a flat inner surface ( 60 ). 
     The foreign matters of high temperature reflect in the groove  61  in plural times, and are solidified in the recess of the groove  61  with anchor effect, whereby the foreign matters are collected in the groove  61  as a deposition  65 , which is spread in the groove. In this case, the grooves  61  are formed to be depressed toward the outside from inside of the inner surface  60 , so that the deposition  65  collected in the groove  61  is not likely to contact the laser beam traveling in the torch  34 . 
     The deposition  65  accumulated in the inner surface  60  of the torch  34  (the groove  61 ) is not likely to block the laser beam, thereby keeping the laser output. 
     The grooves  61  may be configured having triangle groove profile, shown in  FIG. 11 , or sawtooth groove profile, shown in  FIG. 12 . In both configurations, at least the face opposing to the machining point of the workpiece  2  is configured as the reflection face  62 , which has the plane where the incident angle of the radial line from the machining point is set in the above-described manner. 
     Additionally, as long as it goes to the base from the tip in the inner surface  60 , the groove  61  may become deeper or the reflection face  62  of the grove  61  may be inclined larger toward the tip. Due to such structures, the incident angles of the foreign matters scattered from the machining point of the workpiece  2  against the grooves  61  disposed at the base side can be adjusted properly. 
     As shown in  FIGS. 13 to 15 , the torch  34  has a tip  70 . 
     The tip  70  is attached to the tip end of the torch  34  to extend toward the workpiece  2 . The tip  70  is made of a material composed of Molybdenum or a material of heat resistance and of durable, and separated from the torch  34 . 
     In other words, the torch  34  has the tip  70  at the tip end thereof, so that the distance between the torch  34  and the workpiece  2  becomes shorter, and thereby there is a smaller area of the opening of the torch  34  through which the foreign matters of high temperature generated at the machining point of the workpiece  2  enter. Thus, the foreign matters scattered over from the machining point of the workpiece  2  are prevented from entering into the torch  34 . 
     As shown in  FIGS. 14 and 15 , the tip  70  includes an opening  71  formed with an inner surface continued from the inner surface of the torch  34  and an assist-gas passage  72  formed as a slit extended outwardly from the periphery of the opening  71 . 
     The extended length of the tip  70  from the torch  34 , namely the clearance between the tip end of the tip  70  and the machining point of the workpiece  2 , is set in response to the influence of the melting heat of the laser machining, the entrance rate of the foreign temperature, and the blowing speed of the assist gas  45 . In this embodiment, the extended length of the tip  70  is set in the minimum length satisfying the above-mentioned conditions. 
     As illustrated in  FIG. 15 , the inner surface of the opening  71  is tapered from the base end to the tip end as the torch  34  is. The inner diameter D 1  of the opening  71  is set in response to the outer diameter of the laser beam (the minimum diameter which provides the clearance between the opening and the laser beam), and set smaller than the inner diameter D of the emitting opening  34   b  of the torch  34 . Thus, without interrupting the laser beam, the opening of the machining head  30  is formed in a small area. 
     As illustrated in  FIG. 16 , the total area of the area S 1  of the opening  71  and the area S 2  of the assist-gas passage  72  is smaller than the opening area S of the emitting opening  34   b  of the torch  34 . 
     As depicted in  FIG. 14 , the assist-gas passages  72  are formed as slit grooves, which are cut radially from the inner periphery of the opening  71 . The bottom of the assist-gas passage  72  has the same diameter as the emitting opening  34   b  of the torch  34  does. 
     The assist-gas passage  72  includes the inner surface continued from the inner surface of the torch  34 . As illustrated in  FIG. 17 , the assist gas  45  flows along the inner surface of the torch  34  and that of the assist-gas passages  72  according to the flow characteristics, and thereby the machining point of the workpiece  2  is surrounded with the inert atmosphere. 
     As described above, at the tip end of the torch  34 , the tip  70  is attached extending the tip end. The tip  70  includes the opening  71  having the inner diameter D 1  corresponding to the outer diameter of the laser beam traveling through the torch  34  and the assist-gas passages  72  opening outward from the outer periphery of the opening  71 . 
     Due to the above-described structure, the opening area of the tip  70  is set smaller than the opening area of the opening  34   a  of the torch  34 , and thereby the foreign matters of high temperature are physically prevented from entering into the torch  34  through the tip  70 . 
     Therefore, the adhesion of the foreign matters to the protection glass  32  is prevented, and the penetration of the foreign matters into the protection glass  32  is avoided. As a result, the transmittance of the protection glass  32  is maintained, and the useful life of thereof is increased. Especially, the present embodiment obtains a noticeable effect compared with the conventional structure in which the opening area is large with respect to the entering opening of the foreign matters. 
     The tip  70  provides the short clearance between the machining head  30  and the machining point of the workpiece  2 , so that the conditions of the assist gas  45  such as flowing speed should be considered. In this embodiment, the tip includes the assist-gas passages  72  having the inner surface continued from the inner surface of the torch  34 . 
     The assist gas  45  blowing along the inner surface of the torch  34  flows along the assist-gas passages  72  in the tip  70 . In short, the assist-gas passages  72  function as the flow passages for the assist gas  45 . Further, the opening area of the tip  70  is enlarged by the assist-gas passages, whereby the passage of the assist gas  45  is enlarged. 
     Therefore, the concentration of the assist gas  45  on the machining point of the workpiece  2  is prevented, so that the increase of blowing speed of the assist gas  45  through the tip  70  is prevented. 
     Additionally, the tip  70  is a separated member from the torch  34 . 
     Thus, there are advantages in selection of the best material for the tip  70  and in machining accuracy for the inner surface of the tip  70  (especially, in the configurations of the opening  71  and the assist-gas passages  72 ). Moreover, the tip  70  is easily applicable to the conventional laser machining apparatus. 
     In this embodiment, the assist-gas passages  72  are formed in the axis direction of the torch  34  and the number of the passages is eight. The assist-gas passages may be formed in alternative configurations such that they have a predetermined passage area and avoid the blocking of the assist gas  45 . 
     The tip end of the opening  71  may be enlarged from the inner periphery of the opening  71  toward the outside of the tip  70 . In this embodiment, the flowing condition of the assist gas  45  is improved. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a laser machining apparatus for welding or cutting a workpiece, especially to a technique of protecting a protection glass composing a part of optical systems of the laser machining apparatus.