Patent Publication Number: US-2022219264-A1

Title: Laser irradiation head

Description:
TECHNICAL FIELD 
     The present invention relates to a laser irradiation head. 
     BACKGROUND ART 
     In relation to a conventional laser irradiation head, for example, WO 2015/136907 (PTL 1) discloses a laser processing head used for laser processing. A laser processing head includes a first protective glass and a second protective glass that are sequentially arranged on an optical axis of a laser beam and transmit the laser beam, a nozzle provided with the first protective glass, and a shield holder provided with the second protective glass and being attachable to and detachable from the nozzle by sliding in a direction orthogonal to the optical axis of the laser beam. 
     CITATION LIST 
     Patent Literature 
     PTL 1: WO 2015/136907 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the laser processing head disclosed in PTL 1, particulates such as spatters or fume generated during laser processing enter a space between the first protective glass and the second protective glass, which may contaminate the protective glasses. 
     Further, also in the laser processing head in which a focus lens and the protective glasses are sequentially arranged on an optical path of the laser beam, there is a possibility that the focus lens and the protective glasses may be contaminated as described above. 
     As a measure for preventing entry of particulates as described above, it is conceivable to provide, in the shield holder, a seal member that seals a gap between the shield holder and the nozzle. However, if an excessive force is applied to the seal member when the shield holder is attached to the nozzle, the seal member may be damaged. In this case, it is not possible to effectively prevent particulates from entering the space between the first protective glass and the second protective glass (alternatively, the space between the focus lens and the protective glasses). 
     Therefore, an object of the present invention is to solve the above problem, and to provide a laser irradiation head that more reliably prevents contamination of a light transmissive member that transmits a laser beam. 
     Solution to Problem 
     A laser irradiation head of the present invention includes a housing provided with a through-hole forming an optical path through which a laser beam travels, a holder provided to be attachable to and detachable from the housing, and a first light transmissive member held by the holder and transmitting the laser beam. The housing is further provided with a holder insertion hole that intersects with the through-hole in a plane orthogonal to a traveling direction of the laser beam and into which the holder is inserted when the holder is attached to the housing. The housing is located upstream of the first light transmissive member in the traveling direction of the laser beam, constituting the inner wall of the holder insertion hole, and has a first plane in which the through-hole is opened. The holder has a second plane facing the first plane. The holder is provided with a groove recessed from the second plane and circulates around the opening surface of the through-hole in the first plane. The laser irradiation head includes a seal member accommodated in the groove and abutting on the first plane, and a biasing member that elastically biases the holder in a direction in which the second plane approaches the first plane. 
     A groove cross section is assumed in which the groove located at a most distal end in an insertion direction of the holder when the holder is attached to the housing is cut by a plane including the insertion direction of the holder and the traveling direction of the laser beam. In this case, the biasing member is disposed to bias the holder after the groove cross section passes the opening surface of the through-hole in the first plane in the insertion direction of the holder when the holder is attached to the housing. 
     In the laser irradiation head configured as described above, since the biasing member is disposed to bias the holder after the groove cross section passes the opening surface of the through-hole in the first plane, no external force caused by a biasing force from the biasing member is applied to the seal member disposed in the groove cross section before the groove cross section passes the opening surface of the through-hole in the first plane. This makes it possible to reduce a load applied to the seal member from the housing when the holder is attached to the housing. As a result, sealing performance between the holder and the housing by the seal member is sufficiently exerted, and it is therefore possible to more reliably prevent contamination of an upstream side of the first light transmissive member and contamination of members upstream of the first light transmissive member. 
     A gap is preferably provided between the seal member and the first plane before the groove cross section passes the opening surface of the through-hole in the first plane in the insertion direction of the holder when the holder is attached to the housing. 
     The laser irradiation head configured as described above can more effectively reduce the load applied to the seal member from the housing when the holder is attached to the housing. 
     The laser irradiation head preferably includes a plurality of the biasing members. The plurality of biasing members includes a first biasing member that biases the holder at a position closer to a distal end in the insertion direction of the holder than the seal member, and a second biasing member that biases the holder at a position closer to a proximal end in the insertion direction of the holder than the seal member. 
     In the laser irradiation head configured as described above, the seal member can be more uniformly in close contact with the first plane between the distal end and the proximal end in the insertion direction of the holder. Accordingly, the sealing performance between the holder and the housing by the sealing member can be further enhanced. 
     The first biasing member and the second biasing member are preferably disposed so as to start biasing the holder at an identical timing when the holder is attached to the housing. 
     The laser irradiation head configured as described above can prevent the seal member from being in partial contact with the first plane when the holder is attached to the housing. Accordingly, the seal member is attached in a more stable posture, and thus the sealing performance between the holder and the housing by the seal member can be further enhanced. 
     The laser irradiation head preferably further includes a second light transmissive member provided in the housing. The first light transmissive member transmits a laser beam from the second light transmissive member. The first plane is located between the first light transmissive member and the second light transmissive member in a traveling direction of the laser beam. 
     In the laser irradiation head configured as described above, it is possible to more reliably prevent contamination of the upstream side of the first light transmissive member and contamination of the second light transmissive member by sufficiently securing sealability of the space between the first light transmissive member and the second light transmissive member by the seal member. 
     Advantageous Effects of Invention 
     As described above, in accordance with the present invention, it is possible to provide a laser irradiation head that more reliably prevents contamination of a light transmissive member that transmits a laser beam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a configuration of a processing machine including a laser irradiation head according to an embodiment of the present invention. 
         FIG. 2  is a partial perspective view of a laser irradiation head in  FIG. 1 . 
         FIG. 3  is a perspective view illustrating a step of attaching a holder to a housing in the laser irradiation head in  FIG. 2 . 
         FIG. 4  is a sectional view of a lower housing and the holder attached to the lower housing in the laser irradiation head in  FIG. 1 . 
         FIG. 5  is a sectional view of the lower housing as viewed in a direction of an arrow on a line V-V in  FIG. 4 . 
         FIG. 6  is a top view of the holder and a first protective glass. 
         FIG. 7  is a bottom view of the holder and the first protective glass. 
         FIG. 8  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line VIII in  FIG. 4 . 
         FIG. 9  is a side view of a plunger. 
         FIG. 10  is a sectional view of the plunger. 
         FIG. 11  is a sectional view illustrating a first step of attaching the holder to the lower housing. 
         FIG. 12  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line XII in  FIG. 11 . 
         FIG. 13  is a sectional view illustrating a second step of attaching the holder to the lower housing. 
         FIG. 14  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line XIV in  FIG. 13 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described with reference to the drawings. In the drawings referred to below, the same or corresponding members are denoted by the same reference signs. 
       FIG. 1  is a schematic diagram illustrating a configuration of a processing machine including a laser irradiation head according to the embodiment of the present invention. Referring to  FIG. 1 , a processing machine  100  is a processing machine capable of additive manufacturing (AM) processing for a workpiece. The AM processing is a processing method of creating a three-dimensional shape on a workpiece by attaching a material, and a mass of the workpiece increases after the AM processing as compared with before the AM processing. 
     Processing machine  100  may be a processing machine capable of only the AM processing for a workpiece, or may be an AM/SM hybrid processing machine capable of the AM processing for a workpiece and subtractive manufacturing (SM) processing for a workpiece. 
     Processing machine  100  includes a laser irradiation head  10 , a laser oscillator  12 , a material powder feeder  13 , a shielding gas feeder  15 , and a workpiece moving stage  16 . 
     Laser irradiation head  10  is provided in a processing area in which the AM processing is performed. Laser irradiation head  10  emits a laser beam. Laser irradiation head  10  discharges material powder M to the workpiece W and irradiates a workpiece W with a laser beam L to perform the AM processing (directed energy deposition (DED)). 
     Laser oscillator  12  oscillates a laser beam used for the AM processing. The laser beam oscillated in laser oscillator  12  is guided to laser irradiation head  10  through a fiber cable or the like. 
     Material powder feeder  13  feeds the material powder used for the AM processing together with a carrier gas toward laser irradiation head  10 . The material powder is, for example, a metal powder such as a stainless alloy and a nickel alloy, or a ceramic powder. As the carrier gas, an inert gas such as argon gas is used. Laser irradiation head  10  includes a powder discharge nozzle  14 . Material powder M fed from material powder feeder  13  is discharged toward an irradiation region of workpiece W with laser beam L through powder discharge nozzle  14 . 
     Shielding gas feeder  15  feeds a shielding gas toward laser irradiation head  10 . The shielding gas prevents oxidation of a metal heated and melted by the laser beam and prevents scattering of the metal. As the shielding gas, an inert gas such as argon or helium is used. A shielding gas G fed from shielding gas feeder  15  is fed to the irradiation region of workpiece W with laser beam L through a nozzle body  23  described later. 
     Workpiece moving stage  16  is provided in the processing area in which the AM processing is performed. Workpiece moving stage  16  faces laser irradiation head  10  in a vertical direction. Workpiece moving stage  16  is provided below laser irradiation head  10 . Workpiece moving stage  16  is able to hold workpiece W. 
     Workpiece moving stage  16  moves workpiece W in a horizontal plane by various feeding mechanisms, guide mechanisms, a servomotor, and the like. By moving workpiece moving stage  16 , a processing point of the AM processing on workpiece W can be moved. 
     In a configuration in which laser irradiation head  10  that emits material powder M and irradiates workpiece W with laser beam L and workpiece moving stage  16  holding workpiece W are moved relative to each other, the processing point of the AM processing on workpiece W can be moved. For example, laser irradiation head  10  may move in air within the processing area, or this configuration may be combined with workpiece moving stage  16 .  FIG. 1  illustrates laser irradiation head  10  that emits the laser beam downward, but an orientation of the laser irradiation head of the present invention is not limited. 
     Next, a structure of laser irradiation head  10  will be described.  FIG. 2  is a partial perspective view of the laser irradiation head in  FIG. 1 .  FIG. 3  is a perspective view illustrating a step of attaching a holder to a housing in the laser irradiation head in  FIG. 2 . 
     Referring to  FIGS. 1 to 3 , laser irradiation head  10  includes an upper housing  22 , a lower housing (housing)  21 , a nozzle body  23 , and a holder  51 . 
     Lower housing  21  is formed of a metal block body. Lower housing  21  as a whole has a rectangular parallelepiped shape. Lower housing  21  has an upward surface  36 , a downward surface  37 , and a sideward surface  35 . Upward surface  36  faces upward. Downward surface  37  is disposed on a reverse side of upward surface  36 . Downward surface  37  faces downward. Sideward surface  35  faces sideways. 
     An upper end of sideward surface  35  is continuous with upward surface  36 , and a lower end of sideward surface  35  is continuous with downward surface  37 . 
     Lower housing  21  is provided with a through-hole  33 . Through-hole  33  penetrates lower housing  21  in the vertical direction. Through-hole  33  is opened in upward surface  36  and downward surface  37 . Through-hole  33  extends in the vertical direction around a central axis  101 . Through-hole  33  has a circular opening shape centered on central axis  101 . 
     The through-hole  33  forms an optical path through which laser beam travels in lower housing  21 . A traveling direction of the laser beam in through-hole  33  is an axial direction of central axis  101 . An optical axis of the laser beam traveling through through-hole  33  overlaps with central axis  101 . 
     Lower housing  21  is further provided with a holder insertion hole  31 . Holder insertion hole  31  intersects with through-hole  33  in a plane orthogonal to the traveling direction of the laser beam. Holder insertion hole  31  is opened in sideward surface  35 . Holder insertion hole  31  extends in a horizontal direction from sideward surface  35 . Holder insertion hole  31  is a bottomed hole provided at a predetermined depth from an opening in sideward surface  35 . 
     Upper housing  22  is connected to lower housing  21 . Upper housing  22  is connected to upward surface  36 . Upper housing  22  is provided with a hole  38  for forming an optical path through which the laser beam travels. Hole  38  extends in the vertical direction around central axis  101 . Hole  38  communicates with through-hole  33  at a lower end of hole  38 . 
     Nozzle body  23  is connected to lower housing  21 . Nozzle body  23  is connected to downward surface  37 . Nozzle body  23  is provided with a hole  39  for forming an optical path through which the laser beam travels. Hole  39  penetrates nozzle body  23  in the vertical direction. Hole  39  extends in the vertical direction around central axis  101 . Hole  39  communicates with through-hole  33  at an upper end of hole  39 . 
     As illustrated in  FIGS. 2 and 3 , holder  51  is provided to be attachable to and detachable from lower housing  21 . Holder  51  is accommodated in holder insertion hole  31 . Holder  51  is inserted into holder insertion hole  31  when attached to lower housing  21 . Holder  51  is inserted in a direction indicated by an arrow  126  in  FIG. 3  (hereinafter, also referred to as “insertion direction of holder  51 ”) through an opening of holder insertion hole  31  in sideward surface  35 . The insertion direction of holder  51  is a direction orthogonal to the traveling direction of the laser beam in lower housing  21  (the axial direction of central axis  101 ). 
     Holder  51  is provided with a hole  55 . Hole  55  penetrates holder  51  in the vertical direction. Hole  55  extends in the vertical direction around central axis  101 . Hole  55  has a circular opening shape centered on central axis  101 . Hole  55  communicates with through-hole  33  at an upper end and a lower end of hole  55 . 
     Hole  55  forms an optical path through which the laser beam travels in holder  51 . A traveling direction of the laser beam in hole  55  is the axial direction of central axis  101 . An optical axis of the laser beam traveling through hole  55  overlaps with central axis  101 . 
     Laser irradiation head  10  further includes a collimation lens  26 , a focus lens  27 , a first protective glass (first light transmissive member)  29 , and a second protective glass (second light transmissive member)  28 . 
     Collimation lens  26  (see  FIG. 2 ) is provided in upper housing  22 . Focus lens  27  is provided in upper housing  22 . Collimation lens  26  and focus lens  27  are provided on the optical path of the laser beam in upper housing  22 . Focus lens  27  is provided downstream of collimation lens  26  in the traveling direction of the laser beam. 
     Collimation lens  26  collimates the laser beam introduced into laser irradiation head  10 . Focus lens  27  condenses the laser beam from collimation lens  26  on a surface of workpiece W. 
     Note that a configuration of condensing the laser beam to a target spot diameter using collimation lens  26  and focus lens  27  is an example. For example, the laser beam may be condensed using one biconvex lens, or the laser beam may be condensed using another method. Further, the laser beam emitted from a fiber cable may be directly applied to the workpiece. 
     Second protective glass  28  is provided in lower housing  21 . Second protective glass  28  is provided to face focus lens  27  in the axial direction of central axis  101 . First protective glass  29  is held by holder  51 . First protective glass  29  is provided to be attachable to and detachable from holder  51 . First protective glass  29  is provided to face second protective glass  28  in the axial direction of central axis  101 . 
     First protective glass  29  and second protective glass  28  are provided on optical paths of the laser beam in holder  51  and lower housing  21 , respectively. First protective glass  29  is provided downstream of second protective glass  28  in the traveling direction of the laser beam. First protective glass  29  is exposed to an external space through-hole  55  in holder  51 , through-hole  33  in lower housing  21 , and hole  39  in nozzle body  23 . 
     First protective glass  29  and second protective glass  28  include a transparent member capable of transmitting the laser beam. First protective glass  29  and second protective glass  28  are provided to prevent particulates such as spatters or fume generated by the AM processing for workpiece W from adhering to optical components such as focus lens  27 . 
     During the AM processing for workpiece W, particulates such as spatters or fume adhere to first protective glass  29 . Meanwhile, first protective glass  29  is held by holder  51  attachable to and detachable from lower housing  21 . By removing holder  51  from lower housing  21 , first protective glass  29  can be replaced or cleaned. At this time, since second protective glass  28  is still provided in lower housing  21 , it is possible to prevent focus lens  27  from being exposed to the external space and being contaminated (double structure of protective glasses) when first protective glass  29  is replaced or cleaned. 
     Although laser irradiation head  10  to which the double structure of the protective glasses is applied has been described in the embodiment, the present invention is not limited to this configuration, and can also be applied to a laser irradiation head that protects an optical component such as a focus lens with a single protective glass. 
       FIG. 4  is a sectional view of the lower housing and the holder attached to the lower housing in the laser irradiation head in  FIG. 1 .  FIG. 5  is a sectional view of the lower housing as viewed in a direction of an arrow on a line V-V in  FIG. 4 . 
     Referring to  FIGS. 4 and 5 , lower housing  21  has an upper surface (first plane)  41  and a lower surface  42 . 
     Upper surface  41  constitutes an inner wall of holder insertion hole  31 . Upper surface  41  constitutes an upper wall of holder insertion hole  31 . Upper surface  41  is formed of a plane. Upper surface  41  is formed of a plane orthogonal to the traveling direction of the laser beam (the axial direction of central axis  101 ). Upper surface  41  is located upstream of the first protective glass  29  in the traveling direction of the laser beam. Upper surface  41  is located between first protective glass  29  and second protective glass  28  in the traveling direction of the laser beam. Through-hole  33  is opened in upper surface  41 . Through-hole  33  upstream of holder insertion hole  31  in the traveling direction of the laser beam is opened in upper surface  41 . 
     Lower surface  42  constitutes the inner wall of holder insertion hole  31 . Lower surface  42  constitutes a bottom wall of holder insertion hole  31 . Lower surface  42  is provided to face upper surface  41  in the axial direction of central axis  101 . Lower surface  42  as a whole is formed of a plane orthogonal to the traveling direction of the laser beam. Through-hole  33  downstream of holder insertion hole  31  in the traveling direction of the laser beam is opened in lower surface  42 . 
     Lower surface  42  has a proximal end region  42 P, an intermediate region  42 Q, and a distal end region  42 R. Proximal end region  42 P, intermediate region  42 Q, and distal end region  42 R are arranged in that order from a proximal end to a distal end in the insertion direction of holder  51  (the direction indicated by arrow  126  in  FIG. 3 ). Through-hole  33  is opened to intermediate region  42 Q. 
     A level difference in the axial direction of central axis  101  is provided between proximal end region  42 P and intermediate region  42 Q. A distance between upper surface  41  and intermediate region  42 Q in the axial direction of central axis  101  is smaller than a distance between upper surface  41  and proximal end region  42 P in the axial direction of central axis  101 . That is, proximal end region  42 P is stepped down from intermediate region  42 Q. 
     A first slope  43  is provided at a boundary between proximal end region  42 P and intermediate region  42 Q. First slope  43  extends obliquely upward from proximal end region  42 P toward intermediate region  42 Q. 
     A level difference in the axial direction of central axis  101  is provided between intermediate region  42 Q and distal end region  42 R. The distance between upper surface  41  and intermediate region  42 Q in the axial direction of central axis  101  is smaller than a distance between upper surface  41  and distal end region  42 R in the axial direction of central axis  101 . That is, distal end region  42 R is one level lower than intermediate region  42 Q. The distance between upper surface  41  and distal end region  42 R in the axial direction of central axis  101  is smaller than the distance between upper surface  41  and proximal end region  42 P in the axial direction of central axis  101 . 
       FIG. 6  is a top view of the holder and the first protective glass.  FIG. 7  is a bottom view of the holder and the first protective glass. 
     Referring to  FIGS. 4 to 7 , holder  51  includes a base  54 , a glass holder  56 , an outer plate  53 , and a handle  52 . 
     Base  54  is accommodated inside of holder insertion hole  31 . Base  54  is formed of a plate material whose thickness direction is the axial direction of central axis  101 . 
     Holder  51  (base  54 ) has a holder upper surface (second plane)  46  and a holder lower surface  48 . Holder upper surface  46  is formed of a plane. Holder upper surface  46  is formed of a plane orthogonal to the traveling direction of the laser beam (the axial direction of central axis  101 ). Holder upper surface  46  faces upper surface  41  of lower housing  21 . Holder upper surface  46  faces upper surface  41  in the axial direction of central axis  101 . Holder upper surface  46  is in surface contact with upper surface  41 . 
     Holder lower surface  48  faces lower surface  42  of lower housing  21 . A gap is provided between holder lower surface  48  and lower surface  42 . 
     Glass holder  56  is attached to base  54  from a side of holder lower surface  48 . Hole  55  penetrates base  54  and glass holder  56  in the axial direction of central axis  101 . First protective glass  29  is held by base  54  by being pressed by glass holder  56  from the axial direction of central axis  101 . 
     Holder  51  (glass holder  56 ) has a second slope  49 . Second slope  49  is provided at a position facing lower surface  42  (distal end region  42 R) in the axial direction of central axis  101 . Second slope  49  extends obliquely upward from the proximal end to the distal end in the insertion direction of holder  51 . 
     Outer plate  53  and the handle  52  are provided outside of holder insertion hole  31 . Outer plate  53  has a plate shape whose thickness direction is the insertion direction of holder  51 . Outer plate  53  is in contact with sideward surface  35  of lower housing  21  in the insertion direction of holder  51 . Base  54  and handle  52  are attached to outer plate  53 . Handle  52  has a grip form, and is gripped by a user when holder  51  is attached to or detached from lower housing  21 . 
       FIG. 8  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line VIII in  FIG. 4 . Referring to  FIGS. 4 to 8 , holder  51  is provided with a groove  47 . Groove  47  has a recessed shape recessed from holder upper surface  46 . Groove  47  circulates around an opening surface of through-hole  33  in upper surface  41  of lower housing  21 . 
     Groove  47  is provided in base  54 . Groove  47  extends on a circumference centered on central axis  101 . 
     Note that a circumferential shape of groove  47  is not limited to a circular shape, and may be, for example, an elliptical shape or a rectangular shape in which adjacent sides are connected while being curved. 
     Laser irradiation head  10  further includes a seal member  61 . Seal member  61  is an annular seal member. Seal member  61  is formed of an elastic member such as rubber. Seal member  61  is accommodated in groove  47 . Seal member  61  is in contact with upper surface  41  of lower housing  21 . 
       FIG. 9  is a side view of a plunger.  FIG. 10  is a sectional view of the plunger. 
     Referring to  FIGS. 4 to 10 , laser irradiation head  10  further includes a plunger (biasing member)  71 . Plunger  71  elastically biases holder  51  in a direction in which holder upper surface  46  approaches upper surface  41  of lower housing  21 . Plunger  71  elastically biases holder  51  upward. 
     Plunger  71  is provided inside of holder insertion hole  31 . Plunger  71  is provided between holder  51  and lower surface  42  of lower housing  21  in the axial direction of central axis  101 . Plunger  71  elastically deforms in the axial direction of central axis  101  to generate elastic force for biasing holder  51  upward. 
     Plunger  71  includes a case body  72 , a ball  73 , and a coil spring  74 . Case body  72  has a bottomed cylindrical shape. Case body  72  is provided such that an axial direction of the cylindrical shape is parallel to central axis  101 . Case body  72  is provided with an opening  72 P. Opening  72 P opens in one direction along the axial direction of central axis  101 . 
     Ball  73  is accommodated in case body  72 , and a part of ball  73  is provided so as to protrude outward through opening  72 P. Coil spring  74  is accommodated inside of case body  72  side by side with ball  73 . Coil spring  74  applies elastic force in the axial direction of central axis  101  (elastic force in a direction in which ball  73  protrudes out of case body  72 ) to the ball  73 . 
     Laser irradiation head  10  includes a plurality of plungers  71  (a first plunger  71 S and a second plunger  71 T). 
     First plunger (first biasing member)  71 S biases holder  51  at a position closer to the distal end in the insertion direction of holder  51  than seal member  61 . Second plunger (second biasing member)  71 T biases holder  51  at a position closer to the proximal end in the insertion direction of holder  51  than seal member  61 . 
     First plunger  71 S is provided closer to the distal end in the insertion direction of holder  51  than central axis  101 . First plunger  71 S is provided closer to the distal end in the insertion direction of holder  51  than through-hole  33 . First plunger  71 S is provided closer to the distal end in the insertion direction of holder  51  than groove  47  (seal member  61 ). First plunger  71 S is provided on a straight line extending from central axis  101  in the insertion direction of holder  51 . 
     First plunger  71 S is provided in lower housing  21 . First plunger  71 S is embedded in lower surface  42  of lower housing  21  such that ball  73  is exposed in a space of holder insertion hole  31 . First plunger  71 S is provided in distal end region  42 R of lower surface  42 . Ball  73  is in contact with holder  51  (glass holder  56 ) in the axial direction of central axis  101 . When ball  73  is pushed into case body  72  while resisting the elastic force of coil spring  74 , first plunger  71 S elastically biases holder  51  upward. 
     Second plunger  71 T is provided closer to the proximal end in the insertion direction of the holder  51  than central axis  101 . Second plunger  71 T is provided closer to the proximal end in the insertion direction of holder  51  than through-hole  33 . Second plunger  71 T is provided closer to the proximal end in the insertion direction of holder  51  than groove  47  (seal member  61 ). 
     Laser irradiation head  10  includes a plurality of (two) second plungers  71 T. The number of second plungers  71 T is larger than the number of first plungers  71 S. The plurality of second plungers  71 T is provided apart from each other in a direction orthogonal to the insertion direction of holder  51 . The plurality of second plungers  71 T is provided so as to be symmetrical about the straight line extending from central axis  101  in the insertion direction of holder  51 . 
     Second plunger  71 T is provided in holder  51  (base  54 ). Second plunger  71 T is embedded in holder lower surface  48  such that ball  73  is exposed in the space of holder insertion hole  31 . Second plunger  71 T is in contact with lower surface  42  of lower housing  21  in the axial direction of central axis  101 . Second plunger  71 T is in contact with intermediate region  42 Q of lower surface  42 . When ball  73  is pushed into case body  72  while resisting the elastic force of coil spring  74 , second plunger  71 T elastically biases holder  51  upward. 
     Referring to  FIGS. 4 and 8 , plungers  71  (first plunger  71 S and second plunger  71 T) elastically bias holder  51  upward, and thus seal member  61  in groove  47  is compressed and deformed by being sandwiched between a groove bottom of groove  47  and upper surface  41  of lower housing  21 . As a result, holder  51  (holder upper surface  46 ) and lower housing  21  (upper surface  41 ) are sealed, and this can prevent particulates such as spatters or fume from entering a space  110  between first protective glass  29  and second protective glass  28 . 
     In the embodiment, first plunger  71 S biases holder  51  at a position closer to the distal end in the insertion direction of holder  51  than seal member  61 , and second plunger  71 T biases holder  51  at a position closer to the proximal end in the insertion direction of holder  51  than seal member  61 . This configuration allows seal member  61  to be more uniformly in close contact with upper surface  41  of lower housing  21  between the distal end and the proximal end in the insertion direction of holder  51 . It is therefore possible to more reliably prevent particulates from entering space  110  between first protective glass  29  and second protective glass  28 . 
     The biasing member of the present invention is not limited to plunger  71 , and may be any member that elastically biases the holder in a predetermined direction. 
       FIG. 11  is a sectional view illustrating a first step of attaching the holder to the lower housing.  FIG. 12  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line VII in  FIG. 11 .  FIG. 13  is a sectional view illustrating a second step of attaching the holder to the lower housing.  FIG. 14  is an enlarged sectional view illustrating a range surrounded by a two-dot chain line XIV in  FIG. 13 . 
       FIGS. 8, 12, and 14  illustrate a groove cross section  47 J obtained by cutting groove  47  along a plane including the insertion direction of holder  51  (a direction indicated by arrow  126  in  FIG. 4 ) and the traveling direction of the laser beam (the axial direction of central axis  101 ), groove  47  being located on a most distal end in the insertion direction of holder  51  when holder  51  is attached to lower housing  21 . 
     Referring to  FIGS. 4, 8, and 11 to 14 , plungers  71  (first plunger  71 S and second plunger  71 T) are disposed to bias holder  51  after groove cross section  47 J passes the opening surface of through-hole  33  in upper surface  41  in the insertion direction of holder  51  when holder  51  is attached to lower housing  21 . 
     Hereinafter, the above arrangement of plungers  71  (first plunger  71 S and second plunger  71 T) will be described in more detail. 
     Referring to  FIGS. 11 and 12 , when holder  51  is attached to lower housing  21 , holder  51  is inserted into holder insertion hole  31  while being placed on lower surface  42  of lower housing  21 . 
     In an initial stage of insertion of holder  51 , groove cross section  47 J moves closer to the proximal end in the insertion direction of holder  51  than the opening surface of through-hole  33  in upper surface  41 , and then moves immediately below the opening surface of through-hole  33  in upper surface  41 . In  FIGS. 11 and 12 , groove cross section  47 J and the opening surface of through-hole  33  in upper surface  41  partially overlap in the axial direction of central axis  101 . This state corresponds to a state before groove cross section  47 J passes the opening surface of through-hole  33  in upper surface  41  in the insertion direction of holder  51 . 
     At this time, a gap is provided between holder upper surface  46  and upper surface  41  of lower housing  21 . A gap is provided between seal member  61  and upper surface  41  of lower housing  21 . 
     As an example, a size C 1  of the gap between holder upper surface  46  and upper surface  41  of lower housing  21  illustrated in  FIG. 12  is greater than or equal to 0.6 mm, and a size C 2  of the gap between seal member  61  and upper surface  41  of lower housing  21  is greater than or equal to 0.3 mm. 
     Holder  51  does not reach first plunger  71 S, and thus first plunger  71 S does not elastically bias holder  51  upward. Second plunger  71 T initially moves outside of holder insertion hole  31 , and eventually moves on proximal end region  42 P of lower surface  42  of lower housing  21 . During this time, second plunger  71 T does not elastically bias holder  51  upward. 
     Referring to  FIGS. 13 and 14 , in an intermediate stage of the insertion of holder  51 , groove cross section  47 J passes the opening surface of through-hole  33  in upper surface  41  in the insertion direction of holder  51 . Groove cross section  47 J moves closer to the distal end in the insertion direction of holder  51  than the opening surface of through-hole  33  in upper surface  41 . 
     After groove cross section  47 J passes the opening surface of through-hole  33  in upper surface  41  in the insertion direction of holder  51 , holder  51  (second slope  49  of glass holder  56 ) comes into contact with first plunger  71 S (ball  73 ). Second plunger  71 T (ball  73 ) comes into contact with first slope  43  of lower surface  42  of lower housing  21 . When ball  73  is pushed into case body  73  along with the insertion of holder  51 , first plunger  71 S and second plunger  71 T elastically bias holder  51  upward. 
     In the embodiment, first plunger  71 S and second plunger  71 T are provided so as to start biasing holder  51  at an identical timing when holder  51  is attached to lower housing  21 . 
     Referring to  FIGS. 4 and 8 , at a final stage of the insertion of holder  51 , holder  51  (glass holder  56 ) rides on first plunger  71 S (ball  73 ), and second plunger  71 T rides on intermediate region  42 Q of lower surface  42  of lower housing  21 . When outer plate  53  of holder  51  abuts on sideward surface  35  of lower housing  21 , the attachment of holder  51  to lower housing  21  is completed. 
     Referring to  FIG. 4 , a length X between groove cross section  47 J and the opening surface of through-hole  33  in upper surface  41  in the insertion direction of holder  51  is larger than a moving length of holder  51  after first plunger  71 S and second plunger  71 T start elastically biasing holder  51  upward. 
     If plungers  71  (first plunger  71 S and second plunger  71 T) elastically biases holder  51  upward in the initial stage of the insertion of holder  51  illustrated in  FIGS. 11 and 12 , seal member  61  may be sandwiched between an opening edge of the opening surface of through-hole  33  in upper surface  41  and a side wall of groove cross section  47 J. Further, seal member  61  moves a long distance along the insertion direction of holder  51  while being pressed against upper surface  41 . As a result, there is a concern that seal member  61  may be damaged, or that the attachment may be done with seal member  61  twisted or greatly worn. 
     Meanwhile, in the embodiment, plungers  71  (first plunger  71 S and second plunger  71 T) are disposed so as to bias holder  51  after passing the opening surface of through-hole  33  in upper surface  41 . This configuration resolves a concern of breakage of seal member  61  as described above, and thus sealing performance by seal member  61  can be sufficiently obtained. As a result, particulates such as spatters or fume can be prevented from entering space  110  between first protective glass  29  and second protective glass  28 , and contamination of first protective glass  29  and second protective glass  28  can be reliably prevented. 
     In the embodiment, the gap is provided between seal member  61  and upper surface  41  before groove cross section  47 J passes the opening surface of through-hole  33  in upper surface  41 . This configuration can effectively prevent damage to seal member  61  by further reducing a load applied to seal member  61  from lower housing  21 . 
     In the embodiment, first plunger  71 S and second plunger  71 T are provided so as to start biasing holder  51  at an identical timing when holder  51  is attached to lower housing  21 . This configuration can prevent seal member  61  from being in partial contact with upper surface  41  of lower housing  21 . Accordingly, seal member  61  is attached in a more stable orientation, and thus sealing performance by seal member  61  can be further expected. 
     In the embodiment, the configuration of laser irradiation head  10  used in processing machine  100  has been described, but the laser irradiation head of the present invention is not limited to such an application. The laser irradiation head of the present invention may be used for, for example, various measurement devices using laser beam. The laser irradiation head of the present invention may be used in a laser processing machine or a powder-bed AM processing machine. 
     It should be understood that the embodiment disclosed herein is illustrative in all respects and not restrictive. The scope of the present invention is defined not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applied to, for example, a processing machine or a measurement device using a laser beam. 
     REFERENCE SIGNS LIST 
       10 : Laser irradiation head,  12 : Laser oscillator,  13 : Material powder feeder,  14 : Powder discharge nozzle,  15 : Shielding gas feeder,  16 : Workpiece moving stage,  21 : Lower housing,  22 : Upper housing,  23 : Nozzle body,  26 : Collimation lens,  27 : Focus lens,  28 : Second protective glass,  29 : First protective glass,  31 : Holder insertion hole,  33 : Through-hole,  35 : Sideward surface,  36 : Upward surface,  37 : Downward surface,  38 ,  39 ,  55 : Hole,  41 : Upper surface,  42 : Lower surface,  42 P: Proximal end region,  42 Q: Intermediate region,  42 R: Distal end region,  43 : First slope,  46 : Holder upper surface,  47 : Groove,  47 J: Groove cross section,  48 : Holder lower surface,  49 : Second slope,  51 : Holder,  52 : Handle,  53 : Outer plate,  54 : Base,  56 : Glass holder,  61 : Seal member,  71 : Plunger,  71 S: First plunger,  71 T: Second plunger,  72 : Case body,  72 P: Opening,  73 : Ball,  74 : Coil spring,  100 : Processing machine,  101 : Central axis,  110 : Space