Abstract:
A machine tool has a cover body with a window, the window including two spaced window plates defining an airtight space therebetween. The space is brought to a pressure other than atmospheric. An indicator is coupled to the cover body and has an indicator portion that is displaced in response to pressure in the space. The indicating portion is visually observable from outside the machine tool, so that the airtightness of the window may be determined by visually observing the position of the indicating portion.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to machine tools, and more particularly to machine tools having windows with two window plates joined together in airtight fashion and an indicator for detecting an airtight state of the window. 
     BACKGROUND OF THE DISCLOSURE 
     Usually, taking into consideration safety and environmental aspects, a machine tool is provided with a cover body for dividing a machining region from an external region and the cover body is provided with a window so that the machining region can be observed from the outside thereof. Japanese Unexamined Patent Application Publication No. 3-166037 discloses a conventional example of such a cover body having a window. The cover body (specifically, a slide door) disclosed in this document is made of a plate-shaped polycarbonate and has a structure in which a glass plate is attached to the inside (the machining-region side) of a portion to be a window thereof. 
     According to this cover body, because the polycarbonate plate constituting the cover body has very high toughness and high impact resistance, for example, even if an unexpected accident in which a tool collides with a workpiece due to an operation error or an error in program generation, it is possible to completely prevent the tool and the workpiece from shooting out of the machining region due to the collision. 
     While a polycarbonate plate has high toughness and high impact resistance as described above, its hardness is not so high, that is, its wear resistance is not so high, and therefore a polycarbonate plate has a disadvantage that, when chips generated by cutting or the like hit its surface, its surface is shaved by the chips and becomes rough, thereby resulting in deterioration in transparency (visibility) therethrough, for example. 
     Therefore, in the above conventional cover body, in order to prevent the surface of the polycarbonate plate from becoming rough due to chips and maintain good visibility therethrough, a structure is employed in which a glass plate having high wear resistance is attached to the inside (the machining-region side) of a portion to be a window of the polycarbonate plate. 
     Thus, according to the conventional cover body having the above-described structure, the glass plate arranged on the machining-region side makes it possible to prevent deterioration in visibility (good observability from the outside), which is caused by chips, and the polycarbonate plate having high toughness and high impact resistance makes it possible to completely prevent the tool and the workpiece from shooting out of the machining region, thereby securing high safety. 
     More recently, it has been found that the polycarbonate plate has, besides the disadvantage that it has low wear resistance, a disadvantage that it has low resistance to coolant and, if it is in contact with coolant for a long time, its high toughness and high impact resistance, which are its advantages, are reduced. 
     Therefore, at present, the cover body except for the window is made of sheet metal and the window employs a structure in which a glass plate and a polycarbonate plate are arranged on the machining-region side and the external side, respectively, and they are airtightly joined together with a certain distance between them for compensating the above-mentioned disadvantage. 
     According to the window having such a structure, the polycarbonate plate is prevented from coming into contact with coolant and it is therefore possible to prevent the toughness and the impact resistance of the polycarbonate plate from being reduced due to coolant. 
     SUMMARY OF THE DESCRIPTION 
     However, even the window having the structure in which the glass plate and the polycarbonate plate are airtightly joined together with a certain distance between them has a problem as described below. 
     That is, although the airtightness of the joined portion between the glass plate and the polycarbonate plate of the window is secured, coolant is not allowed to enter between the glass plate and the polycarbonate plate and therefore the reduction of toughness and impact resistance of the polycarbonate plate due to coolant does not occur, the airtightness has not been fully ensured conventionally. 
     For example, conventionally, a method of detecting the airtightness between the glass plate and the polycarbonate plate in manufacturing the window has not been suggested, and therefore, in the present circumstances, it cannot be said that the airtightness is fully ensured at the time of manufacturing. Further, even if the airtightness is ensured at the time of manufacturing, it is possible that a seal member does not have sufficient resistance to coolant, and in this case there is the fear that the airtightness is lowered by deterioration over time of the seal member. 
     Furthermore, various loads act on the window during machine tool operation. For example, a load acts on the window due to an operation of sliding the cover body and a load also acts on the window due to collision of chips or coolant therewith. Further, a load also acts on the window due to deflection caused by cutting heat or heat transmitted from various sources of heat generation. Such loads deform the window and, if such deformation is repeated for a long time, the airtightness of the joined portion can be lowered. Further, in some cases, an excessive load acts on the glass plate and thereby causes a crack therein and the airtightness is therefore lowered. Thus, in the window having the above-described structure, the airtightness thereof has not been fully ensured conventionally. 
     Although naturally the polycarbonate plate has a determined useful life and the window is to be replaced on a regular basis, there is the fear that, if coolant enters between the glass plate and the polycarbonate plate due to poor airtightness and the polycarbonate plate thereby falls into a situation where it comes into contact with the coolant, the polycarbonate plate deteriorates earlier than scheduled and its toughness and impact resistance are remarkably reduced before the replacement. If the above-described unexpected accident happens in this situation, a serious accident in which the tool or the workpiece penetrates the window comprising the glass plate and the polycarbonate plate and shoots out of the machining region might be caused. 
     Accordingly, machine tool embodiments are disclosed herein that enable the machine tool operator to visibly determine, with his or her eyes, the integrity of the airtight state of the window. 
     To achieve this, certain embodiments disclosed herein relate to a machine tool having a cover body with a window, wherein:
         the window comprises two transparent or translucent window plates which are airtightly joined together with a certain distance between them,   the pressure in a space between the window plates is set to a pressure other than atmospheric pressure, and   an indicator having an indicating portion which is displaced in accordance with pressure is disposed on or connected to the cover body so that the pressure in the space acts on the indicating portion and the indicating portion can be visually observed (detected with eyes) from the outside.       

     According to this disclosure, first of all, the pressure in the space is set to a pressure other than atmospheric pressure, that is, the pressure in the space is increased to a pressure higher than atmospheric pressure or is reduced to a pressure lower than atmospheric pressure. When the space is brought into a pressurized state or a pressure-reduced state in this way, the pressure in the space acts on the indicating portion of the indicator and thereby the indicating portion is displaced and is positioned at a position corresponding to this initial pressure. 
     Thereafter, although the pressure in the space does not vary when the airtight state of the window is good, the pressure in the space varies when the airtightness of the window is not maintained in a good state, that is, the pressure is reduced in the case where the space has been brought into a pressurized state, and the pressure is increased in the case where the space has been brought into a pressure-reduced state. In accordance with this pressure variation, the position of the indicating portion is displaced from its initial position. Therefore, it is possible to judge appropriateness of the airtight state of the space by checking with eyes whether the indicating portion is displaced from its initial position. 
     The indicating portion may be configured to be movable between a visible region which can be visually observed from the outside and a hidden region which is visually obscured (i.e., cannot be visually observed) from the outside, and is configured so that the indicating portion is positioned in the hidden region when the pressure in the space is a pressure within a predetermined range, that is, a pressure with which the airtight state is judged to be good, and the indicating portion is positioned in the visible region when the pressure in the space is a pressure other than the pressure as described above, that is, a pressure with which the airtight state is judged to be bad. 
     When thus configured, while the indicating portion is positioned in the hidden region and cannot be observed from the outside when the airtight state is good, the indicating portion is moved to the visible region and becomes observable from the outside when the airtight state becomes bad. Therefore, it is possible to judge appropriateness of the airtight state by a simple checking operation of checking whether the indicating portion can be observed from the outside. 
     Further, the indicator may comprise a transparent or translucent cylindrical cylinder open at both ends, a piston as the indicating portion which is airtightly and movably disposed in the cylinder, and a biasing member which is disposed on one side or the other side of the piston within the cylinder and which biases the piston toward the one side or the other side, wherein:
         the cylinder is airtightly inserted into a joined portion between the window plates of the window so that one side portion thereof is positioned in a visible region which is located within the space between the window plates and can be visually observed from the outside and the other side portion thereof leads to the outside of the window, and   the piston moves within the cylinder in accordance with the pressure in the space and thereby the position thereof is displaced.       

     According to this indicator, the pressure in the space acts on the piston at the one side thereof and atmospheric pressure acts on the piston at the other side thereof, and a basing force of the biasing member acts on the piston. Thus, the piston is caused to be positioned at a position where the acting force of the pressure in the space, the acting force of the atmospheric pressure and the biasing force are balanced. 
     When the pressure in the space varies, the piston is moved within the cylinder in accordance with this pressure variation and is displaced from the original position (its initial position) to a new position where the three forces are balanced. Therefore, also according to this indicator, it is possible to judge appropriateness of the airtight state by checking with eyes whether the piston is displaced from its initial position. 
     Also in this indicator, the piston may be configured to be movable between the visible region and a hidden region in which the piston is obscured from visual detection from an outside of the machine tool, and is configured so that the piston is positioned in the hidden region when the pressure in the space is a pressure within a predetermined range, that is, a pressure with which the airtight state is judged to be good, and the piston is positioned in the visible region when the pressure in the space is a pressure other than the pressure as described above, that is, a pressure with which the airtight state is judged to be bad. When thus configured, it is possible to judge appropriateness of the airtight state by a simple checking operation of checking whether the piston can be visibly detected from the outside of the machine tool. 
     Alternatively, the indicator may comprise:
         a cylinder which comprises a transparent or translucent cylindrical member open at both ends and which is airtightly inserted into a jointed portion between the window plates of the window so that it communicates with the space between the window plates at one side and communicates with the outside of the window at the other side;   a piston which is airtightly and movably disposed in the cylinder;   an indicating plate as the indicating portion which is pivotably disposed within the space between the window plates; a link mechanism one end of which is engaged with the piston at the one side of the piston and the other end of which is engaged with the indicating plate and which causes the indicating plate to pivot in accordance with the movement of the piston; and   a biasing member which is disposed on the other side of the piston within the cylinder and which biases the piston toward the one side or the other side, wherein:
           the indicating plate is provided so that at least a part thereof is able to pivot within a visible region which can be visually observed from the outside, and   the movement of the piston within the cylinder in accordance with the pressure in the space causes the indicating plate to pivot via the link mechanism, and thereby the position of the indicating plate within the visible region varies.   
               

     According to this indicator, the pressure in the space acts on the piston at the one side thereof and atmospheric pressure acts on the piston at the other side thereof, and a biasing force of the biasing member acts on the piston. Thus, the piston is caused to be positioned at a position where the acting force of the pressure in the space, the acting force of the atmospheric pressure and the biasing force are balanced. 
     When the pressure in the space varies, the piston moves within the cylinder in accordance with this pressure variation and is displaced from the original position (its initial position) to a new position where the three forces are balanced. The indicating plate is pivoted together with the displacement of the piston, and thereby the position of the indicating plate within the visible region varies. Therefore, it is possible to judge appropriateness of the airtight state by checking with eyes whether the position of the indicating plate within the visible region varies from the initial state. 
     It is noted that, in this indicator, the indicating plate may be configured so that the whole of the indicating plate is positioned within the hidden region when the pressure in the space is a pressure within a predetermined range, that is, a pressure with which the airtight state is judged to be good, and a part of the indicating plate may be positioned within the hidden region when the pressure in the space is a pressure other than the pressure as described above, that is, a pressure with which the airtight state is judged to be bad. When thus configured, it is possible to judge appropriateness of the airtight state by a simple checking operation of checking whether the indicating plate can be visually detected from the outside. 
     Further, the indicator may comprise a transparent or translucent cylindrical body one end of which communicates with the space between the window plates and the other end of which is sealed and which is disposed along the vertical direction, and a liquid as the indicating portion which is injected in the cylindrical body. According to this indicator, the position of the liquid within the cylindrical body varies in accordance with a force acting on the liquid, which is produced by the pressure in the space. Therefore, it is possible to judge appropriateness of the airtight state by checking with eyes whether the liquid is displaced from its initial position. 
     It is noted that, also in this indicator, the liquid may be able to be displaced between a visible region which can be visually detected from the outside and a hidden region which cannot be visually detected from the outside, the liquid may be positioned within the hidden region when the pressure in the space is a pressure within a predetermined range, that is, a pressure with which the airtight state is judged to be good, and the liquid may be positioned in the visible region when the pressure in the space is a pressure other than the pressure as described above, that is, a pressure with which the airtight state is judged to be bad. When thus configured, it is possible to judge appropriateness of the airtight state by a simple checking operation of checking whether the liquid can be observed from the outside. 
     Further, the indicator may have a configuration in which the indicator is connected to the cover body in a state of being connected to a communicating pipe communicating with the space between the window plates and the indicating portion is arranged so that it can always be visually detected from the outside. 
     Furthermore, the machine tool may have a pressure adjusting mechanism for adjusting the pressure in the space between the window plates. As for the airtight state of the window, it is in reality difficult to achieve a completely airtight state and leakage can occur within a range where there is no problem in practical use, which is allowable. However, if such leakage is left as it is, the judgment made on the basis of the indicating portion of the indicator is that the airtight state is bad, in the spite of the fact that the pressure variation does not interfere with the airtight state. 
     Therefore, by returning the pressure in the space between the window plates to the initial state by the pressure adjusting mechanism on a regular basis or on an irregular basis, it is possible to cancel the pressure variation within the range where there is no problem in practical use, and therefore appropriateness of the airtight state of the window can be judged more accurately. 
     As described above, it is possible to quickly and accurately determine appropriateness of an airtight state of a window comprising two window plates which are airtightly joined together with a certain distance between them by a simple operation of visibly observing an indicator. Therefore, in a case where a material having low resistance to coolant is used as the window plate disposed on the external side, it is possible to take a proper preventive measure, for example, replacing the window with a new one earlier, and taking such a preventive measure makes it possible to prevent a serious accident in which a tool or a workpiece penetrates the window and shoots out from being caused. 
     The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the whole of a machine tool according to a first embodiment of the present disclosure; 
         FIG. 2  is a front view of a window and an indicator according to the first embodiment; 
         FIG. 3  is a partial cross-sectional view taken along the arrow A-A of  FIG. 2 ; 
         FIG. 4  is an enlarged cross-sectional view of the B portion of  FIG. 2 ; 
         FIG. 5  is a cross-sectional view of an indicator according to a variation of the first embodiment; 
         FIG. 6  is a cross-sectional view of an indicator according to a variation of the first embodiment; 
         FIG. 7  is a cross-sectional view of an indicator according to a variation of the first embodiment; 
         FIG. 8  is a front view of a window and an indicator of a machine tool according to a second embodiment of the present disclosure; 
         FIG. 9  is an enlarged cross-sectional view of the C portion of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view of an indicator according to a variation of the second embodiment; 
         FIG. 11  is a front view of a window and an indicator of a machine tool according to a third embodiment of the present disclosure; 
         FIG. 12  is a partial cross-sectional view taken along the arrow F-F of  FIG. 11 ; 
         FIG. 13  is a front view of a window and an indicator of a machine tool according to a fourth embodiment of the present disclosure; and 
         FIG. 14  is a partial cross-sectional view taken along the arrow G-G of  FIG. 13 . 
     
    
    
     It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and apparatuses or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein. 
     DETAILED DESCRIPTION 
     So that the above features and advantages of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only typical embodiments of this disclosure and therefore are not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
     Hereinafter, specific embodiments of the present disclosure will be described with reference to the drawings. 
       FIG. 1  is a perspective view of the whole of a machine tool according to a first embodiment,  FIG. 2  is a front view of a window and an indicator according to the first embodiment, and  FIG. 3  is a partial cross-sectional view taken along the allow A-A of  FIG. 2 . 
     As shown in  FIG. 1 , a machine tool  1  of this embodiment is specifically an NC lathe, and its structures such as a bed, a headstock, a tool rest, etc. are covered by two fixed covers (cover bodies)  2 ,  3  and a door cover (cover body)  4  which is slidable in the directions indicated by the arrow, and the machine tool  1  has a structure in which a machining region is separated from the outside by the fixed covers  2 ,  3  and the door cover  4 . The fixed covers  2 ,  3  are each made of a metal plate, and the door cover  4  has a window  5  and is made of a metal plate except for the window  5 . 
     As shown in  FIGS. 2 and 3 , the window  5  comprises a glass plate  6  and a polycarbonate plate  7  which are transparent or translucent and have the same rectangular shape, and further comprises a spacer  8 , a seal member  9  and a frame body  10 . The spacer  8  is a rectangular frame-shaped member, and is disposed to be positioned at an outer peripheral edge portion between the glass plate  6  and the polycarbonate plate  7  to define a distance between them. 
     Further, the seal member  9  similarly comprises a rectangular frame-shaped member and is joined to the glass plate  6 , the spacer  8  and the polycarbonate plate  7  to seal the outer peripheral surface of an assembly comprising the glass plate  6 , the spacer  8  and the polycarbonate plate  7 , which are integrally assembled to each other as shown in  FIG. 3 . Furthermore, the frame body  10  is a rectangular frame body having an L-shaped cross section, and is joined to the seal member  9  and the glass plate  6  so that it covers the outer peripheral surface and a side surface of the seal member  9  and a peripheral side surface of the glass plate  6 . 
     Thus, the window  5  has a structure in which the glass plate  6  and the polycarbonate plate  7  are airtightly joined together with a certain distance between them. The window  5  having this structure is disposed between a plate member  4   a  and a plate member  4   b , which constitute the door cover  4  and which are respectively arranged on the external side and the machining-region side, at an opening of the door cover  4  so that the glass plate  6  is positioned on the machining-region side, and the window  5  is fixed in a state where an outer peripheral edge portion thereof is held by the plate members  4   a  and  4   b.    
     Further, the window  5  has through holes  8   a ,  9   a ,  10   a  formed in the spacer  8 , the seal member  9  and the frame body  10 , respectively, so that an opening is formed in an outer peripheral surface of the frame body  10 , and a pressure adjusting device  12  is connected to the through holes  8   a ,  9   a ,  10   a.    
     The pressure adjusting device  12  comprises a pressurizing pump  13 , a pipe joint  15  which is attached to the plate member  4   a  of the door cover  4  in a state of passing therethrough, a pipe joint  16  which is connected to the through holes  8   a ,  9   a  and  10   a , a connecting pipe  17   a  connecting the pressurizing pump  13  and the pipe joint  15 , a connecting pipe  17   b  connecting the pipe joints  15  and  16 , and a valve  14  which is intermediately provided in the connecting pipe  17   a . A space  11  between the glass plate  6  and the polycarbonate plate  7  is pressurized by opening the valve  14  in a state where the pressurizing pump  13  is being driven, and the pipe line of the connecting pipe  17   a  is sealed by closing the valve  14 , thereby maintaining the pressure in the space  11 . 
     Further, an indicator  20  is disposed in the window  5 . As shown in  FIG. 4 , the indicator  20  has a cylinder  21 , a piston  22  and a helical compression spring  23 . The cylinder  21  comprises a transparent or translucent cylindrical member open at both ends, and is disposed so that, at one side thereof, it airtightly passes through the spacer  8 , the seal member  9  and the frame body  10  and is positioned within the space  11  between the glass plate  6  and the polycarbonate plate  7  and the portion within the space  11  can be observed through the polycarbonate plate  7  from the outside. On the other hand, the cylinder  21  communicates with the atmosphere at the other side thereof. 
     Further, the piston  22  is airtightly disposed in the cylinder  21  to be movable in the axial direction of the cylinder  21 , and the helical compression spring  23  is disposed on the other side of the piston  22  within the cylinder  21  and biases the piston  22  toward the one side. It is noted that locking pieces  24 ,  25  are respectively provided in the ends of the cylinder  21 , the locking piece  24  prevents the helical compression spring  23  from pulling out at the other side, and similarly the locking piece  25  prevents the piston  22  from pulling out at the one side. 
     Thus, in the indicator  20 , the pressure in the space  11  acts an end surface of the piston  22  at the one side, and atmospheric pressure and a biasing force of the helical compression spring  23  act on an end surface of the piston  22  at the other side. The piston  22  functions as an indicating portion, and moves within the cylinder  21  in accordance with the differential between the acting force of the pressure in the space  11  and the resultant force of the acting force of the atmospheric pressure and the biasing force of the helical compression spring  23  and stops at a position where the forces are balanced. 
     It is noted that, in this embodiment, the piston  22  is positioned inside the space  8  (that is, in a hidden region) when the pressure in the space  11  is a pressure within an appropriate range (a pressure which is set on the basis of the initial pressure and with which the airtight state of the space  11  is judged to be good), and the piston  22  cannot be observed from the outside. On the other hand, when the pressure in the space  11  is lowered and becomes a pressure out of the appropriate range (a pressure with which the airtight state of the space  11  is judged to be bad), the piston  22  moves toward the one side and becomes observable through the polycarbonate plate  7  and the cylinder  21  from the outside. 
     According to the machine tool  1  of this embodiment having the above-described configuration, first, the space  11  is pressurized to a pressure higher than atmospheric pressure by the pressure adjusting device  12 . When the space  11  has been brought into a pressurized state in this way, the pressure in the space  11  acts on the piston  22  of the indicator  20  and thereby the piston  22  moves toward the other side and is brought into an initial state where the piston  22  is positioned inside the spacer  8 , that is, in the hidden region and obscured from view from the outside. 
     Thereafter, although, in the case where the airtight state of the window  5  is good, the pressure in the space  11  does not vary, in the case where the airtight state of the window  5  is not maintained, the pressure in the space  11  reduces and the piston  22  moves toward the one side in accordance with the reduction and becomes observable through the polycarbonate plate  7  and the cylinder  21  from the outside. That is, the piston  22  moves into a visible region. When the piston  22  becomes visually observable from the outside as described above, a judgment can be made that the airtight state of the window  5  is bad. 
     Thus, according to the machine tool  1  of this embodiment, it is possible to quickly and accurately judge appropriateness of the airtight state of the space  11  by a very simple checking operation of checking whether the piston  22  can be visually observed from the outside. Further, in the case where, similar to this embodiment, a polycarbonate plate having low resistance to coolant is used as the window plate disposed on the external side, it is possible to take a proper preventive measure, for example replacing the window  5  with a new one earlier, and taking such a preventive measure makes it possible to prevent a serious accident in which a tool or a workpiece penetrates the window and shoots out from being caused. 
     It is noted that although the helical compression spring  23  is disclosed above as means for applying a biasing force to the piston  22  toward the one side, the means is not limited thereto, and an indicator  30  is possible in which, for example, as shown in  FIG. 5 , an helical extension spring  31  is provided on the one side of the piston  22  within the cylinder  21  and one end thereof is engaged with the locking piece  25  and the other end thereof is engaged with the piston  22 . Also when thus configured, the piston  22  is biased toward the one side by a biasing force of the helical extension spring  31 . 
     Further, although in this embodiment the pressure adjusting device  12  is provided with the pressurizing pump  13 , the disclosure is not limited thereto and the pressure adjusting device  12  may be replaced by a pressure adjusting device  18  provided with a pressure-reducing pump  19  instead of the pressuring pump  13  and the pressure in the space  11  may be reduced to a negative pressure equal to or lower than atmospheric pressure by the pressure adjusting device  18 . In this case, instead of the indicator  20 , an indicator  35  shown in  FIG. 6  or an indicator  40  shown in  FIG. 7  is used. 
     The indicator  35  shown in  FIG. 6  has a structure in which, instead of the helical compression spring  23  of the above indicator  20 , a helical compression spring  36  is provided on the one side of the piston  22  within the cylinder  21 . In the indicator  35 , an acting force of the negative pressure in the space  11  and a biasing force of the helical compression spring  36  act on an end surface of the piston  22  at the one side and an acting force of atmospheric pressure acts on an end surface of the piston  22  on the other side. The piston  22  moves within the cylinder  21  in accordance with the differential between the resultant force of the acting force of the negative pressure in the space  11  and the biasing force of the helical compression spring  36  and the acting force of atmospheric pressure, and stops at a position where the forces are balanced. 
     It is noted that, in the indicator  35 , the piston  22  moves toward the one side and can be observed through the polycarbonate plate  7  and the cylinder  21  from the outside when the pressure in the space  11  is equal to or lower than an appropriate pressure (which is set on the basis of the initial pressure and with which the airtight state of the space  11  is judged to be good), and, on the other hand, the piston  22  is positioned inside the spacer  8  and cannot be observed from the outside when the pressure in the space  11  increases and becomes a pressure higher than the appropriate pressure (a pressure with which the airtight state of the space  11  is judged to be bad). 
     Thus, in this case, first, the pressure in the space  11  is reduced to a negative pressure lower than atmospheric pressure by the pressure adjusting device  18 . When the space  11  has been brought into a pressure-reduced state in this way, the negative pressure acts on the piston  22  of the indicator  35  and the piston  22  moves toward the one side and is brought into an initial state where the piston  22  can be observed from the outside. 
     When the airtight state of the window  5  is not maintained, the pressure in the space  11  increases and the piston  22  moves toward the other side in accordance with the increase and becomes unobservable from the outside. Thus, when the piston  22  becomes obscured from view from the outside as described above, it may indicate that the airtight state of the window  5  is unacceptable. 
     Further, the indicator  40  shown in  FIG. 7  has a structure in which, instead of the helical extension spring  31  of the above indicator  30 , a helical extension spring  41  is provided on the other side of the piston  22  within the cylinder  21 , and one end thereof is engaged with the piston  22  and the other end thereof is engaged with the locking piece  24 . Also according to this structure, the piston  22  is biased toward the other side by a biasing force of the helical extension spring  41 , thereby achieving the same operation as the indicator  35  does. 
     In the case where the space  11  is brought into a pressure-reduced state, when a leak portion is present in the window  5  and therefore the airtightness thereof is not secured, the outside gas is sucked into the space  11  through the leak portion, and if coolant is present near the leak portion, the coolant is sucked into the space  11  through the leak portion. On the other hand, in the case where the space  11  is brought into a pressurized state, even if a leak portion is present, because the gas within the space  11  is discharged through the leak portion, coolant never enters the space  11  through the leak portion even if present near the leak portion. In this respect, the space  11  may be brought into a pressurized state. 
     Furthermore, in the indicators  20 ,  30 ,  35 ,  40 , the piston  22  may be provided so that it is always positioned within the space  11  and can always be observed from the outside. In this case, it is possible to judge appropriateness of the airtight state of the space  11  by comparing the position of the piston  22  which varies in accordance with the pressure in the space  11  with its initial position. 
       FIG. 8  is a front view of a window and an indicator of a machine tool according to a second embodiment of the present disclosure, and  FIG. 9  is an enlarged cross-sectional view of the C portion of  FIG. 8 . As shown in  FIGS. 8 and 9 , a machine tool  50  according to the second embodiment has a structure in which, instead of the indicator  20  according to the first embodiment, an indicator  55  having a structure different from that of the indicator  20  is provided on the window  5 . 
     As shown in  FIG. 9 , the indicator  55  comprises a shielding plate  56 , an indicating plate  57 , a first link  59 , a second link  60 , a cylinder  61 , a piston  62 , a helical compression spring  63  and other components, and the shielding plate  56  is made of an opaque member having a triangular shape in font view and is disposed at a corner portion of the inner periphery of the spacer  8 . Further, the indicating plate  57  is made of an almost fan-shaped plate member and is disposed on the glass plate  6  side of the shielding plate  56 , that is, behind the shielding plate  56 , and is supported by a support shaft  58  to be able to pivot in the directions indicated by the arrow D-E, and is pivoted between a position where the whole of the indicating plate  57  is positioned behind the shielding plate  56  (the position indicated by the solid line in  FIG. 9 ) and a position where a part of the indicating plate  57  is positioned outside the shielding plate  56  (the position indicated by the two-dot chain line in  FIG. 9 ). 
     The cylinder  61  comprises a cylindrical member open at both ends and is disposed in the window  5  in a state of airtightly passing through the spacer  8 , the seal member  9  and the frame body  10 , and communicates with the space  11  between the glass plate  6  and the polycarbonate plate  7  at its end on one side and communicates with the atmosphere at its end on the other side. 
     Further, the piston  62  is airtightly disposed in the cylinder  61  so that it is movable in the axial direction of the cylinder  61 , and the helical compression spring  63  is disposed on the other side of the piston  62  within the cylinder  61  and biases the piston  62  toward the one side. Further, the cylinder  61  has a locking piece  64  provided therein at its end on the other side and the locking piece  64  prevents the helical compression spring  63  from pulling out at the other side. 
     One end of the first link  59  is fixed to the support shaft  58 , and one end of the second link  60  is engaged with a long hole  59   a  formed in the first link  59  and the other end of the second link  60  is fixed to an end surface of the piston  62  on the one side. The first and second links  59 ,  60  together function as a link mechanism for pivoting the indicating plate  57 . 
     Thus, in the machine tool  50 , first, the space  11  is pressurized to a pressure equal to or higher than atmospheric pressure by the pressure adjusting device  12 . When the space  11  has been pressurized to a pressure equal to or higher than atmospheric pressure, the pressure in the space  11  acts on the end surface of the piston  62  on the one side, and on the other hand atmospheric pressure and a biasing force of the helical compression spring  63  act on an end surface of the piston  62  on the other side, and the piston  62  moves within the cylinder  61  in accordance with the differential between the acting force of the pressure in the space  11  and the resultant force of the acting force of the atmospheric pressure and the biasing force of the helical compression spring  63 , and stops at a position where the forces are balanced. 
     Further, when the piston  62  moves, the indicating plate  57  is pivoted in the directions indicated by the arrows D-E on the support shaft  58  due to the engagement relationship between the first link  59  and the second link  60 . In this embodiment, when the pressure in the space  11  is in the initial state, the indicating plate  57  is pivoted in the direction indicated by the arrow D and the whole of the indicating plate  57  is positioned behind the shielding plate  56  and cannot be visually observed from the outside. 
     When the airtight state of the space  11  is bad and the pressure in the space  11  is therefore lowered, the piston  62  moves toward the one side, and thereby the indicating plate  57  is pivoted in the direction indicated by the arrow E and moves toward the outside of the shielding plate  56 , and the indicating plate  57  becomes partially observable through the polycarbonate plate  7  and the cylinder  61  from the outside. 
     Thus, also according to the indicator  55  of this embodiment, when the indicating plate  57  becomes observable through the polycarbonate plate  7  and the cylinder  61  from the outside, a judgment can be made that the airtight sate of the window  5  is bad. 
     It is noted that, also in this second embodiment, the pressure in the space  11  may be reduced to a negative pressure by the pressure adjusting device  18 . A machine tool  65  used in the case where the space  11  is brought into a pressure-reduced state has an indicator  70  shown in  FIG. 10  instead of the indicator  55 . 
     As shown in  FIG. 10 , in the indicator  70 , the first link  59  is fixed to the support shaft  58  so that the indicating plate  57  is pivoted in the direction indicated by the arrow D when the piston  62  moves toward the one side, and the indicating plate  57  is pivoted in the direction indicated by the arrow E when the piston  62  moves toward the other side. 
     Further, in the cylinder  61 , an helical extension spring  71  is provided instead of the helical compression spring  63 , and one end thereof is engaged with the piston  62  and the other end thereof is engaged with the locking piece  64 . 
     In the machine tool  65 , first, the pressure in the space  11  is reduced to a negative pressure equal to or lower than atmospheric pressure by the pressure adjusting device  18 . When the pressure in the space  11  has been reduced to a negative pressure, the pressure in the space  11  acts on an end surface of the piston  62  on the one side, and on the other hand atmospheric pressure and a tensile force (biasing force) of the helical extension spring  71  act on an end surface of the piston  62  on the other side, and the piston  62  moves within the cylinder  61  in accordance with the differential between the acting force of the pressure in the space  11  and the resultant force of the acting force of the atmospheric pressure and the biasing force of the helical extension spring  71 , and stops at a position where the forces are balanced. 
     Further, when the piston  62  moves, the indicating plate  57  is pivoted in the directions indicated by the arrows D-E on the support shaft  58  due to the engagement relationship between the first link  59  and the second link  60 . Also in the indicator  70 , when the pressure in the space  11  is in the initial state, the indicating plate  57  is pivoted in the direction indicated by the arrow D and the whole of the indicating plate  57  is positioned behind the shielding plate  56  and cannot be visually observed from the outside. 
     When the airtight state of the space  11  becomes sufficiently poor such that the pressure in the space  11  increases to an unacceptable level, the piston  62  moves toward the other side, and thereby the indicating plate  57  is pivoted in the direction indicated by the arrow E and moves toward the outside of the shielding plate  56 , and becomes partially observable through the polycarbonate plate  7  and the cylinder  61  from the outside. 
     Thus, also according to the machine tool  65 , when the indicating plate  57  becomes observable through the polycarbonate plate  7  and the cylinder  61  from the outside, a judgment can be made that the airtight state of the window  5  is insufficient. 
     Further, for the same reasons as those described above, also in the second embodiment, the space  11  may be brought into a pressurized state. 
     Furthermore, in the indicators  55 ,  70 , the indicating plate  57  may be configured so that a part or the whole of the indicating plate  57  can always be observed through the polycarbonate plate  7  and the cylinder  61  from the outside. In this case, it is possible to judge appropriateness of the airtight state of the space  11  by comparing the position of the indicating plate  57  which is pivoted in accordance with the pressure in the space  11  with its initial position. 
       FIG. 11  is a front view of a window and an indicator of a machine tool according to a third embodiment of the present disclosure, and  FIG. 12  is a partial cross-sectional view taken along the arrow F-F of  FIG. 11 . As shown in  FIGS. 11 and 12 , a machine tool  75  according to the third embodiment has a structure in which, instead of the indicator  20  of the first embodiment, an indicator  80  having a structure different from that of the indicator  20  is provided on the window  5 . 
     As shown in  FIG. 12 , the indicator  80  comprises a transparent or translucent cylindrical body  81  having an upside-down L shape, which comprises a horizontal portion and a vertical portion, a cover portion  82  having a similar upside-down L shape for covering the cylindrical body  81 , and a liquid  83  as an indicating portion, which is injected in the cylindrical body  81 . The horizontal portions of the cylindrical body  81  and the cover portion  82  are airtightly joined to each other, and end portions thereof penetrate the plate member  4   a  and are airtightly inserted into a through hole  7   b  formed in the polycarbonate plate  7 . Further, the spacer  8  also has a through hole  8   b  formed to be open to the space  11 , and the through hole  8   b  communicates with the through hole  7   b.    
     An end portion of the vertical portion (lower end portion) of the cylindrical body  81  is sealed and the liquid  83  is injected in the vertical portion of the cylindrical body  81 . On the other hand, the end portion of the horizontal portion of the cylindrical body  81  is open and the pressure in the space  11  acts on the top surface of the liquid  83  through the through holes  7   b  and  8   b . Further, the cover portion  82  has an opening  82   a  formed in an upper portion thereof and the cylindrical body  81  can be visually observed through the opening  82   a.    
     According to the machine tool  75  of the third embodiment, first, the space  11  is pressurized to a pressure equal to or higher than atmospheric pressure by the pressure adjusting device  12 . Thereby, the pressure in the space  11  acts on the top surface of the liquid  83  through the through holes  7   b ,  8   b  and presses the liquid  83  downward. On the other hand, a space below the liquid  83  is sealed and the pressure in the sealed space is increased by the downward movement of the liquid  83 , and the liquid  83  stops at a position where the pressure in the space  11  acting on the top surface of the liquid  83  and the pressure in the sealed space are balanced. 
     Although, as described above, the opening  82   a  is provided in the cover portion  82  and the cylindrical body  81  and the liquid  83  can be visually observed through the opening  82   a , in the initial state where the space  11  has been pressurized, the liquid  83  is positioned below the opening  82   a  and is positioned at a position where it cannot be visually observed from the outside, that is, in a hidden region. 
     When the airtight state of the window  5  becomes bad, the pressure in the space  11  reduces and the liquid  83  moves upward in accordance with the reduction, and as a result thereof the liquid  83  becomes visually observable through the opening  82   a . Thus, when the liquid  83  becomes visually observable from the outside as described above, a judgment can be made that the airtight state of the window  5  is bad. 
     It is noted that, also in the machine tool  75 , the pressure in the space  11  may be reduced to a negative pressure equal to or lower than atmospheric pressure by the pressure adjusting device  18 . In this case, the liquid  83  is sucked and moves upward due to the difference between the pressure in the space  11  acting on the top surface of the liquid  83  through the through holes  7   b ,  8   b  and the pressure in the sealed space below the liquid  83 . The pressure in the sealed space is reduced by the upward movement of the liquid  83 , and the liquid  83  stops at a position where the pressure in the space  11  acting on the top surface of the liquid  83  and the pressure in the sealed space are balanced. 
     In this mode in which the space  11  is brought into a pressure-reduced state, the opening  82   a  is provided in a lower portion of the cover portion  82 , and in the initial state the liquid  83  is positioned in a hidden region located above the opening  82   a . When the airtight state of the window  5  becomes bad, the pressure in the space  11  increases and the liquid  83  moves downward in accordance with the increase, and as a result thereof the liquid  83  becomes visually observable through the opening  82   a . Thus, also in this mode, when the liquid  83  becomes visually observable from the outside, a judgment can be made that the airtight state of the window  5  is bad. 
     It is noted that, in the third embodiment, that the liquid  83  may be a liquid in which air can be hardly dissolved, such as mercury. Further, for the same reasons as those described above, also in the third embodiment, the space  11  may be brought into a pressurized state. 
     Furthermore, in the above-described mode, the vertical portion of the indicator  80  as shown in  FIGS. 11 and 12  may be turned upside down so that the sealed side is placed on the upper side. In this case, the opening  82   a  may be formed in a lower portion of the cover body  82  in the case where the space  11  is brought into a pressurized state, and may be formed in an upper portion of the cover body  82  in the case where the space  11  is brought into a pressure-reduced state. 
     Further, the indicator  80  may be configured so that the liquid  83  can always be observed from the outside. In this case, it is possible to judge appropriateness of the airtight state of the space  11  by comparing the position of the liquid  83  which is displaced in accordance with the pressure in the space  11  with its initial position. 
       FIG. 13  is a front view of a window and an indicator of a machine tool according to a fourth embodiment of the present disclosure, and  FIG. 14  is a partial cross-sectional view taken along the arrow G-G of  FIG. 13 . As shown in  FIGS. 13 and 14 , a machine tool  85  according to the fourth embodiment has a structure in which the indicator  20  of the first embodiment is connected to the space  11  of the window  5  via communicating pipes  88 ,  89 . 
     The space  8 , the seal member  9  and the frame body  10  have through holes  8   c ,  9   c ,  10   c  formed therein, respectively, and a pipe joint  87  is airtightly inserted into the through holes  8   c ,  9   c ,  10   c . Further, a pipe joint  86  is fixed to the plate member  4   a  of the door cover  4  in a state of passing therethrough. One end of the communicating pipe  88  is connected to the pipe joint  87  and the other end thereof is connected to one end portion of the pipe joint  86  which is positioned between the plate members  4   a  and  4   b.    
     One end of the communicating pipe  89  is connected to the other end portion of the pipe joint  86  and the other end of the communicating pipe  89  is connected to one side end portion of the indicator  20  via a joint  90 . 
     Thus, in the machine tool  85 , the indicator  20  is connected to the space  11  via the joint  90 , the communicating pipe  89 , the pipe joint  86 , the communicating pipe  88  and the pipe joint  87 , and the pressure in the space  11  acts on the piston  22  of the indicator  20 . Further, the indicator  20  is disposed outside the door cover  4 . 
     In the indicator  20 , the position of the piston  22  moves in accordance with the pressure in the space  11  and the piston  22  can always be observed from the outside. Therefore, it is possible to judge appropriateness of the airtight state of the space  11  by checking with eyes whether the position of the piston  22  is displaced from its initial position. 
     It is noted that the behavior of the indicator  20  in accordance with the pressure in the space  11  is the same as described above, and therefore detailed explanation thereof is omitted. 
     Further, in the case where the space  11  is brought into a pressurized state, the indicator  30  may be used instead of the indicator  20 . Furthermore, in the case where the space  11  is brought into a pressure-reduced state, the indicator  35  or the indicator  40  may be used instead of the indicator  20 . However, also in the fourth embodiment, for the same reasons as those described above, the space  11  may be brought into a pressurized state. 
     Thus, some embodiments of the present disclosure have been described. However, the mode of embodying the present disclosure is not limited thereto. 
     For example, although the window  5  provided on the door cover  4  of the machine tool  1 ,  50 ,  65 ,  75 ,  85  was exampled in the above embodiments, the present disclosure is not limited thereto and can be applied to a window  5  provided on the fixed covers  2 ,  3 . Further, the type of the machine tool is not limited to the NC lathe as described above. 
     Further, although a window having a glass plate disposed on the machining-region side and a polycarbonate plate disposed on the external side was exampled as the window  5 , the structure of the window is not limited thereto and the present disclosure may be applied to a window  5  having a structure in which a material having high wear resistance is used as the window plate disposed on the machining-region side and a material having high toughness and high impact resistance but having low resistance to coolant is used as the window plate disposed on the external side. Furthermore, the present disclosure can be applied regardless to the materials of the two window plates constituting the window  5  as long as the purpose thereof is to detect the airtight state of the space between the two window plates. 
     Furthermore, as for the airtight state of the window  5 , it is in reality difficult to achieve a completely airtight state and leakage can occur in a range where there is no problem in practical use, which is allowable. Therefore, the pressure in the space  11  may be returned to the initial state on a regular basis or on an irregular basis by the pressure adjusting device  12 ,  18 . In this way, it is possible to cancel the pressure variation within the range where there is no problem in practical use and judge appropriateness of the airtight state of the window  5  accurately. 
     Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the burner assembly and methods for flaring low calorific content gases disclosed and claimed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.