an embodiment of the present invention will be described with reference to the drawings . in the description below , same components will be represented by the same reference number . such components have the same name and function . thus , detailed descriptions thereof will not be repeated . the configuration of a robot system 1 according to this embodiment will be described , in outline , with reference to fig1 . as illustrated in fig1 , the robot system 1 includes a vertically articulated six - axis welding robot 2 , a slider 3 that moves the welding robot 2 itself , a positioner 4 that holds a workpiece w in such a manner that the orientation is changeable , a control device 5 including a teaching pendant 51 , and a personal computer 6 . the welding robot 2 is a vertically articulated six - axis industrial robot , and a welding torch 21 is disposed at the tip of the welding robot 2 . the control device 5 controls the welding robot 2 , the slider 3 , and the positioner 4 in accordance with a teaching program 300 prepared in advance . the teaching program 300 may be prepared using the teaching pendant 51 connected to the control device 5 or may be prepared using an offline teaching system using the personal computer 6 . in either case , the teaching program 300 is prepared in advance before the actual operation . the teaching program 300 prepared by the personal computer 6 is sent to the control device 5 via a medium , etc . in which data is magnetically stored or transferred to the control device 5 by data communication . the personal computer 6 , i . e ., the offline teaching system includes a display capable of displaying graphics as a display device and a keyboard and a mouse as an input device . the offline teaching system has a magnetic storage device or a communication device for taking in cad information of the workpiece w . the present invention is related to a welding - line selecting method of automatically selecting a “ welding line ,” where welding is to be performed by the welding robot 2 , on the basis of a workpiece figure in the three - dimensional cad data displayed on the display screen . the welding line selection is performed by the personal computer 6 , which constitutes the offline teaching system . as illustrated in fig2 a to 2c , and 3 , a welding line is set near a line of intersection of two continuous faces or two faces of different members that are equivalent to continuous faces . in the robot system 1 according to this embodiment , by performing the following steps 1 to 7 , a welding line , where welding is to be performed by a welding robot , can be automatically selected on the basis of a workpiece figure in three - dimensional cad data displayed on the display screen . step 1 ( first - face - group selecting step ): among two faces constituting the workpiece w on which a welding groove is formed , at least one first face ( reference face ) is selected , and the selected first face is assigned to a first face group ( reference face group ). step 2 ( second - face - group selecting step ): among two faces of the workpiece w on which a welding groove is formed , at least one second face ( groove face ) is selected , and the selected second face is assigned to a second face group ( groove face group ). step 3 ( edge - line extracting step ): in association with the edge lines ( outlines defining the faces or edge lines of the faces ) of the faces selected in step 2 ( second - face - group selecting step ), all edge lines having vertices , whose distances to the corresponding faces in the reference face group are zero or substantially zero ( which is a value that can be set appropriately in accordance with the object to be welded ), are extracted . step 4 ( weldable - edge - line extracting step ): among the edge lines extracted in step 3 ( edge - line extracting step ), edge lines existing in a region weldable by a welding robot are selected on the basis of the directions of the normal vectors of the faces in the reference face group and groove face group . step 5 ( weldable - edge - line adding step ): among the edge lines selected in step 4 ( weldable - edge - line extracting step ), the intersection of an edge line that crosses the border of faces is calculated , and a weldable edge line is further selected on the basis of information about the intersection . step 6 ( integrating step ): the edge lines selected in step 4 ( weldable - edge - line extracting step ) and step 5 ( weldable - edge - line adding step ) are integrated into a continuous welding line to prepare a welding line . step 7 ( edge - line displacing step ): among the edge lines extracted in step 3 ( edge - line extracting step ), for an edge line that does not contact a face in the reference face group , the position of the edge line is displaced in accordance with the position and / or distance relative to the reference face group and groove face group . as illustrated in fig4 ( a ) , when a groove face and a reference face are disposed at a substantially 90 degree angle with the groove face not joining the reference face and forming a gap therebetween , the welding line extracted through the process of steps 1 to 5 is an edge line on the groove face side , and a gap is formed with the reference face . when this gap is small , a new welding line is set by projecting the extracted welding line ( edge line on the groove face side ) to the reference face in step 7 ( edge - line displacing step ). when the gap is large , the midline of the gap is set as a new welding line in step 7 ( edge - line displacing step ). as illustrated in fig4 ( b ) , when the groove face and the reference face are disposed substantially parallel to each other with the groove face not joining the reference face and forming a gap therebetween , the midline of the gap is set as a new welding line in step 7 ( edge - line displacing step ). fig5 illustrates the welding line selection described above as a flow chart , and fig6 are diagrams illustrating the welding line selection . in welding line selection , first , in step s 110 in fig5 , a face of a member to be a reference face is assigned . when the reference face extends across a plurality of members , a plurality of reference faces is assigned . depending on the three - dimensional cad data , since a curved face may be approximated by a collection of polygons ( polygon approximation ), when a plane of the same member adjoining the assigned plane is determined to be a continuous face , these faces are also automatically selected . the selected plurality of faces is stored as the reference face group . next , in s 120 , the second face or second faces , if applicable , of the two faces on which the welding line is formed is assigned in a similar manner as in s 110 and stored the groove face group . in s 130 , the distances between every two vertex defining the edge line of faces in the groove face group and every face in the reference face group are calculated , and all edge lines having vertices of which the distance is zero or substantially zero are extracted . if none of the edge lines are extracted , the cylindrical member may be penetrating a flat plane , such as illustrated in fig3 , and thus , an edge line of the reference - face group side may be forming the welding line . in such a case , an edge line is extracted by switching the reference face group and the groove face group . that is , the distances between every two vertices of the edge line of the faces in the groove face group and the reference face group are calculated , and all edge lines having vertices of which the distance is zero or substantially zero are extracted . furthermore , in s 140 , normal vectors perpendicular to the faces in the reference face group and the groove face group are determined . base of the directions of the normal vectors , all edge lines existing in a region weldable by the welding robot 2 are selected from the edge lines extracted in s 130 . when an edge line exists across a weldable region and an unweldable region , by calculating the intersection of the edge line and the regions and dividing the edge line at the determined intersection , only the edge line in the weldable region is selected . for example , when edge lines , such as those illustrated in fig6 ( a ) , are extracted , the direction of the groove face 1 ( normal direction of the groove face 1 ), the directions of the groove face 2 ( normal directions of the groove face 2 ), and the direction of the groove face 3 ( normal direction of the groove face 3 ) are calculated , as illustrated in fig6 ( b ) . as illustrated in fig6 ( c ) , the side on which the normal vectors of the groove faces 1 , 2 , and 3 intersect is determined as a weldable area , and the opposite side is determined as an unweldable area . as illustrated in fig6 ( d ) , the edge lines in the weldable area are separated as weldable edge lines . furthermore , for all of the weldable edge lines selected in s 140 , among the combinations ( four combinations ) of the two vertices of an edge line and the two vertices of another edge line , if any one of the combinations is at a distance of zero or smaller than a value that can be considered near zero , the edge lines are integrated into a continuous welding line in s 150 . when such a combination does not exist , the position of the intersection of extended or shorted edge lines is calculated , and the vertices of the edge lines are moved to the intersection . as a result of such process , edge lines that have been extracted but cannot be welded are removed , and a continuous welding line can be defined as illustrated in fig7 . the welding line information generated in s 150 is corrected in s 160 as described with reference to fig4 . the condition of the workpiece w displayed on the display while actually performing the above - described welding line selection will be described below . fig8 illustrates the display mode of the workpiece w displayed on the display . as illustrated by the wire frames in fig8 ( a ) , the workpiece w includes a flat workpiece w ( 1 ), a substantially u - shaped workpiece w ( 2 ), a workpiece w ( 4 ), which is symmetric to the workpiece w ( 2 ), and a workpiece w ( 3 ) connecting the workpieces w ( 2 ) and w ( 4 ). in the description below , the workpieces w ( 2 ), w ( 3 ), and w ( 4 ) are integrated into one . in the wire frame of the workpiece w , which is illustrated in fig8 ( a ) , curved planes are approximated by a plurality of line segments , as illustrated in fig1 . to select a welding line for the workpiece w on the wire frame , many line segment &# 39 ; s must be selected one by one , which is an extremely time consuming process . in a welding - line preparing procedure , first , a reference face is assigned ( s 110 ). at this time , when the workpiece w ( 1 ) is assigned as the reference face ( for example , the mouse is operated to move a pointer onto the workpiece w ( 1 ), and then the mouse button is clicked ), the display mode of the workpiece w ( 1 ) changes , as illustrated in fig8 ( b ) ( in fig8 ( b ) , the workpiece w ( 1 ) is hatched , but when a color display is used , the color changes ). next , a second face , which is not a reference face , is assigned ( s 120 ). if necessary , multiple faces are assigned at this time . when workpiece w ( 2 ) is assigned as a second face ( for example , the mouse is operated to move a pointer onto the workpiece w ( 2 ), and then the mouse button is clicked ), the display mode ( color ) of the workpiece w ( 2 ) changes , as illustrated in fig8 ( c ) . furthermore , when the workpiece w ( 3 ) is assigned as a second face ( for example , the mouse is operated to move a pointer onto the workpiece w ( 3 ), and then the mouse button is clicked ), the display mode ( color ) of the workpiece w ( 3 ) changes , as illustrated in fig8 ( d ) . furthermore , when the workpiece w ( 4 ) is assigned as a second face ( for example , the mouse is operated to move a pointer onto the workpiece w ( 4 ), and then the mouse button is clicked ), the display mode ( color ) of the workpiece w ( 4 ) changes , as illustrated in fig8 ( e ) . edge lines are extracted ( s 130 ); a weldable edge region is computed and selected ( s 140 ); and the selected edge lines are integrated into a welding line ( s 150 ). in this way , a continuous welding line is displayed , as illustrated in fig8 ( f ) . as illustrated in fig8 ( f ) , a welding line is not prepared in the unweldable region ( region in which the normal vectors do not intersect ) between the workpiece w ( 1 ) and the other workpieces ( workpieces w ( 2 ), w ( 3 ), and w ( 4 )). by simply assigning the reference faces and the groove faces on which a welding line is to be set , a continuous welding line can be assigned efficiently . even when there is a gap , a practical welding target position can be set , improving the welding quality . that is , the operation load borne by the operator preparing an operating path for a welding robot can be reduced , and welding line selection having significantly improved operation efficiency can be provided . next , welding teaching operation ( teaching operation ) will be described below with reference to fig9 . in preparation of the operation , three - dimensional cad data 100 of the workpiece to be processed and robot model data 200 defining the three - dimensional form and the mechanisms of the robot , the positioner , and the slider are installed to the offline teaching system in advance are read in advance . first , in s 100 , the welding line selection described in this embodiment is performed to assign the positions of the welding lines and to prepare welding - line information . in s 200 , the welding conditions of the welding lines and welding torch orientation are set . in s 300 , the positions of the slider 3 and the positioner 4 are set , if required , to set the robot in the operating range without interference . in s 400 , the approaching path at the start of welding and the retracting path at the end of welding are set . by performing such process of s 100 to s 400 , the welding teaching operation ( teaching operation ) is completed . finally , the teaching program ( robot teaching program ) 300 that realizes operation of an actual apparatus is output . the robot teaching program 300 is sent to the control device 5 via a medium , etc . in which data is magnetically stored or transferred to the control device 5 by data communication . the embodiment displayed above is merely an example and should be considered as not limiting . the present invention is not defined by the above description but by the claims and intends to include various modifications equal to the concept of the claims and within the scope of the present invention . for example , an offline teaching system for a welding robot is described above as an example in the embodiment . the present invention , however , may also be applied to welding line assignment of an automatic welding apparatus . the present invention is based on japanese patent application no . 2009 - 031216 filed in the japan patent office on feb . 13 , 2009 , the entire contents of which are incorporated herein by reference .