a thread cutting device of a sewing machine , which is one mode of embodiment of the invention will now be described with reference to fig1 a ( 1 ) to 1b ( 5 ), 4 , 5 , and 7 . similar to the conventional example , the thread cutting device of a sewing machine of the invention is disposed below a slide 6 so as to be flanked by a throat plate ( not shown ). the thread cutting device mainly includes a movable knife 1 , a fixed knife 2 , a front stopper 3 , and a rear stopper 4 . as shown in fig5 the movable knife 1 is accommodated in a groove 5a of the thread cutting device table 5 so as to be slidable . the fixed knife 2 is accommodated in a groove 5b so as to be slidable while superposed on the upper side of the movable knife 1 in the groove 5a . the front stopper 3 and the rear stopper 4 are accommodated side by side so as to be flanked by the movable knife 1 in the groove 5a and the fixed knife 2 in the groove 5b . the slide 6 is arranged so as to cover these members . on the upper surface close to the point of the movable knife 1 is a blade portion 1a . in the middle is a thread pickup portion 1b for picking up a sewing thread . a pin 7 is arranged so as to project from the base end portion of the movable knife 1 . the movable knife 1 can shuttle between a standby position ( fig1 a ( 1 )) and a thread pickup position ( fig1 a ( 3 )) while coupled to a not shown drive means similar to that of the conventional example . as shown in fig7 ( 1 ), a blade portion 2a is arranged on the point of the fixed knife 2 . a corner edge of the blade portion is not pointed unlike that in the conventional example . that is , the corner edge is right - angled or slightly wider - angled than the right angle ( obtuse - angled ). however , since an excessively large angle impairs cutting performance , the angle is set to such a value as to allow a thread to be cut properly . further , an upper corner portion 2f is not pointed , either , i . e ., rounded or chamfered . it may be noted that a smoothly projecting portion 2b is formed on the upper part of the conventional acute - angled point , the smoothly projecting portion 2b projecting frontward from the acute - angled point position as shown in fig7 ( 2 ). at any rate , it is so designed that a sewing thread y is prevented from coming in contact with the blade point before the normal cutting operation is performed . in the base end portion of the fixed knife 2 is an elongated hole 2c that extends along the length of the knife . the pin 7 of the movable knife 1 engages with the elongated hole 2c . the right end of the elongated hole 2c comes in contact with the pin 7 when the front stopper 3 is released from a catching stepped portion 2d of the fixed knife 2 by a cam portion 1c to be described later acting on the free end of the front stopper 3 during the forward movement of the movable knife 1 . the left end of the elongated hole 2c comes in contact with the pin 7 when the rear stopper 4 is released from a catching stepped portion 2e of the fixed knife 2 by a cam portion 1d to be described later acting on the free end of the rear stopper 4 during the backward movement of the movable knife 1 . the front stopper 3 fixes the fixed knife 2 at the standby position until the movable knife 1 completely advances into a needle thread loop , whereas the rear stopper 4 not only positions and fixes the fixed knife 2 at the operating position but also prohibits movement thereof to the standby position . each of the front stopper 3 and the rear stopper 4 is constructed of a small plate body having such a shape as shown in fig4 . the front stopper 3 is reversed from the condition shown in fig4 when it is in a set position . a small hole 4a ( 3a ) is formed in one end of each stopper 4 ( 3 ) so that a shaft 9 ( 8 ) can pass therethrough . in the other end thereof is a latch portion 4b ( 3b ). the catching surface of the latch portion 4b ( 3b ) is inclined at such an angle as to be perpendicular to a line connecting the center a of the small hole 4a ( 3a ) to the center b of the catching surface or at a larger angle so that engagement of the fixed knife 2 with the catching stepped portion 2d ( 2c ) to be described later can be reinforced . the front stopper 3 and the rear stopper 4 are oscillatably supported by the shafts 8 , 9 passing through the small holes 3a , 4a on the table 5 with the small holes 3a , 4a confronting each other on one side of the movable knife 1 and the fixed knife 2 accommodated in the grooves 5a and 5b of the thread cutting device table 5 . further , the free end ( the latch portions 3b , 4b ) sides of these stoppers 3 , 4 are urged by a stopper spring 10 so as to turn toward the movable knife 1 and the fixed knife 2 . the stopper spring 10 has the center thereof supported by a pin 11 that erects on the table 5 . further , the cam portion 1c is formed on the movable knife 1 at a position that is closer toward the rear side than the center and that faces the stoppers 3 , 4 . the cam portion 1c releases engagement of the front stopper 3 with the fixed knife 2 by operating the free end of the front stopper 3 during the forward movement . the cam portion 1d is likewise formed on the rear side so that the cam portion 1d releases engagement of the rear stopper 4 with the fixed knife 2 by operating on the free end of the rear stopper 4 during the backward movement after the sewing threads have been cut . further , the catching stepped portions 2d , 2e respectively engageable with the latch portion 3b of the front stopper 3 and the latch portion 4b of the rear stopper 4 are formed at positions closer toward the front side and at the rear end of the fixed knife 2 . in the thus constructed thread cutting device of a sewing machine , a needle thread y1 and a bobbin thread y2 are cut in the following operation sequence after the sewing operation has been ended . the movable knife 1 makes a forward movement from the standby position ( fig1 a ( 1 )) while driven by the drive means as in the conventional example as shown in fig1 a ( 2 ), and loop spreading is effected during this process . that is , similarly to the conventional example shown in fig3 the movable knife 1 advances into the loop of the needle thread . however , the fixed knife 2 is locked at the initial position until the movable knife 1 reaches the position shown in fig1 a ( 2 ) since the fixed knife 2 has the catching stepped portion 2d thereof engaged with the front stopper 3 . when the movable knife 1 has arrived at the position shown in fig1 a ( 2 ) after having advanced into the loop of the needle thread , the cam portion 1c of the movable knife 1 operates on the front stopper 3 , which in turn unlocks the fixed knife 2 . when the movable knife 1 has further advanced to the position shown in fig1 a ( 3 ), the pin 7 of the movable knife 1 engages with the right end of the elongated hole 2c of the fixed knife 2 , and the fixed knife 2 also further advances so as to follow the movable knife 1 , so that the rear stopper 4 engages with the catching stepped portion 2e to thereby lock the fixed knife 2 . the blade portion 2a of the fixed knife 2 at this time is positioned at the needle swinging center for zigzag stitches ( at the center in the longitudinal direction of the elongated hole ). further , the needle thread and the bobbin thread are caught by the blade portion 2a of the fixed knife 2 . then , during the backward movement of the movable knife 1 toward a position shown in fig1 b ( 4 ), the needle thread y1 and the bobbin thread y2 are picked up by the thread pickup portion 1b . when the movable knife 1 has been brought to a position shown in fig1 b ( 4 ), the blade portion 2a of the fixed knife 2 coincides with the blade surface la of the movable knife 1 , and the needle thread and the bobbin thread are cut in association with the backward movement of the movable knife 1 . when the cam portion 1d of the movable knife 1 making the backward movement operates on the rear stopper 4 , the catching stepped portion 2e of the fixed knife 2 is released from the rear stopper 4 , and the pin 7 of the movable knife 1 engages with the left end of the elongated hole 2c of the fixed knife 2 . as a result , the fixed knife 2 also returns to a position shown in fig1 b ( 5 ) together with the movable knife 1 . at the same time , the front stopper 3 engages with the catching stepped portion 2d of the fixed knife 2 , so that the fixed knife 2 is locked to get ready for a next thread cutting operation . further , when the movable knife 1 makes a backward movement to bring itself to the condition shown in fig1 b ( 4 ) from the condition shown in fig1 a ( 3 ), the sewing threads ( the needle thread and the bobbin thread ) y are brought to a regular thread cutting position without receiving excessive icontact pressure at the rounded or chamfered portion 2f or projecting portion 2b of the fixed knife 2 as shown in fig7 . since the invention is constructed as described above , the following advantages can be provided . that is , the fixed knife is held in the standby position until the movable knife effects loop spreading . therefore , defective thread cutting is eliminated . further , the basic components of the thread cutting device are interposed between the thread cutting device table and the shuttle race slide . therefore , the basic components no longer come off , which in turn allows members such as guide members ( shafts , plates ) for the movable knife and the like and release preventing members ( the spring shown in fig2 ( 1 )) and the like to be dispensed with and contributes to controlling the cost of manufacture under a low level . further , the smoothly projecting portion is formed on the upper portion of the blade point of the fixed knife . therefore , the lengths of the cut needle thread and the cut bobbin thread that extends from the bobbin can be made constant independent of the magnitudes of the amount of bobbin thread wound around the bobbin , the amount of oil in the rotary hook portion , the tension of the bobbin thread , and the like . as a result , the coming off of the needle thread from the needle at the sewing end , and stitch skipping and thread cast - off at the sewing start can be eliminated . hence , neatly finished sewn products can be obtained .