Pfaff 3334 - Page 23

The motion which causes the knives to swing forward and catch the threads
emanates from the knife cam and is transmitted to the knives by the knife
bar tripping lever and the knife bar (Fig. 12).
Fig.12 Thread trimming mechanism
1 = Knife cam positioning flange 6
2 = Lifting lever 7
3 = Knife cam 8
4 = Knife bar 9
5 = Knife bar tripping fork 10
Knife
bar tripping lever
Bearing bracket
Ball joint connection
Locking lever
Bracket
The needle and bobbin threads are cut when the left treadle is depressed.
A detailed description of the thread trimming action is given below:
While the last stitch is being made, the knife cam causes both knives to
swing forward from the inoperative to the stand-by position. During this
motion the tip of the needle thread knife catches the needle thread and
pulls it onto the back edge of its blade (Figs. 13 & 14). The bobbin thread
Is caught by the bobbin thread knife in the same manner.
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From the library of: Superior Sewing Machine & Supply LLC