Patent GB-768810-A (GB)
Title: Thread cutting device for sewing machine
IPC: D05B65/00
Filed: 1955-04-27
Abstract: 768,810. Sewing machines. TRIMLESS, Inc. April 27, 1955 [May 25, 1954], No. 12123/55. Class 112. [Also in Group XXXII] Thread-cutters and thread-end holders. In a sewing machine, means for severing and holding the needle thread are incorporated in a presser foot carried by a presser bar adapted to be raised and lowered, means for severing and holding the bobbin thread are mounted under a throat plate disposed beneath the presser foot, and means are provided, co-operating with a handwheel adapted to reciprocate the needle and with the means for raising and lowering the presser bar, for permitting actuation of the thread severing and holding means only when the needle bar is raised and the presser foot is lowered. The needle 22 is carried by a needle bar 20 reciprocated by means of a handwheel 28 which is grooved for a belt drive. The presser foot 30 is carried by a presser bar 32 connected by a lever 34 and a rod 38 to a treadle, depression of which thereby serves to lift the presser foot. The presser foot comprises a member 58 which is secured to the presser bar and which has a forked lower end. Between the limbs of the latter is pivoted on a horizontal pin 62 a body 64 provided with a needle slot 66. Loosely suspended from body 64 by means of screws 72 is a sole plate 68 having an upturned front end provided with a needle slot 70. Journalled in body 64 is a pin 78 which carries at its upper end a ball-ended arm 96 and which is biased upwardly by means of a spring 98. Fixed to the lower end of pin 78 by means of a screw 88 are two superimposed blades 84, 86 which are movable in a recess 76 provided in the lower surface of body 64. On rotation of pin 78 the blades are carried across the needle slots, the upper blade 86, which has a rounded leading edge, serving to grip the needle thread resiliently between its upper surface and the lower surface of body 64, while the rearwardly offset leading edge of the lower blade 84 co-operates with a bevelled edge of slot 70 to sever the thread. A lever 100 pivoted at 102 to the presser foot member 58 is provided at one end with a slot 104 which engages the ball end of arm 96, and at its other end with a pin 114 engaged in a slot 112 provided at one end of a lever 108. The latter is pivoted at 110 to the head 18 of the machine, and is pivoted at its other end to a rod 120 which is connected by an adjustable coupling 124 to a rod 122 pivoted to the movable core of a solenoid 128 mounted on a bracket 130 secured to the standard 14 of the machine. Energization of the solenoid serves to exert a pull on rod 122, whereby the blades 84, 86 are swung' across the needle slots, the blade-operating system being returned to the inoperative position, upon de-energization of the solenoid, by means of a tension spring 132 acting on rod 120. The throat plate 40 carries a depending pin 160 on which is pivoted a blade 158 formed with a wedge-shaped knife portion 162. An arm 166 carried by blade 158 has an eye 168 which is aligned with the needle opening 164 in the throat plate, and through which the bobbin thread passes. Also mounted under the throat plate is a resilient strip 170 formed with a wedge-shaped free end. When blade 158 is swung clockwise, arm 166 draws the bobbin thread under the strip 170 and the knife portion 162 moves across the opening 164 to sever the thread, the free end of the latter being then gripped resiliently between the cooperating bevelled surfaces of portion 162 and strip 170. Blade 158 is connected by a flexible cable 178 to the movable core of a solenoid 174 mounted on a bracket 176 secured to the bed 12 of the machine, energization of the solenoid serving to exert a pull on the cable for swinging blade 158 to the operative position. Solenoids 128 and 174 are connected in parallel with each other, and the circuit connected to the solenoids contains a pair of switches 42 and 142 which are mounted on opposite sides of the standard 14 and are connected in series with each other. Switch 42 is normally open and switch 142 is normally closed. A lever 48 pivoted at 50 on the standard 14 is provided with a handplate 52 by means of which a projection 54 on the handplate is urged against the periphery of the handwheel 28 to enter a notch 56 in the latter for stopping the machine, the opposite end of lever 48 simultaneously contacting an operating finger 46 for closing switch 42 to energize the solenoids, the needle bar then being in the raised position. Rod 38 carries a collar 144 which contacts an operating finger 146 for opening switch 142 when the treadle is depressed, whereby energization of the solenoids is prevented while the presser foot is raised. Thread controlling. Lever 108, when swung to operate the blades 84, 86 engages a cam surface 154 provided on a rod which passes through the head 18 and which thereby serves to separate the discs of a tension device 24 mounted on the other side of the head, for providing sufficient slack in the needle thread to permit the formation of the subsequent first stitches. Similar slack is provided in the bobbin thread by the action of arm 166. In order to prevent breaking of the bobbin thread, the initial movement of the blade 158 is damped. For this purpose a pin 184, carried by the movable core of solenoid 174, is engaged with the plunger of a dashpot 188 so as to withdraw the plunger when the solenoid is energized. During the initial movement of the plunger, admission of air is controlled by a valve, a port being thereafter uncovered by the plunger to allow the free admission of air. Movement of the solenoid core also serves to stress a tension spring 182. In a modification, the solenoid core is attached to a permanent magnet which adheres to a spring- loaded plunger during the initial. movement of the core, for damping said movement, the magnet separating from the plunger when the spring stress exceeds the magnetic attraction.