Patent GB-984090-A (GB)
Title: Improvements in or relating to thread cutting mechanisms of lockstitch sewing machines
IPC: D05B65/00
Filed: 1962-12-13
Abstract: 984, 090. Sewing machines. G. M. PFAFF A.G. Dec. 13, 1962 (Dec.13, 1961], No. 47049/62. Heading D1G.  A thread-severing member in a lock-stitch sewing machine is rotatable coaxially of the looptaker and is positioned close to the surface of the latter, said member controlling a thread-clamping device and being driven by reduction gearing, the ratio of which determines the number of initial stitches, formed while the thread remains clamped, of the stitch group following the cutting operation. A rotary hook 18 is carried by a shaft 17 journalled in bearings 14, 15 and driven by gearing 12, 13 at twice the speed of an auxiliary shaft 11 driven by a belt 9 at the speed of the arm shaft 5. A thread-cutting device, 33, journalled on bearing 15, is coupled by 2 to 1 reduction gearing 30, 31 to a shaft 25 which is releasably coupled, via 2 to 1 reduction gearing 19, 20, to shaft 11. Device 33, which thereby turns, in the opposite direction to the rotary hook, through a quarter of a revolution for each stitch-forming cycle, comprises a cam disc 41. An arcuate, axially-projecting extension 38 of cam 41 is formed in its leading edge with a triangular projection 35 and a notch 36. A knife blade 39, carried by extension 38, co-operates with a knife blade 40 fixed under the throat plate 50<SP>1</SP>. Cam 41 co-operates with a follower roller 42 on a lever 44 which has a pin-and-slot connection to a thread-clamping bar 47 slidably mounted under the throat plate 50<SP>1</SP>. Shaft 25 is fixed within a hollow shaft 21 which rotates in a bearing 24 and journals gear 20. A sleeve 57, rotatable and slidable on shaft 25, has a peripheral groove 64 engaged by a roller 63 on a lever 60 normally biased by a spring 65 to engage a projection 58 on sleeve 57 with a fixed notch 59. For coupling shaft 25 to gear 20 a solenoid 71 is energized, whereas the plunger 70 actuates lever 60 for engaging diametrically-positioned projections 56 on sleeve 57 with corresponding notches 54, 55 formed in gear 20 and shaft 21 respectively. Solenoid 71 is energized, at the end of a seam, by depressing a button 76 to shift a blade 75 into engagement with a contact 77, blade 75 then being held by a member 78 on a latch bar 79. During the first quarter revolution of cam 41, roller 42 engages a cam position 41.1 to position the wedge-shaped end 51 of bar 47 adjacent the needle hole 50<SP>11</SP>. In the course of the corresponding stitch-forming cycle, projection 35 enters the thread triangle extending from hole 50<SP>11</SP> to the looptaker. The needle thread extending to the work and the bobbin thread are then lodged in the notch 36 to be drawn over the blade 40 and severed thereat by the blade 39. During part of the next quarter revolution, roller 42 engages cam portion 41. 2 to shift wedge 51 to the right of hole 50<SP>11</SP> ,  whereby the needle on its next descent passes through a notch 52 in bar 47. After the needle has ascended, roller 42 engages cam portion 41. 4, whereby bar 47 moves leftwards for clamping the free end of the needle thread between wedge 51 and a leaf spring 50 fixed under the throat plate 50<SP>1</SP>. During the next two needle penetrations, roller 42 remains on cam portion 41. 4 to hold the needle thread clamped while the first two stitches of the new seam are being inserted. When roller 42 re-engages cam portion 41.1 for releasing the thread end, latch bar 79 is engaged by a lobe 81 on a cam 80 rigid with or driven synchronously with cam 41 for releasing blade 75 to de-energize solenoid 71. If it is desired to insert four stitches at the beginning of a new seam while the thread end remains clamped, cam 41 is arranged to rotate once during six stitch-forming cycles, and gear 20 is formed with three equally-spaced notches 54 for co-operation with three corresponding coupling projections 56. Cam 80 may be provided with a second lobe for deenergizing solenoid 71 after the thread has been clamped, button 76 then being re-depressed to initiate formation of a new seam.