a step 29 on the top cover thread spreader 24 as illustrated in fig2 the presser foot 13 has between the needles 18 and 19 an opening 31 through which emerges an air jet 32 which aligns the needle threads 21 and 22 which have been trimmed following sewing against the direction of sewing that is to say the direction in which the workpiece 15 is fed during a sewing operation as indicated by the arrow 33 fig3 is a partially sectional view of the trimming device compressed air is fed from the compressed air line 7 through a bore 34 in the lid 9 into a chamber 35 in the top 11 a control device enables the compressed air supply to be changed over so that the chamber 35 can be supplied with compressed air by way of the pressure line 8 and a bore 36 instead a vane 37 is connected to a drive shaft 38 which is mounted in the holder 5 a counterplate 39 is fastened to the end of the holder 5 facing the workpiece 15 the lower face of the counter plate 39 is aligned with respect to the lower side of the top cover thread spreader 24 in such a way that a thread catching device 41 can swing unimpeded beneath the top cover thread spreader 24 a knife 42 is secured by a screw 43 to the counter plate 39 and is kept at a distance 40 from the counter plate 39 by a spacer ring 44 fig4 shows the top 11 with the chamber 35 and the vane 37 whose range of swing is limited by stop edges 45 and 46 respectively three threaded bores 47 48 and 49 enable the lid to be fastened.