started since at that time electromagnetically actuable clutch 129 is engaged gear belt 113 is driven counterclockwise as viewed in fig6 the clasp carriage 104 is moved downwardly from its upper standstill position during this motion the beam emitted by reflex light barrier 67 senses the ply of material c depending on whether the treated piece is a part of women 39 s or girls 39 slacks or of large or small size the trailing edge of ply c will run out of the beam of reflex light barrier 67 sooner or later whereupon the light beam is reflected by the base surface of groove 19 in consequence brake motor 133 is switched off whereby clasp carriage 104 is stopped in this position which is an initial position for the subsequent engagement of clasps 40 41 as soon as clasp carriage 104 has reached the mentioned initial position the sewing operation is continued at a higher speed the adjusted vertical distance between reflex light barriers 63 64 on the one hand and reflex light barrier 67 on the other hand is so small that the trailing edge of the shorter ply d runs out of the beam of reflex light barrier 64 only a short time after the stopping of clasp carriage 104 whereupon the light beam is reflected by the base surface of groove 20 in response thereto device 66 actuates clasp 41 by de energizing electromagnet 47 due to the following release of compression spring 51 needles 50 are pushed through ply d backed by intermediate plate 18 into groove 20 whereby the trailing end of ply d becomes firmly.