the arc 22 against sliding the needle carrier is then held at a given angle of elevation fig2 b to 2g nevertheless show a preferred different solution in which the arc 22 is fixed with the needle carrier 15 sliding along the arc the arc 22 has a t shaped section with two flanges provided at their top ends with chamfers 100 and 101 wheels 102 and 103 fig2 d are mounted free to rotate relative to the needle carrier 15 and bear against these chamfered ends another wheel 104 of the needle carrier 15 is shaped like a pulley wheel with a peripheral groove that exerts reaction on the base of the t shape of the arc the base is likewise chamfered for adjustment purposes on assembly the axle of the wheel 104 is eccentric by adjusting the position of an eccentric the extent to which the needle carrier carriage clamps the arc can be adjusted to hold the carriage on the arc 22 the carriage which extends generally around the arc is provided with two screw knobs 105 and 106 enabling break pads 107 and 108 to be pressed against the flanks of the arc the knobs are screwed into the carriage fig2 f and 2g also show that the top of the t shape of the arc 22 is provided with a rack 109 a pinion 110 driven by a knob 111 likewise supported by the needle carrier 15 meshes with the rack and enables the needle carrier to be displaced this displacement may be motor driven a clamping screw 112 fig2 e driven by a knob 113 serves to clamp a collar 114.