A machine (4) for tying a length of wire (46) around one or more objects (2) comprising a wire feed mechanism adapted to feed wire (46) from a spool during a first phase; and to withdraw the wire (46) during a second phase, said wire feed mechanism comprising a gripping mechanism (102, 103) including a pair of rollers urged together to grip the wire (46) therebetween and drive it in the appropriate direction, said gripping mechanism (102, 103) being configured such that during said second phase, increasing tension in the wire (46) automatically increases the gripping force on the wire (46).