A rebar tie wire reel for a reinforcing bar binding machine, said rebar tie wire reel comprising: a first flange; a second flange spaced from said first flange; a hub connecting said first and second flanges; and a rebar tie wire wound around said hub, wherein said first flange, second flange and hub are a plastics moulding and said first flange has an outwardly facing major face and an outer periphery (170, 172, 174, 176), characterised in that: said outer periphery is defined by a first edge, a second edge spaced from said first edge, a third edge connecting said first edge to said second edge and a fourth edge connecting said first edge to said second edge and disposed opposite and spaced from said third edge, said major face is provided with a flat surface portion extending between said first and second edges which, in use, is engaged by a movable element of a contact sensor when said first edge, said major face and said second edge successively travel past said movable element, the arrangement being of said first and second edges (170, 172) and said flat surface portion being such that, in use, an amount of rotation of the tie wire reel can be determined using signals trigged by said movable element when during rotation of said rebar tie wire reel, said first edge, said major face and said second edge successively travel past said movable element to cause two said signals to be provided for each 360° of rotation of the rebar tie wire reel. a first flange; a second flange spaced from said first flange; a hub connecting said first and second flanges; and a rebar tie wire wound around said hub, wherein said first flange, second flange and hub are a plastics moulding and said first flange has an outwardly facing major face and an outer periphery (170, 172, 174, 176), characterised in that: said outer periphery is defined by a first edge, a second edge spaced from said first edge, a third edge connecting said first edge to said second edge and a fourth edge connecting said first edge to said second edge and disposed opposite and spaced from said third edge, said major face is provided with a flat surface portion extending between said first and second edges which, in use, is engaged by a movable element of a contact sensor when said first edge, said major face and said second edge successively travel past said movable element, the arrangement being of said first and second edges (170, 172) and said flat surface portion being such that, in use, an amount of rotation of the tie wire reel can be determined using signals trigged by said movable element when during rotation of said rebar tie wire reel, said first edge, said major face and said second edge successively travel past said movable element to cause two said signals to be provided for each 360° of rotation of the rebar tie wire reel. said outer periphery is defined by a first edge, a second edge spaced from said first edge, a third edge connecting said first edge to said second edge and a fourth edge connecting said first edge to said second edge and disposed opposite and spaced from said third edge, said major face is provided with a flat surface portion extending between said first and second edges which, in use, is engaged by a movable element of a contact sensor when said first edge, said major face and said second edge successively travel past said movable element, the arrangement being of said first and second edges (170, 172) and said flat surface portion being such that, in use, an amount of rotation of the tie wire reel can be determined using signals trigged by said movable element when during rotation of said rebar tie wire reel, said first edge, said major face and said second edge successively travel past said movable element to cause two said signals to be provided for each 360° of rotation of the rebar tie wire reel.