Abstract: A roller (10) drives a workpiece (not shown, but to the right) along a fence. An electric motor (16) rotates the roller (10) through chains (18, 19) and a double sprocket (20) freely rotatable on an upright shaft (22). The roller (10) is rotatable in relation to an arm (26). The arm (26) is pivotable in relation to the shaft (22). A coil spring (30) around the shaft (22) is fastened at a lower end to a stop (32) and at an upper end of a collar (34), and urges the roller (10) towards the fence. The force of the spring (30) makes the roller (10) engage a wide workpiece strongly, and so increases its tractive effect.