Abstract: There is disclosed apparatus for machining a workpiece, the apparatus including: a support; a boom mounted to the support; a machining station mounted to the boom; and displacement means provided on the boom for moving the boom relative to the support through engagement with a support surface. The support includes a boom pivot about which the boom is rotatable, the machining station being mounted on the boom remote from the boom pivot. The displacement means includes wheels or rollers having an axis of rotation extending along a radius from the boom pivot. The wheels or rollers have an at least partly tapered contact surface and have a diameter which increases along the axis of rotation away from the boom pivot.