wheels 37 are rotatably mounted within the blast cabinet 31 in this instance there are for such throwing wheels 37 two above and two below the traverse passage for a workpiece as will be described later each throwing wheel 37 is as disclosed in u s pat no 4 723 379 and generally is a one piece bi directional throwing wheel comprising a single side or backplate 38 with integral angularly spaced throwing blades 39 and a central vaned impellor 40 with integral centreplate 41 the centreplate 41 is bolted as indicated at 42 directly onto the output shaft 43 of an electric drive motor 44 the motor 44 is bolted as indicated at 45 onto an annular plate 46 welded to the outside of blast cabinet wall 33 a seal ring 47 to resist abrasive egress is located between the internal diameter of the annular plate 46 and a bush 48 extending through an aperture 49 in the wall 33 and keyed to the motor spindle 43 which bush 48 engages in a recessed seating formation 50 on the backside of the throwing wheel backplate 38 an annular flange 51 is welded onto the inside of the wall 33 around the aperture 49 and spatially surrounds the formation 50 on the throwing wheel backplate 38 to provide a labyrinth path serving also to resist egress of abrasive out of the aperture 49 with the ring seal.