bottom part 1 the annular area 29 is therefore provided in an arrangement corresponding to the arrangements of the rod like elements provided with slots 34 as shown in fig3 7 and 8 the outside diameter of annular area 29 which is somewhat raised is selected so that it can be overlapped with the very slightest movement of cap 33 whereupon this provides additional safety against radial thrust as shown in fig7 disk 28 of the locking body is provided with pass through openings or slots 35 in an arrangement corresponding to the arrangement of pass through openings or slots 34 so that in that rotary position which corresponds to disk 28 being in locked position as shown in fig3 slots 34 are opened for passage of the rod like elements in that rotary position of the cover in which this cover can be removed from the work holder or must be placed thereon disk 28 on the other hand closes off slots 34 as shown in fig8 this means that in this rotary position the shaft of mixer 31 cannot be coupled with hub 12 of pulley 11 disk 28 also ensures that the cover of the work device cannot be removed from the work holder so long as the shaft of mixer 31 is coupled with hub 12 of the pulley 11 indeed disk 28 cannot be rotated as long as the rod shaped projections.