in an end 43 of the lever 41 being lifted and by way of the pivot 23 lifting of the left hand ends of the grid bars 11 in fig4 fig4 contemplates the simultaneous adjustment of the two ends of the grid bars 11 by motors 51 on the left and right of the extraction arm 16 so that the depth of projection can be varied between t min and t max intermediate positions are also possible fig5 shows that different adjustments of the left hand and right hand ends of the grid bars 11 are possible by appropriate control of the associated motors 51 the angle of incidence of the grid bars 11 being variable and if required without simultaneous adjustment of the mean projection depth the possibility of changing the angle of incidence of the grid bars is particularly advantageous since this can increase the skid action of the grid bars as shown in fig6 it is not always absolutely necessary to adjust the two ends of the grid bars in order to change the projection depth instead the right hand end of the grid bar 11 in fig6 can simply be pivoted to the pivot spindle 24 it being unnecessary in this case to provide displaceability of the spindle 24 although the change of the angle of incidence of the grid which can be obtained by moving the.