seconds through appropriate passages in the hub of the compartmented wheel the spray disk 27 may of course also be mounted on flange 37 so that neither the electromagnetic transducer 14 nor the spray disk 27 has to be dismantled when changing the grinding wheel the transducer is then fitted permanently in the machine fig1 shows a variant whereby the electromagnetic transducer 14 with spray valve is arranged in relation to the rotating spray disk 27 so that the axis of the valve motion is parallel to the axis of rotation of the rotating body 4 and the spray jet lies in the axis of the valve motion in the example shown the electromagnetic transducer 14 with the spray valve 25 would have to be swung away in order to change the grinding wheel 4 which is not always practical it may therefore be of benefit to arrange the injection tangentially radially or in some intermediate position instead of frontally to the axis of rotation of the spray disk 27 instead of a compartmented wheel with sectorial chambers the compartments may assume any other shape they may for example be simply drilled holes up to speeds of about 5000 revolutions per minute the transducer and valve unit may employ a commercial solenoid valve especially where water grinding fluid or grinding oils are used as the spray medium at higher speeds electromagnetic transducers 14 must be used with.