deenergized to stop rotation thereof then the motor 84 is energized for rotation of the screw shaft 86 for moving the carriage 44 in a direction away from the grinding wheel 174 thus the dressing roll 76 is moved to a position spaced from the grinding wheel 174 as illustrated in fig3 preferably mechanical and electrical means are employed for moving the grinding wheel 174 and or the dressing roll 76 out of operating position and into operating position when the dressing roll 76 is positioned in spaced relationship from the grinding wheel 174 as illustrated in fig3 and 5 grasping elements 320 are brought into grasping position with respect to the mandrel 170 which supports the grinding wheel 174 the grasping elements 320 may be portions of an automatically operable mechanism such as a robot or the like if the dressing roll 76 is to be removed grasping elements 330 are brought into grasping position with respect to the mandrel 70 which supports the dressing roll 76 as shown in fig3 and 5 fluid is then forced into the chamber 94 of the bearing housing 88 through the conduit 102 and through the passage 100 thus the piston 92 is forced to move in a direction away from the mandrel 170 thus the shaft 90 is moved axially from engagement with the mandrel 170 as illustrated in fig8 the sensor.