Surface grinder for ball of ball valve

A surface grinder, which serves to grind a ball of a ball valve and comprises:

• a single horizontal axis;
• a single vertical axis;
• a ball positioning device, assembled along said horizontal axis of the grinder for positioning and rotating the ball;
• and
• a grindstone set, assembled along said vertical axis of the grinder and having a plurality of grindstones arranged in an circular pattern, wherein each said grindstone has a concave grinding surface and all the grinding surfaces jointly define a semi-spherical concave grinding space, in which said grindstone set is rotated by a power source so as to use the grinding surfaces to grind a spherical surface of the ball wherein, the ball positioning device comprises a power source and a ball positioning shaft, in which the power source rotates the ball positioning shaft and the ball is mounted around an exterior of the ball positioning shaft at an axial flow channel of the ball and wherein, the ball positioning shaft comprises an outer bush and an inner bush, in which the outer bush is fixed to a driving shaft of the power source and has a claw portion constricts toward an axis of the outer bush while the ball is mounted around an exterior of the claw portion at the axial flow channel of the ball and the inner bush has a taper shaft settled in the claw portion so as to expand the claw portion outward in order to close fit the axial flow channel.