Cylindrical grinder is a type of grinding machine which is used to shape the outer side of an object. There are some end units that allows the objects or a work piece to get rotated. Cylindrical grinding machine(Wikipedia) are used to work on the various varieties of shapes. Central axis of rotation must be present in the object.
There are various shapes included as an ellipse, a crankshaft, a cylinder and a cam. In O.D manual grinding machines, the grinding wheel is rotated in the same direction of object and then it can come in contact of the object.
Two surfaces will move in the opposite direction when contact is made in the wheel and object. This will minimize the chance of Jam up and also allows the smoother operation.