Abstract: A portable printer for printing on a roll of paper or label stock is provided having automatic print alignment with the width of the roll. The portable printer has a housing having a compartment for receiving the roll, a cover to access the roll, and a centering mechanism for the roll. The centering mechanism has two rotatable spindle members in the compartment engageable with the opposing ends of the roll's tubular core, and a pair of racks each coupled to one of the spindle members, and to each other by a gear, to enable each of the spindle members to move in opposite directions with respect to a center between the spindle members. The position of centering mechanism is optically, magnetically, or electro-mechanically encoded and a sensor reads the encoded position of the centering mechanism.