1. Field of the Invention
The present invention relates to a device for positioning an original in sheet form on the exposure table of a copying machine.
2. Description of the Prior Art
Many devices are known to those skilled in the art for positioning an original on the exposure table of a copying machine. Several use a pivotable ruler or registration member which is used to position the original. For example, IBM Technical Disclosure Bulletin, Vol. 17, No. 4 (September 1974) at p. 1131, and IBM Technical Disclosure Bulletin, Vol. 21, No. 6 (November 1978) at pp. 2222-23, disclose positioning devices in which a ruler of flexible material is disposed at an angle of about 45.degree. to an end edge of the exposure table of a copying machine. The ruler is slidable in its own plane from downwardly retracted position up to an upright position in which its top part projects above the table surface. In the retracted position, the original can pass; whereas, in the upright position, the original is retained by the top projection. Similarly, Research Disclosure, No. 19805 (October 1980) at pp. 407-08, shows a positioning device having a moveable registration membr which is retractable from the edge of the exposure table.
All of these devices suffer from the same disadvantage that documents, particularly if they are flexible, may slide between the exposure table and the ruler or registration member. If this happens, the document may be clamped in a bad position for copying or worse, damaged. This problem particularly occurs with thinner documents but can occur even with thicker ones. For example, the device of Research Disclosure, No. 19805 (October 1980) at pp. 407-08, has a broad gap between the registration member and the exposure table in which originals may be clamped and damaged.
Other devices have a ruler or registration member which is pivotable about an axis in their longitudinal direction. The ruler pivots, bringing it onto the exposure table, or pivots taking it away therefrom. As with the other prior art devices, there is the risk that the front edge of thin sheets may penetrate between the retaining edge of the ruler and the exposure table at one or more places. If this happens, the original will be in an undesirable position for copying and may be damaged.
One proposed solution, as described in U.S. Pat. No. 4,183,519, is to press a number of discrete fingers resiliently downwards onto the exposure table. These fingers operate as a ruler. There must be a fairly considerable spring force applied to the fingers to prevent the original from sliding underneath. In this case, however, the sharp bottom edge of the fingers scrapes over the exposure table and results in undesired wear, particularly of the bottom edge.
Research Disclosure, No. 18649 (October 1979) at pp. 587-90 (corresponding to U.S. Pat. No. 4,243,316), also discloses a device wherein the ruler pivots. Although the exposure table has a recessed part, the ruler in actuality bears against the transitional edge from the flat exposure table to the upright wall of the recess in order to form a clamping wedge for the documents. The recesses part accommodates with considerable play a reinforcing part of the ruler. The recessed part also serves to receive a moveable guide surface which can pivot in order to be brought into the path of returning documents when copying of the second side is required. The accuracy of the positioning of this device is generally open to criticism while, here again, the front edge of thin sheets may slide too far and end up between the transitional edge and the ruler.
U.S. Pat. No. 4,334,674 discloses a registration device having a magnetic registration member which is located adjacent to the forward edge of a platen and an electromagnet in a cover which is disposed parallel to the magnetic registration member. The end surface of the platen, against which the registration member is disposed, may be vertical or sloped. If the end surface is vertical, light may not reach the leading edge of the original preventing proper exposure. If the end surface is sloped, there exists the disadvantage mentioned above that the leading edge of an original, particularly a thin one, will be fed and clamped in the slit between the registration member and the sloped edge of the platen.