Abstract:
A plotting board includes a base; a top; a pivot that connects the top and the base, the top and the base being rotatable and detachable with respect to each other, the pivot including a magnet wherein at least a portion of the pivot is disposed at a substantially central point on the top.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 USC 119(e) of U.S. provisional patent application 60/595,569 filed on Jul. 15, 2005, which is hereby incorporated by reference. 

   STATEMENT OF GOVERNMENT INTEREST 
   The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
   BACKGROUND OF THE INVENTION 
   The invention relates in general to munitions and in particular to plotting boards used to plot firing missions for a munition. 
   Soldiers may use a plotting board to calculate and plot firing missions for, for example, a mortar weapon system. The method of using the plotting board is known. A known plotting board comprises a base with a vertical pin or axle in its center. A top disk has a central hole and fits over the vertical pin in the base. The top disk also has a retaining clip to keep the disk in place. When the disk is removed from the base, the disk often fractures. Furthermore, because a soldier may slide a ruler across the base when plotting a fire mission, the presence of the vertical pin is an obstacle to effective use of the ruler. Thus, there is a need for a plotting board that is easier to use and disassemble. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a plotting board having a surface substantially free of vertical projections. 
   It is another object of the invention to provide a plotting board having a top that is easily removed and reattached. 
   It is a further object of the invention to provide a plotting board wherein the top is held tightly to the bottom to minimize plotting error. 
   One aspect of the invention is an apparatus comprising a base; a top; a pivot that connects the top and the base, the top and the base being rotatable and detachable with respect to each other, the pivot including a magnet wherein at least a portion of the pivot is disposed at a substantially central point on the top. Preferably, the base and top comprise plates. The pivot includes male and female mating portions with the male portion disposed on one of the base and the top and the female portion disposed on the other of the base and the top. 
   The magnet may be disposed on the base or on the top. In one embodiment, the male and female mating portions comprise cylindrical shapes. In another embodiment, the male and female portions comprise frustro-conical shapes. Preferably, the top plate is substantially transparent and substantially circular. 
   The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
       FIG. 1  is a schematic side view of one embodiment of a plotting board in accordance with the invention. 
       FIG. 2  is a schematic side view of another embodiment of a plotting board in accordance with the invention. 
       FIG. 3A  is a schematic top view of a top of a plotting board. 
       FIG. 3B  is a schematic top view of a base of a plotting board. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In one embodiment, the apparatus of the invention is a plotting board for use in calculating and plotting fire missions. However, the apparatus may be used in other applications as well. 
     FIG. 1  is a schematic side view of one embodiment of a plotting board  10  in accordance with the invention. Board  10  includes a base  12 , a top  14  and a pivot  16  that connects the top  14  and the base  12 . Base  12  and top  14  are preferably plate-like members. In  FIG. 1 , the pivot  16  comprises male and female mating portions  18 ,  20 . The male portion  18  is shown on the top  14  and the female portion  20  is shown on the base  12 . However, the positions of the male and female portions may be reversed. The male portion  18  of the pivot  16  is disposed substantially at the center of the top  14 . The male portion  18  comprises a cylinder  22  and the female portion  20  comprises a cylindrical opening  24  for receiving the cylinder  22 . The top  14  is rotatable and detachable with respect to the base  12 . 
   The pivot  16  includes a magnet  26 . The magnet  26  may be disposed on the base  12 , as shown, or on the top  14 . The magnet  26  may be integral with or separate from the base  12 , top  14 , male portion  18  or female portion  20 . If the top  14  or male portion  18  includes the magnet  26 , then a part of the base  12  or female portion  20  comprises a magnetic material that is attracted to the magnet  26 . Similarly, if the base  12  or female portion  20  comprises the magnet  26 , then a part of the top  14  or male portion  18  comprises a magnetic material that is attracted to the magnet. In this context, “magnetic material” includes any material that is attracted to a magnet, such as ferrous material, and also includes a second magnet. If a second magnet is used, then the poles of the magnet  26  and the second magnet are disposed so that the top  14  is attracted to the base  12 . 
     FIG. 2  is a schematic side view of another embodiment of a plotting board  30  in accordance with the invention. Board  30  comprises a top  32 , a base  34  and a pivot  42  that connects the top  32  to the base  34 . Pivot  42  comprises male and female mating portions  38 ,  40 . The male portion  38  is shown on the base  34  and the female portion  40  is shown on the top  32 . However, the positions of the male and female portions may be reversed. The male portion  38  comprises a frustrum of a cylinder and the female portion  40  comprises a similarly shaped mating opening. A magnet  36  is shown in the top  32 . Shapes other than cylindrical and frusto-conical may be used for the male and female portion of the pivot. 
     FIG. 3A  is a top view of a top  14  of a plotting board  10 . For use as a plotting board, the top  14  is substantially transparent and preferably circular. The top  14  includes suitable scale markings  28  on its outer circumference.  FIG. 3B  is a top view of a base  12  of a plotting board  20 . Base  12  is preferably made of honeycomb aluminum. Base  12  includes a rectangular grid  44  on the portion of its surface that is covered by the top  14 . Base  12  may also include suitable scale markings  46  around all or a portion of the outer periphery of the top. 
   The magnetic pivot of the invention provides a means to easily attach and reattach the top to the base without fracturing the top. The magnetic pivot further provides a secure connection between the top and the base to minimize errors in calculations. In addition, the magnetic pivot does not protrude vertically above the top so that a ruler can slide smoothly over the plotting board. 
   While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.