Drafting instrument

A drafting instrument for forming circles comprising:

• a support member having an upper surface, a lower support surface and an inner edge forming a circular opening of given diameter;
• a bearing surface formed on said inner edge of said support member;
• a circular disk member of a diameter slightly less than said given diameter so as to be rotatably received on said bearing surface in a position vertically spaced from said support surface;
• an incremental circle guide slot having first and second edges formed in said disk member, said slot located to begin adjacent to a center point of said disk, to extend along a line parallel with a radii of said disk member and to terminate at a point adjacent to the circumference of said disk;
• said slot having a plurality of wedge-shaped notches of equal size and equal spacing and which include generally straight circumferentially extending surfaces facing said center point and forming an acute angle at their apexes with intersecting surfaces of said notches, said notches being arranged along at least one of said first and second edges;
• said first and second edges being spaced at a width sufficient to receive and positively locate a forming point in said apexes so that said point is disposed entirely within one of said notches and is contacted only by said circumferentially extending and intersecting surfaces;
• and
• whereby upon exertion of pressure in a direction outward from said center point, said disk is rotated and a circle of selected size is accurately formed.