Rule protractor

A drafting quadrant comprising first and second similar flat arms, each arm having inner and outer parallel straight edges, said first arm having a circular lower disc at an outer end and said second arm having a similar upper disc at an outer end, wherein said discs are concentric and have opposing adjacent inner faces and spaced parallel outer faces forming coplanar extensions of said arms, said discs being axially secured together with a pivot pin to form a pivotable joint, including a nut threadedly mounted on said pin, and in abutment with the upper disc for locking purposes wherein the outer face of the upper disc is graduated in degrees with the 0 degree marking in alignment with the inner edge of the first said arm, said discs being one half the thickness of each arm and wherein each said outer edge is tangent to each disc whereby the periphery of the discs form a continuous smooth arcuate transition between the said outer edges, each arm including an arcuate guide offset from the inner face of each disc, wherein said guides slidingly abut the periphery of the adjacent disc for guiding pruposes, in further combination with a longitudinal sliding notch in the first said arm contiguous with the periphery of the upper disc including a wedge slidably mounted in said notch having a bevelled upper surface, said upper disc having a frustroconical inner surface complementary and opposite to said bevelled surface whereby movement of said wedge radially towards said upper disc causes said surface to abut said frustroconical surface of said upper disc thereby jamming the discs frictionally against the wedge and the pivot pin further preventing relative rotation.