Knife pen for program-controlled plotters

A program controlled plotter for cutting material, said plotter comprising:

• a base on which said material is mounted so that said material is movable in a first plane;
• carriage means for holding plotter pens, said carriage means being movable relative to said first plane;
• and
• a knife carryable in said carriage means, said knife having a blade removably mounted in a shaft, said shaft having an upper end and a lower end, said lower end of said shaft having a notch for receiving said blade, said lower end of said shaft having a threaded portion for receiving a nut, said nut retaining said blade in said notch, said upper end of said shaft having an opposing pair of flats disposed so that said nut may be loosed and tightened by use of two wrenches applied to said nut and said pair of flats so that said blade is removable, said shaft also having an upper groove disposed in said upper end and a lower groove disposed in said lower end, said knife also having an annular shell, said shell having a first land on an upper end and a second land on a lower end, said shell having an upper bearing mounted on said first land and a lower bearing mounted on said second land, for retaining and supporting said shaft in said shell by means of an upper snap ring in said upper groove of said shaft riding on said upper bearing, and a lower snap ring in said lower groove of said shaft riding on said lower bearing so that said shaft rotates in said shell, said shell having a flange compatible with said plotter pens for mounting in said carriage means, so that said knife cuts said material on said base by being moved by said carriage means in conjunction with motion of said material on said base under control of a program.