The mechanism is based on the four-bar link-gear linkage BCD for enveloping parabolas with the addition of a double guiding element consisting of two sliders, 4 and 5, connected together by turning pair P. Point P describes the space centrode of slider 2. Sliders 2 and 4 are connected by sliding pairs to cross- shaped slider 6 which has guides perpendicular to each other. When slider 1 moves along axis x-x of fixed guides N, point D of cross-shaped slider 6 describes a parabola and rule t-t, secured rigidly to slider 2, envelops the parabola. Point C is placed at the focus of the parabola. Parabolas of various sizes can be obtained by adjusting pivot C along slot E.
$1111$LG,Ge$