Cavalieri\'s ellipse drawing tool
Consider triangle RTV where VT=p is the right side of the ellipse to be drawn and VR=a is the major axis of the same ellipse. When frame SSBB slides along RT, groove SS remains perpendicular to VR and intersects the straight lines TR and VR in Q and H respectively. Groove QA is 45° slanted respect to SS, fixed respect to frame SSBB and it forces a pin in A to slide along groove VR. Right angle VPA is steered by the pin in A and groove SS allong which vertex P slides; at the same time side VP is forced to slide through V. When frame SSBB moves, point P describes therefore an ellipse.
Proof (ita) (pdf)