The Reaper's motion pattern was worked out with Legos. It is based on 2 pairs of meshing gears, each pair carrying a rod. The rods are fixed to the lower gears axially with pivoting bearings , and to the upper gears with axial sliders which are also free to pivot. The left and right gear / rod systems are phased 180 degrees, so as to produce opposite motions.
The design was fine-tuned and shop drawings produced using CAD software.
The gearmotor powers the lower axel by way of a set of miter gears. This motor arrangement gives symmetry and allows for future versions to have motor angle itself motorized by a hidden linear actuator.
Motor electrical power goes through the 2 legs, so no wires are visible.
Reaper has all ball bearing construction (axels, arm pivots, arm sliders) for long life and smooth operation.