SG90 Scotch-Yoke Flapping Wing Rig

This is a desktop animatronic mechanism. One SG90 micro servo turns a short crank with a 624 ball-bearing roller on the end. The roller runs in a horizontal slot in a vertical slider (a Scotch yoke), which turns the servo's 0-180° sweep into a straight 27 mm up-and-down stroke. Two drive links carry that stroke up to a pair of rockers that pivot on shoulder posts. Each rocker works as a lever and throws a bolt-on bat-wing frame through about ±37°. Both wings are driven by the same slider, so they always move together. It's for anyone making animatronic birds, bats or dragons who wants a plain, bolt-together flapping mechanism instead of a tangle of linkages.

Every pivot is a real bolt: M4 bolts at the shoulders and the roller, and M3 bolts with heat-set inserts everywhere else. The frame is a flat 5 mm spine plate with the servo pocket, the slide channel and both shoulder posts printed on it. It stands upright in a socketed display base. The servo's shaft height and horn size vary between brands, so the crank has a raised boss for the roller, and you set the roller's depth in the slot with M4 washers. The frame's slide channel also allows about ±1.5 mm of vertical error in where the servo shaft ends up.

Assembly order:

1. Press M3 heat-set inserts into the spine plate's rail tops (4), the slider tab (2) and each rocker (2 each). Press M4 inserts into both shoulder posts.
2. Push the SG90 into the pocket from the back, slide it to the top end and fix the tabs with the servo's own screws. Drill 1.5 mm pilot holes through the tab holes first.
3. Centre the servo at 90°. Seat the single-arm horn in the crank's back channel and screw or glue it, then press the horn onto the spline so the crank pin points along +X (horizontal).
4. Fit the M4 nut into the crank's pocket. Stack the 624 bearing and washers on the M4x20 bolt and thread it into the crank pin.
5. Drop the slider into the channel so the roller sits in the slot, then bolt on both retainer strips (M3x8). The slider should slide freely.
6. Bolt one link to each slider pin (M3x10 with threadlocker, snug but free to turn). Mount the rockers on the shoulder posts (M4x16 plus a washer), with their inner arms pointing inward. Join each link to its rocker with M3x16 and a nyloc nut, nut on the back.
7. Bolt the wings to the rockers (M3x8). Mount the second wing face-down so it's a mirror of the first. Drop the spine into the stand and bolt it with 2x M3x20.

Tuning: sweep the servo between 0° and 180°. It flaps about 1-1.5 times per second, limited by the SG90's speed. If the wings aren't level at 90°, move the horn one spline tooth. Shim the roller with washers until it runs in the middle of the slot's depth. Keep each wing under about 15 g. For a smaller, gentler flap, limit the sweep to 30-150°.