Gimbal rig for a model plane: three SG90 servos with 2:1 gearing

A three-axis gimbal that holds a model plane for photos and video: yaw, pitch and roll, each driven by an SG90 through a 2:1 gear pair. gimbal_assembled.obj is the whole thing as it goes together — base, yoke, pitch frame, cradle and the three servos with their gears.

The gearing is the point of this version. A z16 pinion on the servo drives a z32 wheel on the axis, so the torque doubles and the servo's full 180° becomes ±45° at the output. Every axis turns on two M3 screws in printed bushings, one on each side, so the servo only turns the gear and no longer carries the weight of the model. The gears clear each other over the whole travel — checked position by position in 9° steps.

The platform sits 33 mm above the roll axis, so the centre of rotation is right at the fuselage: a body up to 40 mm wide takes 45° of roll and 45° of pitch at the same time without touching anything.

Parts — the STL files are already laid out for printing, no supports:

• base.stl — 130 x 130 x 34 mm, base with the yaw servo pocket
• yaw_yoke.stl — 39 x 73.1 x 63 mm, yaw yoke, printed pad down
• yaw_wheel_z32.stl — 33.9 x 33.9 x 10 mm, yaw wheel with hub
• pitch_frame.stl — 38 x 27.5 x 54.3 mm, pitch frame, printed bottom down
• pitch_wheel_z32.stl — 32.2 x 28.4 x 13 mm, pitch wheel sector with hub
• pitch_axis_spacer.stl — 9 x 9 x 7.6 mm, spacer on the pitch axis
• roll_cradle.stl — 51.6 x 39.5 x 13 mm, roll cradle with the platform, printed on its side
• roll_wheel_z32.stl — 33.9 x 33.9 x 5 mm, roll wheel
• servo_pinion_z16.stl — 18 x 18 x 5.6 mm, servo pinion, print three

PLA or PETG, 0.16-0.2 mm layers, 3 perimeters, 30% infill; the gears at 50-100%. Pinions go on the bed with the horn pocket down.

Hardware. M3 screws cut their own thread in the plastic, so the holes are 2.6 mm: M3x30 for the yaw axis, 2 x M3x10 for the yaw wheel, M3x20 and M3x12 for the pitch axis, 2 x M3x16 for the pitch wheel, M3x25 and M3x10 for the roll axis, 2 x M3x8 for the roll wheel, plus the stock SG90 screws for the servo ears. Take a stock single-arm horn and cut it to 6 mm from the centre so it drops all the way into the gear pocket.

Centre every servo (1500 us) before its gear goes on, hold the output axis in the middle as well, and let the teeth mesh there. Do not overtighten the axes: everything should swing from a finger push.

In the firmware the output turns the other way and at half the speed:

axis  = -(servo - 90) / 2
servo = 90 - 2 * axis

Travel is ±45° on all three axes — limit it in code.

SG90 bodies wander a little between makers, so if the horn does not drop in, the pocket is a parameter in the generator script. STEP sources exist for every part — ask and I will post them.