SG90 Tendon-Driven Robot Hand

This is a desktop robot hand with four fingers and an opposable thumb, driven by five SG90 micro servos in a forearm deck. Each finger has two lines. The flexor tendon runs along the palm side and the extensor runs along the back. Both wrap in opposite directions on one twin-groove spool, so when the servo turns, one line pulls in and the other pays out the same length. The servo drives the finger closed and drives it open again, so no rubber bands are needed. It's aimed at hobby roboticists who want a hand that actually grips: it works for gesture demos, EMG/flex-sensor mirroring, or as the end of a larger arm project.

The fingers and thumb print lying on their side with print-in-place pin joints (5 mm pins in 6 mm bores), so each one comes off the bed already jointed. They attach to clevises on the palm with M3 bolts as knuckle pins. The palm bolts onto the front of the deck. A guide comb at the wrist sets the tendons to two fixed heights (flexors 17 mm, extensors 5 mm above the deck). From there they run over slotted ribs on top of the palm and through grooves in its underside to each knuckle. The thumb lines turn 90° through a curved guide into a side clevis, so the thumb curls up and across to meet the fingertips.

Assembly order:

1. Print the forearm deck (printed upside down). Drop the five SG90s into their pockets from the top, tabs on the deck, and fix them with M2 screws and nuts. Route the cables out through the rear notch.
2. Centre every servo (90°), fit the stock single-arm horns, then screw a tendon spool onto each horn with two M2 self-tappers through the base slots.
3. Bolt the wrist guide comb to the deck (I3), then bolt the palm onto the front of the deck (I4).
4. Pin each finger and the thumb into its palm clevis with an M3x20 bolt and nylock nut. Snug the nut, then back it off until the joint swings freely.
5. For each digit, cut about 60 cm of 0.5 mm line. Tie the middle at the fingertip tie window, run one end down the flexor bore and the other down the extensor bore, through the palm guides and the comb, and on to the spool.
6. Set the servo to 0° with the finger straight. Wind the flexor into the lower groove and the extensor into the upper groove in opposite directions. Thread each through its tie hole and knot it inside the spool bore with both lines just taut.

Tuning notes: the 16 mm spool drum gives about 25 mm of tendon travel over 180°. That curls a finger roughly 80° at each of its three joints. The thumb needs only about 130° of servo travel. If a finger goes slack at one end of its travel, re-tie that line a little tighter rather than over-tensioning both. Constant tug-of-war tension stalls the SG90s. Nothing stops a joint from bending backwards (hyperextension), so the extensor line alone sets how straight the finger goes. Power the servos from a separate 5 V 3 A supply, not the microcontroller's 5 V pin.