Claw
Single-servo gripper with a parallel-jaw linkage. The cheapest reliable manipulator on the field — covers servo geometry, hard stops, and compliant fingers.
Before you start
Pick the smallest servo that can produce the grip force you need. A goBILDA Dual-Mode servoFTC-legal smart servos like the goBILDA 2000-series or Axon let you read position via PWM and have enough torque (~3 N·m) for most game pieces. Cheap analog servos chatter and slip. is the usual choice.
1Sketch the closed and open jaw positions
On the top plane, draw both jaw positions overlaid. The pivot point and link length together set the servo swing angleMost hobby servos give you ~180° of usable travel, but you want to use only the middle ~120° — the ends are unreliable and at full travel the servo can't hold against external force. — keep it under 120°.
Lock in the maximum game-piece width plus 5–10 mm clearance for the open position.
2Build the four-bar linkage
Two equal-length links pivoted at each end keep the jaws parallel through their full travel. Sketch the bar geometry, then extrude one jaw and mirror for the other.
Identical jawsMake them mirror copies of the same part — never two slightly different parts. When one breaks you reprint the same file twice instead of figuring out which one was the left. save you reprinting time later.
3Mount the servo and horn
Place the servo with its mate connector. Use a servo horn that bolts to one of the jaws — the servo arc translates directly into jaw rotation.
Locate the servo so the link hits its endpoints just inside the servo's full range — that's how you build mechanical hard stops without overloading the servo.
4Add compliant fingertips
Print or mold a TPU finger pad on each jaw. Compliance fixes alignment errorsRigid jaws need millimeter-perfect alignment with the game piece. A 1-2mm TPU pad takes up that slop and keeps the grip force consistent across the piece's width. — even a sloppy approach grabs the piece cleanly.
If you need to grip a slippery piece, add a shallow V-groove in the pad to cradle it.
5Test the grip in the assembly
Drive the servo angle in Onshape's Play view. The jaws should sweep cleanly between open and closed without the links binding or the horn hitting anything.
If the linkage binds anywhere, walk the geometry back to the sketch and adjust the pivot positions — don't try to fix it downstream.
Your claw is a self-contained subassembly. Bolt it to an arm, a slide, or a turret and it just works — the linkage's geometry doesn't care what's above it.