NEMA17 Rack-and-Pinion Parallel Gripper

This is a parallel-jaw gripper driven by a NEMA17 stepper, for a robot arm end-effector, a pick-and-place gantry or a bench test rig. A 24-tooth module 1.5 pinion on the motor shaft meshes with two racks, one above it and one below. When the shaft turns, the two jaws move toward or away from each other by the same amount. Each jaw carriage rides on two 8 mm rods through LM8UU linear bearings, so the fingers stay parallel and all side load goes into the rods instead of the motor. The finger faces go from 4 mm apart when closed to 56 mm when open. That is 26 mm of travel per jaw, or about 83° of pinion rotation. Because it uses a stepper, you can position the jaws precisely and hold a grip with the motor's holding torque. Every printed part is designed around verified hardware (NEMA17 31 mm pattern, LM8UU, 608-free 8 mm rod, M3 heat-set inserts, 2020 T-nuts).

The frame plate is the foundation. The motor bolts to its back face, and the shaft comes through the 22.4 mm pilot hole. Two identical rod end blocks bolt to the front face at the ends and clamp the rods with pinch screws. The two jaw carriages are identical: the right one is the left one turned 180° about the plate normal. Each carriage holds two LM8UU and has one drive rack bolted underneath and one V-groove finger bolted to its front face. The racks are identical too, one turned 180°, so each passes under the opposite carriage with 2 mm of clearance. An optional wrist bracket bolts to the back of the plate and puts the gripper on the end of a 2020 extrusion with two M5 T-nuts.

Assembly order:

1. Press 4 M3 heat-set inserts into the end blocks, 2 into each rack, 2 into each carriage front face and 4 into the wrist bracket.
2. Bolt the NEMA17 to the back of the frame plate with 4x M3x10, with the pilot boss sitting in the 22.4 mm hole.
3. Press 2 LM8UU into each carriage from the outer face until they seat against the inner lip.
4. Bolt one rack under each carriage with 2x M3x20 from the counterbored front face.
5. Slide both carriages onto the two 140 mm rods, left carriage rack up and right carriage rack down, then slip the end blocks onto the rod ends.
6. Bolt the end blocks to the plate from the back with 4x M3x10, then tighten the rod pinch screws.
7. Move both jaws to the fully open position against the end blocks. Slide the pinion onto the shaft so it meshes with both racks, and lock it with the M3 set screw on the shaft flat.
8. Bolt the fingers to the carriage fronts with 4x M3x12, then bolt on the wrist bracket and fit it to your 2020 extrusion.

Tuning notes: The pinion stack needs at least 18 mm of motor shaft sticking out past the faceplate. Measure your motor before printing. For homing without a switch, drive the jaws open at low current until the carriages stall against the end blocks, which act as hard stops, then set zero. Keep the driver current modest when gripping, because PLA teeth will strip before a NEMA17 stalls. The racks sit 0.15 mm further from the pinion than nominal to allow for FDM backlash. If the mesh binds, shim the end blocks out slightly. If it rattles, sand the rack backs.