This gripper design uses a novel worm gear approach, without requiring a hard-to-find (and expensive) worm gear. The worm is a length of 1/4" 20 bolt; the gears are standard 1"-diameter 64-pitch aluminum spur gears (hobby stores have these for about $1 apiece). Turning the bolt opens and closes the two fingers of the gripper.
Construct the gripper by cutting two 3" lengths of 1/2" × 1/2" × 1/16"-thick aluminum channel stock. Using a 3" flat mending “T” plate as a base, attach the fingers and gears to the “T” as shown below. The spacing of the holes is critical and depends entirely on the diameter of the gears you have. You may have to experiment with different spacing if you use another gear diameter. Be sure the fingers rotate freely on the base but that the play is not excessive. Too much play will cause the gear mechanism to bind or skip.
Secure the shaft using a 1-1/2" × 3/8" corner angle bracket. Mount it to the stem of the “T” using an 8/32 × 1" machine screw and nut. Add a #10 flat washer between the “T” and the bracket to increase the height of the screw shaft. Mount a 3-1/2"-long 1/4" 20 machine bolt through the bracket. Use double nuts or locking nuts to form a free-spinning shaft. Reduce the play as much as possible without locking the bolt to the bracket. Align the finger gears to the bolt so they open and close at the same angle.
To activate the fingers, attach a motor to the base of the bolt shaft. The prototype gripper uses a 1/2"-diameter 48-pitch spur gear and a matching 1"-diameter 48-pitch spur gear on the drive motor. Operate the motor in one direction and the fingers close. Operate the motor in the other direction and the fingers open. Apply small rubber feet pads to the inside ends of the grippers to facilitate grasping objects. The finished gripper is shown below.