This is a simple robotic end effector modeled after the Aluminum Sparkfun Robot Claw. The effector uses a parallogram design so the effector grips stay parallel at all times. The design has an impressive opening range. I have included the Inventor file so you can make changes; just show one "body" at a time and export so you get individual STL files.
I just needed something simple to hold and drop things on command. If you wanted to use this to pickup objects, then the effector tip (Clamp) should be made thicker or at least into a cross; I just dipped mine in rubber cement and that was good enough.
The servo is a Tower Pro SG90; you can pick these up at Amazon or E-Bay. They are very light and small, but they have a lot of jitter - for a more serious project, I would change the opening to use an E-flite S60 or for more strength something larger. In order to combat the jitter, I just turn off the servo after a move is completed, but that won't work if you're lifting something and need to continually grip it.
Initially, I tried to get away with struts just on the front/top; however, the gripper didn't align very well on closing; adding struts on the back made the design much more stable.
All the bolts are 0.5 inch #4-40; I used a bit of red thread lock on the nuts to keep them from loosening during use. Between the various layers, I added teflon/PTFE washers (B006GHXLWA) available on Amazon.
Front View: https://youtu.be/qHRINylgXGA
Back View: https://youtu.be/muxDoD2NcbI
Printed in PLA with CuraEngine; no supports required.
Print one Gripper_Base.stl, two Gripper_Clamp.stl (Left & Right), one Gripper_LGear.stl (Left), one Gripper_RGear.stl (Right), four Gripper_Strut.stl (Front & Back, Left & Right), and one Gripper_SupportStrut.stl (Back under Right gear).
I trimmed a dual sided servo horn (the mounting holes line up for the second holes, so trim through the third or fourth holes). Clean the Left Gear hole until it fits snugly. The servo is a Tower Pro SG90.
Mount the servo through the Back, screw it down. Move the servo to closed position and mount the Left Gear straight up and down (closed position); screw down the gear/servo horn to the servo.
Bolts are round head 1/2 inch #4-40 and washers are PTFE (B006GHXLWA). You need 7 bolts & nuts and 29 washers.
Start with a bolt, add a washer, then add the Right gear and another washer; thread through the base. Make sure the gears mesh properly. On the back add another washer, the long support strut and another washer; secure with a nut (not too tightly but no wobble either).
Attach the clamps to the gear arms. For the Left gear, I used a washer on top, then the gear arm, then three washers (for alignment), then the clamp, then a washer and bolted them down snugly. For the Right gear, I used a washer, then the gear arm, another washer, then the clamp arm, then a washer, then the gear support strut, then a washer and bolted it snugly.
Finally, add the struts between the base and the clamp. Washer, strut, washer, then Base/Clamp, washer, strut, washer and bolt snugly.
Be sure to test that the servo can move the gripper; if not, you may have to loosen some of the nuts. Finally, apply some thread lock - otherwise the nuts will loosen quite quickly as you use the gripper.
I dipped my effector tip in rubber cement.