There is a post of a robotic gripper by a guy named haqbany. This guy has the parts posted as makerbot .thing files. when you open the .stl file contained in these files, it appears to be very large. I have taken these files and resized them so that you should be able to build this with standard 3mm screws. I have not yet built this myself, so I have not verified that things will fit together, but doing some measurements in cad, it looks like it should all fit together.
Here is a list of the part .stl files, what they are and the quantities needed for the build.
.stl file | Description | quantity to print
1_1.stl gripper finger 4
2_1.stl Finger arm 16
3_1.stl 4 way pivot 1
5_1.stl Finger pull arm 4
6_1.stl Finger arm knuckle 4
7_1.stl Upper wrist plate 1
8_1.stl Lower wrist knuckle plate 1
I think it would be best to use nyloc nuts on this with all screws and nuts being stainless steel.