This is a quite robust 3d-printable machine vise. It's 100% 3d-printed - no screw or other piece of hardware is needed to hold the individual pieces together. Pieces clip and snap together.

It's a demonstration that it is very important to consider forces and material properties already when designing a mechanical object. Hints about proper design are presented in my video:

https://youtu.be/mziT7KV-fRI

The video also includes explanations how to assemble this vise.

For 3d-printing it's recommended to print 3 shell layers. 35% infill is sufficient. All the forces for clamping a workpiece are only active at the outer surface - more infill doesn't make the vise more robust. Layer height is 0.15 mm (maybe screws are better printed with 0.1mm layers). No support material needed. Printed with 0.4 mm nozzle.

Assembly: https://jig.space/view?jig=1216 (thanks to http://www.thingiverse.com/greenlig)
Designed with Fusion 360: http://a360.co/2juhSUG

If there are difficulties to print the threaded rods vertically, I've designed a version of the sliding jaw with horizontally printed rods: http://www.thingiverse.com/thing:2123529