These four locks grip the leveling thumbscrews of the Printrbot LC to prevent undesired rotation as the build platform jiggles in operation.
I recommend a dab of hot glue on the heads of the leveling machine screws (under the Y carriage) to keep them from rotating.
All four locks are included in the STL file. Drop them down over the thumbscrews to lock them in place. They are easily removed for further adjustment.
These locks fit my Printrbot LC. OpenSCAD source provided so you can modify for machines of slightly different dimensions.
Note that front and rear locks have different dimensions. Left locks are mirror images of right locks. Front locks are positioned +X in the STL file, but will be installed -Y on your Printrbot.