One of my favourite Mindstorms models is the H25 robot arm that is part of the LEGO Mindstorms education set. (The project for controlling the robot arm is available here)
When I downloaded the plans for the H25 robot arm I had to learn that LEGO added some extra parts to the education set that are missing in the consumer set.
I changed the design a little and now you can build it with the parts in the 31313 set.
For the model in the image I used a yellow bar that istn´t part of the standard set. The building instructions solely contain parts from the official inventory list!
The building instruction can be found HERE.
The instructions were automatically generated by LEGO Digital Designer (LDD) and are far from being perfect. I suggest you download LDD and view the model there.
****** UPDATE *****
It seems my set came with an extra Z12 gear. Therefore the mode uses 3 Z12 gears. The set just contains 2… 🙁
****** UPDATE 2*****
Paul has shared his version of th erobot arm in the comments.
You can download this version here.