I couldn't find drawings or dimensions for the little cog/drive on the Tower Pro SG90 servo, so I printed 8 different designs with different sized "teeth" to see which fit best... this is the result!
This basic "cap" serves as a template or building block for integrating in to whatever structure you want to move with your servo. It includes space for a recessed screw to fix it to the servo.
It has been printed via Shapeways highly accurate laser sintering, in order to get the detail in the 21 0.3mm teeth.
It fits snugly over the top of the drive, and the teeth mesh perfectly so it won't slip when rotating. My other model "Meccano SG90 Cap" is an example of this same design integrated in to a "disc" with meccano spaced holes: that was the design I printed for testing.