Step 5: Building the y-axis
The solution I came up with for the Y-axis was a rack and pinion system. The rack is essentially a toothed rectangular bar that is driven my a toothed circular "pinion" gear connected to a hobby servo motor. By picking the length of the rack and the number of teeth on the circular pinion gear I could now control the throw distance of the "finger". A rack and pinion system is another common way to convert circular motion to linear motion. In fact, it is used widely in the ShopBot system. See the attached pictures for the rack and pinion design. I'm fairly happy with the compactness and adjustability of this design when using different brand servos and different size pinion gears.