I need some advice of a mechanical nature. I'm in the design phase of building my first camera, and I'm confused about how the focus and lock-down knobs work on the focusing rail. I'm implementing a rack and pinion focusing mechanism, but I'm not sure how to add two focusing knobs to the ends of the shaft, while at the same time providing some way to 'lock' the shaft so it can't turn? Can anyone shed some light on this for me?