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?
On the two field cameras I own the shaft had a threaded end with a knob/nut on it opposite the focusing knob. Tightening this will squeeze the bed and lock down movement of the shaft. The alternate would be to have a threaded lock screw at right angles to the shaft to immobilize it or a cammed lever that will wedge the standard in place.
It's a tricky thing to incorporate the lock in the knob and spool (shaft), so consider another way. For example, you could put a threaded hole in the front/rear standard by the rail and have a nylon ball in the shaft backed up by a bolt with knob at the end (they sell the knobs w/bolts at ACE Hardware). A tiny bit of pressure on the shaft seats the nylon ball just enough. You should not need much pressure at all.