DIY: Focusing mechanism help

[Curtis Nelson]

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?

Thanks,

Curtis

[David Vickery]

Hello Curtis,
The way that the Koronas and others work is that the knob on the outside of the shaft is attached to it and does not turn. The shaft turns when you turn that knob. The knob that is on the inside moves on that part of the threaded shaft and when you lock the focus it simply compresses the camera and base together.

[Ernest Purdum]

Do you know what a "collet" is? Picturee a bushing with the outside tapered, with several cuts extending about three quarters of the length of the bushing. If you run a nut with an internal taper onto the collet, it tightens. This would be a very elegant way of providing a shaft lock. Their usual purpose is holding metal-working cutters in place. They are common in sizes from 1/8" to huge.

If you wind up with a good arrangement, you can congratulate yourself. There have been lots of focus locking devices that don't work very well.

[Calamity Jane]

When I built my latest 4x5, I used a threaded rod as my focus mechanism because threaded rod is readily available and a WHOLE LOT cheaper than rack and pinion.

(http://www.geocities.com/diannebest/camera2/camera2.index.html)

My camera is a flat-bed design with thumbscrews that lock both the front and rear standard to the rails (by pinching the bottom "follower" against the bottom of the rail). The rear standard has a wooden nut underneath that follows along on the threaded rod and there is a wing nut back-centre on the bed to adjust the focus.

With the front standard set for "rough focus", I tighten the thumbscrews on the front standard and use the threaded rod for fine focus. If I wanted to, I could tighten the thumbscrews on the rear standard for more rigidity but I don't find it necessary.

The only down-sides of this arrangement are #1 - the rear standard can not come forward past the centre of the bed (because of the nut underneath), and #2 - I don't have a "quick release" on the threaded rod so every once in awhile I have to "wind the rear standard" back to the mid position.

I am very happy with this arrangement. Not only was it cheap and easy, NOT require any great deal of precission, but it works very well.

[Seamus]

Try looking here:

http://www.srv.net/~vail/camera.htm

and here (.pdf)

http://masterview.net/manuals.html

for some ideas. Also, lots o' links here:

http://home.online.no/~gjon/camerabuilders.htm

[Curtis Nelson]

Thanks for all the help. Seamus, the link to the masterview manuals was very informative.

Other suggestions are still welcome.

[jongrep]

Try looking here:

http://www.srv.net/~vail/camera.htm

and here (.pdf)

http://masterview.net/manuals.html

for some ideas. Also, lots o' links here:

http://home.online.no/~gjon/camerabuilders.htm

New URL: http://jongrepstad.com/

My new camera builders page wil be available there by the end of October.

Jon Grepstad