This article applies to the Speed or Crown Graphic with a top-mounted rangefinder. When you change lenses, you can quite easily remove the rangefinder cam and replace it with a cam for the new lens.
It is not difficult to make a cam for another lens. (Focusing with a telephoto lens/cam combination is not as fast as with the normal lens- you need to be more careful that the two rangefinder images are exactly together.)
If you've never taken the cam out, look carefully at the camera and the accompanying diagram before acting. It's one of those things that's easy if you know how, but difficult if you don't. It's nothing like a Linhof rangefinder cam -- the Graphic cam is much smaller and in a different place on the camera.
Find the spring-loaded cover under the rangefinder housing it's between the bellows and the underside of the rangefinder housing. Hold it open. The camera will need to be partially upside-down. Rack the focus back and forth and watch the cam moving right and left in a slot. (It's a piece of 0.8mm thick steel about 25mm long by 10mm wide and, no doubt, easy to lose.) A follower slides across the surface of the cam, which is the long side nearest you (that is, nearest the front of the camera).
Diagram 1 shows roughly what you'll see under the spring-loaded cover. The arrows show the direction that the components will move as you focus back towards infinity.
To remove the cam, rack the focus forwards, past the limit of rangefinder focusing, which will move the cam to the left. Press the cam against the spring then slide the cam out towards you.
The factory cam will have the lens focal length stamped on the under side. Note that the two ends are angled in slightly and there are two 'feet' on the side opposite the surface. The high (or left-hand) end of the cam's surface is the position of the follower when the camera is focused on infinity, and the low end is the close-focusing position.
To replace the cam, first check that the shaft at the end of the plunger tube hasn't moved close to the spring shaft. If it has, tilt the camera to the left to allow the plunger (which is filled with ball-bearings) to move away, or push the two shafts apart. Then slide the cam between them and into the slot. It will be held in place by the spring.
A cam for a longer focal length lens will have a profile with less of a slope than the one for the normal lens. It will look something like Diagram 2.
I used 0.8mm thick brass sheet to make mine. It can be cut with tin snips (just!). If the sheet metal that you use is too thick, the cam will wedge in the slot.
If you have some aluminium sheet as well, you could make a prototype to get the dimensions approximately right before starting with the brass, which is slower to work with. I suspect that aluminium would be too soft for the permanent cam.
You can get an idea of the profile of the new cam by looking at the existing cam. The distance along the surface is proportional to the distance that the lens panel is racked out, and the height of the cam affects the movement of the rangefinder mirror.
Although it would be possible to calculate what the profile should look like, I think it is just as easy to work out the correct profile by trial and error.
Start by cutting the new cam to the same length as the old, with the same height (or a little more) at the infinity end and with a very shallow slope. Use a file and fine emery paper on the surface of the cam to make it smooth so the follower won't 'grab' when you're testing the rangefinder.
When you fit a cam for a telephoto lens to the camera you will probably need to pull the follower towards you to get the right hand end of the cam into position.
To test the high end of the cam, fit the lens and the cam to the camera and, using a loupe on the ground glass, carefully focus on an object that's at a considerable distance. Now check the rangefinder images. If you need to rack the lens out (that is, focus closer) to get the images to coincide then you need to remove more metal from the high end of the cam. If you need to rack back then you've taken off too much metal. (Either start with a new piece of metal or, if it's only slightly too small, punch the left-hand 'foot' of the cam with a hammer and screwdriver to expand it slightly.) While you're checking the high end of the cam, check that the low end of the cam is high enough, using the principle mentioned above.
Keep filing or emery papering until the infinity focus is correct. Be careful not to take off too much metal - 0.03 mm makes a noticeable difference. Accurate calipers can help with this process. You might find it easier to make the final adjustments by taking a small amount of metal off the 'foot'.
Next, repeat the process at the right-hand end for focusing up close. When the rangefinder is correct for close and for distant focusing, try the middle distance. The slope may need to be slightly concave. The cam for my 127mm lens is concave by about 0.5 mm but the cam for my 270mm isn't.
A cam for a wide-angle lens would, of course, have a steeper slope than the one for the normal lens.Brian Oakes