Making Graflex Super Graphic RF Cams

Donn Cave

I made a rangefinder cam this morning, for a 200mm lens on a Super Graphic. Not perfect, but ``good enough for government work'' as they say. I have an official cam for 135mm, so I just cut a copy out of 22ga. sheet metal, with the rangefinder bearing edge a bit higher and flatter to match the longer lens ... used a hacksaw, an ordinary file, and a sharpening stone to smooth the edges. (The bearing edge needs to be somewhat smooth, or the rangefinder pivot arm will hang up the cam.) I'll probably make another one as a copy for my 135mm official cam, in case I lose it.

If anyone is inspired to try this themselves (or already has) I hope you will pass along your observations. I figure there are probably better materials than I'm using - 22ga. sheet metal is somewhat thinner and softer than the official cam, which I guess is stainless steel. I started with the idea that I'd use some easily worked material for mockups, like plastic cards or wood tongue depressors or something, but I never did find anything that worked; it's probably easier to skip the mockup step anyway.

In theory, there ought to be a couple of ways to get the correct measurements without any experimentation. If you could accurately measure the position of the rangefinder apparatus, then you could just move the arm to convergence at various focus distances and record the results - the position of both the tube and the arm. Or, there's surely a way to express these dimensions as a mathematical function, maybe using precise measurements of an existing cam to derive the parameters. Neither of those ideas seemed practical in my case, though, so I just guessed. (I don't have very good measuring equipment, but if people who have decent calibrated calipers or something want to make measurements of their cams, please send them to Graflex.Org and for inclusion in this article.) Measure relative to the point of contact with the rangefinder tube, i.e. the left rear corner when in service, and specify which Graphic model.

If guessing at the dimensions, it makes sense to start large and whittle it down, but don't do that with the large end - the rangefinder arm travel appears to be limited past the infinity point, and if the large end is too large you'll run into that limit and possibly bend something out of alignment if you try to force it. I guess this kind of thing is the main worry, though I did make that mistake with no apparent ill effects (the original cam still works).

The Super Graphic cam is a little less than 1/16in thick, 11/16in wide and 1 1/2in long; it has holes and cutouts, but as far as I can see they're not significant.

The Top-Rangefinder model Pacemaker Speed and Crown Graphic cams are not interchangable with teh Super Graphic cams, but I'm told they are similar enough that the same procedure ought to work.

References