Graflex.org

[Pork Rind]

I'm doing some top mount rangefinder recalibration work after replacing the beam splitting mirror with a fresher one. I've been able to successfully get the moving mirror adjustment made such that the rangefinder, the marks on the bed and the ground glass all agree. But there's a step where things go sideways...

Once I've got the movable mirror exactly where I want it, it's time to tighten the 3/16th inch set screw to lock the mirror shaft in place. So far, doing that invariable alters the mirror position such that my infinity target is no longer aligned. Rinse and repeat.

My best luck so far has been to offset the mirror position by the amount I'm guessing tightening the screw will move it, and am very close now but not perfect. Anyone have any tips that might help? Searching finds many discussions, but none that I've seen that have addressed this specific issue.

[William Hallett]

I haven't dealt with a top mount rangefinder before, but I can offer a general suggestion for anything with set screws. Tightening a set screw often leaves a burr on the shaft that makes it difficult to re-tighten the screw in a new position. Try removing the piece from the shaft to see if this is the case; if it is, gently remove the burr with a fine file and try again.
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WilliamH

[Pork Rind]

[William Hallett]

You would be better off to first see if there is a burr present, and if so target it rather than rubbing down the whole surface. If you don't have a file, you can support the emery paper on a thin wooden stick.
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WilliamH

[alt.kafka]

To set the mirror accurately, you'll want to minimize the amount both the amount the mirror rotates, and that the set screw turns when tightening. I found that the answer was to make sure there was a good amount of friction on the set screw already when I was adjusting the mirror, so neither turns freely. This made both the mirror adjustments finer, and also gave the set screw less room to affect the position. There was still a very slight amount of setting and guessing, but it was at a bare minimum.




IIRC that shaft is stainless. You should not need to turn the set screw tight enough to leave a burr. I guess you could try to buff it out somehow, or break the edge. It'll be an ad hoc fix, no matter what. If the screw is putting some leverage on a flat spot, IDK, maybe just a speck of epoxy. That could make things worse just as easily as it could make things better.

[Pork Rind]

Thanks. What eventually got me to good alignment was a lot of the above. Keeping a fair amount of friction from the set screw helped, plus reconsidering how much tightening force to apply. It's such a small screw it was easy to over tighten given the long driver I needed to use. Just tightening it a lot less on my last attempt was what did the trick.