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How to cut a mirror?
Old 10-07-2018   #1
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How to cut a mirror?

Hi there, I'm attempting to replace the frameline mirror in my Bessa R3A. For those who aren't familiar, inside many rangefinders is a mirror that is angled at 45 degrees to the frameline image and viewfinder. This mirror reflects the framelines into the viewfinder. I have a couple of spare mirrors from broken cameras, but I can't seem to cut these to the right size. The problem is that it's only about a centimeter tall and 3 centimeters wide, so attempting to score and snap causes them to shatter. Moreover, the mirror has to have a hole in the center to allow the rangefinder patch into the viewfinder. Any tips? Perhaps a cheap rangefinder camera you know of has a similar type of setup?
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Old 10-07-2018   #2
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Sounds like quite a challenge. I sourced some front surface mirrors and beam splitter material a year or two back when it was on special to have on hand for rangefinder repairs, but am yet to use it. Not really looking forward to the challenges youíve outlined when I eventually have to size it.

Given the issues youíve outlined I wonder if gently clamping the piece you want to preserve to support it might assist? What Iím thinking of is Eg sandwiching the piece you want between a couple of small softwood blocks with good flat and smooth surfaces and sharp edges. Youíd need to align them very carefully and accurately along the inside edge of your score line top and bottom, and probably protect the glass surfaces from direct contact with the wood (perhaps a couple of layers of lens tissue top and bottom?). Obviously you would have to use the minimum clamping pressure you can get away with because unless it was applied perfectly evenly across the entire mirror surface, I could envisage clamping too firmly might easily break it. But done carefully it could provide sufficient support top and bottom to prevent the piece failing as you take off the excess material?

Regards the hole needed. On the one hand pre-drilling the hole needed might weaken the piece being clamped and sized. On the other, given the small dimensions of the finished mirror, it might be easier than sizing it first and then having to drill the hole. I suspect youíll want to start off with a pin vice or at least a hand drill rather than an electric drill, because youíll have to go quite slowly and gently. All up itís a tricky business. Let us know how you get on.
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Old 10-07-2018   #3
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Cosina should still have spares since the Bessa R3a was only discontinued in October 2015.

Cosina's website kind of sucks and doesn't look like it has any info on ordering parts.
Ugly Cameras
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Old 10-07-2018   #4
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If the mirror you have is close to the right side try a diamond hone and sand the mirror down to shape. I did this to make an internal IR dust cover filter for a sd Quattro and it works well.

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Old 10-07-2018   #5
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AS to the hole, wouldn't it be sufficient to polish the mirror coating off? No need for a hole in the glass.
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Old 10-07-2018   #6
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Originally Posted by retinax View Post
AS to the hole, wouldn't it be sufficient to polish the mirror coating off? No need for a hole in the glass.
No, it will flare like crazy if there is glass in the way, particularly if it is uncoated. The hole is necessary. It will be easier and cheaper to try to get a new Cosina made mirror. Did the old one decoat/desilver or break?

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Old 10-07-2018   #7
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Depending on what equipment (tools) you have (and how precise you need to be), you can probably cut it with a diamond cutoff wheel for a Dremel. Similarly, diamond burr could make the hole. Maybe a carbide one would also. You can get the diamond wheel at Harbor Freight for a fraction of the cost of the 'official' Dremel version, and they also have sets of carbide bits there. They might have diamond bits also, but to be honest I haven't looked for them. You need to set this up, I don't think you could get a good job handholding a Dremel for this, and handholding might not even work.

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Old 10-08-2018   #8
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I appreciate all the replies! I think the best course if action is to buy broken rangefinders ans salvage their mirrors. Unfortunately, I can score and cur thw mirrors to size, but drilling the hole just causes it to shatter no matter how careful. Even using diamond bits made FOR glass causes it to shatter.
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Old 10-08-2018   #9
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Have you tried drilling first then cutting? The larger sized glass should be a little stronger. Maybe tape both sides too as well as having a very flat backing plate?

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Old 10-08-2018   #10
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This all depends upon the question of WHY.
If you are set on drilling the hole yourself, good luck.
This operation is hard enough to do correctly without scratching the half-silver coating and getting to size correctly. I've done it right twice and has more failures due to the glass age, imperfections, tiny weaknesses, etc. I've never drilled a hole through a half-silvered mirror though, so that's awesome if you can get that done.
Aside from that, I'd also recommend just buying the correct part. If you take the cost of all your salvaged RFs, the tool cost and the cost of your time and effort, you are going to have a thousand dollar mirror by the time you're done successfully.
Unless the whole point of this exercise is to do every step yourself, then it's just down to being a craftsman who happens to be working on a rangefinder part. The latter course of action is admirable and you'll have a piece you can really be proud of when you're done but in the former case, this is not the way to save money or time.
Like building one's own large reflector telescope from hand grinding raw glass blanks, silvering the mirrors, precisely setting up the chassis and getting everything dialed in. Ultimately satisfying when you're done but if you just want to look at the stars, a good used Newtonian with a couple eyepieces cost pennies in comparison.

Phil Forrest
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Old 10-08-2018   #11
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I'm guessing here, but might it work to make up a 'sandwich' with an oversized mirror blank in the middle, so that the surface is protected, and then get it cut to size, and the hole formed, by waterjet?

The tolerances on waterjet can be a 0.1mm, but you could always grind the outside down to fit. The sandwich might be: perspex/rubber/mirror/rubber/perspex clamped at the edges.

I'm sure there'll be somebody on here, who knows what they're talking about, who can advise.
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