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Thread: How do you cut delicate mirrors?

  1. #1
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    Feb 2007
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    Default How do you cut delicate mirrors?

    I just received some hot mirrors. They are about 30mm square, which is way too big for my needs. They are dichroic, maybe 2mm thick. I only need maybe 10mm square.

    Any suggestions on how to cut them? If I could cut them into quarters that would probably do fine.

    Also, I have many other ordinary front surface mirrors that are too big to use in my projects. Whenever I've tried to cut them, the results have not been pretty. These are mirrors salvaged from scanners, laser printers, etc.

    I tried scoring with a regular glass cutter, and then snapping along the score, but I couldn't get them to break cleanly, and it's very difficult to do while protecting the coating.

  2. #2
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    Thumbs up How to cut a mirror if you don't have a lapidary saw.

    I have sucessfully cut first-surface mirrors up to 3/8 of an inch thick with a dremel tool and one of the thin oxide cutting wheels. You need to set the dremel on SLOW speed and keep it moving across the glass. If you stay in one spot or have the dremel set too fast, you'll build up enough heat that the glass will fracture, and you'll have a mess.

    Watch carefully for signs that it's getting hot. If you see any sparks, or even a hint of an orange glow where the wheel is touching the glass, even if it's just for a split-second, YOU ARE OVERHEATING IT! Stop for several seconds to let the glass cool. Then start again while making sure you keep the disc moving across the surface. (Again, make sure the dremel is on low speed.) Don't push down hard on the tool either; slow and easy is the only way this works.

    If you go slow and move the cutting disc back and fourth, you'll eventually end up carving a groove into the glass. It might look black and charred; that's OK. When you get the groove deep enough (say, 1/5 the thickness of the glass or so), you can snap the mirror along the groove and it will break cleanly.

    In fact, if you're lucky, after you've been cutting for a while you might notice that the glass has already started to split along the groove. (Especially if you accidently got it too hot...) If that is the case you can stop cutting and just snap it the rest of the way.

    When I've done this the coating on the mirror was undamaged except for the area immediately around the groove. (We're talking about a band that is less than 1 mm wide where there is damage - on either side of the cut.)

    When you're done, be sure to wash the mirror carefully (don't wipe it! use running water instead) to remove any glass dust that might scratch the surface. Then you can clean the mirror and mount it any way you want.

    Since your mirrors are so thin, you will need to be extra careful with regard to heat. Go really slow! The dremel tool will do it, but it might take a few minutes. Be patient, and you'll be rewarded with a clean cut.

    Adam

  3. #3
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    Feb 2007
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    Does the groove need to be straight? Or rather.. how straight does the groove need to be?

  4. #4
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    Quote Originally Posted by Buffo View Post
    I have sucessfully cut first-surface mirrors up to 3/8 of an inch thick with a dremel tool and one of the thin oxide cutting wheels.
    Nice. I tried something like it once, with a small diamond dusted cutting wheel. Worked, but I never tried it on anything fine and exotic. I guess it could work ok. The sharp edges of a diamond dusted needle file might also work. Messy, and hard to control accurately unless you figure out some kind of frame to work it in, but one thing it can't do is get too hot.

  5. #5
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    Quote Originally Posted by pixpop View Post
    I just received some hot mirrors. They are about 30mm square, which is way too big for my needs. They are dichroic, maybe 2mm thick. I only need maybe 10mm square.

    Any suggestions on how to cut them? If I could cut them into quarters that would probably do fine.

    Also, I have many other ordinary front surface mirrors that are too big to use in my projects. Whenever I've tried to cut them, the results have not been pretty. These are mirrors salvaged from scanners, laser printers, etc.

    I tried scoring with a regular glass cutter, and then snapping along the score, but I couldn't get them to break cleanly, and it's very difficult to do while protecting the coating.
    Iv had luck with a glass cutter in the past. I would be concerned using a dremel as the particles sprayed up during cutting are abrasive and have the potential to scratch up the coatings..
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  6. #6
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    Then put a protective tape over the lens surface.
    CLICKY!!!

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  7. #7
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    I've heard that if you wet the glass before cutting it will make it easier to break.

  8. #8
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    Cool

    Dave has a good point about the abrasive dust created. It doesn't really "spray up", but it does get all over the surface of the mirror. I didn't think to cover the mirror surface with tape when I did it. Instead, I just washed the mirror with water after I was finished. The coating survived just fine. (I think the tape idea is the better way to go; good thinking Laser Ben!)

    As far as using water to cool/lubricate the cut... I don't know if that will work with the dremel tool cutting wheels. I guess it should, but I've never tried it. (Though often times glass and ceramic are cut with a wet saw, so it might work better...)

    I have tried using the metal cut-off wheels that have been impregnated with diamond dust, and they don't work nearly as well as the rust-colored oxide cutting wheels. For some reason the metal wheels get dull *really* fast. The oxide wheels, on the other hand, seem to work vey well. Just remember to go slow enough that you don't build up a lot of heat. This is hard for me, because usually I use those cutting wheels to cut sheet metal. Since heat doesn't matter when cutting metal, I normally have a good shower of sparks coming off the wheel when I'm using it, and the wheel itself is reddish-orange around the rim from the heat.

    As far as how straight the cut has to be - that depends... If you're talking a couple straight lines that intersect, that should be OK. But if you want to carve a complex curve, then you're going to want to cut the groove as deep as possible before you try to snap the mirror along the groove. Remember that glass is very brittle, and once the crack starts it will run very fast through the rest of the mirror. You want to be sure that the crack will follow the groove as it runs through the mirror.

    On a really thin mirror, you probably won't have much trouble making the crack follow a curved path. But I woulnd't want to try and carve a circle out of a 3/8 inch thick mirror (or thicker) unless I had the groove cut down through at least 1/3 to 1/2 the thickness of the mirror before I tried to snap it the rest of the way.

    Adam

  9. #9
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    I will be needing to cut a mirror "rod" that is 18" long and a 1/2" square, itr is dichroic and a don't want to damage the coatings. I need to cut it into 1" long pieces. Would the dremel thing work for something this thick? Also, how deep fo I need to go in order to break it off cleanly. This is a 1 or a kind mirror and irreplacable.
    CLICKY!!!

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  10. #10
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    I would recommend a professional glass shop, then. Do you have a picture of this thing? (I'm curious )

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