I thought i might post this and get feed back as well, here is a quick and messy way i use to cut a Dichro. I started with a damaged one that reflects UV and passes visible. The Dichro i used for this also had damage to the coatings aside from a hunk broken out of it, a spider like pattern that i think was caused by fungus growing on it while it was still in a TV light engine (at lest i think that's what i got it from).

Step one. I used a flat aluminum plate i had laying around that i did not care if i cut lines in to it. I then put a few layers of packing tape over it so i don't scratch the dichro, then i taped both sides of the dichro with Kapton tape, i have found that it does not leave goo behind and contains larger chips of glass. and protect the surface on both sides.
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Second. I taped the dichro to the aluminum and made a well with clay to form a pool when filled with water. Then use a Diamond cutting wheel for a Dremel tool, mark out lines, add water and carefully cut along the line, i usually make a light scoring first just in case the glass cracks, if the glass cracks then it would form under the scored line, then make another pass or two cutting though the glass all the way and adding water along the way, running water would be ideal.
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I ended up cutting it into 4 sections counting the little unusable tip i cut off from the larger part. I did not do this at high RPM, I set the speed that had the least vibration when cutting and this what i ended up with after removing the tape
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I have yet to shatter a dichro or mirror using this technique. I hope this was helpful and diffidently curious to how you guys do this as well