cutting mirror glass

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I can't say I'd even know how to cut regular glass, but I've got some mirror glass to cut to about 11x20". Its from a $11 cheesy wood framed ~16x24" new mirror. I measured it at .081" = ~2mm thickness. So there are inches on the sides to cut off. I've bearing-rabetted out 3/8" around the inside of a 1x2" frame I made. Theres about .2" left to the front face I'll paint. I'm gonna back it with 1/4" Birch plywood, and brad it in w/ 1/4" 1/4 round (.500"Diameter).
1) should I worry about the birch ply right up against the mirror paint? Its not pictures.
2) Should I score the back side where the mirror paint is. Or grind it, or get through to the glass somehow, whether straight or not. Theres ~3/8" to the max edge on each side, so I could make a mess of the finish, I think, without affecting the view from the outside.
3)I am assuming no matter what I am going to do I need to score on the glass side
4) When scoring it says to score with one line only. How hard do I need to push? I just bought a $5 "6 wheels" Richard cutter. Then push down (away) from the cut line on the corner of a table?
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Unless it's tempered, from the top (glass) side, make_one_ firm score along your desired cut with the aid of a straightedge. Then take the ball end of the cutter and, starting at one end, gently tap along the bottom (mirrored) side of the score. Take your time. You might still break your glass where you don't wish it to break, and them's the breaks. Tom
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Then take the

do I need a soft (say towel) to pressurize under while doing this?
do I need also to flip it back over to snap it off in one piece.
not sure i'm clear on this method, or part of a method
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<do I need a soft (say towel) to pressurize under while doing this?> No, let the cutoff hang over the bench, and tap from below. Oh, and wear safety glasses.
<do I need also to flip it back over to snap it off in one piece.> No. A good break will come off as one piece.
<not sure i'm clear on this method, or part of a method.> Then practice on some scrap, first. Be patient whilst tapping, increasing power carefully. Tom
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how can I be tapping along from one end and a good break will be one piece. Isn't a tap gonna produce a break, which will be either one piece, or a bunch of pieces. Break little pieces, but try to break one piece? Don't break?
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bent wrote:

That's why you shouldn't tap. You want to snap the throw away part in one piece.
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You may want to consider going to a stain glass shop and see if they will cut it. Otherwise with only 3/8"waste you might want to consider grozzing pliers. Used in stain glass. they are about 1: wide and there is a slight angle in the bill. You would put the center of the pliers on your score line and squeeze firmly. This should give you a running crack along the score. All this said and done the pliers about $15-16 . you spent $5 for a basic glass cutter. Mine are $25-30 . So now you have $20 or so into it. Maybe the glass shop will cut for less, Then again if they break it they will cut you a new one from their inventory. As for sealing the edge from the moisture and turning black, just put a good coat of fingernail polish on the edge and on the mirroring back . Aint nothing cheap about this but wanting to do this without the right tools is kind of harbor freight style. Good luck
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On 5 Mar 2006 19:45:28 -0800, with neither quill nor qualm, "tom"

Nearly every glasscutting book I've read has said that the ball end of the cutter will shatter more glass than it helps score, so don't use it. My experience prior to reading them (33% loss) mirrored theirs. (bad pun intentional.) I don't cut much glass, but I haven't lost a piece of glass since buying a set of running pliers.
I suggest setting the mirror on kraft paper a super clean piece of MDF or other flat table. Score it on the front, move that score line to let the scrap overhang the table, using either glass running pliers or a quick snap to break off the glass scrap. Some silver protectants are plasticized so if you also score the silver on the back, it has less of a tendency to pull off any silver at the cut edges when you break the scrap off.
Also remember: NEVER run the carbide cutter over a previously scored section of glass. It instantly dulls the edge and makes a rough cut which can shatter and run away from you. DAMHIKT. ;)
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writes:

I was a glass cutter in a stained glass studio in an earlier life. I cut glass all day long for a couple of years. This is what I would do:
- Dip the glass cutter's wheel in kerosene. - Holding the glass cutter almost vertical, use firm pressure and make one clean line the length of the cut against a straight edge. - Immediately, with thumbs on top and pointer fingers curled underneath, one hand on each side of the cut line, snap like a cracker. From -- to /\ . It's important that you do this within a few seconds after the cut is made. As I was taught, part of the weakening of the glass is due to heat from crushing the surface with the wheel. The longer you wait, the harder it is to get a clean break. If you have a good clean cut, the break will just run down the line. Tink... and you're done.
Also, practice doesn't hurt. Go to Lowe's, HD, wherever and ask for some scrap glass. A couple three cuts and you'll have it.
Good luck - Robert
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bent wrote:

You need to seal the cut edge of the mirror so moisture (read humidity) doesn't get under the silvering. Otherwise you'll get the black tarnishing working its way in from the edge.
The back of the rabbet will be reflected by the mirror, so its typical that the rabbet is painted black so the reflection won't be distracting.

I don't understand what you're asking. Why would you need to score the back side of the mirror? You always cut from the face. No need to touch the back.

Right.
You're just scratching the surface of the glass, so you don't need a lot of pressure. You can practice on a glass bottle to see how hard you have to press. The cutter should be lubricated with kerosene and it will sing if you're using constant pressure. Keep the movement uniform and unbroken.
Work on top of a towel so you won't scratch the mirror, and snap it on the edge of the table like you mentioned.
R
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How can I do this. Can I use a paint? or a tape? Polyurethane? Sounds important!

I will have a lot of white going on around . Does this work, or should I do black too?
The cutter should be lubricated with kerosene and

I don't have any kerosene. Is lube a must? What about anything else I have, say, methyl hydrate, lacquer thinner, mineral spirits, gas (w/ fill-up), TSP solution, brush cleaner, other?

do you suggest any cracking or prep before a final snap?
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can I just align the score op top of a 1x2" edge and PUSH down, to snap on one piece?
push down firmly against the score until it gives?
I don't want to learn breaking glass. I am looking for my best bet, or I'd take my Richard back. But I took apart the frame so...
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bent wrote:

Yes!
Yes. Use a glove.

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Maybe I'm wimpy, but I've wound up with so much broken glass, I take mine to the local glass experts. They don't mind even if I bring my own, and the cost is minimal ...and expertly done. They do the glass. I do the wood. Sometimes I buy the mirror/glass from them for projects. I definitely let them cut the ovals.
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Don't put the glass on a towel - if you push hard and the towel is soft enough, you'll break the glass. You might get away with it on thick glass, but the stuff you have is very thin.
Use a _flat_ surface and make sure it's clean - no junk under the glass.
When you score the glass, you create a stress concentration along the score. When you tap the glass with the little hammer, small cracks emanate from the score in various directions - mostly through the glass. When enough of these cracks propagate and connect, the glass breaks clean along the score line. When you look at the edge that results, you'll see the uneven surface that forms from all these little cracks. You can sand the edge with fine, hard sandpaper to smooth the edges.
Wear gloves and safety glasses.
Mike
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I'm no expert on cutting glass, so I take it to my local glass supplier and they cut it for me. They usually don't charge me cause they know I'll be back to buy eventually. Cheers, cc
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bent wrote:

Depends on how old the glass is as to how easy it is to cut. I don't like the cutting wheel types and have never had good luck with them. A diamond pointed scorer works much better.
You need to practice on a piece of throw away glass. The trick is to use medium pressure on the wheel and to make one continuous swipe using a good straight edge. When you break it, the score should be up, put the glass on a table with a piece on 1/2" plywood under the large part and the the score just past the free edge. Then use another piece of plywood and push down on the piece you are cutting off; you want it to just bend a little, and then it will snap. Tapping on the glass, especially using that little ball end of the cutter is likely to end of with a mess with a long straight cut.
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Interesting thread since I've been trying to get around to doing the same thing but on a thicker mirror (float glass). But even after reading the whole thread, there seems to be two conflicting paths to accomplish it, both of which I already knew, and neither of which has ever really worked for me. In fact, some of the posts sound like guesses based on having cut some plexiglas once, but ... I'm too ignorant on the subject to know if they're right. And, if "tapping" isn't right, why does nearly every glass cutter sold come with a ball shaped on the opposite end? Guess I'll go Google.
Pop

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Pop wrote:
<snip>

For cutting curves "tapping" with the ball end is appropriate. For a straight cut it is easier to snap the glass over a straight edge.
--
Jack Novak
Buffalo, NY - USA
  Click to see the full signature.
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On Mon, 06 Mar 2006 12:22:34 -0500, with neither quill nor qualm,

(Why do men have nipples, Pop? Why do ballots come with Reps and Dems listed on them? They're also quite inappropriate and useless, but inertia keeps things going. <sigh> Besides, it's expensive to change the dies.)

You've obviously never used running pliers, Yack. The curved tips work really great. http://www.glassworld.com.au/membersarea/Projects/Page2.htm Cheap source: <http://cgi.ebay.com/Stained-Glass-Supplies-8-Running-Pliers-has-Extra-Jaws_W0QQitemZ7396356555QQcategoryZ4770QQrdZ1QQcmdZViewItem
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