Restorations by Rick Kreuziger
Restorations by Rick Kreuziger
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Posted - 04/18/2004 :  17:23:16  Show Profile  Reply with Quote
Some interesting information about laminated & tempered glass from
Steve Parson...

Yes, a glass shop will cut laminated glass. It is very difficult to cut without cracking it. I worked in an auto glass factory for ~5 yrs and we had to cut laminated samples from production windshields (not "old" glass) every shift for quality tests for government regulations. The people who cut them did this every day, and had been doing it for years. They did not have a 100% success rate w/o cracking it- it was probably between 50-75%. That is what you should expect. If you bring a windshield to a glass shop, I would doubt very much that they will guarantee that they can cut it without cracking it- unless they use a water jet or another new method that did not exist 10 yrs ago. At the time I worked in auto glass the samples were cut with an old fashioned hand held glass cutter. A better method, like a water jet or laser, may exist now.

Even so, the only reason you should want to cut a windshield is because you are chopping a top. If you expect to make a Javelin windshield fit a hornet, good luck. In reality, this discussion is pointless because bent laminated glass is not universal- the geometry is different between models, it is not just the "size".

Tempered glass *will* explode when you cut it- it is supposed to by
design. Just try to drill a hole in it. BOOM! Or, pick up a phone book and call a reputable glass shop. They will tell you the truth which is that tempered glass cannot be cut. Yes, maybe you can sand .06" off the side, but you cannot cut it.

Tempered glass is created by rapid cooling after the glass is heated to about 1100F. The cooling induces stress into the glass. The center of the piece is in tension- it is like wild horses that want to run away. The edges, or perimeter, only a small portion- say 1" all the way around, of the piece is the compressive force, or fence, that holds it together. The compressive force is present on the edges because it is cooled more rapidly than the center- just like a pizza out of the oven. If you remove the fence, or even just open a gate, where do the horses go?

If glass is not properly tempered, the "fence" is weak, and the glass
can explode on it own over time without any action upon it. It can even happen to cars assembled and sitting in the shipping lot at the factory. I know this because....

I left out the most obvious thing about "cutting" tempered glass:
Glass is not "cut" *anywhere* during manufacturing, and that includes
when it emerges from the "float line" where it is formed from once granular sand (basically) floating on a molten bath of tin (hot!). Glass is *always* scored and snapped just like at the hardware
store and picture frame shop. Of course, in manufacturing, they use CNC cutters (but they still have a little diamond wheel on them just like you can buy in the hardware store) that are extremely accurate, but the glass is still scored and snapped to make it the correct size/shape. Have you *ever* seen a cracked piece of tempered glass? No. Why? Because it will explode! You don't cut glass, you score and snap it, or "crack it" along the score. No cracks exist in tempered glass means it can't be "cut". period

So, how do they cut it to shape and drill holes in tempered glass at the factory? They do all cutting and drilling before it is tempered. Once glass is tempered, that's it, no more changes to it.
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