Depends on the winsheild, curved glass most won't even try to do it themselves, not even the tried and true hardcore "fathers of the wheel". Curved glass goes to the glass shops where they can have the headache. Flat windsheilds have a few different methods from pinch and break to regular glass cutting and if you have the equipment a glass shaping table lathe like the folks who build artful stained glass windows. I have within the past few months have been introduced to the wet stone used to polish and shape thick peices of plexiglass, I havn't attempted trying it with real curved glass from a old junker, but plan to sometime in the future.
When I chopped the top on my 34 chevy P/U, I made a careful template out of heavy cardboard, made sure it fit the new opening exactly, and took my (flat) windshield and template to a local glass shop. They cut and polished the edge while I waited. Great job! Easy and FAST, no screw-ups! The price you ask???? $23.59!
Save the headache, call a few shops, and ask....
I have talked to a few people and they took thiers to a monument ingraver. They ingrave those head stones with some type of small sand blaster. They use that to cut the glass even curved tempered glass. Just a thought.
there are as many methods floating around as there are people doing them. the three major ways are cutting, grinding, and blasting. all three work and the major fault for loss is heat, glass expands with heat and any fracture or flaw (sand pit,rock chip,stress ect) will cause it to crack or run as we call it.
I've tried most methods over the years and my favorites I wouldn't reccomend for the home builder because it takes a feel and touch that is developed over year of screwing up LOL.
I have never tried sand blasting but I think it is probably tha easiest and maybe the safest. don't get in a hurry work it for a while then walk away and let it cool.
A local sandblaster said years ago he had a 90% success ratio and lately its dropped to below 10% and he won't even try anymore.and he's a pro!!
glass hasn't changed in centuries. but the manufacturing of windshields has in several ways
the main thing you have to look for is stess or more like pre-stress. but its not something you can see or feel. the way a windshield is made up involves taking two pieces of annealed or raw glass just like a window in your home and heating it to a tempature that allows it to bend over a form after it is precut to shape. the two pieces are over two forms that ideally allow one piece to fit inside the other. and then they are laminated together and give them the added strength and the lamination keeps things together for the safety factor. in real life the two pieces don't match exactly and so they bend slightly when laminated and that is the pre-stress. if that stress area is hit by a pebble going down the road the break sounds like a rifle crack. when you try to cut one it will do the same thing.
there are some brands of windshields that seem worse than others so if I try to cut one and lose it I try a different brand for the second try. 67-72 chev pick ups are very critical this way. I hope this sheds a little light and I'm sure others will ad there sure fire methods.
my theory is the older thicker windshields were stronger but the new thinner are easier to cut if not pre stressed
Most industrial places use a wet blasting technique that works everytime and doesn't heat the glass, I have had perfectly round holes blasted through 1/2" glass using this method and it works like a charm. Basically it's idiot proof, usually the places that make headstones for cemetaries have these machines.
BTW tempered glass cannot be cut by this method, the blasting media will just rebound.
We parted out a Caddy one time and I cut the roof off of it with a Sawzall...right across center of the windshield and all. The funny thing is I didn't care if the windshield broke or not, but I cut about the top 6" off and it never cracked at all. Now if I was trying to cut it for a project...craaaaaaack!
Oh well, that's why they call it a Saw'z All.