Glass cutting is a fascinating skill that a lot of people try, but quickly gives up. Why is that? My thought is that most people go and buy glass for a certain project, and expect to get e decent result after a few tries. That might work for a handfull, but I would strongly recommend to find a local glass shop and ask for some scrap glass for free or strongly discounted with the soul purpose to practice on. Glass workshops always get a lot of long skinny pieces that is good for nothing but splitting up to short pieces to make them fit in the recycle bin.
I will show you the basic steps of cutting glass, and hopefully give some helpful hints, but the truth is: The most important thing to learn glass cutting is practice, practice and more practice!
Step 1: What You Need
-Glass (obviously!) For practice I would recommend 3 or 4 mm thick glass., which is not hard to work with but also not to thin to break easily. 2mm is typical glass for picture frames, but can be a difficult thickness before you feel comfortable with how much pressure to put on the cutter. 5 mm and thicker gets harder to break, and can take some courage when it gets really thick. 8mm and up, you need to really put weight on the glass to break it open (I'm talking using your body mass to have enough oomph, and I'm 6'2" tall !, that's 190 cm for those outside the US ).
Glasscutter. The picture is showing three, but really just two different kinds. The wooden handled one is easy to find, has spare cutting wheels, but no oil compartment. The two others have oil compartments fitted with a valve at the cutting head that will open up as you push down the cutter to the glass and lets out a steady flow of oil as you cut. Most glassworkers would have both kinds at hand, but as you start I would recommend the wooden one because of price and a few uses the "fancy" ones does not have. (I'll get back to that)
I went online to look for different cutter, and discovered that it's not easy to find my kind of wood handle cutter now. ( I got mine a long time a go when I was still working at a glass shop.) Most the model have the ball head, used for hitting the glass to open up the score made in the glass, a method I try to avoid since in my mind the hands on method has a greater success rate.
Oil/lubricant. You do not want to do "dry cuts" on glass, which I will show in the next step, so you need some kind of lubricant. Glass shops normally has a thin clear oil specially made for cutting glass, but my experience is that they prefer to keep this for themselves. (mostly because it's inconvenient having several people coming by every day asking for a few "drops" for their puny projects) , You can find glass cutting oil on Amazon or other web stores. If you just need a few drops, and your wife has a sowing machine, you could beg her to give you some of the sowing machine oil. It's very similar to glass cutting oil, but gets a bit pricy if you want to do a lot of cutting. Since my wife has not been willing to share her sowing machine oil, and I am not able to purchase from Amazon ( I live in China), I've used WD-40. It works ok, but is not ideal. The bonus is that the long straw you can put on makes it easy to fill the oil compartment of the fancier cutters. You can also use vegetable oil or other oils found in the grocery shop, but they are most times too thick to put in the oil compartment cutters, and more messy and sticky.
Flat work surface. You want the glass to be totally flat, and have good support everywhere. Any bending and tension in the glass will result in the cut/score to run "off track" if you have not already broken the glass in the cutting process. The best surface should be firm and not too soft. you can work on a tile or smooth concrete surface, but it's easy to damage the glass scratching it and chipping it as you move it around on such a hard surface. The best is a wooden surface covered with felt or something like it. this gives the right firmness and good support, makes it easy to slide the glass around without scratching it, and is forgiving if you should be sloppy and bump the corner/edge of the glass into the work surface.
Measuring tape. Most glass jobs have little leeway, and you want to measure well before you cut. Unlike wood, you can't just sand/plane down the glass a tad (unless you happen to have a big glass polishing machine hidden behind the junk in your shop).
Straight/angle. If you want a straight cut, you need a straight! You might be steady on your hand and good at following lines, but if you want a true straight cut, get a straight that you can put your cutter up against. That means that the typical thin steal straight or angle will not work, since the wheel on the cutter will lift the glass cutters head too high up to lean on the straight. Ideally you want something between 1/8-1/4 inch thick.
There are many more things you can buy for working with glass, like circle cutter, suction cups, tongs etc. but we will not go there now, since it's not necessary for just plain cutting.
Step 2: Getting Ready for the Cut
Measure, measure and measure. A lot of jobs I've done are down to the millimeter, so there is no room for failure. your project probably has some more leeway, but it never hurts to be accurate. Now you come to the first factor where practice makes a difference. How much does your cutter differ from the size you want on the glass. My experience is that most cutters are 2 mm from the middle of the wheel to the edge (left or right). My wooden handle one is different from each side, but I never cut with the thick side since that's where the nut for changing cutting wheels is placed.
Next is the lubricant. If you have a cutter with the oil tank in the handle it's just to fill up and go. If you have a more basic cutter you got two ways of applying the lubricant. With a short cut, it should be enough to put some on the cutter, or dip the cutter in some of whatever oil you use. For longer cuts the best would be to put some straight on the glass the whole way where you want to score the glass. Have some oil of choice in a cup, and use a little brush to tip in the oil and brush onto the glass. You dont need much, but be sure to have it on along the whole line. Putting on a lot will just give you extra mess to clean up after you're done cutting.
I did some cuts with and without oil, to show how they look. The single cut is all without oil. On the picture with three lines, you see one without oil, one with oil done by the wooden handle cutter and the last with my self lubricating cutter. The perfect cut should be close to invisible, but I'm a bit rusty.
If you do a dry cut on a thick glass, pressing down hard as you score, there will actually be this crackling sound from the score after you have done it. It's hard to record the sound though, so you'll just have to take my word for it or try yourself.
Step 3: Cutting/scoring
This is it! you are now ready to cut/score the glass. I've always struggle to explain how hard to press the cutter on the glass, but I think I've found an OK way now. This do require a flatt digital scale preferably with a tare function (reset function), but can be done without the tare as long as you know your math.
I placed a practice piece of glass on the scale, and pushed the tare button to zero out the scale. (no tare function, just notice the weight on the glass and add on the desired pressure). The few cuts I did I ended up with a pressure between 90 and 110 OZ. Put the cutter on the glass and try to get a feel of how much pressure to use, as you cut, try to keep about the same pressure and speed the whole way.
Keep the cutter in a straight angle right/left and slightly tilted toward you.
Cut/score only once! If by any chance parts of the line was not scored, you can try to run over that part again., but ideally you should only need to do it all in one move.
Hopefully you now have a nice clean score in the glass and can move swiftly to the next step. I say swiftly, because the score will "harden up" again if you leave it for a couple of minutes.
Step 4: "Breaking" the Glass
Now, you'll find out if you did all the things above right. A nice clean score should make the breaking easy, but for those who have never done it before it can be a little scary thinking: How is that little scratch going to make the glass break open in a perfect straight line?
There are three ways of doing this.
1. "The Shocker", hitting the glass on the opposite side of the score from one end to the other of the glass. This is the last method I would choose, unless the glass is to thick or in a position where the two other methods are not possible. ( I do not have any pictures of this method now, but can add later if someone want)
The disadvantage with this method is that every time you beat the glass you risk that the cut will start traveling to the left or right away from the score, especially if you're not good at hitting right under the score.
2. "The casual"
!!!! Do not try this on any glass thicker than 5 mm if you don't have some experience with this method !!!!!!
Only by the help of your hands, break open the score. Place your two pointer fingers on the underside of the glass one of each side of the score. Put your two thumbs on the top of the glass, one on each side of the score, and just break it open like you would break a chocolate bar.
The danger with this is your hands are surrounding the edge of the glass, and are more exposed to cuts if your not carefull, or have a little mishap.
3. "The Safe" , the safest and most controllable way of braking the glass on most thicknesses of glass. This method was the one my teacher would always tell me to do during my classes unless it was not possible because of reasons mentioned earlier.
This is where the wooden handle cutter of mine come into great use. I place the end of the cutter centered straight under the beginning of the score. An alternative to the cutter can be any wooden or plastic piece 3-5mm / 1/8 inch thick, and 1 cm / 1/2 inch wide. Then place one palm on each side of the cutter, resting them flat and firm on the glass. Then, when ever you have built up enough guts, give the glass a good push straight down towards the surface and it should "pop" apart in the blink of an eye.
Step 5: Thats It! Now, Practice, Practice and Practice!
That's how you cut glass. I hope this has been helpful to you and you have not had to break out any bandaids. When I was working with glass, my goal was always to start the weekend without any on my hands, which was many times dependent on what kinds of jobs I had at the end of the work week. I'm including a little video of me doing a wavy cut, just to show it's not hard when you have done it for a while.
If there is enough response, I will put in my best effort to post more instructables on glass cutting and other glass related instructables.
Untill next time, go break some glass! And remember the whole thing with bad luck breaking a mirror is just superstition! I would know after breaking thousands of them and having a wonderful family and exciting job.