Walking has been described as a controlled state of continually falling down. "Cutting" glass is similar in that the goal is to do something undesirable -- in this case breaking glass -- in a controlled and predicable way.
There are two basic steps to cutting a sheet glass of glass: scoring and breaking. Each provides an opportunity improve your likelyhood of success.
- The diamond (or carbide) wheel should be perpendicular to the glass. The score is start of the break and the glass will tend to continue to break in the same direction. A perpendicular score increases the chance of a perpendicular (square) edge on the glass. The best way to ensure a perpendicular cut is to stand with the shoulder of your cutting hand directly behind where you intend to score so that you are pulling your hand straight back.
- An even depth score tends to break evenly. That means keep consistent pressure on cutter and keep the glass clean. You are also more likely to get an even score on the smoothest side of the glass.
- If you cut a lot of thick glass, buy a glass cutter intended for that purpose. Cutting wheels on thick glass cutters are angled differently and artists who use them usually swear by them.
- Don't be cheap when buying a circle cutter. The more expensive ones really do tend to be much better.
- If you believe cutting oil helps then use it. Lots of glass artists don't use it -- except on thick glass where most do use it. If you fuse, consider turpentine or minerals spirits instead of cutting oil. It is loads cheaper, works as well, and (unlike cutting oil) burns off your glass cleanly and won't cause devitrification.
- On thick glass learn to tap along the score with a chisel to make it "run". When you do it correctly you'll see the break start and spread down the score.
- Use your ears. An experienced glass cutter knows the score is a good one by the "zip" sound of the cutter. Recognizing that sound will help you quickly zoom in on the right pressure.
- Successful breaking is 80% confidence and 20% technique. The more afraid you are of screwing up the expensive piece of art glass the more likely you are going to screw it up. See if your local window glass shop will give you scrap to practice on and practice, practice, practice.
- Develop experience with different breaking techniques. Hand breaks, bench breaks and running plier breaks all have their uses. There's even a spit ball break -- where spit balls are placed on the back side of a score, the glass placed on a hard surface, spit ball down and pressure is applied (usually with thumbs) to both sides of the spit ball. This, of course, only works with transparent glasses.
- Always try to break the glass so that there are equal amounts of glass on both sides of the score. For example, if you have scored a sheet into strips first break it in half, then the halves into quarters and so on. Trimming the edge off of a piece of glass is often difficult because one side of the score has much more glass than the other. Consider trimming a wider piece. You may hate having a larger piece of scrap but it is still better than ruining the whole sheet.
Glass cutting techniques are like chili and religion. There are countless versions and everyone thinks theirs is the best. Develop what works for you and, if it works, don't let anyone tell you it is wrong.