How to Cut Glass
Authors: Pat Mackle; compiled by Irene D'Aloisio
Revised: August 03, 2005
First you need good quality tools. I recommend purchasing "Toyo" brand oil filled cutters. They are a workhorse in the industry. The refillable handle holds oil to lubricate the wheel and shaft. They come in two basic handle designs, regular and pistol grip (pistol grip is usually preferred by people with weaker wrist strength) They also come in two cutter wheel angles. One is sharper for thinner glass and one is less angled for thicker glass. This "hone angle" is very important as it sends the pressure deeper into the glass, or less according to the glass thickness. The less hone angle or flatter the wheel the deeper the pressure fissure will be resulting in an easier and accurate cut. To cut glass from 1/16" up to 1/4" use the appropriate degree hone wheel specified by the manufacturer or catalog. Glass thicker than 1/4" requires a lesser hone angle.
Of equal importance are the glass pliers you choose. Do not scrimp on the quality of your pliers. (This is about the time to toss those funky green flexible plastic running pliers that the neighbor down the street gave you because their attempt at stained glass nearly drove them to drink, mostly because of said pliers) That being said contact a good glazers supply house and ask for a catalog. I happen to use Sommer Maca with warehouses across the US. Their phone in Ca. is 1-800-338-7700. Also there is C .R. Laurence Co. and others. When buying pliers you should get a pair of running pliers and a set of drop jaw pliers, so called because one jaw is set lower than the other for leverage. Running pliers are used to run a straight or curved cut, drop jaws are used to aid the running pliers on long cuts and also for running circle cuts. We ARE talking STEEL PLIERS HERE, no more Pacific Rim plastics!
Apply a line of good cutting oil to the glass where you are going to cut. I use a Q-tip and run the oil along the metal straight edge that I guide the cutter against. You should add oil to the glass, the oil in the cutter handle is mostly there to lubricate the wheel and axel pin, cutting requires more. Not using any oil causes the glass to crunch and crack under the pressure of the wheel dissipating the pressure energy onto the surface instead of deep into the glass, resulting in the break running off into your glass.
NEVER double score your cut. It will damage your wheel. When cutting use a firm even pressure and LISTEN to the cut. It should make a sweet little sizzling sound, not a gritty-crunching-crushing sound that results in whatís called a hot cut. These usually look like a white crystalline chipped gouge. Too much pressure, not good! You will never run this cut! Instead, experiment to see just how a lesser amount of pressure will result in a nice clean cut. Actually a proper cut should barely disturb the surface.
We all know that glass breaks easily, all its strength is in its surface. Thatís why the glass cutter works so well, because it weakens one surface, and when we apply pressure from the opposite side it breaks along the weakened cut. Once you are skilled you can score and break glass up to 1/4" with your bare hands. Thicker glass requires pliers or a breaking tool. I just use a Sharpie Pen or wooden dowel on a flat table.
A piece of 1/4" plate glass is easily cut square with the proper tools and information. Be sure to hold your cutter square. Leaning the cutting wheel will angle the pressure sent into the glass resulting in a flared or sloped edge. Use pliers that are good condition. Make sure that you position your pliers exactly centered on the cut, if you are a little bit off you will get a sharp flare or wiggle in the first inch of your cut which is unsightly and will at best require wet sanding. Also, learn to use the least amount of pressure when you score the glass because the harder you score the deeper the serrated texture will be carried down the cut edge of your glass, this being the surface you intend to glue. A light pressure will leave the entire 1/4" face quite flat and polished looking resulting in a very nice glue joint, no polishing machines required
A word about tapping your cut before breaking it. I don't tap my glass, it usually results in star cracks. Tapping the glass is intended to "vent the cut" in other words run the pressure deeper into the glass. This can be seen as a deeper shining line in the glass. Instead of tapping try turning the scored side down and applying firm pressure directly over the cut with a wooden dowel. The best way to do this on a hard flat surface covered with a dark colored bath towel to allow some "give". As you press you will see the line open, stay behind the running vent line and press along until it ends. If the glass is still joined, carefully flip the glass and use the running pliers to complete the cut. If you get a bad score on one side of the glass and you don't think it will run good, carefully flip the glass over and rescore to line correctly over the other, then run your cut.
Once you are confident on straight and curved cuts, its time to go for those trickier circle cuts, a must for those looking to kiln work their glass. Hope this sheds a little light on glass cutting regular plate glass. There is still curved glass, bottle glass, Pyrex glass and glass tubing but have no fear, you can learn to cut them as well. About the only glass you will never cut is tempered glass, you could be a millionaire if you master that one!
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