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A glass cutter is a tool used to make a shallow score in one surface of a piece of glass that is to be broken in two pieces. The scoring makes a split in the surface of the glass which encourages the glass to break along the score. Regular, annealed glass can be broken apart this way but not tempered glass, since it shatters rather than breaking cleanly into two pieces.
A glass cutter may use a diamond to create the split or more commonly a small cutting wheel is used made of hardened steel or tungsten carbide 4-6 mm in diameter, with its cutting edge ground to a V-shaped profile. Some glass cutters hold a small amount of cutting oil, which both lubricates the wheel and prevents the split in the glass from closing. When properly lubricated a steel wheel can give a long period of satisfactory service. However, tungsten carbide wheels have a significantly longer life than steel wheels and offer other advantages in use, such as greater and more reproducible penetration in cutting and consequently easier parting of the glass.
In the Middle Ages glass was cut with a tool which was nothing more than a sharply pointed rod of iron, heated to a high temperature. The red hot point was drawn along the moistened surface of the glass causing it to snap apart. The fracture was not very accurate and the rough piece had to be chipped or grozed down to the exact shape with the help of a hooked tool called a grozing iron. The present day Steel Wheel Cutter, which is almost universally used, was invented in 1869 by Samuel Monce in Bristol. Connecticut.
At the time the first steel wheel glass cutter was invented in 1869, glass manufacturers produced mostly plain flat glass which was used primarily for windows. Great progress has taken place over the years until now we have hard glass, soft glass, heat-resisting glass, thick glass, thin glass, decorative glass and countless other varieties.With these many variations it becomes necessary to change the penetration action of the wheel to compensate for the difference in the structural composition of the glass. This is accomplished by varying the degree of the bevel which produces a progressively sharper or duller "cut." A soft glass such as plate glass requires a dull cutting while the harder glass requires a sharper wheel. There is a reason for the difference in cutter wheel diameters. The great majority of wheels are 7/32" in diameter. The pressure of the wheel as it rolls over the glass has a bearing on the penetration, and there is a ratio between the pressure and the arc of the wheel that is important. With average hand pressure the 7/32" diameter wheel gives best results. For a duller wheel on soft glass a slightly different pressure is required. This is compensated for with a larger wheel, 1/4" in diameter making it unnecessary for the operator to change the hand pressure. The smaller wheel, 1/8" in diameter, is used for cutting patterns, and for cutting circles. Here the wheel with a small arc is necessary to follow curved lines without dragging.
A computer-assisted CNC semi-automatic glass cutting table is usually used to score large sheets of glass, which are then broken out by hand into the individual sheets of glass (also known as "lites" in the glass industry).
Cutting process 
The glass is first lubricated along the cutting line with a light oil. The cutter is then pressed firmly against the surface of glass and a line is scribed to form a "score" or "cut". The glass is now weakened by this score and the panel is ready to be split along the scored line. The score is then "run" or "opened" to extend the split using running pliers, or by carefully applying pressure (exactly under the score) on the bottom side of the glass using a straight edge or a wedge directly under the score and pressing evenly on each side. Tapping under the score encourages the split, but tends to reduce the smoothness of the broken edge and on thicker glass this may be an issue. Where a tightly-curved edge is required care must be taken as glass will follow the path of least resistance when separating and may not follow the score, so cutting tight corners, small shapes and internal corners may be very difficult to perform. Many internal cuts require specialized equipment to be successful.
After breaking the glass along the score it will reveal the "clean cut" edge. Clean cut edges on thick glass may be "splayed". A splay is where the cut edge is a razor sharp feathering on the edge and extra caution should be used here. At this point the clean cut edge of the glass panel may be ground to remove the sharp edges or the arris.
General purpose glass is mostly made by the float glass process and is obtainable in thicknesses from 1.5mm to 25mm. Thin float glass cuts very easily with a sharp cutter but needs firm support under the panel because of the pressure placed on it when scoring. Thick glass such as 10mm float glass is more difficult to cut and break requiring considerable skill and strength, Different Glass with textured or patterned surfaces may demand specialised methods for scoring and opening the cuts.
Breaking methods 
Three methods are usually used to break the glass along the score. In the first method, the operator uses two hands and holds the edge of the glass, perpendicular to the score, with the forefingers under the glass and the thumbs on top. The operator then rotates both hands outward to break the glass along the line. The second method consists of placing one finger underneath the sheet of glass, directly under the scribed line, and in one smooth motion, lifting the edge of the glass and giving it a small push horizontally. The second method is quicker than the first, but requires more practice. These two methods are usually used when breaking glass on a glass cutting table that lifts the glass gently by forcing air up through the table. The third method is used with smaller sheets of glass, and consists of placing the score along the edge of the table and snapping the overhanging part of the glass in a downward motion. With experience, the break follows the score. However breaking with one finger under the score is dangerous and will pinch the finger as the glass rolls away. Looking along the edge of the cut glass, one can tell which way the cutter was run and which way the glass was run or snapped, by the flow of feathers, in the edge. In most scenarios, glass will break where you want it, so long as it's well lubricated and run before it has a chance to cool off.
Bottle Cutting 
Currently, a very popular art is the cutting and recrafting of common glass bottles. Bottle cutting is slightly more complex than standard flat plane glass cutting, mainly because the cutter is dealing with a round surface. There are slightly different designs of cutters for such projects that allow for the cutter to find center and swing an arm with a steel head around the bottle making a clean score line. There are a few different methods of getting a clean cut from this point. It becomes more difficult than pushing against the glass because it will shatter. There are three popular methods that can be used to make the cut. This includes the rope method, the torch method and the water method.
Rope Method 
The rope method is a common practice for those novice at cutting glass bottles. The idea is to take a rope or piece of string and tie it tightly to the bottle directly into score line. The string is to be then painted with a flammable oil then set on fire. The heat that comes from the burning string will cause the bottle to soften in the score. While the score is still warm, the bottle is then placed under a flow of cold water which will tighten the heated molecules at such a quick rate that the stress of the bottle is released through your score causing a break in the glass. Problems with this method include the stress in the bottle being so great that the bottle fails to break immediately on the score line and will leave large jagged pieces that take a long time to grind down.
Torch Method 
The torch method is very similar to the rope method but requires a much more practiced hand in cutting bottles. The idea here is to use a torch to heat the bottle over the score line until the bottle reaches appropriate temperature to then place under the cold running water. Problems that occur with this method include still large jagged edges much like the rope method, however, much more accurate to the score. An advantage to this method is the lack of material and cost with a quicker and cleaner result.
Water Method 
Although all methods include a cold stream, this is the only method in which to make a cut using entirely water. Problems that occur in the rope method and the torch method is that the glass becomes too hot too quickly making inaccurate breaks with large stress lines and fractures in the glass. With the water method, you can obtain an almost perfect cut and at a faster and cheaper rate than almost all other methods. In this method, boiling water is poured slowly on the score line raising the temperature in the glass at a much slower rate than a fire method and will maintain a much lower temperature. Once the bottle becomes warm, it is then placed under the cold running water for the break. Problems still occur during this method with sometimes jagged edges and stress lines and fractures, however; most methods are a 50%-60% success rate, where as the water method is about a 95% success rate.
See also 
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