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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 as the latter tends to shatter rather than breaking cleanly into two pieces.
A glass cutter may use a diamond to create the split, but more commonly a small cutting wheel made of hardened steel or tungsten carbide 4–6 mm in diameter with a V-shaped profile called a "hone angle" is used. The greater the hone angle of the wheel, the sharper the angle of the V and the thicker the piece of glass it is designed to cut. The hone angle on most hand-held glass cutters is 120°, though wheels are made as sharp as 154° for cutting glass as thick as 0.5 inches (13 mm). Their main drawback is that wheels with sharper hone angles will become dull more quickly than their more obtuse counterparts. The effective cutting of glass also requires a small amount of oil (kerosene is often used) and some glass cutters contain a reservoir of this oil which both lubricates the wheel and prevents it from becoming too hot: as the wheel scores, friction between it and the glass surface briefly generates intense heat, and oil dissipates this efficiently. When properly lubricated a steel wheel can give a long period of satisfactory service. However, tungsten carbide wheels have been proven to have a significantly longer life than steel wheels and offer greater and more reproducible penetration in scoring as well as easier opening of the scored glass.
In the Middle Ages glass was cut with a heated and sharply pointed rod of iron. The red hot point was drawn along the moistened surface of the glass causing it to snap apart. Fractures created in this way were not very accurate and the rough pieces had to be chipped or "grozed" down to more exact shapes with a hooked tool called a grozing iron. Then In 1869 the wheel cutter was developed by Samuel Monce of Bristol, Connecticut, which remains the current standard tool for most glass cutting.
Large sheets of glass are usually cut with a computer-assisted CNC semi-automatic glass cutting table. These sheets are then broken out by hand into the individual sheets of glass (also known as "lites" in the glass industry).
Glass cutters are manufactured with wheels of varying diameters. One of the most popular has a diameter of 5.5 mm (7⁄32 in). The ratio between the arc of the wheel and the pressure applied with the tool has an important bearing on the degree of penetration. Average hand pressure with this size wheel often gives good results. For a duller wheel on soft glass a larger wheel (e.g., 6 mm (1⁄4 in) will require no change in hand pressure. A smaller wheel (3 mm (1⁄8 in)) is appropriate for cutting patterns and curves since a smaller wheel can follow curved lines without dragging.
The sheet of glass is typically lubricated along the cutting line with a light oil. The cutter is then pressed firmly against the surface of glass and a line is briskly scribed to form a "score" or "cut". The glass is now weakened along this line and the panel is ready to be split. Running pliers may then be used to "run" or "open" to the split.
General purpose glass is mostly made by the float glass process and is obtainable in thicknesses from 1.5 to 25 mm. Thin float glass tends to cut easily with a sharp cutter. Thicker glass such as 10 mm float glass is significantly more difficult to cut and break; glass with textured or patterned surfaces may demand specialized methods for scoring and opening the cuts.
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