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The process of separating glass has changed very little for over 100 years. The least expensive and quickest way is to create a fault, or fissure along which a break may be induced.
This technique may be described as a controlled catastrophe. By contrast, sawing glass with diamond media, water jet, or laser may seem less catastrophic, but involves much more expense and time.
Irrespective of the alloying ingredients, glass is an amorphous material lacking grain or other directional characteristics. Often called a super cooled liquid, glass might be likened in many ways to a gelatin such as Jello, although much harder of course.
When any material is cooled, it will contract and when heated, it expands. In proceeding from a hot viscous liquid to a cold solid, the outer surfaces of the glass tend to reach a solid state sooner than the interior because they are exposed to cooler room temperature. As the interior continues to cool, it contracts further, putting the exterior in compression. When glass finally hardens, the interior is in tension and the exterior in compression. The resultant opposing forces are less severe in well annealed glass, and conversely very extreme in tempered glass where the exterior cooling is accelerated by sudden quenching.
The various methods of making glass result in different compression/tension gradients, but these internal forces tend to be in equilibrium or the glass assumes a warped shape. One can generalize by stating "the greater the difference between the compressive forces and tensile forces, the harder the surface "seems" to be. " Another point of relevance is that glass tends to break in tension rather than in compression. The above described characteristics of glass are important to an understanding of the process of scoring and breaking.
Several tools for scoring glass are in use today, but the most common is the wheel. Diamond, in the shape of a pyramid, was once the most prevalent tool and still is used in some foreign countries. The 90 degree edge of a block of tungsten carbide is used in tube scoring. The wheel, however remains the easiest and most cost effective tool for scoring glass.
Today's glass cutting wheels are made of hardened tool steel or tungsten carbide. The surface of glass is relatively hard and the scoring action is abrasive, so wheel life is a function of the hardness and wear resistance of the wheel material. Generally, tungsten carbide wheels can be expected to outlast steel by a factor of ten to twenty.
The geometry of the wheel is significant. The smaller the diameter, the less force required to create a fissure. The typical range of diameters is .125 to .500 inches. The angle at which the apex is honed is critical. The most widely used angle is 134 degrees included, but the range is about 114 to 140 degrees depending on glass thickness. Special angles outside this range are used with the following guide: The harder the glass, the sharper the angle should be. For example, quartz tubing and pyrex may separate easier by using an angle as sharp as 88 degrees. Float glass, in excess of 1/2 inch thick, may score best with wheel angles from 140 to 160 degrees.
Angle selection can best be appreciated by examining what happens when a wheel rolls along the surface. Regardless of the angle, the apex must be a well defined edge, not rounded nor variable in its profile. The wheel generates shearing or tensile forces in the glass at right angles to its rolling direction. These forces tend to be more parallel to the surface of the glass with sharp wheels and more oblique as the wheel angle increases.
A fissure or crack will be created which tends to be in line with the wheel travel and extends into the glass to a depth related to the force applied to the wheel. In flat float glass, this depth would be approximately .012 to .015 inches. If the wheel angle is too sharp and/or the force applied is too great, the glass tends to chip along the score line. This is undesirable not only cosmetically but more importantly, the energy that should be creating the fissure is partially dissipated.
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Good Score |
Too Much Pressure |
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Place the glass on a flat surface and clean it to remove dust, dirt or any foreign objects.
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How you hold the cutter handle is not important as long as you do not tip it side to side. The figure to the right illustrates the classical method of holding it.
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Place the wheel on top of the glass about 1/16 inch from the far edge.
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Apply downward force on the cutter and pull it toward you at a steady speed. Do not slow down or ease off the pressure as you approach the near edge, but run the wheel completely off the glass.
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The amount of pressure you apply takes some practice, but the tendency is to press down too hard. Remember, you are not "cutting" the glass, just scoring it. The ideal score is barely visible, but continuous. If the score line is very chalky and small chips are visible on the glass...you used too much pressure.
The "controlled catastrophe" is completed by bending the glass in such a direction as to place the score line in tension. A typical practice with flat glass is to place a small cylindrical object under the glass and in line with the score at the edge where the score was completed. Apply a modest downward force on both sides of the score; the glass will be seen to separate in a rapid "run" of the break from edge to edge. One can test the fact that glass breaks more readily in tension than in compression. Try bending the glass with the score on the inner arc of the bend and nothing will happen until much higher forces are attained. The break may become a general fracture unrelated to the score line.
Tapping under the terminal end of the score line is a common technique for starting the break. A shock wave is induced which, acting on the notch effect of the fissure, causes a rupture to occur. This often creates a flare in the break because it is difficult to direct the blow exactly under the score line and even more difficult to judge the force of the impact. Another technique frequently used to propagate the fracture is to introduce an intense local flame to the glass. Rapid expansion generates the tensile forces which complete the break.
The most common technique is to use break-out pliers shown at the right. The inside of the upper jaw has two wide-spread projections while the lower jaw has a single projection in the inside middle of the jaw. Insert the jaws over the edge of the glass where the score was finished. Align the white line on the upper jaw directly over the score line. Squeeze the handles gradually and the score will suddenly run the full length of the score. Do not squeeze too aggressively because the jaws will close abruptly and may chip the edge of the glass.
The process of "Glass Cutting" is not cutting at all. While it is a necessary activity in the production of flat glass, it is in many ways more art than science and as such, affords an element of creativity and satisfaction when done well.
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