The Fletcher-Terry Company

SCORING AND BREAKING THICK GLASS

In recent years, the production of thick glass has increased dramatically with architectural and furniture applications. "Thick glass" is 1/2 inch or more, and is produced by most of the float glass plants. The market for replacement glass continues to grow, and is being provided principally by the larger fabricators.

Scoring and breaking thick glass involves the same basic steps as the more common thinner glasses. There are some significant differences, however, that when properly considered, make it possible for any skilled glasscutter to cut thick glass successfully.

Basic Technique
It is appropriate to review some principles of scoring and breaking glass before considering the variations required for thick glass.

Clean the glass surface. Foreign matter may disrupt the wheel as it rolls on the glass.
Select a cutter wheel of the proper included angle and diameter. This will vary with different glass characteristics such as hardness, annealing, and chemistry of the glass. As a guide, a continuous fissure should be produced without creating a flaky surface.
Use a straightedge to guide cutter. Start the wheel on the surface of the glass at the far edge.
Apply constant pressure while pulling the cutter toward you at a steady speed. Do not stop at the near edge, but run the cutter all the way off the glass without slowing down.
Bend the glass downward on both sides of the score, starting at the edge where the score finished. The break will run the length of the glass. It is important to bend the glass symmetrically to prevent the break from running off the score line.

Scoring
Thick glass requires special care in following the above procedures. The surface "acts" softer than thin glass. As a result, it tends to flake more readily during scoring. Therefore, a wheel must be used which has a larger, (blunter) angle, and a larger diameter. The Fletcher (01-266) has a .245" diameter tungsten carbide wheel with a 148 degree angle. The Fletcher (01-275) has a pistol grip handle which is effective for applying the heavier forces required for thick glass, and permits the use of interchangeable carbide wheel units of 154º. Both are ideal for thick glass.

These blunter angles and larger wheel diameters permit more force to be applied during scoring. This is important because the fissure (a crack induced by the wheel) can be deepened with additional force. The greater the depth, the more likely the break will follow the score line. A typical fissure in double strength glass is about .012 inches deep, or about 10% of the thickness. It is deep enough to assure the break will follow the score.

With the recommended wheels and extra pressure, a typical score in thick glass will produce a fissure about .025 inches deep. This is only about 3% of the thickness of 3/4 inch glass. Therefore, there is a greater tendency for the break to veer away from the scoreline during breakout, especially if the bending forces are not equal and symmetrical on both sides of the score.

Not only must the glass be clean, but use of a cut running fluid such as Fletcher EnviroGOLD helps assure a continuous fissure, minimum flaking, and makes the breakout easier.

Breaking out or Running the Score

The picture on the right shows a profile of a break traveling right to left through 3/4" thick glass as photographed through the edge.

The glass is tilted back slightly to show a reflection of the break and the score line on the interior of the upper surface.

Please note the "bullet nose" shape. This means the break is actually running ahead of the score approximately 3/4 of an inch. This increases the tendency for the break to veer away from the score line if the bending is not symmetrically applied on both sides of the score.

Glass is very strong and considerable bending force is required to cause it to break even when scored. If the thickness is doubled, the bending force increases eight times. As an example, a piece of 3/4 inch thick glass, 2 inches wide and 12 inches long, scored in the center, would require a bending force of approximately 65 pounds to break. It would deflect about .005 of an inch at the point of breaking. If the thickness were 3/8 of an inch, or half as thick the force would only be about 8 pounds, or 1/8th as much and the deflection would be about .010 inches, twice as much. These numbers depend on the condition of the surfaces. Scratches may reduce the measurements, but the exponential relationship of thickness and force is valid.

One can imagine the force required if you were to break out a large sheet of 3/4 inch thick glass by trying to bend the entire length at once.

Apply the bending force at the edge where the score was finished. Once the break starts, considerably less force is required to continue its progress the full length of the score.
Apply the bending forces at equal distances from the score line, otherwise the break will run at an angle, causing what is known as a flare. Considerable grinding will be required to create a square edge.
Avoid trying a "bending" break-out close to one edge of a sheet of glass. As the break travels, it will either flare or break out toward the narrow side.

It is very difficult to cut a narrow strip from the edge of the glass. Success varies with the quality of the score and the break-out technique. With the proper tool, it is possible to cut 3/4" glass down to 1-inch squares by repetitively cutting the glass in half. On the other hand it is nearly impossible to break out a strip 1 inch wide from a large piece by the bending method.

The photo at the right illustrates the Fletcher Breaker Tool (06-311). This tool permits the user to apply very high forces to break glass up to 1 inch thick. Since a lead screw is used, the force can be controlled so that when the break occurs, the force ceases and the anvils will not crush the glass. This method produces a clean, reasonably square edge, which greatly reduces the time required to grind. The photograph on page 2 was possible because the breaker tool permitted arresting the break before completely separating the glass. A special cutter with 148º and .245 diameter wheel is supplied with this tool for scoring thick glass.

Tapping is another technique used to break out the score in thick glass. After scoring, the glass is extended far enough off the table to expose the underside. The glazier uses a tapping hammer with a handle 10 or 12 inches long, and a brass ball at the end. He strikes the glass directly under the score line which produces a break 1 or 2 inches long. A series of taps is applied, each of which advances the break. It is necessary to slide the glass across the table to access the underside of the glass from the opposite edge. The first tap should be 5 or 6 inches from an edge to get the break started. After reaching the far edge, go back and extend the break to the edge where it was started. The force of the tap is judgmental and determined by practice.

Tapping does not fully run the score all the way to the bottom surface. A thin unbroken section on the under surface holds the glass together, but it can abruptly break apart unexpectedly. The normal practice is to pull the narrowest side off the table and finish separating the glass using metal nipping pliers with a "down and away" force.

Since tapping runs the score by generating shock waves in the glass rather than bending, it is the most common way to remove a narrow strip. Unfortunately, tapping produces an irregular edge, which requires more grinding than a bending break.

Safety
Whatever break-out method is used, close attention to safety is essential. The weight of thick glass presents hazards beyond those of thin glass. Gloves and safety goggles should be worn and foot protection is recommended, particularly when tapping, because of the possibility of heavy glass falling.

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