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Scoring and breaking out contoured shapes of flat glass is truly an art requiring special techniques. The creation of stained glass designs, for example, involves a great deal of contour cutting. The growing use of thick glass for furniture and other architectural applications has created the need to make many curved, circular, and free form shapes.
The glass cutting wheel does not "cut" glass. As it rolls along the surface, it concentrates stresses which cause a crack or "fissure" along the path of the wheel. This crack provides the fault along which the glass will break. The combination of the right wheel diameter, angle, and force will produce a continuous fissure with little or no flaking at the surface.
The same basic procedures apply in scoring curves as in making straight scores. Do not tilt the wheel to one side. This is harder to do when hand cutting curves. The speed should be as constant as possible. Unfortunately, hand or machine cutting generally require slowing down when going around small radius corners. Recognize the effect of slowing down. The same wheel and pressure will produce a deeper fissure when it travels faster. When it is necessary to slow down, exert additional pressure to keep the fissure at the same depth. This may cause more flaking at the surface, so a compromise is required.
Ideally, a wheel should travel in a straight line. When it rolls on a curved path some "scuffing" is unavoidable. Make an effort to line it up as much as possible with the direction of the score. Machines that are designed for contour scoring will keep the wheel in line with the tangent of the curve. This is true even of a simple beam type circle cutter. Scuffing, or sluing will not only wear the wheel more rapidly, but will also produce a flaky score line.
It is a good idea to use a very small diameter wheel for contour scoring. Its line of contact, or footprint. on the glass is shorter, allowing it to travel around curves with less scuffing. As with straight-line cutting, using a lubricant while scoring will produce a better fissure and consequently, an easier break-out. Commercially available cut running fluids such as the Fletcher EnviroGOLD, are designed for glass cutting.
Simply bending the glass to break out a score will not succeed with contour cutting as it does on straight lines. A break may start on the score line and follow it a short distance. It tends, however, to follow the straight line dictated by the bend. It will soon veer away from the curving score line, or worse yet, break through the finished piece. The following steps are recommended for glass under 1/4" inch thick:
The objective is to cause the crack or fissure produced by hand scoring to continue all the way through the thickness of the glass from surface to surface. There are several ways to do this.
1. A simple technique is to turn the glass over after scoring and place it on a pliable surface such as the carpeting on a cutting table or a stack of newspapers. Place your thumb (wear gloves) directly over the score line which can be seen through the glass. Press down with increasing force until you hear a slight crack. You will be able to see the fissure has been propagated through the glass at the point of pressure. Move along the score line repeating the thumb pressure until the complete score has been deepened.
2. Tap the side opposite the scored side directly under the score. Each tap should generate a short run through the glass. Continue tapping until the entire score has been deepened. Tapping will not produce as square an edge as the method in 1 above because it is difficult to hit the glass precisely beneath the score.
3. The application of a concentrated intense flame along the score will deepen the fissure by causing abrupt, local expansion. Success with flame running is even more artful than the other methods and requires trial and error practice before consistent success can be expected. It is very important to observe safety rules required in the use of gas flame equipment.
It is often desirable to use tapping just to start, then apply either method 1 or 3 to complete the deepening. If the score ends at the edge of the glass sheet, it is easy to use lightweight cut running pliers to start the deepening. Then follow with pressure as mentioned in 1 above.
Flame is practical for glass more than 1/2-inch thick. Tapping can be done on even the thickest glass.
For circle cutting and similar shapes completely within the sheet of glass, it is necessary to remove the waste glass from around the good piece. Make scores outside the finished part and break them out. These are called relief cuts, and are accomplished as follows:
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Make certain the relief cuts are on the side originally scored.
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Start the relief score about 1/8 inch outside the original pattern and score all the way to the nearest edge. Make many relief scores which radiate out from the pattern to the nearest edge all the way around the shape.
In every case, be sure the relief score does not intersect or even reach the original shape score.
Some glass cutters prefer to score relief cuts so they are tangent to the finished shape. Perhaps this gives more certainty that the relief breaks will not run into the good piece. In any case, it is essential the original score is fully deepened before relief cutting, otherwise the relief breaks will surely ruin the job.
At this point conventional break-out methods are used on the relief scores because they are straight and proceed to an edge. It is a good idea to use lightweight cut running pliers. Place the jaws over the relief score at an edge in the normal way and gently apply increasing pressure until the relief score runs to the finished part and stops. Move to the adjoining relief score and repeat. Continue this process all the way around the part. It will probably drop free before all radials break out.
Do not be too vigorous about running the radials, especially the first few. Apply the breaking pressure carefully because heavy bending of the glass will cause the break to run into the good part even though its score was deepened.
Tapping may also be used to run the radials. Here again, do not tap too close to the finished piece.
It is always important to observe safety precautions when scoring and breaking out glass. Wear safety glasses at all times. Use gloves when handling the glass. Be careful breaking out entrapped contour shapes. Since there may be many relief scores, you can not always be certain when or where the material will break free. Keep the glass over a work surface to avoid pieces falling to the floor.
Practice contour cutting and breaking on scrap material until you develop the skills that will assure success.
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