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The principles of scoring glass on an automatic or semi-automatic Cutting Table are basically similar to hand scoring, but quite different controls are available.
The variables which affect the score are the same, wheel angle, wheel pressure, wheel diameter, and speed. Unlike hand cutting, the speed is determined by the machine. The glass being scored dictates the wheel selection and pressure.
Most Cutting tables are covered with a carpet like material which permits glass chips to fall into the
nap rather than scratch the glass. This surface also allows the glass to be moved on the table with efficient air flotation.
The compliance of such a surface, however, presents some difficulty in properly scoring thin glass. The force exerted by the wheel pressure tends to depress the surface under the score and bends the
glass in a concave direction. This adds to the compressive condition of the surface of the glass. Since the wheel is trying to overcome compression of the glass surface and cause a tensile shear, bending of the glass is undesirable.
Some tables used to produce thin precision glass components such as microscope slides, and LED screens utilize machined table surfaces of steel or aluminum to avoid concave bending. The alternative is to reduce the wheel pressure to a bare minimum by using a sharper wheel angle. A smaller diameter wheel also assists in this effort because it makes a smaller "footprint" and requires less force to produce a score comparable to a larger wheel. A typical wheel for this application would have an angle of 114 degrees and a diameter of about .175 inches.
Thicker glass is stiffer and less inclined to flex under the force of the wheel. Although 1/2" glass is twice as heavy as 1/4" it is eight times stiffer. As a result, the soft compliant surface of a cutting table creates no problem in scoring thick glass. On the other hand, the weight of 3/4" glass is so extreme it is essential the table be strong enough not to sag under the weight.
One of the advantages of table cutting versus hand scoring is the control of wheel pressure. When needed, much more force can be applied mechanically. It is desirable to produce as deep a fissure as possible without chipping or flaking the surface. This is particularly important with glass over 1/2" thick, because the breakout is less likely to follow the score line than with thinner glass.
For any given wheel diameter and angle, the greater the force, the deeper the fissure. Excessive force, however, tends to cause surface chipping of the glass. A blunter wheel angle and/or a larger wheel diameter will reduce chipping and flaking of the score line and allow considerably more force to be used. A typical wheel for thick glass will have an angle as large as 162 degrees and a diameter of 1/4". In some cases, even a 1/2" wheel is practical.
Such large angles and diameters require more force. Many machines apply force to the wheel using air cylinders. The applied force can be readily calculated by multiplying the pounds per square inch, as shown on the pressure regulator, times the area of the air cylinder piston in square inches. For example, if the piston is 1 1/2" in diameter, its area is .75 times .75 times 3.1416 or 1.77 square inches. If the regulator pressure reads 25 psi, the maximum force on the wheel would be 44-1/4 pounds. Some of this force is lost in friction within the air cylinder, but the calculation provides a guide.
It is necessary to maintain a reasonably constant force irrespective of the wheel and glass. One should avoid setting the pressure gauge near the low end of its scale. Slight variations in line pressure can
drastically affect the score. When cutting thin glass with a sharp wheel and a low force such as 5 pounds, a cylinder like the one above would require a gauge setting of about 9 psi. Variations of line pressure of 1 psi will change the force almost 2 pounds. It is best to use a smaller cylinder.
The best control of cutting force will result from the use of an adjustable spring. The better cutting tables use an air cylinder to bring the wheel to the glass and spring pressure to apply the cutting force. As long as the air cylinder exerts more force than the spring, the spring is in control of cutting pressure.
This is particularly important because it is difficult to assure a perfectly flat table and uniformly thick glass. As the wheel travels along the surface, minor variations in the height require that the wheel
move slightly up and down. Air cylinders do not generally respond readily to such excursions due to internal friction. This causes the score to vary and may result in no score at all in certain areas. A spring responds instantly to such variations and assures constant force.
A properly lubricated wheel will produce a less flaky score and easier break-out. Most table cutting machines provide pressure lubrication at the score line. As in hand cutting, a recommended fluid is a 50/50 mix of light machine oil and kerosene. If no oil is used the wheel and axle will not be lubricated and the wheel will not roll as freely. In addition, a well lubricated wheel will last longer.
The pillar post which holds the wheel and axle must be aligned in the direction of cutting. If the wheel is cocked at the slightest angle to the score line it will gouge the glass. The result is a chipped scoreline and poorly produced fissure. Break-out will not be as easy and the edge will not be clean and free of vents and cracks.
Tables that are capable of contour cutting control the alignment of the wheel electronically or by permitting the pillar post to swivel with a castering action like the wheel on a desk chair. Such pillar posts are available in a variety of designs. Some swivel a full 360 degrees, while others swivel plus and minus 9 degrees from the line of travel. In either case, certain pillar posts are designed to remain in the same alignment when retracted from the glass so they will be in line with the next score. This is called self-aligning. Others are self-centering which means the alignment of the wheel always returns to a fixed position when lifted from the glass. The application dictates which type pillar post is appropriate, but they are precision made with radial and thrust bearings so swiveling is possible with little or no friction.
Generally speaking, the wheel used for contour cutting should have a smaller diameter than for straight cutting. When a wheel travels around tight comers it is impossible to maintain perfectly straight alignment. A smaller wheel has a shorter "foot print" on the glass which minimizes such misalignment.
A popular wheel for contour cutting is called the "wide track". It is about three times thicker than the regular wheel. This provides more contact with the axle and more resistance to "leaning" as the wheel travels in tight circles.
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