WHEEL ANGLES


How does the wheel angle affect glass cutting?

First let's consider what we are trying to do with a glass cutting wheel. Actually, we do not "cut" glass with a glass cutter. The objective is to create a crack or "fissure" along which we expect the glass to break when we bend it. The idea is to produce a fissure which is continuous, and of uniform depth, without creating a flaky score line full of loose glass chips.

While the wheel angle is only one of several variables which influence the quality of the fissure, it is the best place to start. The other main variables are wheel diameter and cutting pressure.

The figure to the right shows a typical wheel used on single strength (.062" thick) and double strength (.122" thick) glass. The angle is identified as the included angle to which the apex is honed. When downward pressure is exerted on the wheel rolling along the glass, forces are created (F) which radiate down and to the side trying to shear or separate the glass along the surface. These forces are shown with arrows to indicate their direction. If these forces are great enough to overcome the inherent compressive conditions near the surface, a crack (Fissure f) will be generated along the path of the wheel.
The direction of these shearing forces is determined by the wheel angle. The illustration on the left shows a wheel with a large or blunt angle. Notice the direction of the shearing forces. They tend to be directed downward more than to the side. As you can see, it would require a great deal more cutter pressure to create enough lateral force to overcome the compression in glass. This explains why a cutter requires more pressure as it gets older. The apex tends to flatten so its effective angle becomes greater.
The illustration to the right shows a very sharp wheel angle. The shear forces are directed more parallel to the surface of the glass. This suggests it is easier to produce a fissure with a sharp wheel than a dull one. The shear forces are directly opposing the compressive condition near the surface of the glass therefore, requiring less downward pressure to make a crack. A sharp wheel tends to cause chips and a flaky score. If the shear forces (F) run close to the surface of the glass they are more likely to cause a lateral crack which then breaks out to the surface, creating a chip.
You can see these chips leap out of the glass a minute or so after scoring. Again, the compressive condition of glass near the surface literally squeezes the fissure closed, spitting out loose chips. They can be seen lying on top of the glass.

The photographs below will further illustrate the information on page 1. They were taken through a microscope to magnify the view from two different directions, first, from the top of the glass looking down on the score line, and secondly, from the edge looking toward the end of the fissure. A polarizing filter was used to highlight stresses in the glass.



The first picture on the top left is a top view, and the one on the top right is an end view. A proper wheel angle cutter (134º) and pressure was used. Note the score is continuous, and did not produce flakes and chips. The fissure view shows a distinct crack about .012" deep.

This set of pictures shows the effect of using too sharp a cutter, (120º) and too much pressure. Loose chips appear on top of the glass and the wavy lines indicate very high stresses just beneath the surface. The fissure view also shows high concentration of stresses near the surface close to the score line.






A very blunt angle cutter (154º) was used in this set of pictures. The score line is barely visible, while the fissure is quite deep. Additional downward pressure was required to create a score, and the greater pressure produced a deeper fissure.






The illustrations at the left show what happens when you tilt the cutter to one side. It is as though two wheels were used simultaneously. The "B" side presents a much sharper angle to the glass and produces the chips seen above the score in the photo. The "A" side is much blunter and created the high stress seen on the lower side of the score.





Another factor to consider in selecting the proper wheel angle is the "edge". The objective of good glass cutting is to produce an edge which is flat and relatively free of irregularities such as "shark teeth." This photo shows two pieces of double strength glass, one lying on top of the other. It is a view of the edges produced with two different cutting wheels. For reference, each piece of glass is approximately 1/8 inch thick.