The wheel of a glass cutter does not “cut” the glass. 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 below shows a typical wheel used on 2mm and 3mm thick glass. The angle is identified as the included angle to which the apex is honed. This means it is measured from one beveled face of the wheel around through the wheel to the other face. Thus the angle between the wheel and the glass on a 150° wheel will be 15° on each side.
When downward pressure is exerted on the wheel rolling along the glass, forces are created which radiate down and slightly to the side trying to shear or separate the glass along the surface. If these forces are great enough to overcome the inherent compressive conditions near the surface, a crack will be generated along the path of the wheel.The direction of these shearing forces is determined by the wheel angle.
A wheel with a large or blunt angle produces shearing forces that tend to be directed downward more than to the side. 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.
With 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 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.
Based on information from the Fletcher Terry Company