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Hardened steel wheels have been used for glass scoring for about 150 years and are still the most prevalent material employed today. The Fletcher-Terry Company patented the steel wheel, cast iron handle glass cutter in 1868. Millions have been sold and they are still being sold and used today.
Prior to the steel wheel, diamond tipped handles were used, but they are rarely used today in this country.
Wheels made of tungsten carbide have been used in ever increasing quantities over the past 50 years. Their acceptance has grown until they are now considered the "premium" quality tool for glass cutting. There are still far more steel wheel cutters being sold than carbide. But a carbide wheel will last 10 or 20 times longer than steel.
To appreciate the choice of carbide over steel, consider the effect of wear on a glass-cutting wheel. Glass is extremely hard and abrasive. As a cutting wheel rolls on glass it crushes a very shallow path. The fine particles of glass wear off the sharp apex of the wheel. This wear is actually a slow process of dislodging individual grains of the steel.
There is an enormous load on the wheel structure as it tries to separate or pry the surface of the glass apart in a shearing action to produce the fissure. The size of the contact between wheel and glass is extremely small. If the path being crushed is only one or two thousandths of an inch deep, and the wheel diameter and angle are typical of general use, the true load on the apex is at least 100,000 pounds per square inch even though the glazier may exert a cutting pressure of only six to ten pounds. Such a unit load can be very damaging to the apex of the wheel. After a while the apex, which was once sharp and neatly honed to a typical angle of 120 to 140 degrees, gradually becomes rounded and assumes an increasingly larger effective angle. Thicker glass is scored with wheel angles of 148 degrees or more. An experienced glazier frequently carries an old well worn steel wheel cutter and uses it exclusively to score thick glass which requires a blunter wheel.
The superior wear-resistance of carbide over steel can be explained by examining the structure and the way the grains are constituted. Most steel wheels are made of what is called medium to high carbon tool steel. Steel is a combination of carbon and iron and some other additives that make it easier to heat-treat the material. If it is not heat-treated, a steel wheel would not make a score on glass, but would simply flatten under the high unit load. Heat-treating converts the iron and carbon into hard, brittle particles bound together. These particles are the grains that are dislodged by the abrasive action of the glass, resulting in "wear."
Tungsten carbide is produced by mixing finely ground powders of tungsten, a very hard element and cobalt, then molding the mix to the desired shape under extremely high temperature and pressure. The result is a metallic material many times harder than steel. In fact, only sapphire and diamond are harder than tungsten carbide. The particles of tungsten are bound together by the cobalt. The mixture is generally about 90 to 95 percent tungsten, with the balance cobalt. The more cobalt, the less hard and brittle the result. Since the cobalt is softer, it is easier to erode grains away, and thus it wears faster. Conversely, the higher content of tungsten imparts a harder material, but it is more brittle resulting in particles breaking away.
To be wear-resistant, a material needs to be composed of hard grains that are as small as possible. The ideal tungsten carbide for glass cutting has a grain structure far smaller and more tightly packed than tool steel and is therefore more wear-resistant. The use of cut running lubricants is beneficial in extending the life of any wheel, but more so with steel because it is less wear resistant. The price of a tungsten carbide glass cutter is greater than steel. The elements that go into steel are readily available and so widely used that the cost is minimal. Iron and carbon are found everywhere. Tungsten, on the other hand, is not as plentiful. Cobalt, not mined in this country, is considered a critical element.
Steel, even in the hardened state, is easier to machine and grind than carbide. While many types of grinding wheels can be used on steel, only diamond-coated grinding wheels are capable of machining carbide, therefore, manufacturing is more costly.
What does all this mean to the glass shop and glazier? It is a trade-off. Under ideal circumstances, carbide cutting is less expensive than steel. A carbide cutter might cost five or six times as much as steel, but its useful life can be 10 or more times that of steel. On the other hand, steel cutters
may be the best choice where the usage is not continuous and the consequences of wheel wear-out are not critical.
Industrial fabricators and float lines use carbide wheels. A float line producing continuous ribbons of glass does not want to experience the end of life of a cutting wheel. The loss is more than the cost of a wheel; it might mean losing production while a wheel is replaced. It is common practice to replace wheels on a scheduled basis rather than waiting for wear-out.
Glass shop owners who provide glass cutters for their employees face a tricky question. Will the hand cutters get lost or dropped before they wear out? If this is their experience, owners will likely buy steel cutters. On the other hand, if workers have been properly trained to treat their tools as they should, carbide is the answer. In many shops, glaziers have a sense of professionalism and buy their own carbide cutters because they appreciate good tools, know the value of caring for them and enjoy the pride of ownership.
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