Graphite plays a vital role

Without graphite, the diamond drill bit industry may not have been able to reach its present state of development. Graphite is used in both the mold and mold parts for drill bits. Speer Canada machines drill-bit molds — among many other items — at its plant in Kitchener, Ont. The manufacture of the graphite from raw materials is done in the parent company’s facility in St. Marys, PA.

Graphite also plays a vital role in chemical, electrical, aerospace, and metallurgical industries.

The use of graphite in high temperature processes is the result of a number of properties unique to the material.

These properties include: a low coefficient of thermal expansion; it does not melt but sublimes at temperatures over 6,700 degrees F and can be used at temperatures exceeding 5,000 degrees F; it is twice as strong at 4,500 degrees F than at room temperature; it has a high thermal conductivity allowing for rapid heat build up and cooling without localized overheating; it resists wetting by most molten metals, allowing metals to solidify in a mold and not stick when removed, and its low density (about 1.7 gm per cubic cm) results in precision parts being light weight and easy to handle.

One of graphite’s disadvantages is the fact that its production is a very time-consuming multistep process.

The production process begins with the combining of petroleum coke and either coal tar pitch or petroleum pitch. The pitch acts as a binding agent for the coke fill. Crushed and then mixed while heated to about 400 degrees F, the pitch forms a thin film around each coke particle allowing for a firm bond when the mixture is formed.

Forming is usually done by extrusion, although molding and isostatic pressing are also used.

Following the forming step, the product is baked in a gas-fired furnace where the pitch binders carbonize under the 2,000 degrees temperature. The product from this process, which takes between 20 and 50 days, is known as baked carbon.

The baked carbon is then impregnated with pitch in an autoclave. The carbon is heated, a vacuum is drawn to remove air from the pores in the carbon, and the chamber is flooded with warm pitch. The carbon is then baked again to carbonize the impregnated pitch.

The next step, called graphitization, takes 20 to 30 days, and involves heating the material in an electric resistance furnace to temperatures of up to 5,000 degrees F. The result is a uniform crystalline graphite which is easily machined and, in fact, soft enough to whittle with a pocket knife.

The total process time, up to machining, takes between 156 and 226 days.

Graphite’s properties allow it to be machined to very close tolerances, using conventional machining tools, to make drill bitmolds. The mold and its parts can be heated to over 2,000 degrees F within a few seconds.

Graphite can be heated to over 2,000 degrees F by convection or by using high-frequency (up to 10,000 CPS) induction coils. Because of its high diffusivity, only one-third of the energy required to heat steel is needed when using graphite.

Graphite does have some disadvantages in that it is susceptible to oxidation at temperatures over 1,000 degrees F. Oxygen attacks the binder, shortening the useful life of the mold. Although the graphite could be coated to protect it from oxidation or the process could be enclosed in an oxygen- free atmosphere, neither method have proven to be cost effective.

Other characteristics of the graphite industry itself which could be considered less than advantageous, are its large capital requirements, extended process times, and high consumption of petroleum products, natural gas, and electricity.

Despite its disadvantages, graphite will likely always have a place in the manufacture of drill bits owing to its unique properties.