Evolution of Cutting Tool Materials from High Carbon Steel to Cemented Carbide and Beyond.
Cutting tools have come a long way in their materials and developments, especially in the field of machining. In the 19th century, high carbon steel and alloy carbon steel were the best tool materials available for cutting. However, advancements were made with the accidental development of Mushet Steel and the subsequent introduction of high-speed steel (HSS) around 1915, which revolutionized cutting tool technology. Cast alloy cutting tools were then introduced, significantly reducing machining time compared to HSS tools.
In the 1930s, another breakthrough came with the use of cemented carbide as a cutting tool material. Also known as sintered carbide or tungsten carbide, cemented carbide is made up of hard carbide particles cemented together by a binder. Initially, cemented carbide tools were brazed to their tool bodies, but later, indexable inserts became the focus of the industry. Indexable inserts are made by pressing a slurry of cemented carbide into a die, but the pressed inserts are initially soft and porous. Therefore, they undergo a high-temperature heat treatment process called sintering to close the pores and increase their hardness and strength. Some inserts are also ground in a grinding machine to achieve high accuracy and thickness. Additionally, inserts can be coated with a thin layer to further enhance their performance.
Compared to HSS tools, carbide tools are more wear and heat resistant, allowing for higher RPM cutting speed and feed, resulting in faster machining and cost savings. However, carbide tools are also more brittle, requiring better spindles and fixtures. They are typically used in CNC machines, while HSS tools are more commonly used in manual machines. Cermet, a material similar to cemented carbide but with added titanium hard particles, is highly wear resistant and produces excellent surface finishes. However, it cannot withstand high compressive stresses and is recommended for low cutting depths and feeds.
Ceramic tools are mainly used for machining materials that generate excessive wear or heat, such as hardened steel, cast iron, or heat-resistant super alloys. CBN, which stands for polycrystalline cubic boron nitride, is a very expensive material used for machining hardened steel or grey cast iron. It is typically welded to a cemented carbide body in the form of inserts or layers. PCD, which stands for polycrystalline diamond, is used for machining non-ferrous materials like aluminum. However, PCD tools cannot be used for machining steel or cast iron as they burn at high temperatures.
In the past, cutting tools needed to be reshaped and sharpened when they became damaged or worn out. However, with the advent of indexable inserts, they can now be easily replaced with new ones due to their tight tolerances, making them a preferred choice for CNC machines. Indexable inserts have become the main focus of the current course on cutting tool materials and their developments in machining.
In conclusion, the evolution of cutting tool materials has greatly impacted the field of machining, allowing for more efficient and precise machining operations. From the early days of high carbon steel and alloy carbon steel to the advancements of high-speed steel, cast alloy cutting tools, and cemented carbide, the development of cutting tool materials has revolutionized the machining industry. Today, indexable inserts made from cemented carbide, cermet, ceramic, CBN, and PCD are widely used, providing increased wear and heat resistance, higher cutting speeds, and improved surface finishes. The use of indexable inserts with tight tolerances has become a popular choice in CNC machines, further enhancing the precision and efficiency of machining processes.