From the perspective of a machinist, cutting tools may be classified on the basis of material. The material classifications for cutting tools are: carbon tool steels, high speed steels, cast alloys, carbides, and ceramics. Natural and artificial diamonds have been excluded as they fall outside of the normal use of a production shop.

The most important properties of a single point cutting tool are:

1. Strength or toughness - the ability to withstand cutting pressure particularly under adverse conditions such as interrupted cuts.
2. Hot hardness or wear resistance - the ability to retain a sharp cutting point at high temperature.

Figure 1 illustrates that for cutting tools these two properties are diametrically opposed: as strength increases, hot hardness decreases. This fact means that no perfect cutting tool material exists.

Carbon tool steels are the lowest grade of cutting tool material and are in the family of low alloy steels. Plain carbon steels with a carbon content of between 0.90% to 1.40% are alloyed with small amounts of chromium or vanadium to increase the degree of toughness, hot hardness, and wear resistance. These high carbon tool Steels maintain their hot hardness between 350° and 650° F. This low temperature makes them nearly obsolete as a modern cutting tool; however, they can still be used as a makeshift tool for unusual situations.

Soon after the invention of a special tungsten alloy by Taylor and White of Bethlehem Steel Company in 1906, high speed tool steels have virtually replaced carbon tool steels. This cutting tool material revolutionized the art of cutting steel by providing a tool that would not lose its hardness even when working at such a speed that the point of the tool was heated to near redness. The term hot hardness is also referred to as red hardness. High speed steel maintains its hardness up to 1100° Fahrenheit; thus its red hardness property is higher than that of carbon tool steel. High speed steels are more brittle than carbon tool steels but are substantially less brittle than carbides or ceramics. High speed steels are classified by type as either tungsten base (T) or molybdenum base (M) and have several different grades within each type. Partitioning of each subclass is shown along with its characteristics in the following table.

Courtesy of Tool and Manufacturing Engineers Handbook Third Edition

Table 1 is excerpted from a comparison table of all tool and die steels; therefore, performance grading is related to other tool steels and not to other cutting tool materials. High speed steels rate best in toughness when compared to cutting materials in use today. The low material cost and ease of hand grinding tool blanks ensure high speed steel's continued viability as a modern cutting tool.

Cast alloy tool steels are alloys of cobalt or chromium and have less toughness and higher wear resistance than high speed steel. They maintain their red hardness up to about 1400° Fahrenheit. Cast alloy tool steels are so highly alloyed that they are too brittle and must be cast to near final size and shape. They are sometimes used as a tipping tool in the same manner as sintered carbides. Carbides have virtually supplanted cast alloy's use as a cutting tool; nevertheless, their extreme hardness and ability to be welded make these alloys important for the hard facing of tools and parts subject to extreme wear.

Tungsten carbide was discovered in 1896, yet its application as a tool material would wait until the Germans used it for wire drawing dies during the First World War. Carbide tool bit inserts are manufactured out of various powders (tungsten, titanium, columbium, and tantalum) in varying quantities with a binder such as cobalt or nickel. Carbides are extremely brittle and must pressed to size and shape prior to sintering at 2500° to 2900° Fahrenheit. Carbides maintain red hardness up to about 1700° F at which point the binder begins to soften. Carbides are comprised of three basic types: 1. straight tungsten carbide (WC), 2. titanium carbide (TiC) or tantalum carbide (TaC), and 3. composite carbides (WC-TiC-TaC). Manufacturers have created several grades within these types for use under specific circumstances with various metals. To improve resistance to wear and spalling Carbide is manufactured with various coatings; in addition, coatings reduce friction, operating temperatures, and cutting forces. Carbides are the workhorses in today's machine shop. The high cost of carbide is offset by the increased production gained by higher cutting speeds. Standard carbide grades are shown in Table 2; grades C-1 to C-4 are WC types while grades C-5 to C-8 are either TiC, TaC, or a combination WC, TiC, TaC. Grades for a few popular manufacturers are also shown.

Ceramics are made from either alumina or silicon nitride. Because they are refractory materials, ceramics can withstand a great deal of heat. Ceramics are even more brittle than carbide and require extreme rigidity in machinery and associated tooling. Ceramics maintain hot hardness up to 2200° F, yet do not perform well at room temperature. They are also extremely sensitive to thermal shock and are often used without coolant. A new class of whisker reinforced ceramics have been developed to increase toughness. Silicon carbide whiskers are used in the ceramic matrix to provide strength. The expense and sensitivity of ceramics keeps their usage confined to high quality machines charged with high production runs or while cutting difficult materials.