Many types of tool materials, ranging from high carbon steel to ceramics and diamonds, are used as cutting tools in today’s metalworking industry. It is important to be aware that differences do exist among tool materials, what these differences are, and the correct application for each type of material.

A cutting tool must have the following characteristics in order to produce good quality and economical parts:

 

Hardness — harness and strength of the cutting tool must be maintained at elevated temperatures, also called hot hardness (Figure 1.1)

 

Toughness — toughness of cutting tools is needed so that tools don’t chip or fracture, especially during interrupted cutting operations.

 

Wear Resistance — wear resistance means the attainment of acceptable tool life before tools need to be replaced.

 

     The materials from which cutting tools are made are all characteristically hard and strong. There is a wide range of tool materials available for machining operations, and the general classification and use of these materials are of interest here.

 

Tool Steels and Cast Alloys

 

    Plain carbon tool steel is the oldest of the tool materials dating back hundreds of years. In simple terms, it is a high-carbon steel, which contains about 1.05% carbon. This high carbon content allows the steel to be hardened, offering greater resistance to abrasive wear. Plain high carbon steel served its purpose well for many years. However, because it is quickly over tempered (softened) at relatively low cutting temperatures (300 to 500°F), it is now rarely used as cutting tool material except in files, saw blades, chisels, etc. The use of plain high carbon steel is limited to low heat applications.

 

    High Speed Tool Steel: The need for tool materials that could withstand increased cutting speeds and temperatures led to the development of high-speed tool steels (HSS). The major difference between HSS and plain high carbon steel is the addition of alloying elements to harden and strengthen the steel and make it more resistant to heat (hot hardness).

  

    An important point to remember is that none of the alloying elements for either series of HSS is in abundant supply and the cost of these elements is skyrocketing. In addition, U.S. manufacturers must rely on foreign countries for supply of these very important elements.

 

   HSS Surface Treatment: Many surface treatments have been developed in an attempt to extend tool life, reduce power consumption, and to control other factors that affect operating conditions and costs.

 

   Cast Alloys: The alloying elements in HSS - principally cobalt, chromium, and tungsten - improve the cutting properties sufficiently, that metallurgical researchers developed the cast alloys, a family of materials without iron.

 

    A typical composition for this class was 45% cobalt, 32% chromium, 21% tungsten, and 2% carbon. The purpose was to obtain a cutting tool with hot hardness superior to HSS.

 

    When applying cast alloy tools, their brittleness should be kept in mind and sufficient support should be provided at all times. Cast alloys provide high abrasion resistance and are thus useful for cutting scaly materials or those with hard inclusions.

 

Classification of Carbide Tools

 

Cemented carbide products are classified into three major categories:

 

• Wear Grades — used primarily in dies, machine and tool guides, and in everyday items such as line guides on fishing rods and reels. Used anywhere good wear resistance is required.

 

• Impact Grades — also used for dies, particularly for stamping and forming, and in tools such as mining drill heads.

 

• Cutting Tool Grades — the cutting tool grades of cemented carbides are divided into two groups, depending on their primary application. If the carbide is intended for use on cast iron that is a nonductile material, it is graded as a cast iron carbide. If it is to be used to cut steel, a ductile material, it is graded as a steel grade carbide.

 

    Cast iron carbides must be more resistant to abrasive wear. Steel carbides require more resistance to cratering and heat. The tool wear characteristics of various metals are different, thereby requiring different tool properties. The high abrasiveness of cast iron causes mainly edge wear to the tool. The long chip of steel, which flows across the tool at normally higher cutting speeds, causes.