LATHE-CUTTING TOOl MATERIALS

 Cutting tool materials

. qualities of good cutting tool material

. characteristics of cold hardness, red hardness and toughness

. the factors to be remembered when selecting tool material

. the different tool materials

. the characteristics of different tool materials

Cutting tool materials

Tool materials should be :

-harder and stronger than the material being cut

-tough to resist shock loads

-resistant to abrasion thus contributing to long tool life.

Cutting tool material should possess the following qualities.

- cold hardness

-red hardness

-toughness

It is the amount of hardness possessed by a material at normal temparature. Hardness is the property by which it can cut/scratch other metals. When hardness increase, brittleness also increases, and a material, which has tooo much of cold hardness, is not suitable for the manufature of cutting tool.

Red hardness

It is the ability of a tool material to retain most of its cold hardness properly even at very high temparature. While matching, the friction between the tool and the work, the tool and the chips, causes heat to be generated, and the tool loses its hardness, and its efficiency to cut diminishes. If  a tool maintains its cutting efficiency even at increased temaprature during cutting, it can be said that it possesses the red hardness property.

Toughness

The property to resist breakage due to sudden load that results during metal cutting is termed as 'toughness '. This will reduce the breakage of the cutting edges of tools.

The following factors are to be considered, when selecting a tool material.

-material to be machined

-condition of the machine tool (rigidity and efficiency )

-the total quantity of production and the rate of production

-the diemensional accuracy required and the quality of surface finish

-the amount of coolant applied and method  of application

-condition and form of material to be machined

Grouping of tool material

The three groups under which tool materials fall are

-ferrous tool materials

-non ferrous tool materials

-non metallic tool materials

Ferrous tool materials

These materials have iron as their cheif constituent. High carbon steel (tool steel) and high speed steel belong to this group.

Non ferrous tool materials

These do not have iron, and they are formed by alloying elements like tungsten, vanadium and molybdenum stelite belongs to this group.

Carbides

These materials are also non ferrous. They are manufactured by powder metallurgy technique. Carbon and tungsten are the chief alloying elements.

Non metallic materialbit.

These tool materials are made out of non metals. Ceramics and diamonds belong to this group.

Higj carbon steel is the first tool material introduced for manufacturing cutting tools. It has poor red hardness property, and it loses its cutting efficiency very quickly. Alloying elements like tungsten, chromium andvanadium, are used to produce high speed steel tool material. Its red hardness property is more than that of high carbon steel.

High speed steel is used for making solid tools, brazed tools and inserted bits. It is costlier than the high carbon steel. Carbide cutting tools can retain their hardness at very high temparature, and their cutting efficiency is higher than that of high speed steel. Due to its  brittleness and cost, a carbide tool cannot be used as a solid tool. It is used as a brazed tool and throw away tool bit.