CUTTING TOOL MATERIAL

Cutting Tool Material

The importance characteristic of ideal cutting tool material are :

1.Hot Hardness : The material must remain harder than the work material at elevated operating temperatures.

2.Wear resistance : The material must withstand excessive wear even though the relative hardness of the tool-work materials changes.

3.Toughness :   The term "toughness" actually implies a combination of strength and ductility. The material must have a sufficient toughness to withstand shocks and     vibrations and to prevent breakage.

4.Frictional co-efficient :The frictional co-efficient at the chiptool interface must remain low for minimum wear and reasonable surface finish.

5Cost and easeness in fabrication : The cost and easeness of fabrication should have within reasonable limits.

Types of tool materials : The selection of proper tool material depends on the type of service to which the tool will be subjected. No one material is superior in all respects, but rather each has certain characteristics, which limits its field of application.

The principal cutting materials are :

1. Carbon steels

2. Cemented carbide

3. Medium alloy steels

4. Ceramics     

5. High-speed steels

6. Diamonds

7. Satellites

9. Abrasives.

The important cutting tool materials are :

Carbon steels :Carbon steels contain carbon in amounts ranging from 0.80 to 1.5 %. A disadvantage of carbon tool steels is their comparatively low-heat and wear resistance. They lose their required hardness at temperatures from 200 degrees to 250 degree centigrade. Therefore they may only be used in the manufacture of tools operating at low cutting speeds and of hand operated tools. But they are comparatively cheap, easy to forge, and simple to harden.

Medium alloy steel : The high carbon medium alloy steels have a carbon content akin to plain carbon steels, but in addition there is, say upto 5 % alloy content consisting of tungsten, molybdenum, chromium and vanadium. Small additions of one or more of these elements improve the performance of the carbon steels in respect of hot hardness, wear resistance, shock and impact resistance and resistance to distortion during heat treatment. The alloy carbon steels, therefore, broadly occupy a midway performance position between plain carbon and high speed steels. They lose their required hardness at temperatures from.250 degrees to 350 degrees centigrade.

High speed steels : High speed steel (h.s.s.) is the general purpose metal for low and medium cutting speeds owing to its superior hot hardness and resistance to wear. High-speed steels operate at cutting speeds 2 to 3 times higher than for carbon steels and retain their hardness upto about 900 deg. centigrade. There are three general types of high-speed steels; high tungsten and high molybdenum, and high cobalt. Tungsten in h.s.s. provides hot hardness and form stability, molybdenum or vanadium maintains keenness of the cutting edge, while addition of cobalt improves hot hardness and makes the cutting tool more wear resistant.

The general types of high-speed steels are as follows :

1. 18-4-1 high speed steel: This steel containing 18 percent tungsten, 4 percent chromium and 1 percent vanadium.
2. Molybdenum high speed steel: This steel containing 6 percent molybdenum, 6 percent tungsten, 4 percent chromium and 2 percent vanadium have excellent toughness and cutting ability.
3. Cobalt high-speed steels: This is sometimes called super high speed steel. Cobalt is added from 2 to 15 percent to increase hardness and wear resistance. One analysis of this steel contains 20 tungsten, 4 percent chromium, 2 percent of vanadium, and 12 percent cobalt.

Cemented carbides : Cemented carbides are so named because they are composed principally of carbon mixed with   other elements.   The basic ingredient of most cemented carbides is tungsten carbide which is extremely hard. Carbide tools are made by brazing or silver-soldering the formed inserts on the ends of the commercial steel holders. The most important properties of cemented carbides are their very high heat and wear resistance. Cemented carbide tipped tools can machine metals even when their cutting elements are heated to a temperature of 1000 deg. centigrade. They can with stand cutting speed 6 or more than 6 times higher than tools of high-speed steel. Cemented carbide is the hardest manufactured material and has extremely high compressive strength. However, it is very brittle, has low resistance to shock, and must be very rigidly supported to prevent cracking.

There are two types of cemented carbides. They are

1. Tungsten type cemented carbides

2. Titanium tungsten type cemented carbides

Ceramics : The latest development in the metal-cutting tools uses aluminium oxide, generally referred to as ceramics. Ceramic tools have very low heat conductivity and extremely high compressive strength, but they are quite brittle and have a low b ending strength. For this reason, these materials cannot be used for tools operating in interrupted cuts, with vibrations, as well as for temperatures up to 1200 deg. centigrade and can be used at cutting speeds 4 times that of cemented carbides, and up to 40 times that of high-speed steel. They are chiefly used for single-point tools in semi-finish and finish turning of cast iron, plastics and other work, but only when they are not subject to impact loads. To give them increased strength often ceramic with a metal bond, known as "cermets" is used. Because of high compressive strength and brittleness the tips are given a 5 deg. to 8 deg. negative rake for carbon steel and zero rake for cast iron and for non-metallic materials to strengthen their cutting edge and are well supported by the tool holder.

Diamonds : The diamond is the hardest known material and can be run at cutting speeds about. 50 times greater than that for h.s.s. tool, and at temperatures up to 1650 deg centigrade. In addition to the hardness the diamond is incompressible, is of a large grain structure, readily suitable for cutting very hard materials such as glass, plastics, ceramics, and other abrasive materials and for producing fine finishes. The maximum depth of cut recommended is 0.125mm with feeds of about 0.05mm.

Abrasives : Abrasive grains in various form - loose, bonded into wheels and stone, and embedded in papers and cloths - find wide application in industry. They are mainly used for grinding harder materials and where a superior finish is desired on hardened or unhardened materials.

For most grinding operations, there are tow kinds of abrasives in general use, namely aluminium oxide (carborundum) and silicon carbide. The aluminium oxide abrasives are used for grinding all high tensile materials, whereas silicon carbide abrasives are more suitable for low tensile materials and non-ferrous metals