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Write a note on different types of cutting tool materials with their applications.

Mumbai University > Mechanical Engineering > Sem 4 > Production Process II

Marks: 10M

Year: May 2015

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Cutting tool materials characteristics:

  1. Hot hardness: The material must remain harder than the work at elevated operating temperature.
  2. Wear Resistance: The material must withstand excessive wear even though the relative hardness of the tool-work material changes.
  3. Toughness: It actually implies a combination of strength and ductility. The material must have sufficient toughness to withstand shocks and vibrations to prevent breakages.
  4. Cost and easiness in fabrication: The cost and easiness of fabrication should have within reasonable units.

Type of Tool Material -

The selection of proper tool material depends on the type of services to which the tool will be subjected. No material is superior in all aspects, but rather each has certain characteristics which limit its field of application.

The principal cutting materials are:

  1. Carbon steels 2. Medium alloy steels 3. High speed steels 4.Stellites 5. Cemented Carbides 6. Ceramics 7. Diamonds 8.Abrasives

Carbon steels: Carbon steels contain carbon in amounts ranging from 0.08 to 1.5 per cent. A disadvantage of carbon tool steels is their comparatively low heat and wears resistance. They lose their required hardness at temperature from 200 to 250 degree. 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 steels: The high carbon medium alloy steels have a carbon content into plain carbon steels, but in addition there is 5% alloy content consisting of tungsten, molybdenum, chromium and vanadium. Small additions of one or more 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. Therefore alloy carbon steels, occupy a midway performance position between plain carbon steels and high speed steels.

High Speed Steels (H.S.S): It is the general purpose metal for low and medium cutting speeds owing to it superior hot hardness and resistance to wear. High speed steels operate at cutting speeds 2 to 3 times higher than that of carbon steels and retain their hardness till about 900 degree. It is used popularly in hobbing, milling and turning tools. There are three general types of high speed steels; high tungsten, high molybdenum and high cobalt. Tungsten provides hardness and high stability, molybdenum keeps the keenness of the cutting edge, while addition of of cobalt improves hot hardness and makes the cutting tool more wear resistant.

Stellites: It is the trade name of a nonferrous cast alloy composed of cobalt, chromium and tungsten. They cannot be forged to shape, but may be deposited directly on the tool shank in an oxy-acetylene flame, alternatively small tips of cast stellite can be brazed into place. They do not lose their hardness up to 1000 degree and can be operated on double the speed used for H.S.S tools. They are not widely used as the brittle but can be used in non-metal application like rubber and plastic. It requires firm support and is used where wear and abrasions are problems.

Cemented Carbides: They 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. Pure tungsten is mixed with lamp black in the ratio of 94% and 6% by weight at about 1500 degree. The new compound tungsten carbide is then mixed with cobalt until the mass is entirely homogenous. This is made into blocks at very high pressure and then heated in hydrogen. The amount of cobalt used will regulate the toughness of the tool.

Ceramics: The latest development in the metal-cutting tools is the use of aluminium oxide generally referred to as ceramics. They are made by composing aluminium oxide powder in a mold at high pressure. The part is then sintered at about 2200 degree. Hot pressed ceramics are more expensive owing to their higher mould cost. Ceramic tool materials are made in the form of tips that are to be clamped on the metal shanks. Other materials used to produce ceramic tools are silicon carbide, boron carbide titanium carbide and titanium boride.

Diamond: The diamonds which are used for cutting are industrial diamonds which are naturally occurring diamonds consisting of flaws. Therefore they have no value as gemstones. The diamond is the hardest element known and can be operated at about 50 times the cutting speed for HSS tools and can bear temperatures up to 1650 degree. In addition the hardness of diamond is incompressible. It readily conducts heat and has a low coefficient of friction.

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

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