Tool material can be divided into two types:
1. Cutting tool material.
2. Material used for machine tools, gauges, press and press tools, jigs, and fixtures.
1. Types, Properties, and Applications of Cutting Tool Material
Developing cutting tool materials is a continuous process in modern industries. The cutting speed, feed, and depth of cut utilized nowadays have been considerably improved in comparison to the nineteenth century due to the development of science and technology. New dimensions are given to cutting tool materials, their development, etc. as they are very effective in manufacturing the products.
Table of Contents
Different types of cutting tool materials are as under :
1. High carbon steel.
2. High-speed steel.
3. Stellite or cast alloys.
4. Cemented carbides.
The cutting tool material should possess the following three properties :
1. Wear-resistant capacity to maintain the shape and efficiency of the cutting tool.
2. Red hardness capacity to maintain the cutting efficiency of the tool at high temperatures.
3. Toughness for providing resistance to shock and impact that can be taken by the cutting tool during machining operations.
Again it is important to check properties like specific heat and thermal conductivity of tool material. Higher values of both these properties are required, so that the heat generated at the cutting edge of the tool can be dissipated fast. The coefficient of friction of tool material should be less so that the brightness of the surface can be increased and the heat flux rate of heat produced between job and tool remains less. Machinability property is equally important for selecting cutting tool material so that machining can be easily done by such tool material.
1. High Carbon Steel :
Carbon content in this steel is 0.9% to 1.2% which makes the cutting edge of the tool very sharp. It loses its hardness at about 200°C temperature, which means its red hardness value is less.
Applications of high Carbon Steel :
1. It can be used as material for hand tools and tools working at low motion.
2. This type of carbon steel can be forged after heat treatment. Therefore it is used for lathe tools, hand-operated screwing dies and tape, reamers, and broaches. Now desired properties of high-carbon steel can be obtained by alloying.
2. High Speed Steel :
High-speed steel was invented by Tailer, who was an engineer. The H.S.S. 18 : 4 : 1 used nowadays is his invention. This steel contains 18% tungsten, 4% chromium, and 1% vanadium. The hardness of the cutting edge of the tool made from H.S.S. is preserved up to 500°C. The specific classification of H.S.S. is as under :
1. Tungsten steel – contains 14% tungsten.
2. Tungsten molybdenum steel.
3. Tungsten cobalt steel.
Based on experience 1% molybdenum satisfies the need for 2% tungsten and tungsten-cobalt steel is called super high-speed steel, which contains 3 to 5% cobalt, due to its red hardness and wear resistance capacity increase. If vanadium is kept 3%, then a very good finish cut will be obtained and the tool will remain stable against shock and impact.
Applications of High Speed Steel :
1. It can be used as tool material for machines rotating at high speed.
2. For roughing tool for rough machining of forging and casting.
3. Different category of H.S.S. is used in drills to take torsional shear stress.
Stellite is a non-ferrous alloy containing 12 to 19% tungsten, 38 to 40% cobalt, 30 to 35% chromium, therefore having a red hardness value of 800°C. The alloy becomes soft at this temperature but again becomes red and hard after cooling. Its workability and efficiency are best. Its elements remain in their original form as this alloy does not get oxidized at high temperatures. Therefore it can provide a better polished surface and can prevent the formation of a built-up edge on the tool face.
Stellite is used at high speed and temperature due to this property.
Applications of Stellite:
1. As a cutting tool where complete ability is required.
2. Generally is used for machining ductile material at high speed.
Stellite is available in different sections and lengths. It is stuck on the tool holder as it is available in the form of a variety of tips. These tips are fixed on the shank with silver solder.
4. Cemented Carbide :
The cemented carbide is a cutting tool having specific cutting characteristics. It is made by sintering of the tungsten carbide grains embedded in tougher metal. Cemented tools are classified as carbide, nitride, borite, and silicite, but out of them, carbide plays a greater role as a tool due to its properties. All the physical properties are of higher values than steel. According to its properties, it can be said metallic thermal and electrical conductivities are equal to that of metals. The carbide tips are used more as cutting tools due to their basic properties of more hardness and high temperature. At certain temperatures, it has higher values of red hardness in comparison to steel. It contains 55 to 90% carbide particles in the structure of cemented carbide alloy and when the cemented carbide is for metal cutting it contains 80% carbide by volume.
It can be classified broadly into three types :
a. P-grouping: Carbide tool used to cut metal into the form of long chips.
b. K-grouping: Carbide tool used to cut metal into the form of short chips.
c. M-grouping: Carbide tool used to cut metals into the form of chips of medium length.
5. Ceramics :
Aluminum oxide cutting tool material is used in newer experiments. It is called ceramics or cemented oxide. Generally, manufacturers of tools are not producing their commercial composition but those that are already popular are as under :
a. Aluminium oxide: 99 to 99.5% Al203, remaining silica and chromium oxide.
b. Silicon carbide: 99% AI2O3, remaining silica and chromium oxide.
c. Boron carbide: 60% Al203i remaining refractories containing titanium oxide.
d. Titanium carbide: 60% Al203, remaining molybdenum carbide, titanium carbide, and tantalum carbide.
e. Titanium boride: 60% Al203/ remaining titanium boride and molybdenum silisite.
In the borite, the titanium is contained, therefore it is called titanium borite, and molybdenum used in place of titanium makes the easy brazing of the tip. The borite tool performs equally well as compared to the popular carbide tool. The popular ceramics contain 90% Al203 and remaining 19% Cr203, MgO, and Fe304.
The ceramic tools are produced by sintering containing different percentage contents of ceramics in its composition with metals as sintering becomes easy for obtaining metal into a liquid state and the heat conduction coefficient of metal goes up in the presence of carbide. Therefore this tool can be used for grinding at high temperatures and its cutting edge remains in its original form without getting any alteration.
5. Diamond Tools :
The use of diamonds is limited. It is used in powder form for grinding and polishing. It is used in a diamond dresser for grinding wheels and in insert form for drawing dies. It is used for machining of costly plastic materials to obtain the best surface finish and close tolerance of dimensions.
The diamond is precious and costly. Therefore, its use is not viable economically except for certain specific types of machining work. It is popularly used for trueing and dressing of grinding wheels. Diamond bits are used for the machining of bearing metals and other hard steel materials.
Applications of Diamond Tools :
1. Used in special equipment and fixtures.
2. Used in boring turning tool.
3. For truing and dressing of grinding wheels.
4. In lapping paste for valve and valve seat grinding.
Desired Properties of Tool Materials
3. Cutting tool geometry.
4. Toughness and impact resistance.
5. High coefficient of thermal expansion.
7. Wear resistance.
8. Chemically inactive to cutting fluids.
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