page 41

Iw	V3	V2
#1	#2	#3
		V1
		time
where,

V1, V2, V3 = cutting velocities where V3>V2>V1 #1 - In this region the tool point is starting to dull #2 - A typical tool wear region

#3 - This zone is temperature sensitive

•General notes of concern are,

-The main factor in tool wear is temperature

-The main factor in tool life is cutting speed

-Critical temperatures for High Speed Steels are 1150°F and for carbides it is 1600°F

-A higher velocity will increase temperature more than an increase in feed for the same mrr

-A higher feed will increase the tool forces

5.7 CUTTING TOOL MATERIALS

•These materials generally need to withstand high temperatures, high forces, resist corrosion, etc.

•The names used for certain materials will be brand names, and so various manufacturers may be calling the same material, different names.

•The List below shows some commercial tool materials

CBN - Cubic Boron Nitride ceramic -

page 42

HSS - High Speed Steel

PCD - PolyCrystalline Diamond sialon -

WC - Tungsten Carbide

coated WC - Tools coated with Tungsten Carbide

5.7.1 A Short List of Tool Materials

•Carbon Steels

-Limited tool life. Therefore, not suited to mass production

-Can be formed into complex shapes for small production runs

-low cost

-suited to hand tools, and wood working

-Carbon content about 0.9 to 1.35% with a hardness ABOUT 62°C Rockwell

-Maximum cutting speeds about 26 ft/min. dry

-The hot hardness value is low. This is the major factor in tool life.

•High Speed Steel

-an alloyed steel with 14-22% tungsten, as well as cobalt, molybdenum and chromium, vanadium.

-Appropriate heat treating will improve the tool properties significantly (makers of these steels often provide instructions)

-can cut materials with tensile strengths up to 75 tons/sq.in. at speeds of 50-60 fpm

-Hardness is in the range of 63-65°C Rockwell

-The cobalt component give the material a hot hardness value much greater than Carbon Steels

-Used in all type of cutters, single/multiple point tools, and rotary tools

•Stellite

-a family of alloys made of cobalt, chromium, tungsten and carbon

-The material is formed using electric furnaces, and casting technique, and it cannot be rolled, or worked.

-The material has a hardness of 60-62°C Rockwell without heat treating, and the material has good hot hardness properties

-Cutting speed of up to 80-100 fpm can be used on mild steels

-The tools that use this method either use inserts in special holders, or tips brazed to carbon steel shanks

•Tungsten Carbide

-Produced by sintering grains of tungsten carbide in a cobalt matrix (it provides toughness).

-Other materials are often included to increase hardness, such as titanium, chrome, molybdenum, etc.