Finishing Cuts
0.010” for carbides
0.015” for high speed steel
Work surface finish is inadequate
Work dimension outside tolerance
• Flank wear can be discussed as a function of time,
• 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
4.2 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:
HSS: High Speed Steel
PCD: PolyCrystalline Diamond
sialon:
WC: Tungsten Carbide
coated WC: Tools coated with Tungsten Carbide
4.2.1 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.
Compressive strength is high compared to tensile strength, therefore the bits are often brazed to steel shanks, or used as inserts in holders
These inserts may often have negative rake angles
Speeds up to 300 fpm are common on mild steels
Hot hardness properties are very good
coolants and lubricants can be used to increase tool life, but are not required.
special alloys are needed to cut steel
• Ceramics
sintered or cemented ceramic oxides, such as aluminum oxides sintered at 1800°F
Can be used for turning and facing most metals, except for nimonic alloys and titanium. Mild steels can be cut at speeds up to 1500 fpm.
These tools are best used in continuous cutting operations
There is no occurrence of welding, or built up edges
coolants are not needed to cool the workpiece
Very high hot hardness properties
often used as inserts in special holders
• Diamonds
a very hard material with high resistance to abrasion
very good for turning and boring, producing very good surface finish
operations must minimize vibration to prolong diamond life
also used as diamond dust in a metal matrix for grinding and lapping. For example, this is used to finish tungsten carbide tools
• Cemented Oxides
produced using powder metallurgy techniques
suited to high speed finishing
cutting speeds from 300 to 7500 fpm
coolants are not required
high resistance to abrasive wear and cratering
4.3 Tool Life
• Tool life is the time a tool can be reliably be used for cutting before it must be discarded/repaired.
• Some tools, such as lathe bits are regularly reground after use.
• A tool life equation was developed by Taylor, and is outlined below,
• An important relationship to be considered is the relationship between cutting speed and tool life,
• Although the previous equation is fairly accurate, we can use a more complete form of Taylor’s tool life equation to include a wider range of cuts.
4.3.1 The Economics of Metal Cutting
• As with most engineering problems we want to get the highest return, with the minimum investment. In this case we want to minimize costs, while increasing cutting speeds.
• EFFICIENCY will be the key term: it suggests that good quality parts are produced at reasonable cost.
• Cost is a primarily affected by,
tool life
power consumed
• The production throughput is primarily affected by,
accuracy including dimensions and surface finish
mrr (metal removal rate)
• The factors that can be modified to optimize the process are,
cutting velocity (biggest effect)
feed and depth
work material
tool material
tool shape
cutting fluid
• We previously considered the log-log scale graph of Taylor’s tool life equation, but we may also graph it normally to emphasize the effects.
• There are two basic conditions to trade off,
Low cost: exemplified by low speeds, low mrr, longer tool life
High production rates: exemplified by high speeds, short tool life, high mrr
*** There are many factors in addition to these, but these are the most commonly considered
• A simplified treatment of the problem is given below for optimizing cost,
• We can also look at optimizing production rates,
• We can now put the two optimums in perspective,
4.4 References
4.1 Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.
4.5 Problems
Problem 4.1 If a bar of SAE 1040 is to be turned with a high speed steel tool with a feed of 0.015” per revolution, and a depth of 0.050”. Previous experiments have revealed that the following cutting velocities yielded the following tool lives,
90 fpm for 30 min.
80 fpm for 90 min.
75 fpm for 150 min.
a) estimate the cutting speeds to get tool lives of 60 and 120 minutes.
b) calculate the mrr at the two speeds found in part a).
Problem 4.2 Two tools are being compared for their costs. The table below summarizes the details of each tool. Find the economic tool life and cutting speed for each tool, and determine the least expensive tool.
