Cutting Tool Technology
Three principal aspects:
1. Cutting temperature
2. Tool Life
3. Tool Materials
Cutting Tool Classification
1. Single-Point Tools
One cutting edge
Point is usually rounded to form a nose radius
Turning uses single point tools
2. Multiple Cutting Edge Tools
More than one cutting edge
Motion relative to work achieved by rotating
Drilling and milling use rotating multiple cutting edge tools
Cutting Tools
Figure 21.4
(a) A single-point tool showing rake face, flank, and tool point; and
(b) a helical milling cutter, representative of tools with multiple cutting edges.
Three Modes of Tool Failure
1. Fracture failure
Cutting force becomes excessive and/or dynamic, leading
to brittle fracture
2. Temperature failure
Cutting temperature is too high for the tool material
3. Gradual wear
Gradual wearing of the cutting tool
Cutting Temperature
Approximately 98% of the energy in machining is converted
into heat
This can cause temperatures to be very high at the tool-chip
The remaining energy (about 2%) is retained as elastic
energy in the chip
Cutting Temperatures are Important
High cutting temperatures
1. Reduce tool life
2. Produce hot chips that pose safety hazards to the machine
operator
3. Can cause inaccuracies in part dimensions due to thermal
expansion of work material
Cutting Temperature
Experimental methods can be used to measure temperatures
in machining
Most frequently used technique is the tool-chip
thermocouple
Using this method, Ken Trigger determined the
speed-temperature relationship to be of the form:
T = K vm
where T = measured tool-chip interface temperature, and
v = cutting speed
k, m = constant
Tool Gradual Wear
Fracture and temperature failures are premature (early)
failures
Gradual wear is preferred because it leads to the
longest possible use of the tool
Gradual wear occurs at two locations on a tool:
Crater wear – occurs on top rake face
Flank wear – occurs on flank (side of tool)
Tool Wear
Figure 23.1 Diagram of worn cutting tool, showing the principal locations
and types of wear that occur.
Tool Wear vs. Time
Figure 23.3 Tool wear as a function of cutting time.
Flank wear (FW) is used here as the measure of tool wear.
Crater wear follows a similar growth curve.
Effect of Cutting Speed
Figure 23.4 Effect of cutting speed on tool flank wear (FW) for three
cutting speeds, using a tool life criterion of 0.50 mm flank wear.
Tool Life vs. Cutting Speed
Figure 23.5 Natural log-log plot of cutting speed vs tool life.
Taylor Tool Life Equation
Relationship is credited to (F. W. Taylor) vT n C
where v = cutting speed; T = tool life; and n and C are
parameters that depend on feed, depth of cut, work material,
tooling material, and the tool life criterion (standard) used
n is the slope of the plot
C is the intercept on the speed axis at one minute tool life
Typical Values of n and C
Tool material n C (m/min) C (ft/min)
High speed steel:
Non-steel work 0.125 120 350
Steel work 0.125 70 200
Cemented carbide
Non-steel work 0.25 900 2700
Steel work 0.25 500 1500
Ceramic
Steel work 0.6 3000 10,000
Tool Life Criteria (standard) in Production
1. Complete failure of cutting edge
2. Visual inspection of flank wear (or crater wear) by the machine
operator
3. Fingernail test across cutting edge
4. Changes in sound emitted from operation
5. Chips become ribbon-like, and difficult to dispose of
6. Degradation of surface finish
7. Increased power
8. Work - piece count
9. Cumulative cutting time
Tool Materials
Tool failure modes identify the important properties that a tool
material should possess:
Toughness - to avoid fracture failure
Hot hardness - ability to retain hardness at high temperatures
Wear resistance - hardness is the most important property to
resist abrasive ( كشط- )احتكاكwear
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Hot Hardness
Figure 23.6 Typical hot hardness relationships for selected tool materials.
Plain carbon steel shows a rapid loss of hardness as temperature increases.
High speed steel is basically better, while
cemented carbides and ceramics are significantly harder at high temperatures.
High Speed Steel (HSS)
Highly alloyed tool steel capable of maintaining hardness at
elevated temperatures better than high carbon and low alloy
steels
One of the most important cutting tool materials
Especially suited to applications involving complicated tool
geometries, such as drills, and milling cutters
Two basic types
1. Tungsten-type, designated T- grades
2. Molybdenum-type, designated M-grades
High Speed Steel Composition
Typical alloying ingredients:
Tungsten and/or Molybdenum
Chromium and Vanadium
Carbon, of course
Cobalt in some grades
Typical composition (Grade T1):
18% W, 4% Cr, 1% V, and 0.9% C
