Uploaded by Gavin Idol

Chapter 20

Overview of Machining Technology
Theory of Chip Formation in Metal Machining
Force Relationships and the Merchant Equation
Cutting Temperature
(Video) CNC Lathe
Material Removal Processes
 A family of shaping operations, the common feature of
which is removal of material from a starting work part so
the remaining part has the desired geometry
 Machining – material removal by a sharp cutting tool,
e.g., turning, drilling, milling
 Abrasive processes – material removal by hard,
abrasive particles, e.g., grinding
 Nontraditional processes - various energy forms other
than sharp cutting tool to remove material, e.g.,
mechanical, electromechanical, thermal, & chemical
energy forms
 The family
tree of
 Cutting action involves shear deformation of work
material to form a chip
 As chip is removed, new surface is exposed: (a) positive and
(b) negative rake tools
Advantages of Machining
Variety of work materials can be machined
 Most frequently used to cut metals
Variety of part shapes and special geometric features
 Screw threads
 Accurate round holes
 Very straight edges and surfaces
Good dimensional accuracy and surface finish
 Achieve tolerances of +/- .001 in
 Achieve roughness values less than 16 microinches
Disadvantages of Machining
 Wasteful of material
 Chips generated in machining are wasted material
 At least in the unit operation
 Chips are usually recycled
 Time consuming
 A machining operation generally takes longer to shape
a given part than alternative shaping processes such
as casting and forming
Machining in the
Manufacturing Sequence
 Generally performed after other basic manufacturing
processes, such as casting, forging, and bar drawing
 Other processes create the general shape of the
starting work part
 Machining provides the final shape, dimensions,
finish, and special geometric details that other
processes cannot create
Machining Operations
 Most important machining operations:
 Turning
(Video) Shaping Machine
 Drilling
 Milling
(Video) Planing Machine
 Other machining operations:
 Shaping and Planing (Video) Broaching
 Broaching
(Video) Band Sawing Machine
 Sawing
(Video) Circular Sawing Machine
 Single point cutting tool removes material from a rotating
workpiece to form a cylindrical shape
 Used to create a round
hole, usually by means of
a rotating tool (drill bit)
with two cutting edges
 Rotating multiple-cutting-edge tool is moved across work
to cut a plane or straight surface
 Peripheral milling: cutter axis parallel to surface being milled
 Face milling: cutter axis is perpendicular to surface being milled
Cutting Tool Classification
1. Single-Point Tools
 One dominant 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
Cutting Tools
 (a) Single-point tool showing rake face, flank, and tool point
 (b) Helical milling cutter, representative of tools with multiple
cutting edges
Machine Tool
 A power-driven machine that performs a machining
operation, including grinding
 Functions in machining:
 Holds work part
 Positions tool relative to work
 Provides power at speed, feed, and depth that
have been set
 The term also applies to machines that perform metal
forming operations
Cutting Conditions in
 Three dimensions of
a machining process
 Cutting speed v :
primary motion
 Feed f :
secondary motion
 Depth of cut d :
penetration of tool
below original
work surface
Cutting Conditions in
 For certain operations (e.g., turning), material removal rate
RMR can be computed as
RMR = v f d
v = cutting speed (in/s)
f = feed (in)
d = depth of cut (in)
RMR = removal rate (in3/s)
Roughing vs. Finishing Cuts
 In production, several roughing cuts are usually taken on a
part, followed by one or two finishing cuts
 Roughing - removes large amounts of material from
starting work part
 Some material remains for finish cutting
 High feeds and depths, low speeds
 Feeds of .015 - .050 in/rev, & depths of .100 - .750 in
 Finishing - completes part geometry
 Final dimensions, tolerances, and finish
 Low feeds and depths, high cutting speeds
 Feeds of .005 - .015 in/rev, & depths of .030 - .075 in
Orthogonal Cutting Model
 Orthogonal cutting uses a wedge-shaped tool in which the
cutting edge is perpendicular to the direction of speed motion
as the tool is forced into the work material
 Simplified 2-D model of machining that describes the mechanics
of machining fairly accurately
Chip Thickness Ratio
 Chip thickness ratio
r 
r = chip thickness ratio
to = thickness of the chip prior to chip formation
tc = chip thickness after separation
 Chip thickness after cut is always greater than before, so
chip ratio is always less than 1.0
Shear Plane Angle
 Based on the geometric parameters of the orthogonal
model, the shear plane angle  can be determined as:
r cos 
tan  
1  r sin
r = chip thickness ratio
 = rake angle
Shear Strain
 Shear strain in machining can be computed from the
following equation, based on the preceding parallel plate
 = tan( - ) + cot 
 = shear strain
 = shear plane angle
 = rake angle of cutting tool
Orthogonal Cutting
 In a machining operation that approximately orthogonal cutting, the
cutting tool has a rake angle α = 10°. The chip thickness before the
cut to = 0.50 mm and the chip thickness after the cut tc = 1.125 mm.
Calculate the shear plane angle and the shear strain in the
 The chip thickness ratio:
r = to / tc = 0.50 / 1.125 = 0.444
 The shear plane angle:
tan  = (r cosα) / (1-r sinα)
= (0.444 cos10) / (1-0.444 sin10)
= 0.4738
 = 25.4°
 The shear strain:
 = tan( - ) + cot 
= tan(25.4-10) + cot25.4
= 2.386
Actual Chip Formation
 More realistic view
of chip formation,
showing shear
zone rather than
shear plane
 Also shown is the
secondary shear
zone resulting
from tool-chip
Four Basic Types of Chip in
Discontinuous chip
Continuous chip
Continuous chip with Built-up Edge (BUE)
Serrated chip
Discontinuous Chip
 Brittle work materials
 Low cutting speeds
 Large feed and depth of
 High tool-chip friction
Continuous Chip
 Ductile work materials
 High cutting speeds
 Small feeds and depths
 Sharp cutting edge
 Low tool-chip friction
Continuous with Built-Up Edge
 Ductile materials
 Low-to-medium cutting
 Tool-chip friction causes
portions of chip to
adhere to rake face
 BUE forms, then breaks
off, cyclically
Serrated Chip
 Semicontinuous - sawtooth appearance
 Cyclical chip forms
with alternating high
shear strain then low
shear strain
 Associated with
metals at high cutting
Forces Acting on Chip
 Friction force F and Normal force to friction N
 Shear force Fs and Normal force to shear Fn
Resultant Forces
 Vector addition of F and N = resultant R
 Vector addition of Fs and Fn = resultant R'
 Forces acting on the chip must be in balance:
 R' must be equal in magnitude to R
 R’ must be opposite in direction to R
 R’ must be collinear with R
Cutting Force and Thrust
 F, N, Fs, and Fn
cannot be directly
 Forces acting on the
tool that can be
 Cutting force Fc and
Thrust force Ft
Forces in Metal Cutting
 Equations to relate the forces:
F = Fc sin + Ft cos
N = Fc cos - Ft sin
Fs = Fc cos - Ft sin
Fn = Fc sin + Ft cos
 Based on these calculated
force, shear stress and
coefficient of friction can be
 s
Shear Stress
 Shear stress acting along the shear plane
where As = area of the shear plane
As 
t ow
sin 
 Shear stress  equals shear strength S of work material
during cutting
Coefficient of Friction
 Coefficient of friction between tool and chip
 Friction angle related to coefficient of friction as
  tan 
The Merchant Equation
 Of all the possible angles at which shear deformation can occur,
the work material will select a shear plane angle  that minimizes
  45 
 Derived by Eugene Merchant
 Based on orthogonal cutting,
but validity extends to 3-D machining
 To increase shear plane angle ( )
 Increase the rake angle (α)
 Reduce the friction angle (β) or the coefficient of friction (μ)
Shear Stress in Machining
 In a machining operation that approximately orthogonal cutting, the
cutting tool has a rake angle α = 10° and shear plane angle  =
25.4°. The chip thickness before the cut to = 0.50 mm and the chip
thickness after the cut tc = 1.125 mm. Also Fc = 1559 N and Ft =
1271 N. The width of the orthogonal cutting operation w = 3.0 mm.
Determine the shear strength of the work material.
 The shear force:
Fs = Fc cos - Ft sin
= 1559(cos25.4)-1271(sin25.4) = 863 N
 The shear plane area:
 The shear stress:
As = tow / sin
= (0.5)(3.0) / sin25.4 = 3.497 mm2
 = S = Fs / As
= 863 / 3.497 = 247 N/mm2 = 247 MPa
Estimating Friction Angle
 In a machining operation that approximately orthogonal cutting, the
cutting tool has a rake angle α = 10° and shear plane angle  =
25.4°. Determine the friction angle and the coefficient of friction.
 Using the Merchant equation:
 The friction angle:
  45 
β = 2(45) + α - 2
= 2(45) + 10 – 2(25.4) = 49.2°
 The coefficient of friction:
μ = tan β
= tan(49.2) = 1.16
Effect of Higher Shear Plane
 Higher shear plane angle  with a resulting lower shear plane area
 Smaller shear plane angle  with a corresponding larger shear
plane area
Cutting Temperature
 Approximately 98% of the energy in machining is converted into
 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
 High cutting temperatures result in the following:
 Reduce tool life
 Produce hot chips that pose safety hazards to the
machine operator
 Can cause inaccuracies in part dimensions due to
thermal expansion of work material