Post on 03-Apr-2018
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MEMS 0040 Materials and Manufacturing
Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Focus here on conventional machining where a sharp hard tool is used to cutaway material. (not EM beams or particulate streams)
• Definition of: chip, rake face, rake angle, cutting edge and relief angle, shear
plane, angle of shear plane, depth of cut and feed for a simple orthogonal cut.
Orthogonal cutting
depth of cut
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MEMS 0040 Materials and Manufacturing
Machining
Topical Subject: CNC machining
• Computer Numerical Controlled
Machining.
• Automation of machining processes by
merging.
• Computer Aided Design (CAD) and
Computer Aided Manufacturing (CAM) to
link shape to a sequence of processes
and then to machine commands.
• Often uses multiple traditional machining
techniques (e.g. cutting, turning, milling,
drilling) in one “cell”.
• Very close matching to CAD for large
number of components.
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MEMS 0040 Materials and Manufacturing
Machining Advantages and
Disadvantages of Machining
1. Applied to the vast majority
of metals
2. Wide shaping flexibility
3. Dimensional accuracy
tolerances up to +/- 0.025mm.
4. Good surface finish:
Roughness down to 0.4 mm.
This is very important to
surface sensitive properties
such as fatigue.
5. Volume of material
removed is waste.
6. Usually takes a long time
to machine large shape or
volume changes.
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MEMS 0040 Materials and Manufacturing
Machining
Flexibility of Conventional
Machining
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MEMS 0040 Materials and Manufacturing
Machining
Flexibility of Conventional
Machining
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MEMS 0040 Materials and Manufacturing
Machining
Flexibility of Conventional
Machining
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MEMS 0040 Materials and Manufacturing
Machining
Flexibility of Conventional
Machining
• Conventional machining is rarely removed by screening of processes but
can rank low due to cost and time! Depends on number of components.
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MEMS 0040 Materials and Manufacturing
Machining
• There is usually not just the one
motion during machining. The
cutting speed is the primary
motion of the tool but the feed is
the secondary motion (e.g. Drill)
and finally there is the depth of
cut.
• E.g. drill:
• primary spin motion of tool
• Secondary motion is feed along
tool axis.
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MEMS 0040 Materials and Manufacturing
Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Machining Operations
Turning: cutting tool has one edge and
reduces diameter of a spinning work piece.
Tool moves perpendicular to the work
piece axis and feed is parallel to axis
Drilling: create round holes
and drill bit had two or more
cutting surfaces. Tool moves
parallel to its axis.
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MEMS 0040 Materials and Manufacturing
Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Machining Operations
Peripheral milling Face milling
Milling: rotating tools have multiple cutting edges is moved along surface of the
work piece to give a flat surface. If tool spins on axis parallel to work surface it is
called peripheral milling. If the tool rotates on axis perpendicular to the work
surface it is called face milling.
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MEMS 0040 Materials and Manufacturing
Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Material Removal Rate in Machining All these operations involve moving the tool relative
to the work piece and this leads to depth of cut and
feed being important variables for the process.
The final variable is the speed :rotation if the tool
spins.
These can be used to calculate the material removal
rate RMR . Example of turning:
RMR (mm3 /s) = V (m/s) f(mm) d(mm)
Where V is the cutting speed, f is the feed (shouldbe in mm/rev for turning) and d is the depth of cut.
For an orthogonal cut
RMR = v to w
V
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MEMS 0040 Materials and Manufacturing
Machining
• Roughing removes a lot of material but tolerances and surface
finish are not good. Feeds of 0.4-1.25mm and depths of 2.5 to20mm.
• Finishing is used to obtain surface finish and tolerances. Feeds of
0.125 to 0.4mm and depths of 0.75 to 2mm.
• Cutting fluid is always used to cool and lubricate tool and work
piece.
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MEMS 0040 Materials and Manufacturing
Machining
• Chip Formation
• Orthogonal cut model has shown that material removal occurs by shear
on a plane that extends from the cutting edge of the tool through the
material to the surface of the piece. This is called the primary shear.
• Once the material has failed in shear the material being removed (chip)then moves up the rake face with friction occurring between the tool rake
face and the cut surface of the chip. This is called the secondary shear .
• The type of chip formed also depends on the mechanical properties of
the work piece and cutting speed.©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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MEMS 0040 Materials and Manufacturing
Machining
• Chip Formation
Discontinuous chip – occurs when
brittle material such as cast iron aremachined at low speed. The chip
segments due to fractures. The
discontinuous nature of the chip
formation causes high surface
roughness.
Continuous chip – when ductile
materials are cut at high speed withsmall depths and feeds, a long
continuous chip if formed with good
surface finish. If chips become too long
in turning may become tangled with
work piece or tool.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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MEMS 0040 Materials and Manufacturing
Machining
• Chip Formation
Continuous chip with built-upedge - ductile materials at low to
moderate speeds when secondary
friction causes chip material to stick
to rake face. This is called built up
edge.
Serrated chip – These chips are semi
continuous and shear becomes much
localized within shear bands in thematerial. This chip leads to a saw tooth
appearance due to alternate high shear
and low shear behavior. Found in
difficult to machine materials such as
titanium and nickel alloys when they are
machined at high speed.©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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MEMS 0040 Materials and Manufacturing
Machining
Analysis of Orthogonal Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• The friction force between the tool
and the material on the rake face
is F and the associated normal
force is N. This defines the
coefficient of friction:
m = F/N
• The resultant force R can then be
defined at the friction angle b to
the direction of N.
• Therefore
m = tan b
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MEMS 0040 Materials and Manufacturing
Machining
Analysis of Orthogonal Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• R must be balanced by a force R’ in the
work piece
• R’ can be related to (i) the shear force Fs
on the plane between the chip and the
work piece and the (ii) normal force Fn.• Therefore the shear stress on the plane
can be defined:
t = Fs/As
Where the area As is :
As= to w / sinf
Where f is the shear plane angle to the machined surface, to is the
depth of cut and w is the width of the machined surface.
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MEMS 0040 Materials and Manufacturing
Machining
Analysis of Orthogonal Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
These forces are not directly measurable and so
they need to be calculated from the cutting force
Fc which is the force measured in the direction
of cutting at the cutting speed and the thrust
force Ft perpendicular to the surface and is
controlled by the depth of cut. Using the aboveexpressions for t and As and the fact that:
Fs = Fccosf –Ftsinf And so
t = (Fccosf – Ftsinf )/ (tow/sinf)
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MEMS 0040 Materials and Manufacturing
Machining
Analysis of Orthogonal Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
The Merchant equation assumes that the
fracture will occur along the direction of
highest shear and therefore only one
angle of f is possible.
In this direction the derivative of the shear
stress t relative to f will be zero (peak
value) and this leads to:
f = 45 + (a/2) - (b/2)
Where a is the rake angle and b is the
friction angle.
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MEMS 0040 Materials and Manufacturing
Machining
Analysis of Orthogonal Machining
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Can decrease As using shear plane angle f which can be increased by:
(i) increasing the rake angle a (design of tool and machining
practice) .
(ii) decreasing friction angle b by using lubricant.
This reduces the shear force, and power required to cut.
Larger aSmaller a
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MEMS 0040 Materials and Manufacturing
Machining
Machining Energy and Power
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• The power to perform machining can be computed from:
P c = F c v
• where P c = cutting power; F c = cutting force; and v = cutting speed.
• In U.S. customary units, power is traditional expressed as horsepower
(dividing ft-lb/min by 33,000)
• where HP c = cutting horsepower, hp 00033,
v F
HP c c
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MEMS 0040 Materials and Manufacturing
Machining
Machining Energy and Power
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Gross power to operate the machine tool P g or HP g is given by:
or E
P P c g E
HP HP c
g
• E is mechanical efficiency of machine tool (usually ~90%).
• Useful to convert power into power per unit volume rate of metal
removal .
• Called unit power , P u
or unit horsepower , Hpu
or
MR
c
U R
P P =
MR
c
U R
P P =
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MEMS 0040 Materials and Manufacturing
Machining
Machining Energy and Power
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Unit power is also known as the specific energy U
Units for specific energy are typically N-m/mm3 or J/mm3 (in-lb/in3)
w vt
v F
R
P
P U o
c
MR
c
u ===
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MEMS 0040 Materials and Manufacturing
Machining
Machining Energy and Power: correlates with material hardness
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Assumes sharp tool and depth of cut to = 0.25 mm (chip thickness).
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MEMS 0040 Materials and Manufacturing
Machining
Machining Energy and Power
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Reducing high cutting power requirements:
1. Reduce cutting speed V
2. Reduce depth of cut or feed to reduce cutting force Fc
3. Use a more machineable material (e.g. free machining steels: may
cost up to 20% more but contain higher lead, sulfur or phosphorous
impurities that create soft particulates and dry lubricants that aid in
chip formation and breakage.
4. Use effective lubricant to reduce friction angle b.
5. Use higher rake anglea.
Or use cutting tool with more power
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MEMS 0040 Materials and Manufacturing
Machining
Cutting Temperature
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Approximately 98% of the energy in machining is converted into heat.
• This can cause temperatures to be very high at the tool-chip.
• 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
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MEMS 0040 Materials and Manufacturing
Machining
Cutting Temperature
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
• Experimental methods can be
used to measure temperatures in
machining. Most frequently used
technique is the tool-chip
thermocouple
• Using this method the
speed-temperature relationship is
usually:
T = K v m
where T = measured tool-chip
interface temperature, and v =
cutting speed