Transcript IE 243 MANUFACTURING PROCESSES

IE 243
MANUFACTURING PROCESSES
Material Removal Processes
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Definition of Material Removal
Processes
• Material removal processes are shaping operations. The
common feature is removal of material from a starting
workpart so the remaining part has the desired shape.
• Variety of work materials can be machined. Most frequently
applied to metals.
• Variety of part shapes and special geometry features
possible, such as: Screw threads, accurate round holes, very
straight edges and surfaces.
• Good dimensional accuracy and surface finish is possible.
• However material removal processes are time consuming and
wasteful of material.
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Definition of Material Removal
Processes
• There are three categories:
– Machining
– Abrasive operations
– Nontraditional operations
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Material Removal Processes
Classification
• Machining:
– Material removed from the surface of the workpart by
means of sharp cutting tools.
• Examples are turning, milling, drilling, etc.
turning
milling
drilling
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Material Removal Processes
Classification
• Abrasive operations:
– Material removed from the surface of the workpart by
means of hard abrasive particles.
• e.g. grinding
grinding
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Material Removal Processes
Classification
• Nontraditional operations:
– Various energy forms other than sharp cutting tool to
remove material
• e.g. electro-discharge machining, water jet cutting etc.
Electro-discharge machining
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Material Removal Processes
Machining
• Relative motion between the
cutting tool and the workpiece
develops a cutting action.
• Cutting action involves shear
deformation of work material to
form a chip.
• As chip is removed, a new
surface is exposed.
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Material Removal Processes
Machining
• Chip formation due to shear
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Material Removal Processes
Machining
• Some terms and definitions in machining
– There is relative motion between the workpiece and
the cutting tool.
• Primary motion: Cutting motion (defined by cutting
speed)
• Secondary motion: Feed motion (defined by the feed
rate)
• Depth of cut (defines the amount of plunging of the tool
into the workpiece)
Any machining operation involves these quantities.
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Material Removal Processes
Machining
• Most important machining operations are
– Turning
– Milling
– Drilling
• Other machining operations are
– Shaping and planing
– Broaching
– Sawing
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Material Removal Processes
Machining
• Turning
– A single point cutting tool removes material from a
rotating workpiece to form a cylindrical shape.
Q: Define the primary motion, secondary motion, and depth of cut.
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Material Removal Processes
Machining
• Drilling
– Used to create a round hole, usually by means of a
rotating tool (drill bit) that has two cutting edges.
Q: Define the primary motion, secondary motion, and depth of cut.
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Material Removal Processes
Machining
• Milling
– Rotating multiple-cutting-edge tool is moved slowly
relative to work to generate plane or straight surface.
– There are two forms of milling;
• Peripheral milling
• Face milling
Q: Define the
primary motion,
secondary
motion, and
depth of cut.
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Material Removal Processes
Machining
• Classification of the cutting tools
– Single-Point Tools:
• One cutting edge
• Turning uses single point tools
• Point is usually rounded to form a nose radius
– Multiple Cutting Edge Tools:
• More than one cutting edge
• Motion relative to work usually achieved by rotating
• Drilling and milling use rotating multiple cutting edge
tools.
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Material Removal Processes
Machining
• Classification of the cutting tools
– Single-Point Tools:
• One cutting edge
• Turning uses single point tools
• Point is usually rounded to form a nose radius
– Multiple Cutting Edge Tools:
• More than one cutting edge
• Motion relative to work usually achieved by rotating
• Drilling and milling use rotating multiple cutting edge tools.
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Material Removal Processes
Machining
• Analysis
– We know that shear is involved for formation of the chips.
– In order to analyze the physics orthogonal cutting model
will be used.
– Orthogonal cutting model is a simplified 2D model of the
machining operations.
Oblique
cutting
(more
realistic)
Orthogonal
cutting
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Material Removal Processes
Machining
• Analysis
– Basic terms:
• Chip thickness ratio:
t0
r
tc
Chip thickness ratio
is always greater
than 1.
Rake angle may be
positive or negative!
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Material Removal Processes
Machining
• Analysis
– Basic terms:
• Shear plane angle:
r cos 
tan  
1  r sin 
Can be derived using
trigonometric
relations here.
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Material Removal Processes
Machining
• Analysis
– Forces in cutting:
• Forces acting on the chip:
F : Friction force on tool chip interface
(friction coefficient is  )
N : Normal force to friction
Fs : Shear force
Fn : Normal force to shear
R : Resultant of the forces F and N
R : Resultant of the forces Fs and Fn
These forces cannot be measured
directly!
Note that
F  N
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Material Removal Processes
Machining
• Analysis
– Forces in cutting:
• Forces acting on the tool:
Fc : cutting force
Ft : thrust force
R: Resultant of Fc and Ft
These forces can be measured
directly!
Note that R, R, and R must be equal in magnitude
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Material Removal Processes
Machining
• Analysis
– Forces in cutting:
• Force circle:
The circle whose
diameter is the
resultant R, and
thrust and cutting
forces are the
vertical and
horizontal
components of the
resultant R.
Cutting
tool
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Material Removal Processes
Machining
• Analysis
α
– Forces in cutting:
• Using this force
circle one can
derive:
Φ
Cutting
tool
F  Fc sin   Ft cos 
N  Fc cos   Ft sin 
Fs  Fc cos   Ft sin 
Fn  Fc sin   Ft cos 
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Material Removal Processes
Machining
• Analysis
– Power and Energy Relations:
• Power required to perform a machining operation
Pc  Fc v
where Fc is the cutting force, v is the cutting speed.
• Power is traditionally expressed in kW or HP (horsepower)
– 1 HP = 0.746 kW
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Material Removal Processes
Machining
• Analysis
– Power and Energy Relations:
• The efficiency of the machine tool (the machine used for the
machining operation) may not be 100 %, thus not all of the
the power it takes is utilized for the machining. Therefore the
gross power required is:
Pc
Pg 
e
where e is the efficiency of the machine tool.
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Material Removal Processes
Machining
• Analysis
– Specific Energy (u)
• Power required to machine unit volume of material in one unit
of time (power per unit volume rate of metal cut)
Pc
u
MRR
where MRR is the material removal rate (volume removed in
one unit of time).
Unit for u is generally J/mm3.
Computation of MRR requires a simple investigation of the
machining process involved. MRR for turning and milling will
be derived later.
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Material Removal Processes
Machining
• Analysis
– General Conclusions:
• Increasing the rake angle increases the shear plane
angle.
• Increase in shear plane angle means smaller shear
plane and thus smaller shear force.
High shear angle
Small shear angle
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Material Removal Processes
Machining
• Analysis
– General Conclusions:
• However increasing the rake angle decreases the tool
strength. If strong tool are required, negative rake
angles may be preferred.
• Specific energy for a work material under specific
cutting conditions is defined. That is if MRR is known,
power requirement can be inferred directly.
• Approximately 98% of the energy in machining is
converted into heat. This can cause temperatures to be
very high at the tool-chip interface.
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References
K. Tur, Manufacturing Processes Lecture Notes, 2004
E. Kılıç, Manufacturing Technologies, Lecture Notes, 2005
E. Kılıç, Manufacturing Engineering, Lecture Notes, 2005
and
http://www.diamond-mold.com/edm.php
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