Download - Turning, milling
Mustafizur Rahman
Dept. of Mechanical Engineering
NUS
ME 3162
MANUFACTURING PROCESSES
(II)
Topics
1. Introduction to machining and machine tools
2. Tool materials
3. Tool life and tool wear
4. Introduction to rapid prototyping
5. Introduction to laser cutting
References:
1. Fundamentals of metal machining and machine tools by Geoffrey
Boothroyd, McGraw-Hill Book Company
2. Metal cutting theory and practice by A. Bhattacharya, Central
Book Publishers, India
Part 1:
Introduction to Machining and Machine Tools
ME 3162 (II)
Methods to shape a component
1. By putting materials together (+)
2. By moving material from one region to another (0)
3. By removing unnecessary material (-)
1. By putting materials together (+)
E.g. Welding, Rapid Prototyping
Methods to generate a required shape
Methods to generate a required shape
2. By moving material from one region to another (0)
E.g. Rolling, Forging
Forging
3. By removing unnecessary material (-)
E.g. Turning, Milling, drilling
Methods to generate a required shape
Material removal - metal cutting
Metal cutting is an important shaping process whereby the
component shape and size is generated by removing excess material
from the original workpiece by a cutting tool which interferes with,
and moves relative to, the workpiece.
tool
workpiece
Three basic elements:1. The cutting tool
2. The workpiece
3. The machine tool
E.g. Turning and lathe.
Axi-symmetrical parts are
generated on the workpiece
(Element 2) by the turning tool
(Element 1) using the lathe
(Element 3).
Material removal - metal cutting
(Turning)
Three basic elements required:1. The cutting tool for removal of excess material
Material removal - metal cutting (Milling)
2. The workpiece or component to be shaped
3. The machine tool which supports the tool and the workpiece and
provides relative motion, power and associated force to sustain
the interference (cut) and generate the component shape and size.
Three basic elements:1. The cutting tool
2. The workpiece
3. The machine tool
E.g. Milling and machine
Prismatic components are
generated on the workpiece
(Element 2) by the milling
tool (Element 1) using the
milling machine (Element 3)
Material removal - metal cutting
Peripheral Milling Operations
In peripheral milling, the rotation direction
of the cutter distinguishes two forms of
milling
• Upmilling
• Downmilling
Upmilling
• Also referred as conventional milling.
• The direction of motion of the cutter teeth is opposite to
the feed direction when the teeth cut into the work.
• The resultant cutting force acts upwards.
• Tendency is to lift the work piece off the table.
• The undeformed chip thickness is minimum at the start
of the cut and maximum at the end of the cut
• Undeformed chip thickness is the thickness of the layer of
material removed by one cutting edge in one pass.
• Reduced tool life and surface finish.
chip thickness
Downmilling
• Also referred as climb milling.
• The direction of motion of the cutter teeth is in the
feed direction when the teeth cut into the work.
• The resultant cutting force acts downwards.
• The undeformed chip thickness is maximum at the start
of the cut and minimum at the end of the cut
• Fixtures and holding devices are simpler and less costly.
• Less tool wear
• Chip disposal is easier and better surface finish
Material removal - metal cutting (Drilling)
Three basic elements:1. The cutting tool
2. The workpiece
3. The machine tool
E.g. Drilling and machine
A circular cylindrical hole is
generated in the workpiece
(Element 2) by the twist drill
(Element 1) using the drilling
machine (Element 3).
A CNC machine has a tool carousel to store tools which are
automatically located and picked by an automated mechanical arm for
each operation so that multiple operations can be performed on a single
workpiece in a single setup.
Material removal - metal cutting (Multiple operations)
Complex geometric features and multiple operations
Video Show
Machining is an important process because it:
1. Can produce a very wide variety of shapes and sizes of component
2. Can produce shapes and sizes with high dimensional accuracy and
very good surface finish
3. Can be computer controlled and automated
Material removal - metal cutting
If possible, avoid machining; otherwise, minimize the amount of
machining required on the parts, e.g. machining of reference surfaces
on cast parts.
Need for machining includes close tolerances, good surface finish,
special geometric features such as threads, precision holes, cylindrical
sections with high degree of roundness, etc.
Objectives of Machining Processes:
1. The process must be physically feasible, i.e. it should be
possible to remove material from the component by cutting
and arrive at the desired shape, size and surface finish.
2. The process must be technologically as efficient and
economical as possible in producing components
3. The process must be capable of competing with other
manufacturing (shaping) process in producing components.
Satisfying objectives 1 and 2 assist in making machining
more competitive. Objective 3 attempts to ensure that
machining is considered in the light of broader spectrum of
manufacturing processes.
Performance Measures and Criteria for
Assessing Machining Objectives:
1. Tool life, or time to tool failure, is required to be ‘infinite’
(ideally) or maximum (in practice).
2. Forces and power are required to be ‘nil’ or minimum.
3. Shape and size variation are required to be ‘nil’ or minimum.
4. Surface finish/roughness are required to be ‘nil’ or minimum
5. Component production rate are required to be ‘infinite’ or
maximum.
1. Tool material properties – chemical, physical, etc.
2. Tool geometry – rake angle, clearance angle, etc.
3. Cut geometry (thickness, width, shape).
4. Cutting speed.
• Machine tool variables – rigidity
• Cutting fluid used
• Cost and time variables, component dimensions
Variables affecting machining performance
1. Tool material properties – chemical, physical, etc.
2. Tool geometry – rake angle, clearance angle, etc.
3. Cut geometry (thickness, width, shape).
4. Cutting speed.
• Machine tool variables – rigidity
• Cutting fluid used
• Cost and time variables, component
dimensions
Variables affecting machining performance
kr
ap
ap : depth of cut
kr : major cutting edge angle
Cutting motions
Primary motion (-C)
Seconday feed motion (-Z)
Two directions of motions: rotation of workpiece about its axis and feed of tool parallel to its axis
Rotational motion of the workpiece at V relative to the tool:
dnVw
Feed of tool at Vf relative to the
workpiece.
Cutting motions
VeV
Vf
d
where nw, rotational speed of spindle
and d is diameter of the workpiece
t
Undeformed chip thickness
ac ap
Back engagement
(depth of cut)
f
Feed engagement
Major cutting edge
angle kr
Tool and cut geometries
ac = f sin k r
ac
f
kr
dnVw
2
mww
av
ddnV
where nw, rotational speed of spindle
dw, original diameter
dm, machined diameter
Tool and cut geometries
nw
Seconday feed motion (-Z)
Chip cross-section area Ac of the layer of the material being removed is
approximately given by
Ac = f ap
where f is the feed per revolution
f = Vf /nw
f
Tool and cut geometries
dw = dm + 2ap
Material removal rate:
f
Tool and cut geometries
nw
Secondary feed motion (-Z)
)(
2
pmwp
mwwp
avp
avcw
adnfa
ddnfa
Vfa
V AZ
Machining Time
tm= (lw / f.nw) [nw = (V /π .dw)]
= (lw / f ) x ( π x dw / V)
Removal rate and machining time
mwppmwpw dnafadnafZ ....).(...
Material removal rate
When the depth of cut ap is small compared to the diameter of the machined surface dm,
Power required
If energy required to remove unit volume of material, is ps; then power Pm required to perform any machining operation,
Pm= ps Zw
Electrical power consumed
If efficiency of the machine tool motor and drive systems is m, the electrical power Pe consumed by the machine tool,
Pe = Pm /m
Power required
Other turning operations
Vertical Milling
where Vf is the feed speed
nt is rotational frequency
N is number of teeth
t
f
n
Vf
Feed per revolution
N
fa f Feed per tooth
Vf
nt
ap
acma
x
dt
Vf
Vertical Milling
Vertical Milling
Vertical Milling
If tool axis is aligned to
that of the workpiece the
maximum undeformed
chip thickness
acmax = af
dt /2 dt /2lw
where is given by,
af
acmax
sin
sinmax
N
f
aa fc
t
e
t
et
d
a
d
ad
21
2/
2/cos
The maximum undeformed chip
thickness acmax (measured
normal to the direction of
primary motion
(dt/2
) -
ae
Vertical Milling
For small ae / dt
2
2
)/(/2
cos1sin
tete dada
)1(2
sinmax
t
e
t
e
t
f
c
d
a
d
a
Nn
V
N
fa
t
e
t
f
cd
a
Nn
Va
2max
t
f
n
Vf
af
acmax
(dt/2
) -
ae
Material removal rate
fpew VaaZ
where ae is the depth of cut and ap is the width of
the workpiece
Vertical Milling
t
e
d
a21cos
Travelling distance is given by
)(2 etew adal
f
etew
mV
adalt
)(2
(dt/2
) -
ae
Vertical Milling
Thus, the machining time
2)/(/2)2/(sin)2/( tetett dadadd
where, lw is the length of workpiece
Drilling
rc kf
a sin2
t
wm
nf
lt
.
Drilling
Most common is a twist drill which has two
cutting edges. On each edge,
where kr, is the major cutting edge angle
The machining time tm is given by
where lw is the length of the drilled hole and
nt is the rotational frequency of the tool
dm
kr
If an existing hole of diameter dw is
enlarged to dm
4
).( 22
twm
w
nddfZ
4
....
4
22 tm
fmw
ndfVdZ
Drilling
dm
kr
Metal removal rate Zw (cross-sectional area
of hole feed speed Vf)