Transcript Document 7131533

Chapter 20
Fundamentals of
Machining/Orthogonal
Machining
(Review)
EIN 3390
Manufacturing Processes
Spring, 2011
20.2 Fundamentals
Variables in Processes of Metal Cutting:
• Machine tool selected to perform the processes
• Cutting tool (geometry and material)
• Properties and parameters of workpiece
• Cutting parameters (speed, feed, depth of cut)
• Workpiece holding devices (fixture or jigs)
FIGURE 20-1
The
fundamental
inputs and
outputs to
machining
processes.
20.2 Fundamentals
7 basic chip formation processes:
shaping,
turning,
milling,
drilling,
sawing,
broaching,
grinding (abrasive)
FIGURE 20-2 The
seven basic
machining
processes used in
chip formation.
20.2 Fundamentals
Responsibilities of Engineers
Design (with Material) engineer:
• determine geometry and materials of products to
meet functional requirements
Manufacturing engineer based on material decision:
• select machine tool
• select cutting-tool materials
• select workholder parameters,
• select cutting parameters
20.2 Fundamentals
Cutting Parameters
Speed (V): the primary cutting motion, which relates the
velocity of the cutting tool relative to the workpiece.
For turning: V = p(D1 Ns) / 12
where, V – feet per min, Ns – revolution per min (rpm),
diameter of surface of workpiece, in.
D1
Feed (fr): amount of material removed per revolution or
per pass of the tool over the workpiece. In turning, feed is
in inches per revolution, and the tool feeds parallel to the
rotational axis of the workpiece.
Depth of Cut (DOC): in turning, it is the distance that the
tool is plunged into the surface.
DOC = 0.5(D1 – D2) = d
FIGURE 20-3 Turning a
cylindrical workpiece on a
lathe requires you to
select the cutting speed,
feed, and depth of cut.
20.2 Fundamentals
Cutting Tool is
a most critical component
used to cut the work piece
selected before actual values for speed and feeds are
determined.
Figure 20-4 gives starting values of cutting speed, feed for a
given depth of cut, a given work material, and a given
process (turning).
Speed decreases as DOC or feed increase
Cutting speed increases with carbide and coatedcarbide tool material.
FIGURE 20-4 Examples of a table for selection of speed and feed for turning. (Source: Metcut’s
Machinability Data Handbook.)
20.2 Fundamentals
To process different metals, the input parameters to the
machine tools must be determined.
For the lathe, the input parameters are DOC, feed, and the
rpm value of the spindle.
Ns = 12V / (p D1) = ~ 3.8 V/ D1
Most tables are arranged according to the process being
used, the material being machined, the hardness, and the
cutting-tool material.
The table in Figure 20-4 is used only for solving turning
problems in the book.
20.2 Fundamentals
DOC is determined by the amount of metal removed per
pass.
Roughing cuts are heavier than finishing cuts in terms of
DOC and feed and are run at a lower surface speed.
Once cutting speed V has been selected, the next step is to
determine the spindle rpm, Ns.
Use V, fr and DOC to estimate the metal removal rate for the
process, or MRR.
MRR = ~ 12V fr d
where d is DOC (depth of cutt).
MRR value is ranged from 0.1 to 600 in3/min.
20.2 Fundamentals
MRR can be used to estimate horsepower needed to
perform cut.
Another form of MRR is the ratio between the volume of
metal removed and the time needed to remove it.
MRR = (volume of cut)/Tm
Where Tm – cutting time in min. For turning,
Tm = (L + allowance)/ fr Ns
where L – length of the cut. An allowance is usually
added to L to allow the tool to enter and exit the cut.
MRR and Tm are commonly referred to as shop equations
and are fundamental as the processes.
20.2 Fundamentals
One of the most common is turning:
workpiece is rotated and cutting tool removes
material as it moves to the left after setting a
depth of cut.
A chip is produced which moves up the face of
the tool.
FIGURE 20-5 Relationship of
speed, feed, and depth of cut in
turning, boring, facing, and
cutoff operations typically done
on a lathe.
20.2 Fundamentals
Milling:
A multiple-tooth process.
Two feeds: the amount of metal an individual tooth
removes, called the feed per tooth ft, and the rate at
which the table translates pass the rotating tool, called
the table feed rate fm in inch per min.
fm = ft n Ns
where n – the number of teeth in a cutter, Ns – the rpm
value of the cutter.
Standard tables of speeds and feeds for milling provide
values for the recommended cutting speeds and feeds and
feeds per tooth, fr.
FIGURE 20-6 Basics
of milling processes
(slab, face, and end
milling) including
equations for cutting
time and metal
removal rate (MRR).
FIGURE 20-7 Basics of the drilling (hole-making)
processes, including equations for cutting time and
metal removal rate (MRR).
FIGURE 20-9 (a) Basics of the
shaping process, including
equations for cutting time (Tm ) and
metal removal rate
(MRR). (b) The relationship of the
crank rpm Ns to the cutting velocity
V.
FIGURE 20-10 Operations and machines used for machining cylindrical surfaces.
FIGURE 20-10 Operations and machines used for machining cylindrical surfaces.
FIGURE
20-11
Operatio
ns and
machine
s used to
generate
flat
surfaces.
20.3 Energy and Power in Machining
Power requirements are important for proper
machine tool selection.
Cutting force data is used to:
properly design machine tools to maintain
desired tolerances.
determine if the workpiece can withstand
cutting forces without distortion.
Cutting Forces and Power

Primary cutting force Fc: acts in the direction of the cutting
velocity vector. Generally the largest force and accounts for
99% of the power required by the process.

Feed Force Ff :acts in the direction of tool feed. The force
is usually about 50% of Fc but accounts for only a small
percentage of the power required because feed rates are
small compared to cutting rate.

Radial or Thrust Force Fr :acts perpendicular to the
machined surface. in the direction of tool feed. The force is
typically about 50% of Ff and contributes very little to the
power required because velocity in the radial direction is
negligible.
FIGURE 20-12 Oblique
machining has three measurable
components of forces acting on
the tool. The forces vary with
speed, depth of cut, and feed.
FIGURE 20-12 Oblique
machining has three measurable
components of forces acting on
the tool. The forces vary with
speed, depth of cut, and feed.
Cutting Forces and Power
Power = Force x Velocity
P = Fc . V (ft-lb/min)
Horsepower at spindle of machine is:
hp = (FcV) / 33,000
Unit, or specific, horsepower HPs:
HPs = hp / (MRR) (hp/in.3/min)
In turning, MRR =~ 12VFrd, then
HPs = Fc / 396,000Frd
This is approximate power needed at the spindle to remove a
cubic inch of metal per minute.
Cutting Forces and Power
Specific Power
Used to estimate motor horsepower required
to perform a machining operation for a
given material.
Motor horsepower HPm
HPm = [HPs . MRR . (CF)]/E
Where E – about 0.8, efficiency of machine to overcome friction
and inertia in machine and drive moving parts; MRR –
maximum value is usually used; CF – about 1.25, correction
factor, used to account for variation in cutting speed, feed, and
rake angle.
Cutting Forces and Power
Primary cutting force Fc:
Fc =~ [HPs . MRR . 33,000]/V
Used in analysis of deflection and vibration
problems in machining and in design of
workholding devices.
In general, increasing the speed, feed,
depth of cut, will increase power
required.
In general, increasing the speed doesn’t
increase the cutting force Fc. Speed has
strong effect on tool life.
Cutting Forces and Power
Considering MRR =~ 12Vfrd, then
dmax =~ (HPm . E)/[12 . HPs V Fr (CF)]
Total specific energy (cutting stiffness) U:
U = (FcV)/(V fr d) = Fc/(fr . d) =Ks (turning)
20.6 Mechanics of Machining
(statics)
Assume that the result force R acting on the back
of the chip is equal and opposite to the resultant
force R’ acting on the shear plane.
R is composed of friction force F and normal
force N acting on tool-chip interface contact area.
R’ is composed of a shear force Fs and normal
force Fn acting on the shear plane area As.
R is also composed of cutting force Fc and tangential
(normal) force Ft acting on tool-chip interface contact
area. Ft = R sin (b - a)
FIGURE 20-20
Free-body diagram
of orthogonal chip
formation process,
showing equilibrium
condition
between resultant
forces R and R.
20.6 Mechanics of Machining
(statics)
Friction force F and normal force are:
F = Fc sina + Ft cosa ,
N = Fc cosa + Ft sina, and
b = tan-1 m = tan-1 (F/N),
Where m - force F and friction coefficient, and b – the angle between
normal force N and resultant R. If a = 0, then F = Ft , and N = Fc .
in this case, the friction force and its normal can be directly
measured by dynamometer.
R = SQRT (Fc2 + Ft2 ),
Fs = Fc cosf - Ft sinf , and
Fn = Fc sinf + Ft cosf,
Where Fs is used to compute the shear stress on the shear plane
20.6 Mechanics of Machining
(statics)
Shear stress:
ts = Fs/As,
Where As - area of the shear plane,
As = (t w)/sinf
Where t – uncut ship thickness and w – width of workpiece.
ts = (Fcsinf cosf - Ft sin2f )/(tw) psi
for a given metal, shear stress is not sensitive to variations
in cutting parameters, tool meterial, or cutting
environment.
Fig. 20-22 shows some typical values for flow stress for a
variety of metals, plotted against hardness.
20.7 Shear Strain g & Shear Front
Angle f
Use Merchant’s chip formation model, a new “stack-ofcards” model as shown in fig. 20-23 is developed. From
the model, strain is:
g = cosa/[sin(f + y) cos(f + y -a)]
where f - the angle of the onset of the shear plane, and y the shear front angle.
The special shear energy (shear energy/volume) equals
shear stress x shear strain:
Us = t .g
20.7 Shear Strain g & Shear Front Angle f
Use minimum energy principle, where will y take on
value (shear direction) to reduce shear energy to a
minimum:
d(Us)/dy = 0,
Solving the equation above,
y = 450 - y + a/2 , and
g = 2cosa/(1 + sina),
It shows the shear strain is dependent only on the rake
angle a.
20.8 Mechanics of Machining
(Dynamics)
Machining is a dynamic process of large strain
and high strain rate.
The process is a closed loop interactive
processes as shown on fig. 20-24.
FIGURE 20-24 Machining
dynamics is a closed-loop
interactive process that creates
a force-displacement response.
20.8 Mechanics of Machining (Dynamics)
Free vibration is the response to any initial condition or
sudden
change.
The
amplitude
of
the
vibration
decreases with time and occurs at the natural frequency
of the system.
Forced vibration is the response to a periodic
(repeating with time) input. The response and input occur
at the same frequency. The amplitude of the vibration
remains constant for set input condition and is linearly
related to speed
20.8 Mechanics of Machining (Dynamics)
Self-excited vibration is the periodic response to the
system to a constant input. The vibration may grow in
amplitude and occurs near natural frequency of the
system regardless of the input.
Chatter due to the
regeneration of waviness in the machining surface is the
most common metal cutting example.
20.8 Mechanics of Machining (Dynamics)
Factors affecting on the stability of machining
Cutting stiffness of workpiece material (machinability),
Ks
Cutting –process parameters (speed, feed, DOC,
total width of chip)
Cutter geometry (rake asd clearance angles, insert
size and shape)
Dynamic characteristics of the machining process
(tooling, machining tool, fixture, and workpiece)
20.8 Mechanics of Machining (Dynamics)
Chip formation and regenerative Chatter
In machining, chip is formed due to shearing of
workpiece material over chip area (A = t x w), which
results in a cutting force.
Magnitude of the resulting cutting force is predominantly
determined by the material cutting stiffness Ks and the
chip area such that F c = Ks t w.
The direction of the cutting force Fc in influenced mainly
by the geometries of rack and clearance angles and
edge prep.
FIGURE 20-27 When the
overlapping cuts get out of
phase with each other, a variable
chip thickness is produced,
resulting in a change in Fc on the
tool or workpiece.
20.8 Mechanics of Machining (Dynamics)
Factors Influencing Chatter:
Cutting stiffness Ks
Speed
FEED
DOC: The primary cause and control of chatter.
Total width of chip
Back rack angle
Clearance angle
Size (nose radius), shape (diamond, triangular,
square, round) and lead angle of insert
Effects of Temperature

Energy dissipated in cutting is converted to
heat, elevating temperature of chip, workpiece, and
tool.

As speed increases, a greater percentage of
the heat ends up in the chip.

Three sources of heat:
◦ Shear front.
◦ Tool-chip interface contact region.
◦ Flank of the tool.
FIGURE 20-31 Distribution of
heat generated in machining to
the chip, tool, and workpiece.
Heat going to the environment
is not shown. Figure based on
the work of A. O. Schmidt.
FIGURE 20-32 There are three main sources of heat in metal cutting. (1) Primary shear zone. (2)
Secondary shear zone tool–chip (T–C) interface. (3) Tool flank. The peak temperature occurs at the
center of the interface, in the shaded region.
FIGURE 20-32 There are three main sources of heat in metal cutting. (1) Primary shear zone. (2)
Secondary shear zone tool–chip (T–C) interface. (3) Tool flank. The peak temperature occurs at the
center of the interface, in the shaded region.
Effects of Temperature

Excessive temperature affects
◦ Strength, hardness and wear resistance of
cutting tool.
◦ Dimensional stability of the part being
machined.
◦ Machined surface properties due to thermal
damage
◦ Machine tool, if too excessive.
FIGURE 20-33 The typical relationship of temperature at the tool–chip interface to cutting
speed shows a rapid increase. Correspondingly, the tool wears at the interface rapidly with
increased temperature, often created by increased speed.