Chapter 20 Fundamentals of Machining/Orthogonal Machining (Part I Review) EIN 3390 Manufacturing Processes Spring, 2012 - PowerPoint PPT Presentation

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Chapter 20 Fundamentals of Machining/Orthogonal Machining (Part I Review) EIN 3390 Manufacturing Processes Spring, 2012

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... depth of cut) Workpiece holding devices (fixture or jigs) ... For the lathe, the input parameters are DOC, feed, and the rpm value of the spindle. – PowerPoint PPT presentation

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Title: Chapter 20 Fundamentals of Machining/Orthogonal Machining (Part I Review) EIN 3390 Manufacturing Processes Spring, 2012


1
Chapter 20Fundamentals of
Machining/Orthogonal Machining(Part I Review)
EIN 3390 Manufacturing ProcessesSpring, 2012
2
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)

3
FIGURE 20-1 The fundamental inputs and outputs to
machining processes.
4
20.2 Fundamentals
7 basic chip formation processes shaping,
turning, milling, drilling, sawing,
broaching, grinding (abrasive) Single point
include turning, facing, boring, shaping,
planning, fly cutter milling, some modes of deep
hole drilling and other variations of lathe
operations such as cutoff, recessing plunge or
form turning. The rest of the machining
processes are multiple points and include
drilling, milling, broaching, sawing, filing and
many forms of abrasive machining.
5
FIGURE 20-2 The seven basic machining processes
used in chip formation.
6
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

7
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), D1 diameter of
surface of workpiece, in. 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
8
FIGURE 20-3 Turning a cylindrical workpiece on a
lathe requires you to select the cutting speed,
feed, and depth of cut.
9
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 coated- carbide tool material.

10
FIGURE 20-4 Examples of a table for selection of
speed and feed for turning. (Source Metcuts
Machinability Data Handbook.)
(for workpiece)
AISI for in
ISO for mm
11
FIGURE 20-4 Examples of a table for selection of
speed and feed for turning. (Source Metcuts
Machinability Data Handbook.)
(for workpiece)
AISI for in
ISO for mm
12
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.
13
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.
14
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.
15
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.
16
FIGURE 20-5 Relationship of speed, feed, and
depth of cut in turning, boring, facing,
and cutoff operations typically done on a lathe.
17
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.
18
FIGURE 20-6 Basics of milling processes (slab,
face, and end milling) including equations for
cutting time and metal removal rate (MRR).
19
FIGURE 20-7 Basics of the drilling (hole-making)
processes, including equations for cutting time
and metal removal rate (MRR).
20
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.
21
FIGURE 20-10 Operations and machines used for
machining cylindrical surfaces.
22
FIGURE 20-10 Operations and machines used for
machining cylindrical surfaces.
23
FIGURE 20-10 Operations and machines used for
machining cylindrical surfaces.
24
FIGURE 20-10 Operations and machines used for
machining cylindrical surfaces.
25
FIGURE 20-11 Operations and machines used to
generate flat surfaces.
26
FIGURE 20-11 Operations and machines used to
generate flat surfaces.
27
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28
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29
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.
30
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.

31
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.
32
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.
33
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.

34
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35
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.

36
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 doesnt increase
    the cutting force Fc. Speed has strong effect on
    tool life.

37
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)
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