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					        Chapter 20

  Fundamentals of
Machining/Orthogonal
     Machining
       (Part I)
  EIN 3390   Manufacturing Processes
              Fall, 2011
20.1 Introduction
Machining is the process of removing unwanted
material from a workpiece on the form of chip.

If the material is metal, then the process is often called metal
cutting or metal removal.

US industries annually spend well over $100 billion to perform
metal removal operations because the vast majority of
manufactured products require machining at some stage in
the production ranging from relatively rough or non-precision
work, such as cleanup of casting or forging, to high-precision
work involving tolerance of 0.0001 in. or less and high-quality
finishes.
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:
    1) shaping,
    2) turning,
    3) milling,
    4) drilling,
    5) sawing,
    6) broaching, and
    7) 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),   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
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 coated-
       carbide tool material.
 FIGURE 20-4 Examples of a table for selection
 of speed and feed for turning. (Source: Metcut’s
 Machinability Data Handbook.)




(for workpiece)




 AISI
 for “in”

  ISO
  for “mm”
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 machining process 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 working 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, ft.
                        24.

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 Operations
and machines used to
generate flat surfaces.
FIGURE 20-11 Operations
and machines 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 (for turning),
 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)
HW for Chapter 20
Review Questions:
3, and 5 (page 557)

Problems (Page 558):
1. a, b, c, d
 Please use fig. 20-4 to find the required speed
    and feed rate.
3.

				
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