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MACHINING

VIEWS: 35 PAGES: 42

									        Turning Process II



Dr. Chana Raksiri
Introduction
Tool geometry
   the designations for a right-hand cutting tool. Right-hand
    means that the tool travels from right to left.
Tool geometry
   Rake angle is important in controlling both the direction
    of chip flow and the strength of the tool tip.
   Side rake angle is more important than the back rake
    angle, although the latter usually controls the direction
    of chip flow.
   Cutting-edge angle affects chip formation, tool
    strength, and cutting forces to various degrees.
   Nose radius affects surface finish and tool-tip strength.
    The smaller the nose
   radius (sharp tool), the rougher the surface finish of the
    workpiece and the lower the strength of the tool.
Material-removal rate
   The material-removal rate (MRR) in turning is the
    volume of material removed per unit time with the units
    of mm3/min.
   d. The volume of this ring is the product of the cross-
    sectional area (f)(d) and the average circumference of
    the ring, where
                             Do  D f
                    Davg   
                                2
Material-removal rate
   Since there are N revolutions per minute, the removal
    rate is
              MRR  Davgd f N           23.1a
   Note that Eq. (23.1a) also can be written as

         MRR  d f V                         23 .1b 
    where V is the cutting speed.
   Since the distance traveled is l mm, the cutting time is
              l
          t                          23.2
             fN
Material-removal rate
   The cutting time does not include the time required for tool
    approach and retraction.
   The foregoing equations and the terminology used are
    summarized in Table 23.3.
Forces in turning
   shows the forces acting on a cutting tool in turning. Fc is
    the cutting force, Ft is the thrust or feed force (in the
    direction of feed), and Fr is the radial force that tends to
    push the tool away from the workpiece being machined.
Forces in turning
   The cutting force acts downward on the tool tip and,
    thus, tends to deflect the tool downward and the
    workpiece upward.
   The product of the cutting force and its radius from the
    workpiece center determines the torque on the spindle.
   The product of the torque and the spindle speed
    determines the power required in the turning operation.
Forces in turning
   The thrust force acts in the longitudinal direction. It also
    is called the feed force because it is in the feed direction
    of the tool.
   The radial force, acts in the radial direction and tends to
    push the tool away from the workpiece.
Roughing and finishing cuts
  In machining, the usual procedure is to first perform one
   or more roughing cuts at high feed rates and large depths-
   of-cut (and therefore high material-removal rates) but with
   little consideration of dimensional tolerance and surface
   roughness.
  These cuts then are followed by a finishing cut, at a lower
   feed and depth-of-cut in order to produce a good surface
   finish.
Tools materials, feeds and cutting speeds
   Specific recommendations regarding turning process
    parameters for various workpiece materials and cutting
    tools are given.
         Example 1 Material removal rate and cutting fluid force in turning




   A 150-mm-long, 12.5-mm-diameter 304 stainless-steel rod is
being reduced in diameter to 12.00 mm by turning on a lathe. The
spindle rotates at N = 400 rpm, and the tool is traveling at an axial
   speed of 200 mm/min. Calculate the cutting speed, material-
  removal rate, cutting time, power dissipated, and cutting force.
                              Solution
  The cutting speed is the tangential speed of the workpiece. The
maximum cutting speed is at the outer diameter, and is obtained from
                           the expression
            V  Do N

            V
                 12.5400  15.7 m/min
                     1000
    The cutting speed at the machined diameter is

             V
                 12.00400  15.1 m/ min
                      1000
     From the information given, note that the depth-of-cut is
                      200
                  f       0.5 mm/rev
                      400
and the feed is
                   12.5  12.0
                d              0.25 mm
                        2

    According to Eq. (23.1a), the material-removal rate is
                            then
  MRR   12.250.250.5400  1924 mm 3/min  2 106 m3/min
       Equation (23.1b) also can be used, where we find
MRR=(0.25)(0.5)(15.7)(1000)=2×10–6 m3/min. The actual time to cut,
                   according to Eq. (23.2), is
                          150
                    t             0.75 min
                       0.5400
  The power required can be calculated by referring to Table 21.2
  and taking an average value for stainless steel as 4 W–s/mm3.
                Therefore, the power dissipated is

                   Power 
                           41924  128 W
                                  60
  Since 1 W = 60 N-m/min, the power dissipated is 7680 N-m/min.
The cutting force, is the tangential force exerted by the tool. Power is
the product of torque, T, and the rotational speed in radians per unit
                              time; hence,


                        7680
                   T             3 .1 N - m
                      2 400

                   Fc   
                          3.11000  506 N
                           12.25 / 2
                Drilling, Drills and Drilling Machines
   When inspecting various large or small products, note that
    the vast majority have several holes in them.
   Hole making is among the most important operations in
    manufacturing, and drilling is a major and common hole-
    making process.
                                  Drills
   shows the two common types of drills:
   (a) Chisel-point drill. The function of the pair of margins is
    to provide a bearing surface for the drill against walls of
    the hole as it penetrates into the workpiece. Drills with four
    margins (double-margin) are available for improved drill
    guidance and accuracy. Drills with chip-breaker features
    also are available.
   (b) Crankshaft drill. These drills have good centering
    ability, and because chips tend to break up easily, these
    drills are suitable for producing deep holes.
Drills
                                  Drills
   The capabilities of drilling and boring operations are
    shown in Table 23.10.
                                 Drills
Twist drill
   Two spiral grooves (flutes) run the length of the drill, and
    the chips produced are guided upward through these
    grooves.
   The grooves also serve as passageways to enable the
    cutting fluid to reach the cutting edges.
   Drills are available with a chip-breaker feature ground
    along the cutting edges.
   This feature is important in drilling with automated
    machinery where a continuous removal of long chips
    without operator assistance is essential.
                               Drills
Other types of drills
  shows the Various types of drills and drilling and reaming
   operations.
                             Drills
Other types of drills
  A step drill produces holes with two or more different
   diameters.
  A core drill is used to make an existing hole larger.
  Counterboring and countersinking drills produce
   depressions on the surface to accommodate the heads of
   screws and bolts below the workpiece surface.
                               Drills
Other types of drills
  Spade drills have removable tips or bits and are used to
   produce large-diameter and deep holes.
  Fig 23.1 shows various types of drills.
                                 Drills
Gun drilling
  Developed originally for drilling gun barrels, gun drilling is
   used for drilling deep holes and requires a special drill.
  Fig 23.22(a) shows the gun drill showing various features.
   (b) Schematic illustration of the gun-drilling operation.
Drills
                    Material-removal rate in drilling
   The material-removal rate (MRR) in drilling is the volume
    of material removed by per unit time. For a drill with a
    diameter D, the cross-sectional area of the drilled hole is
    πD2/4.
   The velocity of the drill perpendicular to the workpiece is
    the product of the feed, f (the distance the drill penetrates
    per unit revolution), and the rotational speed, N, where
    N=V/πD.
   Thus,
                          Thrust force and torque
   The thrust force in drilling acts perpendicular to the hole
    axis; if this force is excessive, it can cause the drill to bend
    or break.
   An excessive thrust force also can distort the workpiece,
    particularly if it does not have sufficient stiffness (for
    example, thin sheet-metal structures), or it can cause the
    workpiece to slip into the workholding fixture.
   The thrust force depends on factors such as (a) the
    strength of the workpiece material, (b) feed, (c) rotational
    speed, (d) drill diameter, (e) drill geometry, and (f) cutting
    fluids.
         Example Material-removal rate and torque in drilling
A hole is being drilled in a block of magnesium alloy with a 10-mm
  drill bit, at feed of 0.2 mm/rev, and with the spindle running at
  Calculate the material-removal rate and the torque on the drill.
                          Solution
 The material-removal rate first is calculated from Eq. (23.3):
         102 
 MRR              0.2800  12,570 mm 3 / min  210 mm 3 / s
            4 
Referring to Table 21.2, let’s take an average unit power of 0.5
 Ws/mm3 for magnesium alloys. The power required is then



           Power  210 0.5  105 W
                               Solution
  Power is the product of the torque on the drill and the rotational
speed, which in this case is (800)(2π)/60 = 83.3 radians per second.
             Noting that W=J/s and J=Nm, we find that

                      105
                   T       1.25 N  m
                      83.8
Drilling machines
Drilling machines
                       Reaming and Reamers


    Reaming is an operation used to (a) make an existing hole
     dimensionally more accurate than can be obtained by
     drilling alone, and (b) improve its surface finish.
    The most accurate holes in workpieces generally are
     produced by the following sequence of operations:
1.   Centering
2.   Drilling
3.   Boring
4.   Reaming
                           Reaming and Reamers
   For even better accuracy and surface finish, holes may be
    burnished or internally ground and honed.
   A reamer is a multiple-cutting-edge tool with straight or
    helically fluted edges that remove very little material.
   Fig 23.26(a) shows the Terminology for a helical reamer
    and (b) Inserted-blade adjustable reamer.
   Hand reamers are straight or have a tapered end in the
    first third of their length.
Reaming and Reamers
                         Reaming and Reamers
   Various machine reamers (also called chucking reamers,
    because they are mounted in a chuck and operated by a
    machine) are available in two types:
    (1) Rose reamers have cutting edges with wide margins
    and no relief.
    (2) Fluted reamers have small margins and relief with a
    rake angle of about 5°.
   Reamers may be held rigidly (as in a chuck), or they may
    float in their holding fixtures to ensure alignment or be
    piloted in guide bushings placed above and below the
    workpiece.
                          Tapping and Taps
   Internal threads in workpieces can be produced by
    tapping. A tap is a chip-producing threading tool with
    multiple cutting teeth.
   Fig 23.27(a) shows the terminology for a tap. (b) Tapping
    of steel nuts in production.
                           Tapping and Taps
   Tapered taps are designed to reduce the torque required
    for the tapping of through holes.
   Bottoming taps are for tapping blind holes to their full
    depth.
   Collapsible taps are used in large-diameter holes; after
    tapping has been completed, the tap is collapsed
    mechanically and is removed from the hole without
    rotation.
   Chip removal can be a significant problem during tapping
    because of the small clearances involved.
                           Tapping and Taps
   Tapping may be done by hand or with machines such as
    (a) drilling machines, (b) lathes, (c) automatic screw
    machines, and (d) vertical CNC milling machines
    combining the correct relative rotation and the longitudinal
    feed.
   Tap life can be determined with the same technique used
    to measure drill life.
   Self-reversing tapping systems also have been improved
    significantly and now are in use with modern computer-
    controlled machine tools.

								
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