Materials Removal Processes Machining by nikeborome

VIEWS: 30 PAGES: 18

									Chapter Six
              Materials Removal Processes
                       (Machining)
6.1 Theory of Material Removal Processes
6.1.1 Machining Definition
Machining is a manufacturing process in which a cutting tool is used to remove excess
material from a work part so that the remaining material is the desired part shape.

6.1.2 When To Consider Machining
Reasons why machining is important commercially and technologically include the following:

   1. Machining can be applied to a wide variety of work materials.
   2. Machining can be used to generate any regular geometry.
   3. Machining can produce dimensions to very close tolerances of less than
      (0.025mm).
   4. Machining is capable of creating very smooth surface finishes.
   5. More economical if the number of parts is small.

6.1.3 Limitations and Disadvantages
Machining has some limitations or disadvantages:

   1. It produces waste material.
   2. It takes longer time compared to other methods.
   3. Depends on the cutting conditions, it can have adverse effect on surface
      quality.

6.1.4 Types of Machining and Machining Operations
   1. Cutting (Single and multiple points).
   2. Abrasive (Grinding).
   3. Non-traditional (Ultrasonic, Laser, EDM, ECM).

6.1.5 Machining Operations
There are many kinds of machining operations, each of which is capable of generating a
certain part geometry and surface texture. The differences are based on the shape formed,
cutting tool, cutting conditions, tolerances, and surface roughness produced.
The most common machining operations are the followings:

Turning                                         Shaping
Drilling                                        Planning
Reaming                                         Breaching
Boring                                          Sawing
Milling                                         Parting
Facing                                          Threading

See Fig. (6.1)




                                          Fig. (6.1)

6.1.6 Machining Variables
There are two kinds of machining variables:

   1. Independent Variables:

These include the tool material, tool shape, work part material, cutting conditions, machine
tool, and fixture.

   2. Dependent Variables:

These include the type of chip formed, force and energy consumed, wear, temperature,
surface finish, and use of cutting fluid.
6.1.7 Types of Chips
   1. Continuous chip: when relatively brittle materials are machined at low cutting
      speed, the chips often from into separated segments. (See Fig. (6.2.a)).
   2. Continuous chip: when ductile materials are cut at high speeds, long
      continuous chips are formed. (See Fig. (6.2.b))
   3. Continuous chip with built-up-edge: when machining ductile materials at low
      to medium cutting speeds, adherence of work material to the tool will occur.
      (See Fig. (6.2.c)).




                                              Fig. (6.2)


6.1.8 Cutting Tool & Temperatures
Of the total energy consumed in machining, nearly all of it (approximately 98%) is converted
into heat; this will raise the temperature of cutting tool, which will have the following effects:

   1. Strength, hardness, and wear resistance of cutting tool will
      decrease.
   2. Quality (toughness and surface finish) will decrease.
   3. Internal thermal stresses will increase causing damages.

The sources of heat are:
   1. Plastic deformation in the shearing process.
   2. The friction between the tool and the chips.

Effect of the cutting conditions on the temperature:
The cutting temperature will increase as the strength of the work piece, cutting speed, depth
of cut, feed rate, coefficient of friction increase.

6.1.9 Tool Wear
There are three possible modes by which a cutting tool can fail in machining:

   1. Fracture failure: when excessive cutting force.
   2. Temperature failure: when cutting temp- is too
      high.
   3. Gradual wears: loss of tool shape and cutting
      efficiency.

Variations in the cutting conditions such as force, temperature, tool and work piece material,
tool geometry, and cutting fluid will cause tool wear.

Types of tool wear
See Fig. (6.3)

   1. Crater wears: concave section, by the action of the
      chip sliding against the surface.
   2. Flank wears: from rubbing between newly work
      surface and the land face.
   3. Chipping (catastrophic)




                                           Fig. (6.3)

6.2 Machining Operations
6.2.1 Cutting Processes & Machine Tools For Producing
Round Shapes
There is a set of operations in which the part is rotating while it is being machined to produce
part that is round in shape.

A variety of other machining operations can produce a round shape part. See Fig. (6.4).

   •   Turning: a single-point tool removes material from the surface of a rotating
       cylindrical work piece, (e.g. straight, taper, profiling, and grooving.).
   •   Facing: produce flat surface at the end of the part.
   •   Taper turning: creating a conical geometry.
   •   Threading: creating threads in the cylinder.
   •   Drilling: producing holes in a cylinder shape.
   •   Cut off/parting: cutting the end of the part.
   •   Knurling: producing a regular crosshatched pattern in the part surface.




                                           Fig. (6.4)

6.2.2 Turning & Turning Machine
The tool is fed linearly in a direction parallel to the axis of rotation. See Fig. (6.5).
Turning is traditionally carried out on a machine tool called a lathe, which provides power to
turn the part at a given rotational speed, and to feed the tool at a specified rate and depth of
cut.
                                          Fig. (6.5)

6.2.2.1 Cutting Parameters in Turning
N: The rotational speed of the work part (rev/min)
f : Feed (mm/rev) on (in./rev).
v: Feed rate or linear speed (mm/mm) or in/mm).
DOC: Depth of cut= (D0-Df)/2 -d.
D0: initial diameter.
Df: final diameter.

Work holding methods are illustrated in Fig. (6.6).




                                          Fig. (6.6)
6.2.2.2 Turning machine (The Engine Lathe)
Fig. (6.7) is a sketch of an Engine Lathe showing its principal components.

   •     The headstock: contains the drive unit.
   •     The tailstock: to support the other end of the work part.
   •     The tool post: fastened to the cross-slide.
   •     The carriage: to feed the tool parallel to the axis of rotation.
   •     Bed - Bead screw- Chucks - Feed rod.




                                              Fig. (6.7)

6.2.2.3 The Cutting Tool
Tool selection and turning parameters are selected depending on the inputs.

Inputs

Work piece material              Toughness
Work piece geometry              Hot hardness
Lot size                         Wear resistance
Quality required                 Chemical Stability.

6.2.3 Drilling
Drilling is usually performed with a rotating cylindrical tool that has two cutting edges on its
working end. The tool is called twist drill, or drill bit. See Fig. (6.8).

The rotating drill feeds into the stationary work part to form a hole whose diameter is
determined by the drill diameter.
Drilled holes are either through holes, or blind holes. See Fig. (6.9).




                                            Fig. (6.8)




                                            Fig. (6.9)


6.2.3.1 Operations Related to Drilling

   •   Reaming: to slightly enlarge a hole. (Fig (6.10.a)).
   •   Tapping: to provide internal screw thread. (Fig. (6.10.b)).
   •   Counter boring: to provide a stepped hole. (Fig (6.10.c)).
   •   Countersinking: to provide a cone shaped stepped hole (Fig
       (6.10.d)).
   •   Centering: to drill a starting hole. (Fig. (6.10.e)).
   •   Spool facing: to provide a flat-machined surface (Fig. (6.10.f)).
                                            Fig. (6.10)

6.2.3.2 Drilling Machine (Drill Presses)
The drill press is the standard machine tool for drilling; the most basic type is the upright drill.
See Fig. (6.11).




                                                                    Fig. (6.11)

6.2.4 Milling & Milling Machines
Milling is a machining operation in which a work part is fed past a rotating cylindrical tool
with multiple cutting edges.

Milling is compared to turning and drilling by the following table:
                    Turning                 Milling`                     Drilling
Cutting tool        Single-cutting edge     Multi-cutting edge           Double-cutting edge.
Rotation            W.P is rotating         cutting tool is rotating     cutting tool is rotating
Axis of rotation    Parallel to feed        Perpendicular to feed        Parallel to feed

 6.2.4.1 Milling Cutting Conditions
 See Fig. (6.12).

       1. Cutting speed, N (RPM)
       2. Feed rate, f (mm/min)
       3. Depth of cut, d (mm)




                                           Fig. (6.12)

 Fig. (6.13) shows a cycle of impact force and thermal shock on every rotation.




                                                               Fig. (6.13)




 Milling cutter and tool geometry is shown in figure (6-14).
                                           Fig. (6.14)

6.2.4.2 Types of Milling Operations

6.2.4.2.1 Peripheral Milling (Plain Milling)

The axis of the tool is parallel to the surface being machined. (See Fig. (6.15a)).




                                           Fig. (6.15)
Types of Peripheral Milling:

   •   Slab milling: the cutter width extends beyond the work piece on
       both sides. Fig. 6.16a)).
   •   Slotting: the width of the cutter is less than the work piece width.
       Fig. (6.16b)).
   •   Side milling: the cutter machines the side of the work piece. Fig.
       (6.16c)).
   •   Straddle milling: the cutter machines the both sides of the work
       piece. Fig. (6.16d)).




                                           Fig. (6.16)

Directions of rotations (up and down milling):

   1. Up milling: (Conventional Milling), the
      direction of motion of the cutter teeth is
      opposite the fed direction. Fig. (6.17.a).
   2. Down milling: (climb milling), the
      direction of motion is the same as feed
      direction. Fig (6.17.b).




                                           Fig. (6.17)

6.2.4.2.2 Face Milling

The axis of the cutter is perpendicular to the surface being milled. (See Fig (6.15.b)).

Types of Face Milling:

   •   Conventional: the diameter of the cutter is greater than the work
       part width. (See Fig. (6.18a)).
   •   Partial: the cutter overhangs the work part on only one side. (See
       Fig. (6.18b).
   •   End milling: the cutter diameter is less than the work piece width.
       (See Fig. (6.18c).
   •   Profile: the outside periphery of a flat part is cut. (See Fig (6.18d)).
   •   Pocket Milling. (See Fig. (6.18e)).
   •   Surface contouring: a ball-nose cutter creates a three-dimensional
       surface. (See Fig. (6.18f)).




                                           Fig. (6.18)

6.2.4.3 Milling Machines
Milling machines are classified into two main categories; horizontal and vertical.
Fig. (6.19)
6.2.4.3.1 Horizontal Milling Machines

(See Fig. (6.19a)).

Characteristics of horizontal milling machines:

   1.   Horizontal spindle.
   2.   Suitable for peripheral milling.
   3.   Use stud arbor to mount milling cutters.
   4.   Use V-block, table and vice to hold a work piece. See Fig. (6.20).

6.2.4.3.2 Vertical Milling Machines

(See Fig. (6.19b)).

Characteristics of vertical milling machines:

   1.   Vertical spindle.
   2.   Suitable for face milling.
   3.   Use cullet chuck to hold a cutting tool.
   4.   Use V-blocks, table and vice to hold the work piece. (See Fig.
        (6.21)).

(Drawing from papers (Collet, table, V-block) Vertical milling, fig. (6-21).

6.2.5 Shaping & Planning
6.2.5.1 Shaping
Shaping is one of the metal removal processes in which slots, notches, keyways are deformed.
The speed motion is accomplished by moving the cutting tool along the work piece in linear
motion. (See Fig. (6.22a)).

6.2.5.2 Planning
The same as shaping, but the speed motion is accomplished by moving the work part. (See
Fig (6.22b)).
                                          Fig. (6.22)




               Fig. (6.23)                                           Fig. (6.24)

6.2.6 Grinding & Grinding Machines
Grinding is the most important of the abrasive processes, which involves material removal by
the action of hard, abrasive particles that are usually in the form of a bonded wheel. Grinding
is considered as finishing operation.

6.2.6.1 Grinding Elements
   1. The grinding wheel: consists of abrasive
      particles and bonding material, which is
      usually made of carbide, oxide, or
      nitride-based material and rotates in a
      high speed. (See Fig. (6.25)).
   2. Work piece: which is usually semi-
      finished product fixed on grinding
      machine table. (See Fig. (6.25)).

6.2.6.2 Temperature at The Work Surface
Because of the size effect, plowing and rubbing (See Fig. (6.26)) of the abrasive grits against
the work surface, the grinding process is characterized by high temperatures and high friction,
which cause in surface burns and cracks.




                                          Fig. (6.26)
6.2.6.3 Grinding Operations & Grinding Machines

  •   Surface grinding: is normally used to grind plain flat surfaces. (See
      Fig. (6.27)).
  •   Cylindrical grinding: is used for rotational parts, external and
      internal. (See Fig. (6.28)).
  •   Center less grinding: the work piece is not held between centers,
      and is often used for high production work. (See Fig (6.29), (6-30)).




                                        Fig. (6.27)




                                        Fig. (6.28)
                                        Fig. (6.29)




                                        Fig. (6.30)

A surface grinding machine with horizontal spindle and reciprocating worktable is shown in
Fig. (6.27).


Notes
   1. The chip deformed is relatively small
      because of high cutting speed and small
      depth of cut.
   2. The sparks observed in metal grinding
      are actually glowing chips because of the
      exothermic reaction of the hot chips with
      oxygen in the atmosphere.

								
To top