Chapter 22 - MACHINING OPERATIONS AND MACHINE TOOLS:

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ME 350 Lecture 5 Chapter 22 23

• Chapter 22 - MACHINING OPERATIONS AND MACHINE
  TOOLS
• Turning and Related Operations
• Drilling and Related Operations
• Milling
• Machining Centers and Turning Centers
• Broaching

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Turning

• Single point cutting tool removes material from a
  rotating workpiece to generate a cylinder
• Performed on a machine tool called a
• Variations of turning performed on a lathe
• Facing
• Contour turning
• Chamfering
• Cutoff
• Threading

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Chamfering
Cutoff

• Cutting edge cuts an angle on the corner of the
  cylinder, forming a "chamfer"

Tool is fed radially into rotating work at some
location to cut off end of part
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Facing
Contour Turning
Threading
Tool is fed radially

• Tool follows a contour that is other than
  straight

Pointed form tool is fed at a large feed rate,
thus creating threads
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Engine Lathe
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Turret Lathe

• Tailstock replaced by turret that holds up to
  six tools
• Tools rapidly brought into action by indexing the
  turret
• Tool post replaced by four-sided turret to index
  four tools
• Applications high production work that requires
  a sequence of cuts on the part

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Multiple Spindle Bar Machines

• More than one spindle, so multiple parts machined
  simultaneously by multiple tools. Example
• After each machining cycle, spindles (including
  collets and workbars) are indexed (rotated) to
  next position

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Boring

• Difference between boring and turning
• Boring is performed on the _________ diameter of
  an existing hole
• Turning is performed on the _________ diameter of
  an existing cylinder
• Boring machines
• Horizontal or vertical - refers to the
  orientation of the machine spindles

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Reaming
Tapping

• Slightly enlarges a hole
• Provides better
• Improves

Used to provide internal screw threads on an
existing hole. Tool called a
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Radial Drill

• Large drill press designed for large parts

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Milling

• Machining operation in which work is fed past a
  rotating tool with multiple cutting edges
• Axis of tool rotation is perpendicular to feed
• Two forms

(a) (b)
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Slab Milling
Slotting

• Basic form of peripheral milling in which the
  cutter width extends beyond the workpiece on both
  sides

Width of cutter is less than workpiece width,
creating a slot in the work
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Face Milling
End Milling
Profile Milling
Cutter diameter is less than work width, so a
slot is cut into part

• Cutter overhangs work on both sides

Form of end milling in which the outside
periphery of a flat part is cut
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Machining Centers

• Highly automated machine tool can perform
  multiple machining operations under CNC control
  in one setup with minimal human attention
• Typical operations are
• Other features
• Automatic tool changing
• Pallet shuttles
• Automatic workpart positioning

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Mill-Turn Centers

• Highly automated machine tool that can perform
  the operations

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Broaching

• Moves a multiple tooth cutting tool linearly
  relative to work in direction of tool axis

Examples of internal broaching
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• Chapter 23 CUTTING TOOL TECHNOLOGY
• Tool Life
• Tool Geometry
• Cutting Fluids

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Three Modes of Tool Failure

1. Cutting force is excessive and/or dynamic,
   leading to brittle fracture
2. Cutting temperature is too high for the tool
   material
3. Preferred wearing of the cutting tool

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Preferred Mode

• Longest possible tool life, wear locations
• Crater wear location
• Flank wear location

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Tool Wear vs. Time

• Tool wear as a function of cutting time. Flank
  wear (FW) is used here as the measure of tool
  wear. Crater wear follows a similar growth curve.

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Effect of Cutting Speed

• Effect of cutting speed on tool flank wear (FW)
  for three cutting speeds, using a tool life
  criterion of 0.50 mm flank wear.

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Tool Life vs. Cutting Speed

• Log-log plot of cutting speed vs tool life.

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Taylor Tool Life Equation

• where v cutting speed
• T tool life and
• n and C are parameters that depend on feed,
  depth of cut, work material, and tooling
  material, but mostly on material (work and tool).
• n is the
• C is the _______ on the speed axis at one
  minute tool life

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Example Problem

• A tool run at 160m/min lasts for 5 min. If the
  tool is run at 100m/min it lasts for an average
  of 41 min.
• What is C and n?

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Tool Near End of Life

• Changes in sound emitted from operation
• Chips become ribbon-like, stringy, and difficult
  to dispose of
• Degradation of surface finish
• Increased power required to cut
• Visual inspection of the cutting edge with
  magnifying optics can determine if tool should be
  replaced

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Desired Tool Properties

• Toughness - to avoid fracture failure
• Hot hardness - ability to retain hardness at high
  temperatures
• Wear resistance - hardness is the most important
  property to resist abrasive wear

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Hot Hardness

• Plain carbon steel shows a rapid loss of
  hardness as temperature increases. High speed
  steel is substantially better, while cemented
  carbides and ceramics are significantly harder at
  elevated temperatures.

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Typical Values of n and C

• Tool material n C (m/min) C (ft/min)
• High speed steel
• Non-steel work 0.125 120 350
• Steel work 0.125 70 200
• Cemented carbide
• Non-steel work 0.25 900 2700
• Steel work 0.25 500 1500
• Ceramic
• Steel work 0.6 3000 10,000

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Tool Geometry

• Two categories
• Single point tools
• Used for turning, boring, shaping, and planing
• Multiple cutting edge tools
• Used for drilling, reaming, tapping, milling,
  broaching, and sawing

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Single-Point Tool Geometry
(a) Seven elements of single-point tool geometry
and (b) the tool signature convention that
defines the seven elements.
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Holding the Tool

• Three ways of holding and presenting the cutting
  edge for a single-point tool (a) solid tool
  (typically HSS) (b) brazed cemented carbide
  insert, and (c) mechanically clamped insert, used
  for cemented carbides, ceramics, and other very
  hard tool materials.

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Common Insert Shapes

• Common insert shapes (a) round, (b) square, (c)
  rhombus with two 80? point angles, (d) hexagon
  with three 80? point angles, (e) triangle
  (equilateral), (f) rhombus with two 55? point
  angles, (g) rhombus with two 35? point angles.

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Twist Drills

• The most common cutting tool for hole-making
• Usually made of high speed steel

Standard geometry of a twist drill.
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Twist Drill Issues

• Along radius of cutting edges cutting speed
• Relative velocity at drill point is _______, (no
  cutting takes place) a large thrust force must
  deform the material
• Problems
• Flutes must provide sufficient clearance to allow
  chips to be extracted
• Rubbing between outside diameter of drill bit and
  hole. Delivery of cutting fluid to drill point
  is difficult because chips are flowing in
  opposite direction