ME 350 – Lecture 5 – Chapters 23 & 24

1
ME 350 Lecture 5 Chapters 23 24

• Ch 23 - CUTTING TOOL TECHNOLOGY
• Ch 24 - ECONOMIC AND PRODUCT DESIGN
  CONSIDERATIONS IN MACHINING

2
Three Modes of Tool Failure

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

3
Preferred Mode

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

4
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.

5
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.

6
Tool Life vs. Cutting Speed

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

7
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

8
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

9
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

10
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.

11
Coated Carbide Tool
Photomicrograph of cross section of multiple
coatings on cemented carbide tool (photo courtesy
of Kennametal Inc.)
12
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

13
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

14
Single-Point Tool Geometry
(a) Seven elements of single-point tool geometry
and (b) the tool signature convention that
defines the seven elements.
15
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.

16
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.

17
Twist Drills

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

Standard geometry of a twist drill.
18
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

19
Cutting Fluids (Lubricants and Coolants)

• Function is to improve cutting performance
• Improve chip
• Reduce
• Improve surface
• Types of cutting fluids
• Generally water based
• more effective at cutting speeds that are
• Generally oil based
• more effective at cutting speeds that are

20
Machinability Criteria in Production

• Tool life longer tool life for the given work
  material means better machinability
• Forces and power lower forces and power mean
  better machinability
• Surface finish better finish means better
  machinability
• Ease of chip disposal easier chip disposal
  means better machinability

21
Tolerances and Surface Finish

• Tolerances
• Machining provides high accuracy relative to most
  other shape-making processes
• Closer tolerances usually mean higher costs
• Surface roughness in machining determined by
• Geometric factors of the operation
• Work material factors
• Vibration and machine tool factors

22
Effect of Cutting Conditions
End Cutting Nose Radius Feed
Edge Angle
23
Ideal Surface Roughness

• where
• Ri theoretical arithmetic average surface
  roughness
• f feed
• NR nose radius

24
Work Material Factors

• Built-up edge effects
• Damage to surface caused by chip
• Tearing of surface when machining ductile
  materials
• Cracks in surface when machining brittle
  materials
• Friction between tool flank and new work surface

25
Effect of Work Material Factors
To predict actual surface roughness, first
compute ideal surface roughness, then multiply by
the ratio from the graph
26
Vibration and Machine Tool Factors

• Related to machine tool, tooling, and setup
• Chatter (vibration) in machine tool or cutting
  tool
• Deflections of fixtures
• Backlash in feed mechanism
• If chatter can be eliminated, then surface
  roughness is determined by geometric and work
  material factors

27
How To Avoid Chatter

• Add stiffness and/or damping to setup
• Operate at speeds that avoid frequencies close to
  natural frequency of machine tool system
• Reduce feed (and sometimes depth)
• Change cutter design
• Use a cutting fluid

28
Determining Feed

• Select feed first, speed second
• Determining feed rate depends on
• Tooling harder tool materials require lower
  feeds
• Is the operations roughing or finishing?
• Constraints on feed in roughing
• Limits imposed by forces, setup rigidity, and
  sometimes horsepower
• Surface finish requirements in finishing
• Select feed to produce desired finish

29
Optimizing Cutting Speed

• Select speed to achieve a balance between high
  metal removal rate and suitably long tool life
• Mathematical formulas available to determine
  optimal speed
• Two alternative objectives in these formulas
• Maximum
• Minimum

30
Maximum Production Rate

• Maximizing production rate minimizing cutting
  time per unit
• In turning, total production cycle time for one
  part consists of
• Part handling time per part Th
• Machining time per part Tm
• Tool change time per part Tt/np, where np
  number of pieces cut in one tool life (round
  down)
• Total time per unit product for operation
• Tc Th Tm Tt/np

31
Cycle Time vs. Cutting Speed
32
Minimizing Cost per Unit

• In turning, total production cycle cost for one
  part consists of
• Cost of part handling time CoTh , where Co
  cost rate for operator and machine
• Cost of machining time CoTm
• Cost of tool change time CoTt/np
• Tooling cost Ct/np , where Ct cost per
  cutting edge
• Total cost per unit product for operation
• Cc CoTh CoTm CoTt/np Ct/np

33
Unit Cost vs. Cutting Speed
34
Comments on Machining Economics

• As C and n increase in Taylor tool life equation,
  optimum cutting speed
• Cemented carbides and ceramic tools, compared to
  HSS, should be used at speeds
• vmax is always greater than vmin
• Reason Ct/np term in unit cost equation pushes
  optimum speed to left in the plot
• As tool change time Tt and/or tooling cost Ct
  increase, cutting speed should be reduced
• Disposable inserts have an advantage over
  regrindable tools if tool change time is
  significant

35
Product Design Guidelines

• Design parts that need no machining
• Use net shape processes such as precision
  casting, closed die forging, or plastic molding
• If not possible, then minimize amount of
  machining required
• Use near net shape processes such as impression
  die forging

36
Product Design Guidelines

• Machined features such as sharp corners, edges,
  and points should be avoided
• They are difficult to machine
• Sharp internal corners require pointed cutting
  tools that tend to break during machining
• Sharp corners and edges tend to create burrs and
  are dangerous to handle